Chapter 11 Management of irrigated vertisols

429 Chapter 11

MANAGEMENT OF IRRIGATED VERTISOLS N. AHMAD

11.1. INTRODUCTION

It is well established that Vertisols are highly fertile soils but very often their productive capacity is limited by lack of water for plant growth; this is so because most of them occur in climates with deficient rainfall and/or marked dry seasons. Without irrigation, farming activities have to be carefully regulated for the cropping cycle to fit into the annual rainfall distribution. However, in many cases, the rainfall is unreliable in the amount received and its distribution pattern especially at the commencement of the rainy season, which particularly affects crop establishment. In any event, many Vertisols occur in rainfall regimes in which the total amount received would be insufficient to sustain good crop growth even if its distribution were dependable; this emphasizes the need for supplemental irrigation. In situations where there is a more reliable rainfall, farming activities are confined to the rainy season or immediate post-rainy season, with little activity during the dry season which can last for several months. In all these situations, the availability of supplemental irrigation can greatly increase soil productivity, extend the annual duration of cropping and enable farmers to exploit more closely the agricultural potential of these naturally fertile soils. Since Vertisols usually occur in large land masses in generally drier climates, the availability of irrigation water in close proximity is a problem and usually it has to be conducted from outside the area where it is to be used. The development of the conveyance systems usually requires substantial civil engineering inputs, are costly to install and require a good level of expertise to operate and maintain. These are all scarce resources in developing countries with the result that the needed irrigation facilities are very inadequately developed. In some locations there is the availability of ground water (i.e. Barbados, Jamaica, India from the author's experience) but the quality of water is variable from season to season and requires close monitoring. The source of water is also costly to exploit as it requires pumping often from deep wells and the water may even require pre-treatment prior to use. 11.2. METHODS OF IRRIGATION APPLICABLE TO VERTISOLS

There are three main categories of irrigation practices (Dargan et al., 1981) and we must consider their relative suitability for Vertisols. These are as follows:

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(a) Surface methods which include flood, basin, furrow, dyked furrow and border confinement of water; by these methods, water is appHed at intervals to allow the crop to utilize as much as 50 percent or more of the available water in the root zone before the next irrigation. (b) Overhead methods which include the use of sprinklers and perforated pipes. The water is applied above the ground surface as a spray somewhat resembling rainfall which is developed by flow of water under pressure through small orifices or nozzles. (c) Drip-irrigation which supplies the quantity of water needed on an almost daily basis by means of drippers or emitters strategically placed along a surface water distribution plastic line to emit water where it is needed with respect to the crop to be irrigated. For the surface methods, there is a degree of inflexibiUty in managing the systems as to frequency of irrigation and amount of water to apply. Further, the methods are only suitable for gently sloping lands usually up to 1 percent slope for close growing crops. Where soil salinization is a problem, these are the methods by which enough water can be added to effect both crop water supply and leaching at the same time. Surface irrigation is most widespread for Vertisols in which generally basin irrigation is used for rice cultivation and some form of furrow irrigation for other crops. The overhead irrigation methods must meet the evapo-transpiration requirements of the soil/plant complex, considering the rate of apphcation and water storage capacity. In these methods, water can be applied uniformly and the rate of application can be kept low enough to prevent runoff and perfectly level ground is not necessary. However, there are problems with the methods when water with high salt content is used both in causing direct crop damage through leaf injury and by being incapable of applying enough water to effect leaching as well. Sprinkler systems are costly to install and must be justified in terms of improvement in crop yields, water use economy and soil preservation. Surface and sprinkler methods are the two traditional forms of irrigation and their good and bad points for use on Vertisols must be considered. In a Vertisol requiring irrigation, cracking would have started and the major avenue of entry of water into the soil would be through these cracks (Fig. 11.1). When cracked soils are irrigated by surface flooding, water enters and fills the cracks first. The subsequent movement of water into the soil between the cracks is slow and water then infiltrates laterally into the inter-crack soil. The soil then swells and the cracks eventuaUy are closed by the expanding soil mass. The presence of cracks greatly increases the surface area for the infiltration process. While some water will infiltrate through the soil surface, the contribution of this to sub-soil wetting may be minimal due to the extremely low level of hydraulic conductivity. In practice, the surface of the soil will become wet quickly during irrigation, the soil will swell and seal and there would be little further entry of water by infiltration. In contrast, the water entering the soil through cracks would slowly wet the soil to a greater depth which is needed by the crop being irrigated. The ability of Vertisols to develop cracks even at high moisture contents and the affinity for water adsorption

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Fig. 11.1. Surface soil conditions in the furrow just before the application of irrigation water (Sudan): The crop is grown on a ridge and furrow land layout in which the furrow is used for flood irrigation. Between irrigation cycles, the soil in the furrow dries out and cracks begin to appear. On contact with irrigation or rain-water, these cracks are first filled.

and swelling, are variable among the Order and are important properties in this context. In considering the above situation, Warkentin (1982) proposed that applying water at high rates over short periods of time which is possible by surface methods is preferable to faciUtate adsorption by the soil. He made the point that slow application rates such as in sprinkling systems would cause soil swelling and a decrease in water intake. Likewise in a cracked Vertisol, high intensity rainfall would lead to more water adsorption through soil cracks and less runoff which is the opposite to the usually accepted relationship of increased runoff with increased intensity of rainfall. In this regard, Krantz et al. (1978) showed lower runoff from a Vertisol compared to an Alfisol at Hyderabad in short, high intensity rainfall incidents. Luthin (1982) expressed the opposite view that low rates of application may be beneficial as they may not lead to as rapid destruction of soil structure compared to faster application rates. According to him, the water from the sprinkler is cooler than the soil and contains a large amount of air in solution. When the water infiltrates into the warm soil, a structure, sometimes called a vesicular structure, develops. Also according to him, experience with irrigation of clays in the Imperial Valley in southern California indicates that the vesicular

