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Soil structure transformations over the growing season - A micromorphological approach
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Soil structure transformations over the growing season - A micromorphological approach N.W. Hall Crop and Environment Research Centre, Harper A d a m Agricultural College, Newport, Shropshire, TFlO 8NB, UK ABSTRACT Hall, N.W.,1994. Soil structure transformations over the growing season - A micromorphological approach. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Roc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 659-667.
The study investigates the frequency of occurrence of structures over the growing season in order to understand which structural attributes change in abundance over the season given that soil structure is a complex mixture of soil aggregate and air space characteristics. Thin sections were prepared of arable soils on five different parent materials. The soils were Rendolls on Chalk, Typic Hapludalfs on clayey drift, Vertic and Mollic Haplaquepts on alluvium, and Psammentic Hapludalfs on sandstone. Structure types were delineated on sections at the 1:l scale and classified using four differentiating characteristics, namely degree of ped development, porosity, ped size (void size if apedal) and void type, and their percentage area measured. The results show that peds become smaller (>50% peds <5 mm2 and >80% <40 mm2) and move into closer packing (10 - 20% voids) but do not loose their identity over the season. Sections showing a lack of ped development become increasingly less common over the season. The results show the role of improved ped packing through consolidation but with shrinkage and swelling maintaining structure particularly on the Chalk. The paper integrates these findings with those published elsewhere on the influence of the length of time under tillage on structure and on calcium carbonate content, soil colour and interrelationships between structure and erosion. While aspects of good structure are not fully understood, structure changes over the season. This shows the need to optimise management to meet competing needs of agronomy, economy and sustaining the soil resource.
INTRODUCTION There are two objectives in the physical management of arable soils; namely, creating a suitable seedbed, usually through tillage, and maintaining the number of coarse pores over the growing season (Wild, 1988, pp. 435-441). The latter is achieved by good soil management practices assisted or detracted from by intrinsic soil properties within a given climatic environment.
N.W. HALL
660 Table 1 Management classes sampled in the study. Class 1. Class 2. Class 3. Class 4.
Traditionally cultivated arable land during seed bed preparation and early stages of crop production. Winter crops sampled before mid November and spring crops before June. As 1 above but at the end of the cycle of crop production or after harvesting but before autumn tillage. Sampled August and September. Direct drilled cereals early in the annual cycle. As 3 but late in the cycle.
Table 2 The classification of soil structure types (from Hall,1990). A structure whose degree of ped development was class 1, whose porosity was class 2, whose ped size was class 2 and whose void type was class B would be denoted 1-2-2-B.
Classes
Differentiating Characteristics ~~
Degree of ped development
Class 1 Class 2 Class 3 Class 4 Clearly defined Partially Secondary peds No peds peds developed peds
Porosity
Class 1 High
Class 2 Medium
Ped size (or void size in the absence of peds)
Class 1 Large
Class 2 Small
Void type
Class B Compound oacking voids
Class c Curvo planar voids
~~
~
~~
Class 3 Low
Class D Planar voids
Class E Vughs
This paper attempts to elucidate structure transformations by investigating structural characteristics associated with arable land early in the annual cycle of crop production compared with late in the cycle.
THE SITES STUDIED AND METHODS USED Two study areas were chosen which had a range of soils and management histories and had presented erosion problems. One is on the Berkshire Chalk Downland (National grid reference SU 554803) and the other in Oxfordshire (SU 454960). The Downland soils comprise silty clay loam Rendzina soils and clay loams on drift (Jarvis, 1973) which approximate to Typic Rendolls and Typic Hapludalfs (Soil Survey Staff, 1975). In Oxfordshire the soils are light sandy loams, sandy loam terrace soils and clayey alluvium which correspond to Psammentic Hapludalfs, Mollic Haplaquepts and Vertic Haplaquepts. Ninety six vertically orientated
66 1
SOIL STRUCTURE TRANSFORMATIONS OVER THE GROWING SEASON
Table 3 Table of results for structure --2-B (i.e. occurence or non-occurence of small peds and compound packing pores. Management classes (listed in Table 1) Structure -2-B
1
2
3
4
TOTAL
Occurrence Non occurrence TOTAL
10 17 27
32 4 36
2 4 6
9 6 15
53 31 84
Table 4. Contingency table for structure --2-B. Frequencies for this structure late in the season versus all other management classes. Expected frequencies are given in parenthesis. E.g. the expected frequency of occurence in traditionally cultivated soils later in the season is 36 x 53/84 which is 23. In this example structure --2-B is strongly associated with the management class group. Management clases (listed in Table 1) Traditional cultivation Late in the season Occurrence Non occurrence TOTAL
32 (23) 4 (13)
36
All other classes 21 (30) 27 (18 48
Total
53 31 84
