Interactions between earthworms and their soil environment

So11Btol Bmthem. ‘401 13. pp 191 to 197. 1981 Printed tn Great Brttam All rtghts reserved

0038-0717/81/030191-07102.00/0 Copyrtght 0 1981 Pergamon Press Ltd

INTERACTIONS BETWEEN EARTHWORMS THEIR SOIL ENVIRONMENT

AND

I. ABBOTT* and C. A. PARKER Department of Soil Science and Plant Nutrition, University of Western Australia, Nedlands, Western Australia 6009 (Accepted 29 September 1980) Summary-Laboratory experiments were used to study the effect of food quantity and quality on the biomass of earthworms, and the influence of earthworms on plant growth and infiltration of water into soil. Earthworms with the most food gained weight faster than those with little or no supplementary food. The latter also failed to become reproductively mature. Earthworms lost weight on a nitrogenpoor diet, but this was not rectified by supplementing such food with inorganic nitrogen added to the soil 2 weeks before the worms. Ryegrass grown in soil in which earthworms (Allolobophoratrapezoides) had been kept grew more slowly than in soil which had no previous worm activity, perhaps indicating that earthworms had converted relatively-available organic N into less available forms. Microscolex dubiusgave the fastest infiltration rates of water into soil when clover mulch was present. With Eisenia joetida there was little effect of worm density on infiltration rates; the highest density significantly increased infiltration but only when clover hay had been mixed in the soil. The surface casting behaviour of the two earthworm species varied with the placing of the food offered.

INTRODUCTION

The effects of Allolobophora trapezoides on plant growth in a wheatbelt soil were studied in a glass-

In many parts of the world earthworms are a major component of the biomass of soil animals. Because of their relatively large size and characteristic feeding behaviour, certain species have significant impacts on soil structure, soil fertility, plant growth and crop yields (Edwards and Lofty, 1972, 1978). Despite the demonstrated ability of earthworms to increase soil productivity, relatively little attention has been paid to the converse, namely, how soil fertility affects the abundance of earthworms. This interplay between food quality of soils for earthworms and the effect that earthworms in turn have on soil structure and plant growth is an important one. Characteristically, soils in the wheatbelt of Western Australia have a low natural fertility in the sense that crop plants grow poorly without added fertilizer. In these soils, earthworms should be absent or rare (Stockdill, 1969), and such soils should have a poorer structure and crop yield than similar ones with many earthworms. A survey of portion of the wheatbelt in 1979 showed that earthworms are widespread, but their occurrence is related to land use. Earthworms are present more frequently in pasture soils than cultivated ones (Abbott and Parker, 1980) which may result from the higher quantity and quality of organic matter in pasture soils than those cultivated (Clarke and Russell, 1977). We attempted to understand the role of the organic matter status of soils in developing a high density of earthworms by laboratory experiments which examined the effect of food quantity and quality on the total biomass of Eisenia foetida, and food quality on the growth rates of individual Microscolex dub&.

house experiment. Infiltration of water into cultivated soils is very much slower than into virgin soils in the Western Australian wheatbelt (Abbott et al., 1979). We have hypothesized that this results mainly from the disappearance of the large, burrowing soil animals after virgin bushland is cleared, cultivated and stocked. This deterioration in soil structure could perhaps be overcome partly by the introduction and maintenance of common large soil animals, such as ants, termites or beetles, but because earthworms are easier to culture, we have so far examined only the influence of these invertebtrates on infiltration of water into wheatbelt soils. We studied the effect of various densities of earthworms (E. foetidu) on the infiltration of water through soil and the effect of a surface mulch of clover on the soil in increasing the activity of earthworms (M. dubius) and thereby improving infiltration. The choice of earthworm species used in each experiment was determined by the availability of species at the time the experiments were set up. The numbers used and the weights chosen were determined largely by the material available.

* Present address: Institute of Forest Research and Protection, Hayman Road, Como, Western Australia 6152.