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structure which develops is conducive to improved infiltration rates. It is doubtful whether this will occur in very high activity clay soils. Other disadvantages of sprinkler irrigation over the surface methods are the loss of water to the atmosphere as mist and increased runoff due to the impact of the water on the soil surface; this leads to accelerated soil seahng and development of soil saUnity due to inadequate leaching which is an integral feature of the method. In Jamaica, the use of sprinkler irrigation in sugar cane cultivation on Vertisols also using water high in salt content was responsible for extensive soil salinization (Shaw, 1965, 1966, 1982). The drip method of irrigation is relatively new and has been successfully used on Vertisols especially for tree crop cultivation. Addition of fertilizers can be built into the system which allows for their much more efficient use by the crop. With good quahty water, crop yields with drip irrigation are equal to or slightly better than other methods (Dargan et al., 1981); however, with poor quality water, yields are generally better with drip due to the continuous high moisture content and the daily replenishment of water lost by evapo-transpiration. Other benefits such as saving of water and greater efficiency in plant nutrient use may be large enough to justify the cost of installation of the system. Salts added in the water are concentrated in the periphery of the area wetted by the emitters and are not widely distributed in the soil which may be an advantage where salinity is a problem. 11.3. PROBLEMS ASSOCIATED WITH IRRIGATION

The normal hazards which are associated with irrigation use in soil management such as soil salinization, leaching requirement, soil drainage and related soil erosion apply to all soils. However, there are certain problems which are more specific to irrigation of Vertisols, the more important of these being — water use regulation and efficiency, — water quality and its monitoring, — water conducting systems. 11.4. WATER USE REGULATION AND EFFICIENCY

In water use regulation or budgeting for Vertisols, there are special problems compared to other soils. These problems are associated with two main factors, i.e. what constitutes the available water capacity of the soil, and the rooting depth of the crop. For other soils, the available soil water range is taken as the difference between field capacity and wilting point and the assumption is made that the water over this range is equally available. In Vertisols, because of the very low hydrauhc conductivity when the soil is wet, field capacity measurements may over-estimate this value since the conventional 2 days for draining between saturating the soil and sampling for determining soil water content may not be enough (Farbrother, 1986). The rooting depth of the soil is also difficult to establish in a Vertisol and of course in calculating irrigation needs, this is an important parameter in determining the depth of soil which must be wetted at each irrigation. Plant roots in a Vertisol grow

MANAGEMENT OF IRRIGATED VERTISOLS

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where they may, i.e. along the walls of cracks, around peds rather than through them (Cooper et al., 1989) and therefore extract water unevenly from the soil. The rate of water transmission through the soil to these absorption surfaces is not fast enough even in a Vertisol at near field capacity for the crop to maintain turgidity at the hottest periods of the day (Hudson, 1967). In Australia, it has been appreciated that before the air-dry moisture content is reached, the plant available water capacity (PAWC) is attained and the difference between this point and the air-dry moisture content is designated as the soil evaporation deficit (SED) (Fig. 11.2). The lower storage limit which is the gravimetric water content after plant water extraction in the field (Coughlan et al., 1986; Yule, 1986; Thorburn et al., 1989) has to be determined biologically. Another problem is the determination of bulk density which is needed in calculating the volumetric water storage capacity. This is not constant in Vertisols (Ahmad, 1983; Yule, 1986) although Yule and Ritchie (1980) and Yule (1984) proposed a method which relates moisture content and shrinkage to bulk density and allows accurate bulk density calculation at all water contents. After a long dry period, very large soil water deficits can develop in the surface soil—for example an SED of 40 mm over 0-100 mm depth of soil (Fig. 11.2). Thus large falls of rain or irrigation are needed to replenish the SED before plant available water is stored. Farbrother (1986) has considered some of these problems in scheduling irrigation on Vertisols and have related them to the experience at the Gezira Project in Sudan over many years. The crop for the period 1925-1962 was irrigated at 14 day intervals at a rate of 7.1 mm/day for the 14 day period. In the 38 years, over 126 field trials have been conducted testing factorial combinations of intervals and rates and any significant differences between the treatments were only very

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rarely obtained and then only between the most extreme combinations. Experiments in which irrigation intervals ranged from 7 to 28 days and rates from 2.5 to 10 mm/day produced no significant differences in yield of cotton (Farbrother, 1986). Farmers now allow 14 days at a drying phase and do not take into account the period during which the irrigation is actually done which is usually over 7 days. The standard interval is now therefore between 21 and 25 days depending on the variable time which it takes to complete watering. In the final analysis the farmer must also benefit from his experience of irrigating crops on these soils in scheduling irrigation, and therefore this important matter should be decided on the basis of scientific, technological and practical knowledge and experience of the soil scientist, the irrigation engineer and the farmer involved. With respect to water use efficiency in Vertisols, the effects can be variable and it depends on cracks and cracking and surface seahng. A medium amount of cracking is beneficial as explained earlier by increasing the by-pass flow of water directly into the cracks, where the water, either from rainfall or irrigation, is stored where it is needed. However, if the soil is allowed to dry out too much with the development of wide and deep cracks, an important amount of the apphed water may be channelled to wet layers of the soil too deep to benefit plant uptake. Cracking through unlined irrigation channels and embankments can also lead to substantial leakage of irrigation water out of the area being irrigated and this will continue until the soil on these embankments is saturated, swollen and self-sealed. In good irrigation management these extremes should be avoided; however, they may become unavoidable during fallow periods when it is not necessary to irrigate the fields and irrigation channels and embankments are allowed to dry out and cracks develop. Seahng of the soil surface early in the irrigation phase can prevent further infiltration of water into the soil, making runoff losses very important, thus reducing water use efficiency. The method of irrigation being practised is important in this regard as explained earlier. Vertisols vary in the ease with which surface sealing can occur (Webster, 1985) (Fig. 11.3). Those high in silt content and which have high exchangeable Na and/or Mg are prone to surface crust formation while those developed on calcareous parent materials with high Ca saturation and often higher organic matter contents, are more resistant to surface structure disintegration on contact with water. This problem is also greatly ameliorated by the use of mulches (Fig. 11.4). When irrigation of Vertisols is planned, all these factors should be considered in designing and managing the system. 11.5. WATER QUALITY AND ITS MONITORING

The underlying problem here is the prevention of soil salinization. A Vertisol which has been salinized by faulty irrigation presents an extremely difficult prospect to amehorate the problem since uniform leaching will not take place due to the water transmitting characteristics of the soil. Therefore, prevention of the problem is of strategic importance, the cardinal point of which is the quality of the irrigation water with respect to salinity and sodicity.

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Fig. 11.3. Crust development in Vertisols with low organic matter content and high exchangeable Na and/or Mg (Scotland District, Barbados): a strong, thick crust develops which cracks on drying; seedlings can only emerge through these cracks.