mammoth thin sections were prepared omitting visible wheelings over a three year period and representing the management classes shown in Table 1. The thin sections measured 8 x 10 cm and were taken from surface samples (0 - 10cm). Areas of like structure were delineated on overlays at the 1:l scale. Each area was then measured and classified using the system shown in Table 2. Each structure (c. 2 - 4 per section) was classified by visual comparison with reference sections which illustrate modes and ranges for each structure type (Hall, 1980). The definition of terms follows Bullock et al. (1985). Curvo-planar voids are transitional between compound packing voids and planar voids. RESULTS The structure type frequency data obtained were tested using the Chi square test. The null hypotheses used were based on the general expression that the frequency of occurrence (number of sections in which a structure type occurs) of a structure for a management class was similar to the frequencies expected from the number of times a structure occurred and the number of times each management group was represented. Table 3 shows an example table of results which was converted into the 2 x 2 contingency table shown in Table 4. This was
N.W. HALL
662
TRADITIONALLY CULTIVATED Strong relationships
?:j
Increasing incidences over season
-
Weaker relationships
1-2-2-B
Strong relationships
4-3-
-Decreasing incidences over season Weaker relationships
4 Observed > expected
DIRECT DRILLED Statistical association at end of season only.
Observed < expected
Fig. 1. Changes is soil structure over the growing season. Arrows denote changes from early to late in the cycle of crop production. Probability 76 for rejecting the null hypothesis
1 represents 2 3 4 A " I, I,
>99.9 99.9 - 99 99 - 95 95 - 90
Dot Size Each thin section was divided into areas of like structure, which were then measured. Dot size represents the mean area for each structure. 0 > 50%meansectionarea < 50%meansectionarea 0 no rating due to low frequency
4 0
Other structures tested and showing no associations or dissociations: 1---, 2----, ---B, --D, 1-2--, 2--C, 2--D, 2--B/C or D, --1-D, 2-2-2-, 2-2--, 2-3--
SOIL STRUCTURE TRANSFORMATIONS OVER THE GROWING SEASON
663
repeated for all structures and management classes which met the Chi square requirements. This provided 45 Chi square probabilities for rejecting the null hypothesis for tilled land and 17 direct drilled land. Structures whose probability for rejecting the null hypothesis exceeded 90% and changed between early and late season are shown in Fig. 1. Strong relationships are where probability levels show large changes. This is illustrated by structure 2-2-2-C (Le. partially developed peds with medium porosity, small peds and curvo-planar voids) which changed from an infrequent structure early in the season at the 95 - 99% level to a frequent structure late in the season at the 99 - 99.9% level. Approximately half of all structural associations tested gave statistically significant associations which changed between early and late in the cycle of crop production. Fig. 1 includes information on the mean area (as a percentage of each section) occupied by a structure. Mean areas less that 50% tend to be mainly in the range 20 - 45% and more than 50% tend to be in the range 52 - 70%. In view of the large range of area values, areas are not discussed at length. A more intensive study would be needed. However, Fig. 1 suggests that structures which become more abundant over the season tend to involve areas less than 50% unlike structures whose incidence decreased over the season which tend to occupy large areas of thin sections. Fig. 2 illustrates examples of the structures. In traditionally cultivated soils, the stronger relationships show that peds have become smaller. Ped size class 2 corresponds to more than 50% of peds less than 5 mm2 and more than 80% less than 40mm2. In addition peds move into closer packing - porosity class 2 corresponds with 10 - 20% voids with curvo-planar voids and to a lesser extent compound packing voids. The weaker changes show the increasing importance over the season of clearly defined peds with combinations of medium porosity (30 - 50%), low porosity (<30%), small peds and compound packing voids. Structures decreasing in abundance over the season comprise larger peds with large porosities (90%) and also structures with no peds together with low porosities or these structures with small voids and planar voids or vughs. The less marked changes show a decreasing incidence of no clear peds and/or vughs. A comparison between direct drilled and traditionally cultivated fields shows that none of the structures were associated specifically with direct drilling except the individual differentiating characteristics of curvo-planar voids or vughs. These structures were associated with direct drilling at the end of the season. DISCUSSION AND INTERPRETATION Individual differentiating characteristic classes do not reveal structural changes. The changes over the season involve two or more differentiating characteristics. In addition, the more significant changes include all the four differentiating characteristics showing that combinations of class groups within degree of ped development, porosity, ped size (or void size) and void type are important. Changes over the season involve both classes for soil solids together with classes for air spaces. This shows the importance of measuring a range of soil properties in soil studies. The results show that while peds become smaller and move into closer packing, pedality is maintained at least at the partially developed ped level. Structures with no clear peds become less abundant showing a maintenance of soil structure over the season unlike in some other soils (Dexter, 1976). The Downland site and sandy loam soils are
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664
2-2-2-c/--2-c i.e. Partially developed peds, medium porosity smallpeds, curvoplanar voids. These structures are particularly associated with the end of the growing season.