EXPERIMENTAL METHODS AND RESULTS Food quantity and biomass of Eisenia

Three treatments were randomized in three blocks. Into nine 250 ml conical flasks was placed a 3: 1 sand:loam soil which had been air-dried and sieved (~4 mm). Treatments were: 200 g soil per flask (A); 190 g soil + log crushed sheep dung per flask (B); and 199 g soil + 1 g crushed sheep dung per flask (C). The soil in each flask was wetted to 60% of its waterholding capacity and the flasks covered with a fine muslin. Ten days later five E. foetidu Sav. were

191

I ABBOTT and C. A. PARKER

192 os-

E g \ 0

-

_

E

.o f f

-

E

o-

gor -

no dung

I

K) Weeks

Fig. 1. Effect of food quantity on changes in body weight and reproductive condition of Eiseniafoerida. Arrow Indicates that 9 g of dung was added to the treatment with 1 g of dung at week 6. Standard errors of the means are shown as vertical bars.

weighed individually (after being briefly submerged in water and dried with tissue) and added in groups to each flask. Every week, for 14 weeks, the soil from each flask was emptied, the worms weighed individually, the presence or absence of a clitellum noted, and the worms and soil were returned to the flask. The total N content of the sheep dung used was 1.619,,. Hence. the total nitrogen content of the added food was as follows: 1 g, 16mg N: 9g, 145 mg N; log,

161 mg N.

Earthworms given most food (log dung) gained weight faster than those given 1 g dung or no dung (Fig. 1). After 6 weeks 9 g more dung was added to the treatment with 1 g dung. Within 4 weeks, these worms increased their biomass to as much as those in treatment B. The presence of a clitellum was used as a measure of the reproductive capacity of each worm. Worms in treatment A failed to develop a clitellum at any time over the 14 weeks (Fig. 1). Most worms in treatment B had a clitellum by week 3. Earthworms in treatment C did not have a clitellum until after week 6. when extra food was provided, but one had developed by week 10.

The only treatment in which Microscolex gained weight initially was that containing 2.5 g clover hay (Fig. 2) but after 36 days worms had already lost weight. The slopes of the lines during the first 36 days showed that worms in the treatments without straw, and with 0.5 g oat straw lost weight faster than the treatments with 0.5 g clover hay and 2.5 g oat straw (Fig. 2). The total N present in the clover hay and oat straw used was 2.32 and 0.279; respectively. Hence, the total nitrogen added in the straw or hay for the five treatments was 0; 12; 58; 1; 7 mg. During this experiment, faecal pellets (casts) were deposited on the soil surface. After 36 days, these were easy to separate from the rest of the soil. In 12 ran-

Food quality and biomass of Microscolex Five treatments were randomized in five blocks. To each of twenty five 500 ml conical flasks 400 g of airdried soil (< 2 mm) from Greenhills, Western Australia, was added. The soil treatments were: no straw added; 0.5 g chopped clover hay added (=0.25 g dried stems and 0.25 g of dried leaves); 2.5 g chopped clover hay (= 1: 1 mixture of stems and leaves); 0.5 g chopped oat straw; or 2.5 g chopped oat straw. Soils were wetted to 60”, of their water-holding capacity after 2 days. After 8 days earthworms (M. dubius) were weighed individually and one worm placed at random m each flask. Periodically, 36. 65, 100 and 126 days later, the flasks were emptied and each live worm weighed then the soil and worm were replaced.

0’

’

0

1

36

65

loo

126

DOYS

Fig. 2. Effect of food quality on changes in body weight of Microscolex dubius. The five food regimes used are 0, no addition; x. OSg chopped clover hay added; O. 2.5 g chopped clover hay added; 0. OSg chopped oat straw added; A. 2.5 g chopped oat straw added. Standard errors of the means are shown as vertical bars.

193

Earthworm: soil interactions

aool

0

0.5

IO

20

Weight of straw or hay added, g

Fig. 3. EtTect of food quality and nitrogen fertilizer on changes in body weight of Eiseniafoetida. 0 = no basal nutrient added. No = no fertilizer applied; N1 = 40mg N/flask (1OOppm); N2 = 80 mg N/flask (200 ppm). The actual N levels present per treatment are written beside each treatment. Significant differences between means are indicated by the use of arrows and level of significance.

domly-selected flasks the mean proportion of soil that had been converted to faecal pellets was 9% (range l-15%).