The parameters for assessing irrigation water quality have been known for a long time (e.g. USDA, 1954) and there is therefore no excuse for using poor quality water without the necessary precautions on these soils. The leaching requirement concept does not apply to Vertisols as they do for other soils due to lack of uniformity in water movement through the soil to effect leaching. The principles outUned by Ramdial (1971) for leaching of salts in Vertisols in Jamaica (later in this chapter) are pertinent in this context. It can well be that for any unit of leaching, more water may be needed for Vertisols than for other soils. In any event, since water has to be conveyed often from long distances to irrigate Vertisols, it is often too precious a commodity to be used for leaching of salts; therefore, prevention of sahnization by constant monitoring of water quality and of the salinity status of the soil and consequent modifications and adjustments in the appUcation of water are needed in any irrigation project. If it is unavoidable to use water of fairly high salinity on Vertisols, the method of irrigation must be considered in which the drip method should be preferred. 11.6. WATER CONDUCTING SYSTEM

In Vertisols, the water conducting system, i.e. the water courses and embankments are particularly vulnerable due to the ease of slumping of the soil along

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Fig. 11.4. The effect of mulches on surface crusting: the soil surface shown in this figure is from the same site as in Fig. 11.3 with the exception that it had a surface mulch of about 3 cm thick of bagasse.

the sides of streams and gully type erosion which can resuh (Fig. 11.5). The cracking also affects the embankments and other water retaining and channelling structures. The design of these structures should be given special attention particularly with respect to their foundations. In the case of water conducting channels these should be concrete-lined, if possible, or the water conducted in pipelines. Even so, deterioration results—in the case of concrete lining—by the swelling and shrinking of the soil behind and under the Hning which can cause eventual shatter of the lining or displacement of sections of it. In the Gezira Project in Sudan (Fig. 11.6), water courses have been kept quite well for many years by keeping the channels filled with water as much as possible which prevents drying, cracking and slumping of the soil along stream banks; also, vegetation is maintained along the stream edges. As mentioned above, excessive cracking of the embankments is also a problem, leading to leakage of water or breach in the embankments. All these problems require experienced supervision and timely maintenance if the system is to function efficiently. 11.7. EXAMPLES OF IRRIGATION USE ON VERTISOLS

Forms of irrigation are in use in different parts of the world on Vertisols and clay soils with vertic properties to enhance production. Almost in every case.

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Fig. 11.5. Deterioration of a drainage canal through accelerated stream-bank erosion (Belize): soil cracking along the bank of drainage ways can lead to severe soil slumping and deterioration of water-ways; keeping water in the canals is a good way of preventing this (Fig. 11.6), but is not always possible.

supplemental irrigation only is practised except where the soils are used for rice cultivation under full irrigation. The following are examples: 11.7.1.

Australia

Loveday (1984) and Probert et al. (1987) reviewed the main aspects of irrigation management on Vertisols in Australia which usually employs some form of flood method. Farm design and land preparation are the keys to controlling surface water. Land levelling is considered essential in preparing land for irrigation and modern cut-and-fill operations enable construction of larger bays for flood irrigation and longer furrows with even grade. They pose several questions for which there are no ready answers about the appropriate slope gradients and length of bay or furrow. As with all Vertisols, water acceptance is initially rapid when the soil is dry and cracks are present but rapidly decreases as the soil wets and swells. There may be little advantage in slope gradients and lengths which allow extended times for infiltration. In fact for crops such as cotton which is sensitive to waterlogging and poor aeration, there can be real disadvantages. The soil profile can only be recharged efficiently when cracks have appeared, and this will largely determine the scheduling of irrigation.

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Fig. 11.6. A main canal in the Gezira Irrigation Scheme, Sudan: in this project the water courses are well protected by keeping them filled with water at all times and by allowing vegetation to grow along the banks.

Although reduced tillage may be the direction in which rainfed agriculture is headed, it is recognized that irrigated cropping will require some tillage to loosen soils in order to form the necessary ridges and furrows needed for water conductance. Irrigated cropping is also likely to involve the passage of heavy machinery so that some compaction may be unavoidable and tillage will be needed to relieve it. Even the tillage operation itself may damage rather than improve soil structure, particularly when carried out at inappropriate water contents. Where irrigation is practised, it will often be difficult to till soils at the optimal moisture content since the surface soil may be at ideal levels but the sub-soil may be too wet. This is a problem that has been recognized especially with the cotton crop which is sensitive to waterlogging and poor aeration (Hodgson and Chan, 1984) and has led to attempts to dry the profile to depth by growing crops which root vigorously in the sub-soil (Hodgson et al., 1986). 11.7.2.

Barbados

Sugar cane is the main rainfed crop produced in Barbados and in the past, its production was the dominant form of land use. However, in recent years, it has become necessary to diversify due to economic reasons and there is an increasing development of vegetable and improved pasture production based on partial irrigation. Over 85 percent of the island has a porous calcareous rock strata.

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Further, drainage from agricultural land is directed underground through strategically located drainage "wells" to which surface drainage flows. Runoff is therefore minimal and with an average rainfall of over 1200 mm, there is substantial recharge of ground water which is also the source of water for domestic use. There is, however, the tendency for increased exploitation of the same resource for irrigation through pumping, sprinkHng and drip. The water is of good quality up to the present, meeting limits of purity for potable water. However, the danger exists of over-exploitation, salinization and chemical pollution due to the increased use of agricultural chemicals in the accompanying developments in vegetable production. The irrigation techniques used allow for no leaching of accumulated salts which can be accomplished only partially by rainfall and in the system used, the salts are only recycled. 11.7.3.