Classification 4-3-2-D 1- 1- 1 -B/l--B i. e. clearly developed i.e. No clear peds, peds, large porosity, low porosity small large peds, planar voids. compound packing voids. The clearly developed peds together with the compound packing voids are associated with soils late in the season. These characteristics plus high porosity and large peds are dissociated from soils late in the season.
---E i.e. Vughs
These structures become less common over the season due to fragmentation. Vughy structures do the same but become more common in direct drilled land later in the season.
Fig. 2. Illustrations of the structures reported in Fig. 1. Frame width 10 mm. well suited to sequential direct drilling (Cannell et al., 1979) reflecting their ability to maintain seedbed characteristics through shrinkage over the season as shown in other soils by MackieDawson et al. (1989). Adequate rooting should assist in water utilisation on the shallow Chalk soils even though upward movement of water is important on these soils (Gregory, 1989).
SOIL STRUCTURE TRANSFORMATIONS OVER THE GROWING SEASON
665
Fig. 3. Soil structure transformations. Fig. 3 illustrates pathways of structural transformation through fragmentation with consolidation and fragmentation with dispersion possibly producing similar endpoints. The results for within season changes at the Downland site can be compared with structural associations between long term tilled land (>70 years) and more recently tilled land ( ~ 3 5 years)(Hall, 1990). That study showed that clearly defined peds with compound packing voids or medium porosity are positively associated with long term tilled land. Thus as time elapses within the season, the structures move towards the closer packing of small peds with maintained pedality. This has similarities with the long term effects of tillage on Chalk soils. At the Downland site increasing calcium carbonate contents together with changing soil colour of these calcareous soils focuses on the maintenance of pedality at the partially isolated ped level. The results support the stabilising role of calcium carbonate in structure W m e r and Greenland, 1976) and the development of small peds which may be assisted by the lower tensile strength where exchange surfaces are highly dominated by calcium (Dexter and Chan, 1991). The direct drilling results perhaps demonstrate that as these are surface samples they are disturbed through the action of drilling. Hence these soils have similarities with traditionally cultivated land. This is similar to Ap material shown by Pagliai et al. (1984) for a calcareous clay loam. That work demonstrated differences in structure between no till and conventionally tilled treatments but no specific structure exclusive to either. Differences were more in amounts of different voids rather than type of void per se. MAFF (1992) has indicated the need for information on erosion. Relationships between structure types and the incidence of a water erosion fabric on these soils (Hall, 1987) can be applied to these results. In the 1987 study 25 structures occurred in association with an erosion fabric. 10% of these became more abundant over the season, for example partially
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N.W. HALL
developed peds with medium porosity and small peds together with curvo-planar voids. However, 44% become less common over the season for example, structures with the vughly void class. This suggests that the erosion hazard is greatest early in the season supporting the findings of Mutter and Burnham (1990) for Chalk soils. Structures favouring erosion tended to have large mean areas although their incidence decreased over the season. This suggests that erosion could focus or be initiated at specific points within fields later in the season. Visible wheelings, which were not sampled here, are likely to contain fewer peds and more planar voids (Bresson and Zambaux, 1990) and be the focus of water erosion. Avoiding erosion will require good soil and machinery management including tyre selection (Rusanov, 199 1). Field observations and judicous interpretation of profile characteristics would seem to be important in utilising the soil's ability to maintain or improve its structure in an environment of seasonal variability and climatic change (Varallyay, 1990). CONCLUSIONS The results demonstrate the importance of both soil and void components and of using more than one parameter in soil structure studies. Peds became smaller over the season and pedality is maintained at the partially developed ped level as peds move into closer packing. This contrasts with the long term effects of tillage on the Chalk soils where increasing calcium carbonate contents through erosion are associated with even more strongly developed pedality and compound packing voids. Within season structural changes suggest that the water erosion hazard decreases over the season which focuses on the need for appropriate machinery management practices. ACKNOWLEDGEMENTS I am indebted to Harper Adams Agricultural College for supporting this programme, Dr J.B. Dalrymple of Reading University for supervising the original research funded by The Ministry of Agriculture Fisheries and Food. REFERENCES Bresson, L.M. and Zambaux, C., 1990. Micromorphological study of compaction induced by mechanical stress for a Dystrochreptic Fragiudalf. In: L.A. Douglas (Editor), Soil Micromorphology: A Basic and Applied Science. Proc. VIII Int. Working Meeting on Soil Mircromorphology, San Antonio, Texas, July 1988. Developments in Soil Science 19, Elsevier, Amsterdam, pp. 33-40. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., and Tursina, T., 1985. Handbook for Soil Thin Section Description. Waine Research Publications, Wolverhampton, U.K., 152 pp. Cannell, R.Q., Davies, D.B., Mackney, D and Pidgeon, J.D., 1979. Suitability of soils for sequential direct drilling of combine-harvested crops in Britain: a provisional classification. In: M.G. Jarvis and D. Mackney (Editors), Soil Survey Applications. Soil Survey Technical Monograph 13, Harpenden, Hertfordshire, U.K. Dexter, A.R., 1976. Internal structure of tilled soils. J. Soil Sci., 27: 267-278.