The difference between treatment by

means was assessed

(x1 - x~)/\ (2/3)52 Food quality and biomass of Eisenia Air-dried virgin sandy soil from Kojonup, Western Australia (0.03% total N) was sieved (c4mm) and 400 g added to each of forty two 500 ml conical flasks. The 14 treatments used, replicated 3 times, are shown in Fig. 3. The straw or hay applied was buried just below the surface of the soil. Nitrogen was applied in treatments Nr and N2 (Fig. 3) as ammonium nitrate. Basal nutrients (gl- ‘) applied to all but one treatment were: K2S04, 48.1; ZnSO*.7H,O, 1.5; CuS04~SH20, 1.5; (NH& Mo70z4.4HZ0, 0.03; and 1.33ml of the mixed solution was added per 400g of soil. Phosphorus was added as 0.11 g KH2P04 per 400g soil equivalent to 1008 kg of superphosphate ha-t. The concentrations of these nutrients added are adequate for the growth of oats in this soil (A. D. Robson, personal communication). Each flask was wetted to 60% of the soil water-holding capacity, covered with fine muslin, and allowed to stand for 2 weeks to allow fungal decomposition of straw to occur. Three weighed earthworms (E. foetida) of similar weight were added to each flask. Worms were reweighed 30 days later. The change in total worm weight was calculated for all 42 flasks (xtj for i treatments, j flasks). The mean proportional change in worm weight per treatment The variance within treatments

= Xi = f 2 xij. J was calculated

as

and compared with a r-statistic with 28 d.f. We expected that supplementing the N content of oat straw would improve the growth of earthworms. Instead N depressed growth (Fig. 3). This is not due to NHf toxicity because at low levels of added straw growth was not significantly reduced. Competition by soil microorganisms for readily-available carbon sources seems the most likely explanation for this result. Allolobophora

and plant growth

Air-dried soil collected at Greenhills in December 1977 was mixed and then divided into four buckets, A, B, C and D. Clover straw (10 g) was added to the surface of each bucket. No earthworms were added to buckets C and D; 11 worms (Allolobophora) with a combined weight of 5.31 g were added to bucket A; and 12 worms of the same species, combined weight of 5.13 g, were added to bucket B. The buckets were kept in a constant temperature room (2o”C), and watered weekly to about 60% of the soil water-holding capacity. Ninety nine days after setting up, the soils were searched carefully for worms. In bucket A, nine worms with a combined weight of 3.62g were recovered and in bucket B six worms weighing collectively 2.31 g were present. These earthworms were not returned to the soils. All of the soils were air-dried. Soil A was mixed with soil B, and soil C mixed with soil D. Sixteen cm dia pots lined with plastic bags. and containing 3 kg of soil, were placed in root cooling tanks (temperature 20°C) in a glasshouse. The treatments were paired (*worm-worked soil) and replicated six times. Soils were watered daily to field

194

I. ABBOTTand C. A. PARKER

Table 1. Mean fresh weight (g) + SE and total N (%) * SE of tops of Lolium rigidum L. after 45 days and a further 69 days in soil with and without earthworms. Sample size = 6 Age (days) Fresh weight of tops Worms added No worms added % Total N content of tops Worms added No worms added

45

114

12.7 + 0.6 17.4 * 1.0

17.1 + 0.6 22.1 * 0.9

3.9 * 0.1 4.5 + 0.1

0.7 * 0.01 0.7 f 0.01

capacity. Twenty-seven days later 20 seeds of Wimmera Ryegrass (Lolium rigidum L.) were planted in each pot; these were thinned to ten seedlings/pot 10 days later. Tops were harvested to 3 cm above soil level after 45 and 114 days, and were weighed immediately fresh and dried for total N analysis by the Kjeldahl method. The fresh weight of leaves of ryegrass was significantly greater (paired 2-tailed r-test, P < 0.01) in the soils to which no earthworms had been added (Table 1). The N content of tops after 45 days was significantly greater in the leaves of the plants that grew without worms (same test, P < O.OS),but there was no significant difference after a further 69 days (P > 0.05). The total amount of N in the leaves at both harvests combined was calculated to be 604 mg (in woil with worms) and 923 mg (in soil without worms). This suggests that 300mg N or approximately 10% of the N in the soil in which the grass grew had been immobilized by earthworms, perhaps being converted to an organic form of N less available for uptake by the grass. Effect of Microscolex water