Guyana

The area of high activity clay soils in Guyana is located along the Atlantic coastUne from the Suriname border in the east to the Pomeroon River in the west. The main crops grown and for which irrigation is used are sugar cane and rice. There is likely to be over 100,000 ha of land under partial irrigation in Guyana. The water source is from inland fresh water swamps and rivers and there is an abundance of good quality water which is renewable and sustainable. The lands involved are flat and of depressed topography which is generally below the level of high tides. Irrigation is usually by gravity from the inland swamps which are developed as reservoirs by empolderment. This source is supplemented by pumping from rivers. Flood irrigation is practised for sugar cane and water management for rice cultivation is carried out by standard practice as dictated by the various stages of cropping. Due to factors such as a natural high rainfall (over 2000 mm/yr), flat terrain and therefore very restricted natural external drainage and the length of water conveyance lines, an intricate network of drainage and irrigation channels are needed to effectively manage water use. In the system which has been developed, about 20 percent of the land surface is taken up by water courses and the overall cost of developing lands is very high (Fig. 11.7). However, in the long run, the high productivity of the soils which is assured by the irrigation make such developments cost effective. In irrigation use in Guyana, drainage is given adequate importance and this is achieved by conveying the drainage water through an intricate system of field drains and eventually to canals leading to the ocean front, at which point there are sluice gates which are opened at low tides for gravity drainage. Flooding by high tide sea water is prevented by constructed dykes or naturally developed sand banks aided by mangrove vegetation along the sea front. From experience, the opportunities for natural drainage are not enough in the wet seasons and therefore there are pumps strategically located along the coast to facilitate drainage on a continuous basis when necessary. With the availability of water, flood fallowing is routinely practised as a soil rejuvenating technique (Chapter 10, this volume)

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Fig. 11.7. Land layout for drainage and irrigation in clay soils in Guyana for the commercial production of sugar cane: an intricate system of water-ways is needed to drain or irrigate the fields as needed; the same basic layout for overall drainage and irrigation is also used for rice production.

and water-ways are used to transport the harvested sugar cane crop to the processing factories. No soil degradation results from the system of irrigation and drainage which is carried out. The water used is of such good quality that there is no salinization or pollution and soil erosion is at a minimum caused by the system of land layout involving cambered beds and ridges (Gumbs, 1982) to aid external drainage. The excessive use of water can cause leaching of plant nutrients which in fact is indicated by the accumulation of large amounts of exchangeable and fixed NH4-N in the sub-soils of traditional sugar cane growing areas (Hardy and Rodrigues, 1951) and the lush growth of weeds along water causes which must be controlled. The same soil, land and water resources as in Guyana also occur in Suriname but they are not utilized to the same extent; where developed, the pattern and nature of utilization is the same. 11.7.4. India In India the expansion of irrigation has been one of the key aspects of government's policy in achieving self-sufficiency in agriculture. As a reflection on this, the net irrigated area has increased from about 20 m ha in 1954 to about

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45 m ha at present (Abrol and Seghal, 1994). The large expansion of the irrigated area during this period has been achieved through water conveyed in canals. In almost all cases, the groundwater table which was several metres deep prior to the introduction of irrigation, rises when irrigation begins to 2 m and even shallower (Murthy et al., 1982). Where the ground water is within 2 m from the ground surface it contributes significantly to evaporation from the soil surface and causes soil salinization (Abrol and Seghal, 1994). In most canal irrigated areas the problems of soil deterioration through accumulation of salts have reached serious dimensions. According to one estimate, nearly 50 percent of canal irrigated areas are affected by salt problems due to lack of or inadequate artificial and/or restricted natural drainage, inefficient use of irrigation water and socio-political reasons (Abrol and Seghal, 1994). Salinity problems have also increased where ground water of poor quality has been used for irrigation in the absence of good quality irrigation water. It is estimated that 1.42 m ha of Vertisols and related soils are now salt affected. From the Indian experience it has been observed that indiscriminate irrigation has resulted in salinization of great areas of deep, Typic-Chromusterts in parts of Ahmednagar, Akola and Sural Districts (Murthy et al., 1982) among other areas. It has been assessed that unless proper drainage facilities are provided and the irrigation carried out according to water needs of the crop and infiltration rates and leaching requirements of the soil, irrigated agriculture will not be economical or profitable and will lead to rapid soil degradation. As pointed out earlier, however, the soil factors are difficult to assess. The most serious soil salinity hazard arises where ground water is used with variable and unpredictable quality and this is not monitored adequately to adjust the irrigation practices accordingly. In areas with higher rainfall, supplemental irrigation using surface stored or harvested water can be very beneficial (Virmani et al., 1982). Dry periods even during the rainy season are typical and the availability of stored water for supplemental irrigation in these periods is important in reducing risks and to improve production. According to Virmani et al. (1982), it is not only stabilization of the rainy season crop which can be ensured with the "life saving" irrigation but it can also facilitate an extension of the cropping season as well. For the post-rainy season crop, there is generally adequate available water stored in the deeper layers of the soil profile but the soil surface is usually quite dry. This seriously impairs the opportunity of establishing crops at this time and where they are planted, the stand is often quite poor. In such cases, supplementary irrigation would not only help in stand establishment, but also in efficient utilization of the sub-soil water. Experience in the use of harvested water for irrigation of Vertisols is limited and according to Virmani et al. (1982), its economics will depend on numerous factors such as location moisture content and distribution in the profile, drought stress and its intensity and duration and socio-economic factors including the value of the crop. For sequential crops of chickpea, wheat, sorghum and sunflower in many of the Vertisols in India having dependable rainfall, supplemental irrigation with harvested water offers a possibility not only of making the post rainy season

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crop a success but also of improving the overall use efficiency of water stored in the soil. Irrigation at this level and with surface water harvested and stored, also poses no serious soil salinization problems. Adaptive research along these lines has been carried out at ICRISAT and the All India Coordinated Dryland Project, both located at Hyderabad. 11.7.5. Israel In Israel, Vertisols, as with all other agricultural soils, have to be almost fully irrigated. The Vertisols are widely used for tree crop cultivation, i.e. citrus, avocado and mango. The crops are planted centrally on small raised beds and irrigated by trickle irrigation or mini-sprinklers. As is commonly known, water for irrigation is in short supply in Israel and so there is the greatest economy and efficiency in its use. For instance, water is generally conveyed in pipes and if by open ditches, they are concrete fined to eliminate seepage losses. With aU the measures adopted, it is estimated that water use efficiency exceeds 90 percent. Fertilizers are usually added with the irrigation water and this method of plant nutrient application reduces waste, minimizes pollution and increases the efficiency of uptake by the crop. Drip irrigation requires about one-third less water than furrow irrigation for maximizing yield and it is particularly advantageous over furrow and sprinkler irrigation if poor quality water is used. The dispensing of water by emitters in drip irrigation in amounts actually needed and placed where the crop can efficiently use it, has added benefits in crop management. For instance, the rate of growth and size of the tree crops can be controlled and this makes crop management operations such as harvesting, spraying and pruning much easier. Also, since water is not appUed over the entire soil surface in an essentially dry climate, weed growth is severely restricted, which further makes use of water and plant nutrients more efficient (Fig. 11.8). With the quality of irrigation water which is used and the method of application, there are no hazards to the crop or soil. Irrigation systems are electronically controlled and scheduling is done based on the needs of the crop at the particular stage of development and evapo-transpiration losses. 11.7.6. Jamaica In Jamaica substantial areas of Vertisols in different parts of the country are being irrigated for the production of sugar cane, banana, tree crops, pasture, vegetable and food crops and ornamental horticulture. However, the main area is in the southern plains of the Parishes of St Catherine and Clarendon. Surface water obtained from a river (Rio Cobre) with an extensive catchment area consisting of shallow soils on limestone, as well as ground water from shallow to medium depth wells obtained from limestone aquifers, are the main water sources. The underlying limestone extends into the Caribbean sea so that the deep ground water is saline and the relatively fresh water occurs as lenses over the saline water. This situation makes the ground-water quite vulnerable to salinity intrusion when over-exploited. The overall water distribution system is managed by a government