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Dexter, A.R. and Chan K.Y., 1991. Soil mechanical properties as influenced by exchangeable cations. J. Soil Sci., 42: 219-226. Gregory, P.J., 1989. Depletion and movement of water beneath cereal crops grown on a shallow soil overlying Chalk. J. Soil Sci., 40: 513-523. Hall, N.W., 1980. An Integrated Macro and Micro-pedological Approach to the Study of Soil Structure. Ph.D. thesis, University of Reading, U.K. Hall, N.W., 1987. An application of micromorphology to evaluating the distribution and significance of soil erosion by water. In: N. Fedoroff, L.M. Bresson and M.A. Coutry (Editors), Soil Micromorphology. Proc. VII Int. Working Meeting of Soil Micromorphology, Paris, July 1985. Association Franqaise pour 1'Etude du Sol, Plaisir, France, pp. 437-443. Hall N.W., 1990. Micromorphology and complementary assessments of soil structure description and their relationship to length of time under tillage and calcium carbonate contents. In: L.A. Douglas (Editor), Soil Micromorphology: A Basic and Applied Science. Proc. VIII Int. Working Meeting on Soil Mircromorphology, San Antonio, Texas, July 1988. Developments in Soil Science 19, Elsevier, Amsterdam, pp. 53-60 Jarvis, M.G., 1973. Soilsof the Wantage and Abingdon District. Memoirs of the Soil Survey of Great Britain, England and Wales, Harpenden, U.K., 200 pp. Mackie-Dawson, L.A., Mullins, C.E., Fitzpatrick, E.A. and Court, M.N., 1989. Seasonal changes in the structure of clay soils in relation to soil management and crop type. 1. Effects of crop rotation at Cruder Bay, N.E. Scotland. J. Soil Sci., 40: 269-281. MAFF, 1992. Ministry of Agriculture, Fisheries and Food, Research Strategy Requirements Document 1992-94. MAFF, London, pp. 29-30. Mutter, G.M. and Burnham, C.P., 1990. Plot studies comparing water erosion on Chalky and non calcareous soils. In: J. Boardman, I.D.L. Foster and J.A. Dearing (Editors), Soil Erosion of Agricultural Land. John Wiley and sons, Chichester, U.K., pp. 15-23. Pagliai, M., La Marca, M., Lucamante, G. and Genovese, L., 1984. Effects of zero and conventional tillage on the length and irregularity of elongated pores in a clay loam soil under viticulture. Soil Tillage Res., 4: 433-444. Rimmer, D. and Greenland, D.J., 1976. Effects of calcium carbonate on the swelling behaviour of a soil clay. J. Soil Sci., 27: 129-139. Rusanov, V.A., 1991. Effects of wheel and track traffic on the soil and on crops growth and yield. Soil Tillage Res., 19: 131-143. Soil Survey Staff, 1975. Soil Taxonomy. U.S. Dept. Agric. Handb. 436, U.S. Gov. Printing Office, Washington, D.C., 754 pp. Varallyay, G.Y., 1990. Influence of climatic change on soil moisture regime, texture, structure and erosion. In: H.W. Scharpenseel, M. Schomaker and A. Ayoub (Editors), Soils of Warmer Earth. Developments in Soil Science, 20, Elsevier, Amsterdam, pp. 39-49. Wild. A., (Editor), 1988. Russells Soil Conditions and Plant Growth. 11th ed., Longman, New York.