and clover hay on injltration of

Soil collected from Greenhills was mixed with yellow sand (3: l), air-dried and placed in 13 cm pots, each with a small wire-gauze covered hole in the base. Pots contained 1.42 kg of soil and were wetted to 60% of the soil water-holding capacity. There were three treatments (no worms added, 1 worm added, 1 worm + 5 g moistened clover leaf added to surface), with five replicates. Infiltration was measured 34 days later with a hollow cylinder (9 x 9 cm dia) pushed gently into the surface of the soil. This cylinder was filled with water; the number of seconds that it took the water to fall each centimetre was recorded with a stopwatch. Infiltration of the first centimetre of water was fastest in the treatment containing an earthworm and a surface clover mulch (mean rf: SE = 39.4 f 5.8 s), and slowest in the treatment without a worm or clover mulch (79.0 + 9.4 s), and intermediate in the treatment with a worm but with no clover mulch (54.0 f 3.3 s). (Analysis of variance F2.s = 6.62, P < 0.025 and Student-Neuman-Keuls test). Faecal pellets were brushed off the surface of each pot that had a worm present and were weighed before infiltration times were measured. The mean weight of pellets produced on the surface was 53.7 f 4.8 g for pots lacking mulch compared with 40.3 + 3.8 g in

pots with mulch. There were significantly more pellets in the soil without clover mulch, suggesting that earthworms cast differently when mulch is available. Worms also lost weight in the treatments without clover mulch (mean body wt + SE = 0.51 & 0.04 g. based on 5 worms) and gained weight in the treatment with clover mulch (0.76 + 0.08 g, 5). Eisenia, water

distribution of clover hay, and injltration of

Soil collected from cultivated land at Kodj-Kodjin was air-dried, sieved (~4 mm) and placed in 13 cm pots each with six wire-gauze covered holes in the base. Each pot contained 1.37 kg of soil which was watered to 60% of its water-holding capacity. The nine treatments, replicated five times, were: no addition of finely-ground clover hay, addition of 5 g of hay to the surface of the pot, addition of 5 g of hay thoroughly mixed with the soil, with one of the following in turn: no worms added, two worms added, four worms added. Infiltration was measured (as described above) 30 days later. The infiltration times have been analysed separately because there was a difference in water content between the treatments with clover on the surface and the remainder (Table 2). Only one pair of treatment means differed significantly (Table 2). Infiltration into soil mixed with clover was faster with worms present than when worms were absent. Worms did not increase infiltration of water into soil with clover on the surface. Worms gained weight more quickly when clover was mixed in the soil (Fig. 4), irrespective of the density of worms per pot. The casting behaviour of worms differed in soil in which clover was mixed. because worms deposited more faecal pellets on the surface than they did in soil with no clover or with the clover on the surface only (Fig. 4). This result cannot be explained by differences in the weight of worms per pot (mean weights in treatments with no clover, clover on surface, clover mixed in soil at density of 2 worms per pot = 0.58, 1.34, 14Og respectively; at density of 4 worms per pot = 1.19, 2.39. 3.72 g respectively). DISCUSSION

Food requirements of earthworms and their injluence on plant growth Soil that has passed through the gut of earthworms changes so that nutrients are in a more available form for uptake by plants (Lunt and Jacobson, 1944; Satchell, 1958; Barley and Jennings, 1959; Sharpley and Syers, 1976); hence earthworms should promote plant growth. Several experiments purporting to demonstrate that earthworms do this have been shown by Barley (1961) to be inconclusive through poor experimental design, but properly conducted studies have confirmed the beneficial effect of earthworms on plant growth. Most of these studies (e.g. Waters, 1952; Hopp and Slater, 1948) have been conducted in soils in which the food supply of earthworms has not been limiting. In the experiment in which ryegrass grew with earthworms, the earthworms probably reached a situation where food was limiting because Wimmera ryegrass grew poorly on this soil and N content levels