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Fig. 11.8. Use of drip irrigation in three crop production (Israel): The emitters are located along a plastic water line aligned along the ridge on which the crop is planted and the crop is fertigated as needed; water and plant nutrient use are very efficient and in addition there is good weed control and the growth of the trees can be controlled.

agency and the water is made available to farmers on a constant flow basis at subsidized cost. There are many inefficiencies in the conveyancing and distribution of the water and the nature and causes of these inefficiencies have been the subject of several studies (FAO, 1974; JICA, 1987; Melamed, 1989; Ahmad, 1990); the best estimate is that water use efficiency is between 40 and 50 percent. There is a major difference in water quality between the surface and ground water sources as shown in Table 11.1. From the data the ground water with a much higher soluble Na and Na absorption ratio would have much greater potential for soil salinization. The irrigation technique commonly used at present is furrow irrigation for sugar cane (Fig. 11.9), sprinkling for pastures and drip for tree crops and banana. On the Vertisols, the early use of sprinkling on sugar cane using ground water has been particularly conducive to soil salinization (Fig. 11.10). The overall tendency is to overuse irrigation water in the wet season when surface water is unlimited but in the dry season there is a deficiency and the trend is to under-irrigate at this time. Because of this shortage, in recent years there has been the tendency to increase pumping and use of the ground water which has been done rather indiscriminately and without adequate monitoring of water quality. The consequences have been grave and there has been a high degree of

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445

Fig. 11.9. Furrow irrigation of sugar cane (Jamaica); due to several factors, the water use efficiency is low and the level of irrigation fluctuates with water availability.

salt accumulation in some of the Vertisols over a period of time. The overall level of availability of water, its salt content and low hydrauUc conductivity of the soils, are all in favour of salt accumulation (Shaw, 1982). Low rainfall in recent years and therefore reduced leaching, has also added to the problem. The main management poUcies to cope with the problem of soil salinization are the leaching of accumulated salts by storm rains, land forming and appropriate land shaping and conscious management of irrigation. It was observed that rainfall and irrigation incidents are inter-related in leaching of salts (Shaw, 1965, 1966) and research has shown that cracking of the soils on drying was also an important factor in leaching. Ramdial (1971) defined the mechanism of salt removal, the sequence being drying and cracking of the soil, migration of the soluble salts to the walls of the cracks and removal and flushing of the salts by rainfall or low sahnity water. Following this sequence has resulted in significant lowering of salt content within 1 year. However, the effect depends on adequate drainage being available and in field practice, this is not always the case. Land forming and land shaping are used to improve surface and internal drainage to aid the leaching process. It has been shown (Ramdial, 1971) that the increase in salts is more rapid and more intense on ridges than in furrows and leaching in the rainy season is more rapid from banks than from furrows. According to Shaw (1982), ridging also allows the maintenance of improved

446

N. AHMAD

Fig. 11.10. Soil salination resulting from irrigation with water of low quality and with inadequate field drainage (Jamaica): there is a strong development of salt crust on the soil surface in the newly planted sugar cane crop; the growth of the crop is severely affected by the high level of salinity.

physical condition of the tilled soil in the root zone which in turn removes more water from this zone and facilitates cracking for more effective leaching. Management of the irrigation water also helps leaching and reduces salt accumulation. The use of large furrows and irrigating through alternate furrows have been shown to be beneficial. According to Shaw (1982), large furrow streams allow the water to be more efficiently used and tend to cause a greater removal of the salts concentrated at the top of the ridges. The practice also reduces deep percolation and the likely attendant increase in salt content in the sub-soil. Alternate furrow irrigation minimizes the surface salt concentration and also increases water application efficiency. Other cultural techniques such as using more salt tolerant crop varieties and the continuous incorporation of crop residues to improve the physical condition of the soil are also considered helpful. 11.7.7. Kenya According to Ikitoo (1989), the most productive Vertisols in Kenya are irrigated. Gravity fed surface irrigation methods (basin or furrow) have been used at the various irrigation schemes in the country with considerable success. At Mwea,

447

MANAGEMENT OF IRRIGATED VERTISOLS TABLE 11.2 Crop production on some Vertisols under irrigation in Kenya^ Crop

Irrigated area

Irrigation method

Current production area (ha)

Mwea irrigation scheme Ahero pilot scheme West Kano pilot scheme Bunyala irrigation scheme Hola irrigation scheme

Rice Rice Rice Rice Cotton

Basin Basin Basin Basin Furrow

6000 600 500 200 850

Average annual yield Rice (kg/ha)

Seed cotton (kg/ha)

5500 3000 4000 5800

— — — —

—

2000

^Adapted from Ikitoo (1989).

annual rice yields of 5-6 t/ha are obtained and at Hola, seed cotton yields of about 2t/ha are normal. Other irrigated areas include West Kano, Ahero and Bunyala. In Table 11.2, the irrigation schemes, the methods of irrigation used, the area irrigated in each case and crop yields are given. In these schemes, irrigation water is more than adequate for crop water requirements during the growing period and it is possible to provide the water when necessary and in the required amounts. At Mwea, it was shown that high irrigation water efficiency to minimize uneven distribution and underground and lateral seepage are required for maximum yields of rice (Owido et al., 1980; Njihia et al., 1984). The farming system entails the construction of level basins of about 0.5 ha each which are separated by levees or bunds to control water. Cropping starts with the growing of seedhngs in nurseries for 4 weeks, transplanting the seedlings in the basins after the land had been tilled and puddled by rotavating, and maintenance of the water level throughout the cropping season. One crop is produced annually, but with suitable measures, double cropping would appear quite possible. Rotavating is done in March-August, transplanting in July-August and harvesting DecemberJanuary. Limitations to expansion of irrigation of Vertisol areas include high cost and unavailability of irrigation water in many locations. In Kenya, surface water of good quality is used for irrigation and there are no reports of soil salinization or other forms of land degradation resulting from irrigation of the Vertisols. 11.7.8.