Soil structure transformations over the growing season - A micromorphological approach N.W. Hall Crop and Environment Research Centre, Harper A d a m Agricultural College, Newport, Shropshire, TFlO 8NB, UK ABSTRACT Hall, N.W.,1994. Soil structure transformations over the growing season - A micromorphological approach. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Roc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 659-667.
The study investigates the frequency of occurrence of structures over the growing season in order to understand which structural attributes change in abundance over the season given that soil structure is a complex mixture of soil aggregate and air space characteristics. Thin sections were prepared of arable soils on five different parent materials. The soils were Rendolls on Chalk, Typic Hapludalfs on clayey drift, Vertic and Mollic Haplaquepts on alluvium, and Psammentic Hapludalfs on sandstone. Structure types were delineated on sections at the 1:l scale and classified using four differentiating characteristics, namely degree of ped development, porosity, ped size (void size if apedal) and void type, and their percentage area measured. The results show that peds become smaller (>50% peds <5 mm2 and >80% <40 mm2) and move into closer packing (10 - 20% voids) but do not loose their identity over the season. Sections showing a lack of ped development become increasingly less common over the season. The results show the role of improved ped packing through consolidation but with shrinkage and swelling maintaining structure particularly on the Chalk. The paper integrates these findings with those published elsewhere on the influence of the length of time under tillage on structure and on calcium carbonate content, soil colour and interrelationships between structure and erosion. While aspects of good structure are not fully understood, structure changes over the season. This shows the need to optimise management to meet competing needs of agronomy, economy and sustaining the soil resource.
INTRODUCTION There are two objectives in the physical management of arable soils; namely, creating a suitable seedbed, usually through tillage, and maintaining the number of coarse pores over the growing season (Wild, 1988, pp. 435-441). The latter is achieved by good soil management practices assisted or detracted from by intrinsic soil properties within a given climatic environment.
N.W. HALL
660 Table 1 Management classes sampled in the study. Class 1. Class 2. Class 3. Class 4.
Traditionally cultivated arable land during seed bed preparation and early stages of crop production. Winter crops sampled before mid November and spring crops before June. As 1 above but at the end of the cycle of crop production or after harvesting but before autumn tillage. Sampled August and September. Direct drilled cereals early in the annual cycle. As 3 but late in the cycle.
Table 2 The classification of soil structure types (from Hall,1990). A structure whose degree of ped development was class 1, whose porosity was class 2, whose ped size was class 2 and whose void type was class B would be denoted 1-2-2-B.
Classes
Differentiating Characteristics ~~
Degree of ped development
Class 1 Class 2 Class 3 Class 4 Clearly defined Partially Secondary peds No peds peds developed peds
Porosity
Class 1 High
Class 2 Medium
Ped size (or void size in the absence of peds)
Class 1 Large
Class 2 Small
Void type
Class B Compound oacking voids
Class c Curvo planar voids
~~
~
~~
Class 3 Low
Class D Planar voids
Class E Vughs
This paper attempts to elucidate structure transformations by investigating structural characteristics associated with arable land early in the annual cycle of crop production compared with late in the cycle.