195

Earthworm: soil interactions

Table 2. Infiltration of water into soil containing one of three levels of clover hay and one of three levels of earthworm inoculation (Eiseniafoetida). Infiltration measured 30 days after setting up of experiment

Nil

Clover treatment: Mean time (s) &SE for the water level in a column to drop 2 cm Mean soil moisture (%) f SE

Number of worms added 2 Nil Mixed

Mixed

’

Nil

Mixed

4

184 f 17 ab

284 + 14 a

172 f 21 ab

202 + 72 ab

164 + 28 ab

178 f 20 b

8.9 k 1.2 a

9.4 f 0.4 a

8.0 &-0.3

8.0 f 0.4

8.1 + 0.3 a

8.3 If: 0.3 a

Nuiber 0 Surface

Clover treatment: Mean time (s) *SE for the water level in a column to drop 2 cm Mean soil moisture (%) f SE

of worms aadded 2 Surface

4 Surface

216 k 34 a

213 & 85 a

239 f 72 a

13.7 + 1.2 a

10.8 f 0.7 b

11.2 + 0.6 b

Means in the same row followed by the same letter are not significantly different (P > 0.05) by Student-Neuman-Keuls

test. were lower than those in soil in which no earthworms

had been present. Thus, under certain conditions, earthworms could reduce plant growth, possibly by converting available organic N to less available forms. They did not deplete total soil N through increasing their own body N because 8 of the initial 23 worms died and the survivors lost weight (including an estimated 16 mg N). The results of the second experiment support the idea that available protein is important in determining the distribution and abundance of earthworms (Satchell, 1967) because worms increased in weight only on the highest protein food. This advantage lasted for about 40 days. No treatment involving still higher levels of protein, such as young clover leaf was used. Other work has shown that when soil N contents are increased, numbers and biomass of earthworms (Satchell, 1967; Tisdall, 1978) and their reproduction increased (Evans and Guild, 1948); nevertheless the N content of food is not the only important factor (Barley, 1959b). The experiment which examined the effect of food quality on biomass of Eisenia showed how much

Clover : Worms/~t:

nil

top mixed 2

nil mpmixd

4

extra N needs to be added to make oat straw suitable as food for earthworms. The N fertilizer depressed

growth of the earthworms compared to the treatments with oat straw without fertilizer. Probably, this was due to enhanced competition by micro-organisms for the energy-rich straw. It is well known that straw added to soil may temporarily reduce availability of nitrogen to plants. It is likely that insufficient time was allowed for microbial decomposition to produce protein-rich organic matter from N supplemented straw. Earthworms and infiltration of water

There is ample evidence that earthworm activity in soil increases infiltration rates of water (e.g. Slater and Hopp, 1947; Guild, 1955; Satchell, 1958; Stockdill and Cossens, 1967; Ehlers, 1975). Their tunnels also promote root growth (Edwards and Lofty, 1978). We have shown, however, that the presence of one M. dubius increased the infiltration of water if clover mulch was present; presumably worms move to the surface to feed on the clover and thus create more vertical tunnels. This suggests that Microscoles

nil top mkad

2

nil

top mixed

4

Fig. 4. Effect of placing of clover-hay on changes in body weight and surface casting of Eisenia fuerida. Standard errors of the means of surface casts are shown as vertical bars.

196

1. ABBOTTand C. A. PARKER Table 3 Extrapolations from experiments of weight of surface casts produced per area per year per worm casts (kgm-* yr-‘)

Earthworm species Mlcroscoiex dubzus Microscolex dubius without clover mulch Mlcroscolex dubius with clover mulch Eiseniafoetidu no addition of clover (density = 2 worms) Eiseniu foetidu with clover on surface (density = 2 worms) ~isenjufoer~du with clover mixed with soii

2-20

40 30 5 4

(density = 2 worms)

11 7 6

Eisenia foetidu no addttion of clover (density = 4 worms) Eiseniafoetida with clover on surface (density = 4 worms) Eiseniufoetida with clover mixed with soil

(density = 4 worms)