Sudan

It is likely that the largest area of irrigated Vertisols in the world is in Sudan, in the geographical area known as the Central Clay Plain where the rainfall received is too low or marginal and unreliable for crop production. Advantage has been taken of the Nile River system, the drainage basin of which extends into

448

N. AHMAD

the humid areas of East and Central Africa and therefore well supplied with water. At Khartoum in Central Sudan, the Nile branches into the White Nile and the Blue Nile. As these tributaries diverge, the land in between has a gentle even gradient from the Blue Nile to the White Nile. The Blue Nile is a highly seasonal river with a ratio of peak flow to low flow of only 5:2 (Blokhius, 1993). Early in this century a vast irrigation project was planned which was designed to irrigate the lands between these rivers, taking advantage of the natural difference in gradient, thus facilitating irrigation by gravity flow. A diversion dam and reservoir were completed at Sennar on the Blue Nile in 1925 along with the internal water conveyance system to irrigate an area of 470,000 ha, the project centred at Gezira and known as the Gezira Irrigation Project (Information and Public Relations Department, Sudan, Gezira Board, 1981). Between 1958 and 1962, the scheme was enlarged by the Manaqil Extension and Gezira Phase V, which, together, occupy an additional 400,000 ha. For many years, the main cash crop in the Gezira-Manaqil was long staple cotton, but more recently there has been a shift towards medium staple cotton in response to increased demand, and food crops such as wheat and groundnut. Traditionally, cotton was cultivated in rotations including the cereal crop millet and the forage legume crop Dolichos lablab (Blokhius, 1993). The GeziraManaquil Scheme is under the management of a tripartite partnership: the Sudan Gezira Board, the Sudan Government and the tenant farmers (ILO/UNDP, 1976). The Guneid Pump Scheme is on the Blue Nile east bank opposite the Gezira-Manaqil Scheme and it irrigates an area of almost 36,000 ha. The main cash crop is sugar cane. The New Haifa or Khashm el Girba Scheme is irrigated by a reservoir in the Atbara river above the town of Khashm el Girba. Irrigation of the first phase of the Scheme was started in 1965; at present some 200,000 ha are irrigated. Over most of the irrigable area, cultivation is in rotations similar to those of the Gezira-Manaqil, but there is also large-scale cultivation of sugar cane. The Roseires Dam Project, which is now being implemented, wifl ultimately provide for the irrigation of extensive areas in the Rahad-Dinder-Blue Nile Gezira, along the Blue Nile and Rahad east banks, and in the northern Gezira from a reservoir in the Blue Nile above Er Roseires (Blokhius, 1993). In northern Kenana, the Kenana Sugar Corporation includes an integrated sugar plant and factory with a planned capacity of 350,0001, and an irrigated area on the clay plain that will ultimately have an extent of 120,000 ha.The high capital coefficient, as well as organizational and logistical problems, throw doubts on the ultimate profitability of this gigantic enterprise (Oesterdiekhof, 1982). Another large enterprise in the Roseires Project is the Rahad Scheme, on the east bank of the river Rahad. In 1985 an area of 120,000 ha was irrigated. Irrigation from a dam in the Rahad is restricted to August, when the river is in flood; otherwise irrigation is by water from the Blue Nile, pumped from a site near Meina el Mek, from where it is led over a distance of 80 km by a canal (Jansen and Koch, 1982). The two main crops are medium staple cotton and groundnut.

MANAGEMENT OF IRRIGATED VERTISOLS

449

On both banks of the Blue and White Nile are numerous pump irrigation schemes. These schemes were started by private enterprise, but were nationalized in 1968 and are now managed by the Agrarian Reform Corporation (ARC). In 1975 (ILO/UNDP, 1976) the ARC was responsible for 62 schemes, with a gross area of almost 120,000 ha along the Blue Nile between 40 km north and 20 km south of Sennar, and for 186 schemes, with a gross area of 175,000 ha along the White Nile, over a length of 380 km upstream from Jebel Auliya. Along the Blue Nile, and on islands in the river, there is intensive cultivation of vegetable, food and fruit crops during the dry season using irrigation. These fertile lands, known as ' ' g e r f , are flooded annually when the river is in flood. Figure 11.11 shows the location of the existing main irrigation schemes within the Central Clay Plains. The Sudan Gezira experience is a highly successful example of established irrigation agriculture on Vertisols (Abdulla, 1989). The Gezira system depends on ridges and furrows for irrigation and drainage, the introduction of a leguminous crop in the rotation (for biological N fixation) and the mineraUzation of the crop residues in the fallow phase. The fallow is an energy phase, where the land is not cultivated, but allowed to be desiccated by solar energy. In this phase, extensive shrinkage cracks and fissures develop, improving soil structure and water penetration in these otherwise impermeable soils. The use of mineral N fertilizers allows the introduction of wheat and groundnut in the rotation and the agriculture system has become more intense (75% cropping in Gezira and 100% cropping in Manaqil). The old Gezira rotation was cotton-sorghum + half-legume-fallowfallow. Now the rotation in Gezira is cotton-wheat-groundnut/sorghum-fallow. In Manaqil, it is cotton-wheat-groundnut/sorghum-fallow. Both the four-course and the three-course rotations receive about 260kgN/ha as urea. No other fertilizers are being used. However, with the intensification of the rotation and diversification of crops, the yields of the various crops have started to deteriorate faster than expected. Phosphorus fertilizer is now being tried on all crops in the rotation and there is a very good yield response. Experience in the Gezira indicates that the yield potential of Vertisols can only be reaUzed by adopting a balanced fertilizer nutrient policy. Since the Gezira is one of the most successful irrigation projects in the world which has been in operation for the past 70 years, the quality of irrigation water as a factor in its success is significant. Table 11.3 shows some of the properties of the water in the main irrigation canals of the Manaqil and Gezira parts of the area, as well as the water of the Blue Nile at the intake (Sennar Dam) for June (hot and dry) and October (cooler). As the data show, with water of this quality, there is little danger of soil salinization and therefore it can be used almost without precaution. There are plans to develop another irrigated area of almost 2 m ha in Northern Sudan with water made available by damming the Nile; it is intended that wheat would be the main crop; however, it is not certain what percentage of the soils in the proposed irrigated area are Vertisols.