THE SITES STUDIED AND METHODS USED Two study areas were chosen which had a range of soils and management histories and had presented erosion problems. One is on the Berkshire Chalk Downland (National grid reference SU 554803) and the other in Oxfordshire (SU 454960). The Downland soils comprise silty clay loam Rendzina soils and clay loams on drift (Jarvis, 1973) which approximate to Typic Rendolls and Typic Hapludalfs (Soil Survey Staff, 1975). In Oxfordshire the soils are light sandy loams, sandy loam terrace soils and clayey alluvium which correspond to Psammentic Hapludalfs, Mollic Haplaquepts and Vertic Haplaquepts. Ninety six vertically orientated
66 1
SOIL STRUCTURE TRANSFORMATIONS OVER THE GROWING SEASON
Table 3 Table of results for structure --2-B (i.e. occurence or non-occurence of small peds and compound packing pores. Management classes (listed in Table 1) Structure -2-B
1
2
3
4
TOTAL
Occurrence Non occurrence TOTAL
10 17 27
32 4 36
2 4 6
9 6 15
53 31 84
Table 4. Contingency table for structure --2-B. Frequencies for this structure late in the season versus all other management classes. Expected frequencies are given in parenthesis. E.g. the expected frequency of occurence in traditionally cultivated soils later in the season is 36 x 53/84 which is 23. In this example structure --2-B is strongly associated with the management class group. Management clases (listed in Table 1) Traditional cultivation Late in the season Occurrence Non occurrence TOTAL
32 (23) 4 (13)
36
All other classes 21 (30) 27 (18 48
Total
53 31 84
mammoth thin sections were prepared omitting visible wheelings over a three year period and representing the management classes shown in Table 1. The thin sections measured 8 x 10 cm and were taken from surface samples (0 - 10cm). Areas of like structure were delineated on overlays at the 1:l scale. Each area was then measured and classified using the system shown in Table 2. Each structure (c. 2 - 4 per section) was classified by visual comparison with reference sections which illustrate modes and ranges for each structure type (Hall, 1980). The definition of terms follows Bullock et al. (1985). Curvo-planar voids are transitional between compound packing voids and planar voids. RESULTS The structure type frequency data obtained were tested using the Chi square test. The null hypotheses used were based on the general expression that the frequency of occurrence (number of sections in which a structure type occurs) of a structure for a management class was similar to the frequencies expected from the number of times a structure occurred and the number of times each management group was represented. Table 3 shows an example table of results which was converted into the 2 x 2 contingency table shown in Table 4. This was
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662
TRADITIONALLY CULTIVATED Strong relationships
?:j
Increasing incidences over season
-
Weaker relationships
1-2-2-B
Strong relationships
4-3-
-Decreasing incidences over season Weaker relationships
4 Observed > expected
DIRECT DRILLED Statistical association at end of season only.
Observed < expected
Fig. 1. Changes is soil structure over the growing season. Arrows denote changes from early to late in the cycle of crop production. Probability 76 for rejecting the null hypothesis
1 represents 2 3 4 A " I, I,
>99.9 99.9 - 99 99 - 95 95 - 90
Dot Size Each thin section was divided into areas of like structure, which were then measured. Dot size represents the mean area for each structure. 0 > 50%meansectionarea < 50%meansectionarea 0 no rating due to low frequency
4 0
Other structures tested and showing no associations or dissociations: 1---, 2----, ---B, --D, 1-2--, 2--C, 2--D, 2--B/C or D, --1-D, 2-2-2-, 2-2--, 2-3--
SOIL STRUCTURE TRANSFORMATIONS OVER THE GROWING SEASON
663
repeated for all structures and management classes which met the Chi square requirements. This provided 45 Chi square probabilities for rejecting the null hypothesis for tilled land and 17 direct drilled land. Structures whose probability for rejecting the null hypothesis exceeded 90% and changed between early and late season are shown in Fig. 1. Strong relationships are where probability levels show large changes. This is illustrated by structure 2-2-2-C (Le. partially developed peds with medium porosity, small peds and curvo-planar voids) which changed from an infrequent structure early in the season at the 95 - 99% level to a frequent structure late in the season at the 99 - 99.9% level. Approximately half of all structural associations tested gave statistically significant associations which changed between early and late in the cycle of crop production. Fig. 1 includes information on the mean area (as a percentage of each section) occupied by a structure. Mean areas less that 50% tend to be mainly in the range 20 - 45% and more than 50% tend to be in the range 52 - 70%. In view of the large range of area values, areas are not discussed at length. A more intensive study would be needed. However, Fig. 1 suggests that structures which become more abundant over the season tend to involve areas less than 50% unlike structures whose incidence decreased over the season which tend to occupy large areas of thin sections. Fig. 2 illustrates examples of the structures. In traditionally cultivated soils, the stronger relationships show that peds have become smaller. Ped size class 2 corresponds to more than 50% of peds less than 5 mm2 and more than 80% less than 40mm2. In addition peds move into closer packing - porosity class 2 corresponds with 10 - 20% voids with curvo-planar voids and to a lesser extent compound packing voids. The weaker changes show the increasing importance over the season of clearly defined peds with combinations of medium porosity (30 - 50%), low porosity (<30%), small peds and compound packing voids. Structures decreasing in abundance over the season comprise larger peds with large porosities (90%) and also structures with no peds together with low porosities or these structures with small voids and planar voids or vughs. The less marked changes show a decreasing incidence of no clear peds and/or vughs. A comparison between direct drilled and traditionally cultivated fields shows that none of the structures were associated specifically with direct drilling except the individual differentiating characteristics of curvo-planar voids or vughs. These structures were associated with direct drilling at the end of the season. DISCUSSION AND INTERPRETATION Individual differentiating characteristic classes do not reveal structural changes. The changes over the season involve two or more differentiating characteristics. In addition, the more significant changes include all the four differentiating characteristics showing that combinations of class groups within degree of ped development, porosity, ped size (or void size) and void type are important. Changes over the season involve both classes for soil solids together with classes for air spaces. This shows the importance of measuring a range of soil properties in soil studies. The results show that while peds become smaller and move into closer packing, pedality is maintained at least at the partially developed ped level. Structures with no clear peds become less abundant showing a maintenance of soil structure over the season unlike in some other soils (Dexter, 1976). The Downland site and sandy loam soils are
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2-2-2-c/--2-c i.e. Partially developed peds, medium porosity smallpeds, curvoplanar voids. These structures are particularly associated with the end of the growing season.