23

inoculated in clover leys in the Western Australian wheatbelt areas could improve long-term drainage. Another species, E. foetida, thrived on clover mulch (Fig. 4), but surface clover mulch did not improve infiltration. This may mean that Eisenia concentrates its feeding activities nearer the surface than Nicroscolex. successfully

Earthworms

and their castings

The main mechanism of tunnelling by earthworms is by ingesting soil (Dexter, 1978), which is voided on or below the surface as casts of faecal pellets. The penultimate experiment indicated that earthworms (Micruscolex dubitts) lost weight in the absence of a clover mulch but produced more casts. This was because they consumed more soil in attempting to obtain more food (Darwin, 1881). It is to be expected that the production of casts may improve soil stability, but the absence of suitable food must eventually lead to a decrease in numbers and biomass of earthworms. E. foetida differed in its casting behaviour according to the placing of the clover. There was little difference in the weight of surface castings produced between the treatments without clover and with surface clover; yet having the clover mixed into the soil resulted in significantly more surface castings than in the other treatments (Fig. 4). Because we have no measures of the weight of sub-surface casts produced in the treatments, we are unable to discuss this finding further. The total weight of surface castings gathered from these experiments have been extrapolated to an annual basis and converted to kg of surface castings produced per mz per year per worm. The lower range of our values agrees well with values of 2-5 kg me2 yr- ’ found by Darwin (1881), mainly in England, and of 5 kg mm2 yr- ’ found in Tennessee forests (Pratt, 1978). Values of 35 kg m- * yr- ’ are sufficient to result in soil turnover to a depth of 35cm within 60-70 yr (Pratt, 1978). Possible sign~~cance Australian

of eur~hwor~

in the

Western

~he~tbe~~

Current land management in the Western Australian wheatbelt seldom favours proliferation of earthworms. The 3- to 4-yr rotation (wheat-pasture-pasture) is losing favour to a 2-yr rotation (wheat-pasture) or even continuous wheat. The legume component of such pastures is usually sparse especially in the drier parts

of the region. Barley (1959a), in a climatic region (M~iterranean) similar to the Western Australian wheatbelt, showed that very few earthworms were found in soils cropped on a 2-yr rotation for 30yr. Only on longer rotations that include a substantial legume component were large populations of earthworms found. Normal cultivation reduces earthworm populations (Evans and Guild, 1948), and it is the length and quality of the ley that determines the rate of recovery to pre-ploughing levels (Barley 1959a personal observations). With direct drilling, there is little physical disturbance of the soil. Other studies (Edwards, 1975) and limited local observations suggest that earthworms increase under this regime. probably because physical disturbance is minimal and organic matter levels are higher (Russell, 1977) than with conventional cultivation.

Acknow~edgemenfs-This work was partially supported by both Wesfarmers Co-op Ltd and the Australian Research Grants Committee. We thank Maurice Barnes, Bruce Halbert. George Regazzo and Bill Short for supplymg solI and earthworms. We are grateful to L. K. Abbott and A. D. Robson for reading the manuscript, W. J. Simmons for the nitrogen determinations, and K. Upton for technical assistance. J. Henstridge advised on the statistlcal analysis of the experiment dealing with the effect of food quality on the biomass of Eisenia.

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BARLEYK. P. (1959b) The influence of earthworms on soil fertility. II. Consumption of soil and organic matter by the earthworm Allolobophoru culiginosu (Savigny). Austruliun Journui of Agricult~rul Research 10, 179-185.

197

Earthworm: soil interactions BARLEYK. P. (1961) The abundance of earthworms in agri-

cultural land and their possible significance in agriculture. Advunces in Agronomy 13,249-268. BARLEYK. P. and JENNINGSA. C. (1959) Earthworms and soil fertility. III. The influence of earthworms on the availabilit; of nitrogen. Australian Journal of Agricultural Research IO. 364-370. CLARKEA. L. and’Rvss~~~ J. S. (1977) Crop sequential practices. In Soil Factors in Crop Production in a SemiArid Environment (J. S. Russell and E. L. Greaten, Eds), pp. 279-300. University Press, Queensland. DARWIN C. (1881) The Formation of Vegetable Mould Through Habits.

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