450

N. AHMAD

MCPS schemes MCPS schemes, with incidental irrigation irrigation schemes.

Fig. 11.11. Irrigation schemes and mechanized crop production schemes (MCPS) in the Central Clay Plain area (after Blokhuis, 1993).

77.7.9. The United States Detailed information on the use of irrigation in Vertisols is scarce. According to Burnett (1989) there is little ground water available for supplemental irrigation in the semi-arid and sub-humid portions of the United States Vertisol region. Some surface impounded water is used to irrigate rice in the Gulf Coast area. Since crops suffer from moisture deficits in almost every growing season, the potential for small scale water impoundments for farm size supplemental irrigation has been considered. In Central Texas, it was estimated that between March and June each

451

MANAGEMENT OF IRRIGATED VERTISOLS TABLE 11.3

Chemical properties of the irrigation water in the Gezira and Manaqil canals and of the water in the Blue Nile^ Chemical properties

Total salts (ppm) pH Na (m.e./lOOg) Mg (m.e./lOOg) Ca (m.e./lOOg Sodium adsorption ratio

El Gezira

Manaqil

June^

October

June

October

June

October

196 8.1 0.58 0.74 1.66 0.53

100 7.9 0.17 0.46 0.88 0.21

199 8.2 0.58 0.75 1.68 0.52

87 7.8 0.19 0.38 0.90 0.24

198 8.2 0.58 0.75 1.68 0.53

87 7.9 0.19 0.39 0.90 0.24

Nile

^Source of data is the Soils Section of the Ministry of Agriculture, Republic of Sudan. ^June represents conditions in the dry season; October the cool season.

year up to 50 mm of runoff can be expected and this water can be captured in small impoundments and could be used to alleviate crop moisture deficits during critical growth stages. In situations where the surface topography would allow the easy construction of water impoundment structures, i.e. small dams, supplemental irrigation may be successfully used. Site specific information regarding runoff probability, slope and other factors must be taken into account in determining whether supplemental irrigation with small impoundments is feasible. Stewart et al. (1983), Unger and Stewart (1988) and Stewart (1989) have proposed a limited irrigation dryland farming system (LID) for the conjunctive use of rainfall and limited irrigation of graded furrows (Fig. 11.12). The main feature of this system is that it actually adjusts during the crop growing season the amount of land irrigated since more land can be irrigated during the above average rainfall years with a limited amount of irrigated water than below average years. The design uses a limited water supply to irrigate a larger area than could be conventionally irrigated. While this is only one of several systems being used by farm operators, it serves as a good example of how water use efficiency can be increased. The system as proposed and being used consists of a graded furrow field, 600 m long on 0.3-0.4 percent slope and divided into three water-management sections (Fig. 11.12). The upper half of the field is managed as fully irrigated, the next one-quarter as tail-water-runoff section that uses furrow runoff from the fully irrigated section. The lower one-quarter is managed as a dryland section capable of receiving and utilizing any runoff resulting from either irrigation or rainfall on the wetter, fully-irrigated and tail-water sections. Plant densities can be reduced down the field to alleviate stress and furrow dams (dykes) can be placed at regular intervals throughout the length of the field. Irrigation water is applied in alternate furrows. In experimental conditions, the system tended to reduce all water losses

452 Management practice

N. AHMAD Water management sections

Irrigation water

Fully irrigated

Planting density

Tail water runoff

1 Dryland

Furrow dams

Fig. 11.12. Schematic diagram of the Hmited irrigation dryland (LID) system (after Stewart, 1989).

Other than transpiration. The more efficient use increased grain yield to 154 kg/ha for each 10 mm added irrigation, compared to 92 kg/ha for each 10 mm added under conventional irrigation practices. According to McKee and Hajek (1973) some cotton is irrigated in the south-west Texas (soil unit V9) along the Rio Grande River, presumably utilizing water from this river. Where Vertisols are irrigated in the southern United States, the system used incorporates adequate soil conservation measures and there are no reports of soil salinization resulting (Unger and Stewart, 1988). Therefore, irrigation use under normal conditions does not lead to soil degradation. 11.7.10. West Africa There are apparently no highly successful irrigation projects on Vertisols in West Africa. In Ghana, the once planned development of irrigation of the Accra Plains with water diverted at the hydro-electric project at Akosombo and the develop-

MANAGEMENT OF IRRIGATED VERTISOLS

453

ment of large scale cultivation of sugar cane and other crops did not materialize. In Nigeria the planned South Chad Irrigation Project in the Firki Plains of Ngala in which it was proposed to irrigate over 2000 ha for the production of wheat and rice could not be satisfactorily implemented due to a fall in the level of water in Lake Chad in recent years, which was intended to be the source of water for the project. The major installations are in place but lack of water is preventing the project from functioning. The other irrigated area in Northern Nigeria is on the main Vertisols where the Savannah Sugar Company is producing irrigated sugar cane. However, the level of efficiency is low and the irrigation practices can be improved (Dr W. Ekwue, Personal communication). In summary, only relatively small areas of Vertisols are being irrigated at present throughout the world, yet the soils present great potential for increased productivity through the application of irrigation. Irrigation can result in high levels of production and would enable farmers to realize the full potential of these naturally fertile soils. The most successful irrigation projects are in Sudan in which water from the Nile River system is utilized, the water being of excellent quality. In Vertisols, the standard irrigation techniques and scheduling need special consideration. The importance and relevance of water quality in deciding on the method of irrigation is of special importance. Due to extremely low wet hydraulic conductivity which is characteristic of Vertisols, leaching of accumulated salts is severely restricted and takes place unevenly and gradually the water table rises in situations where the soils are continuously irrigated; this eventually leads to increases in salt concentrations in the surface layers. Some form of land layout to facilitate external drainage is important in any irrigation project on these soils. The normal considerations used in scheduUng irrigation on other soils are not particularly applicable to Vertisols due to soil cracking, uneven wetting and difficulties in establishing the rooting zone and the range of available water in the soil. In this regard, two points are important in deciding when to irrigate, one of these being that cracking should commence before an event since this is the main means for water to enter the soil; the other is the experience of the farmer in crop management in knowing when the crop can benefit from irrigation.