Classification 4-3-2-D 1- 1- 1 -B/l--B i. e. clearly developed i.e. No clear peds, peds, large porosity, low porosity small large peds, planar voids. compound packing voids. The clearly developed peds together with the compound packing voids are associated with soils late in the season. These characteristics plus high porosity and large peds are dissociated from soils late in the season.
---E i.e. Vughs
These structures become less common over the season due to fragmentation. Vughy structures do the same but become more common in direct drilled land later in the season.
Fig. 2. Illustrations of the structures reported in Fig. 1. Frame width 10 mm. well suited to sequential direct drilling (Cannell et al., 1979) reflecting their ability to maintain seedbed characteristics through shrinkage over the season as shown in other soils by MackieDawson et al. (1989). Adequate rooting should assist in water utilisation on the shallow Chalk soils even though upward movement of water is important on these soils (Gregory, 1989).
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Fig. 3. Soil structure transformations. Fig. 3 illustrates pathways of structural transformation through fragmentation with consolidation and fragmentation with dispersion possibly producing similar endpoints. The results for within season changes at the Downland site can be compared with structural associations between long term tilled land (>70 years) and more recently tilled land ( ~ 3 5 years)(Hall, 1990). That study showed that clearly defined peds with compound packing voids or medium porosity are positively associated with long term tilled land. Thus as time elapses within the season, the structures move towards the closer packing of small peds with maintained pedality. This has similarities with the long term effects of tillage on Chalk soils. At the Downland site increasing calcium carbonate contents together with changing soil colour of these calcareous soils focuses on the maintenance of pedality at the partially isolated ped level. The results support the stabilising role of calcium carbonate in structure W m e r and Greenland, 1976) and the development of small peds which may be assisted by the lower tensile strength where exchange surfaces are highly dominated by calcium (Dexter and Chan, 1991). The direct drilling results perhaps demonstrate that as these are surface samples they are disturbed through the action of drilling. Hence these soils have similarities with traditionally cultivated land. This is similar to Ap material shown by Pagliai et al. (1984) for a calcareous clay loam. That work demonstrated differences in structure between no till and conventionally tilled treatments but no specific structure exclusive to either. Differences were more in amounts of different voids rather than type of void per se. MAFF (1992) has indicated the need for information on erosion. Relationships between structure types and the incidence of a water erosion fabric on these soils (Hall, 1987) can be applied to these results. In the 1987 study 25 structures occurred in association with an erosion fabric. 10% of these became more abundant over the season, for example partially
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developed peds with medium porosity and small peds together with curvo-planar voids. However, 44% become less common over the season for example, structures with the vughly void class. This suggests that the erosion hazard is greatest early in the season supporting the findings of Mutter and Burnham (1990) for Chalk soils. Structures favouring erosion tended to have large mean areas although their incidence decreased over the season. This suggests that erosion could focus or be initiated at specific points within fields later in the season. Visible wheelings, which were not sampled here, are likely to contain fewer peds and more planar voids (Bresson and Zambaux, 1990) and be the focus of water erosion. Avoiding erosion will require good soil and machinery management including tyre selection (Rusanov, 199 1). Field observations and judicous interpretation of profile characteristics would seem to be important in utilising the soil's ability to maintain or improve its structure in an environment of seasonal variability and climatic change (Varallyay, 1990). CONCLUSIONS The results demonstrate the importance of both soil and void components and of using more than one parameter in soil structure studies. Peds became smaller over the season and pedality is maintained at the partially developed ped level as peds move into closer packing. This contrasts with the long term effects of tillage on the Chalk soils where increasing calcium carbonate contents through erosion are associated with even more strongly developed pedality and compound packing voids. Within season structural changes suggest that the water erosion hazard decreases over the season which focuses on the need for appropriate machinery management practices. ACKNOWLEDGEMENTS I am indebted to Harper Adams Agricultural College for supporting this programme, Dr J.B. Dalrymple of Reading University for supervising the original research funded by The Ministry of Agriculture Fisheries and Food. REFERENCES Bresson, L.M. and Zambaux, C., 1990. Micromorphological study of compaction induced by mechanical stress for a Dystrochreptic Fragiudalf. In: L.A. Douglas (Editor), Soil Micromorphology: A Basic and Applied Science. Proc. VIII Int. Working Meeting on Soil Mircromorphology, San Antonio, Texas, July 1988. Developments in Soil Science 19, Elsevier, Amsterdam, pp. 33-40. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., and Tursina, T., 1985. Handbook for Soil Thin Section Description. Waine Research Publications, Wolverhampton, U.K., 152 pp. Cannell, R.Q., Davies, D.B., Mackney, D and Pidgeon, J.D., 1979. Suitability of soils for sequential direct drilling of combine-harvested crops in Britain: a provisional classification. In: M.G. Jarvis and D. Mackney (Editors), Soil Survey Applications. Soil Survey Technical Monograph 13, Harpenden, Hertfordshire, U.K. Dexter, A.R., 1976. Internal structure of tilled soils. J. Soil Sci., 27: 267-278.