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Astatke, A. and Mohammed Saleem, M.A., 1992. Low cost animal drawn implements for better utilisation of Vertisols in Ethiopian Highlands. In: Reports and Papers on the Management of Vertisols (IBSRAM/AFRICALAND), Network Document No. 1, IBSRAM, Bangkok, Thailand. Blokhuis, W.A., 1993. Vertisols in the Central Clay Plain of the Sudan. Thesis, The Agricultural University of Wageningen, The Netherlands. Buckley, D.D., 1970. Irrigation water quaUty on the St Catherine and Clarendon Plains, Jamaica. Project Report, Department of Soil Science, University of the West Indies, St Augustine, Trinidad, West Indies. Burnett, E., 1989. Land and water management practices for Vertisols. In: Management of Vertisols for Improved Crop Production. IBSRAM Inaugural Workshop, ICRISAT, Hyderabad, India, pp. 133-146. Cooper, P.J.M., Keatinges, J.D.H. and Kukula, S., 1989. Influence of the environment on the management and productivity of cereals on a Vertisol at Jindiress, Syria. In: Management of Vertisols for Improved Agricultural Production. An IBSRAM Inaugural Workshop, ICRISAT, Hyderabad, India, pp. 195-212. Coughlan, K.J. McGarry, D. and Smith, G.D., 1986. The physical and mechanical characterisation of Vertisols. In: Management of Vertisols under semi-arid conditions. IBSRAM Proceedings No. 6, IBSRAM, Bangkok, Thailand, pp. 89-106. Dargan, K.S., Singh, O.P. and Gupta, I.C., 1981. Crop production in salt affected soils. Central Soil Salinity Research Institute, Karnal, India. Oxford and IBH Publishing Co., New Delhi, India. FAO, 1974. The Rio Cobre basin, Jamaica Development and management of water resources. FAO, Rome. Farbrother, H.G., 1986. Supplementary irrigation. In: Management of Vertisols under arid conditions. IBSRAM Proceedings No. 6, IBSRAM, Bangkok, Thailand, pp. 267-284. Gumbs, F.A., 1982. Soil and water management features in Trinidad and Guyana. Trop. Agric. (Trin.), 59: 76-81. Hardy, F. and Derraugh, L.F., 1947. The water and air relations of some Trinidad sugar-cane soils. Trop. Agric, 24: 76-87. Hardy, F. and Rodrigues, G., 1951. The nitrogen enigma of the sugar cane soils of British Guiana. Sugar Association of the Caribbean, Inc., Port of Spain, Trinidad, pp. 97-100. Hodgson, A.S. and Chan, K.Y., 1984. Deep moisture extraction and crack formation by wheat and safflower in a Vertisol following irrigated cotton rotations. In: J.W. Garity, E.H. Hoult and H.B. So (Editors), The Properties and Utilisation of Cracking Clay Soils, Reviews in Rural Science 5. University of New England, Armidale, NSW, AustraHa, pp. 299-304. Hodgson, A.S., McGarry, D., Chan, K.Y. and Daniells, I.G. 1986. The effect of wet cultivation and water-logging on physical properties of an Australian Vertisol and its abihty to grow cotton. Trans. 13th Int. Congr. Soil Sci. Hamburg, Vol. II, pp. 79-80. Hudson, J . C , 1967. AvailabiUty of soil water with reference to sugar cane growing in clay soils in Barbados. Ph.D. Thesis, Library, The University of the West Indies, St. Augustine, Trinidad, W.I. Ikitoo, E.C., 1989. Some properties of Vertisols in Kenya and their current level of management for crop production. In: Vertisol Management in Africa. IBSRAM Proceedings No. 9, pp. 193-208.

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1965 Animal Report, Sugar Research Department, Jamaica, pp. 22-25. Shaw, M.E.A., 1982. Aspects of management of salinity on swelling clay soils in Jamaica. Trop. Agric. (Trin.), 9: 167-172. Stewart, B.A., 1989. Limited irrigation—dryland farming systems for efficient water management in semi-arid environments. In: Management of Vertisols for Improved Agricultural Production. IBSRAM Inaugural Workshop ICRISAT, India, pp. 101112. Stewart, B.A., Musick, J.T. and Dusek, D.A., 1983. Yield and water use efficiency of grain sorghum in a Umited irrigation-dryland system. Agron. J., 75: 629-634. Thorburn, P.J., Coughlan, K.J., Gardner, E.A. and Yule, D.F., 1989. Soil water restrictions to land use on Vertisols in Queensland, Australia. In: Vertisol Management in Africa. IBSRAM Proceedings No. 9, IBSRAM, Bangkok, Thailand, pp. 77-96. USDA, 1954. Diagnosis and Improvement of Saline and AlkaU Soils. Handbook No. 60. Government Printer, Washington, DC. linger, P.W. and Stewart, B.A., 1988. Conservation techniques for Vertisols. In: L.P. Wilding and R. Puentes (Editors), Vertisols: their Distribution, Properties, Classification and Management. Texas A & M University, College Station, Texas, pp. 165-181. Virmani, S.M., Sahrawat, K.L. and Burford, J.R., 1982. Physical and chemical properties of Vertisols and their management. In: Vertisols and Rice Soils of the Tropics. Symposia Papers II, 12th International Congress of Soil Science, New Delhi, India, 8-16 February 1982, pp. 80-93. Warkentin, B.P., 1982. Clay soil structure related to soil management. Trop. Agric. (Trin.), 59: 82-91. Webster, J.L., 1985. A comparative study of the effect of some chemical ameliorants on Scotland clays of the Scotland District Barbados. Ph.D. Thesis, Library, The University of the West Indies, St. Augustine, Trinidad, W.I. Yule, D.F., 1984. Measured and predicted field shrinkage and swelling. In: J. W. McGarity, E.H. Hoult and H.B. So (Editors), Properties and Utilisation of Cracking Clay Soils. Reviews in Rural Science No. 5, University of New England, Armidale, NSW, AustraUa, pp. 136-140. Yule, D.F., 1986. Water management of Vertisols in the semi-arid tropics. In: Management of Vertisols under Semi-arid Conditions. IBSRAM Proceedings No. 6, IBSRAM, Bangkok, Thailand, pp. 107-124. Yule, D.F. and Ritchie, J.J., 1980. Soil shrinkage relationships of Texas Vertisols I. Small cores. Soil Sci. Soc. Amer. J., 44: 1285-1291.