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Dexter, A.R. and Chan K.Y., 1991. Soil mechanical properties as influenced by exchangeable cations. J. Soil Sci., 42: 219-226. Gregory, P.J., 1989. Depletion and movement of water beneath cereal crops grown on a shallow soil overlying Chalk. J. Soil Sci., 40: 513-523. Hall, N.W., 1980. An Integrated Macro and Micro-pedological Approach to the Study of Soil Structure. Ph.D. thesis, University of Reading, U.K. Hall, N.W., 1987. An application of micromorphology to evaluating the distribution and significance of soil erosion by water. In: N. Fedoroff, L.M. Bresson and M.A. Coutry (Editors), Soil Micromorphology. Proc. VII Int. Working Meeting of Soil Micromorphology, Paris, July 1985. Association Franqaise pour 1'Etude du Sol, Plaisir, France, pp. 437-443. Hall N.W., 1990. Micromorphology and complementary assessments of soil structure description and their relationship to length of time under tillage and calcium carbonate contents. In: L.A. Douglas (Editor), Soil Micromorphology: A Basic and Applied Science. Proc. VIII Int. Working Meeting on Soil Mircromorphology, San Antonio, Texas, July 1988. Developments in Soil Science 19, Elsevier, Amsterdam, pp. 53-60 Jarvis, M.G., 1973. Soilsof the Wantage and Abingdon District. Memoirs of the Soil Survey of Great Britain, England and Wales, Harpenden, U.K., 200 pp. Mackie-Dawson, L.A., Mullins, C.E., Fitzpatrick, E.A. and Court, M.N., 1989. Seasonal changes in the structure of clay soils in relation to soil management and crop type. 1. Effects of crop rotation at Cruder Bay, N.E. Scotland. J. Soil Sci., 40: 269-281. MAFF, 1992. Ministry of Agriculture, Fisheries and Food, Research Strategy Requirements Document 1992-94. MAFF, London, pp. 29-30. Mutter, G.M. and Burnham, C.P., 1990. Plot studies comparing water erosion on Chalky and non calcareous soils. In: J. Boardman, I.D.L. Foster and J.A. Dearing (Editors), Soil Erosion of Agricultural Land. John Wiley and sons, Chichester, U.K., pp. 15-23. Pagliai, M., La Marca, M., Lucamante, G. and Genovese, L., 1984. Effects of zero and conventional tillage on the length and irregularity of elongated pores in a clay loam soil under viticulture. Soil Tillage Res., 4: 433-444. Rimmer, D. and Greenland, D.J., 1976. Effects of calcium carbonate on the swelling behaviour of a soil clay. J. Soil Sci., 27: 129-139. Rusanov, V.A., 1991. Effects of wheel and track traffic on the soil and on crops growth and yield. Soil Tillage Res., 19: 131-143. Soil Survey Staff, 1975. Soil Taxonomy. U.S. Dept. Agric. Handb. 436, U.S. Gov. Printing Office, Washington, D.C., 754 pp. Varallyay, G.Y., 1990. Influence of climatic change on soil moisture regime, texture, structure and erosion. In: H.W. Scharpenseel, M. Schomaker and A. Ayoub (Editors), Soils of Warmer Earth. Developments in Soil Science, 20, Elsevier, Amsterdam, pp. 39-49. Wild. A., (Editor), 1988. Russells Soil Conditions and Plant Growth. 11th ed., Longman, New York.