Soil & Tillage Research, 16 (1990) 329-336 329 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Effec t of P l an t Roots on Soi l S t r e n g t h

S.T. Willatt ~* and N. Sulistyaningsih 2

1School of Pure and Applied Sciences, The University of the South Pacific, Suva (Fiji) 2Soil Science Department, University of Jember, East Java (Indonesia)

(Accepted for publication 22 February 1990)

ABSTRACT

Willatt, S.T. and Sulistyaningsih, N., 1990. Effect of plant roots on soil strength. Soil Tillage Res., 16: 329-336.

Roots of plants have been shown, under some circumstances, to increase soil strength. To test whether this applies to rice-growing soils, a pot experiment was conducted using a soil from East Java, Indonesia, to which a series of puddling-irrigation treatments was applied consistent with rice-growing soils and preparation techniques in the East Java region. Plants were grown in half of the pots. Measurements included the bearing capacity and shear vane resistance of soil 45 and 70 days after emergence, and root weight and plant height at Day 70.

The results showed that in most cases rice roots increased both the bearing capacity and shear- ing resistance. A linear relationship between bearing capacity and shearing resistance was found, and agreed with published data for dry soils. The implication of the results for traffic in wet soils is discussed.

INTRODUCTION

Tillage rating systems have been proposed by Lal (1985) for tropical soils, for tilled and no-till systems in dryland agriculture, and also for wetland agri- culture. In the case of wetland agriculture, Willatt and Tranggono (1989) sug- gested some modifications to the wetland system as a result of experiments performed in wetland soils measuring changes in bearing capacity with time in rice paddies in Indonesia. In their experiments (see also Tranggono, 1988), they considered other factors which may influence strength, i.e. aging {which has been studied by Utomo and Dexter (1981) ), a water content effect even though the soil was always wet in their experiments, and the potential effect of roots.

Roots are known to be important in stabilizing soils (Tisdall and Oades, 1980) and increasing the shearing resistance of soils (Waldron, 1977; Waldron and Dakessian, 1982). Waldron (1977) and Waldron and Dakessian (1982)

*Formerly: School of Agriculture, La Trobe University, Bundoora, 3083 Vic., Australia.

0167-1987/90/$03.50 © 1990 Elsevier Science Publishers B.V.

330 S.T. WILLATT AND N. SULISTYANINGSIH

showed in laboratory experiments that a range of plants (pine, oak, lucerne and grasses) increase the shearing resistance of soil. In their experiments they grew the plants at field water contents, but measured shearing resistance at water contents near to saturation, i.e. the water content at which soils on slopes are most unstable in relation to slumping and therefore erosion. One aspect which was not measured in these experiments was bearing capacity and whether this measurement of strength is influenced by roots.

Knowing whether puddled soil strengthens with age or with root growth is important because of traffic in paddy soils during the growing period of rice and also at harvest. Weeding and fertilizer applications are usually carried out manually so people move across the land. Prior to harvest, carried out man- ually so people move across the land. Prior to harvest, the land is usually drained to allow the soil to increase in bearing capacity for the major traffic at harvest, which is usually manual in most of Indonesia. In his experiments with two Indonesian soils, Tranggono (1988) showed that bearing capacity increased with time from ploughing when these soils were puddled (a Regosol (loam 6- 15 kPa and a Vertisol (clay) 4-21 kPa). The time between the two readings was 10 weeks; after draining the values increased to 28 and 30 kPa, respectively.

When the soil was prepared for growing rice without puddling, the decrease to minimum bearing capacity after flooding took 3-4 weeks, but the value was then approximately equal to that of the puddled soil and increased in strength slowly with time.

The question raised, but not considered, was whether roots have any effect on the increase in bearing capacity. Rice culture relies on puddling of some form, but the methods vary with location. A series of "puddled" treatments was set up in pots and rice was grown in them to study the effect of roots on soil strength, i.e. bearing capacity, the importance of which has been indicated. Shearing resistance was also measured as information on such a measurement is available for wet (though not puddled) soil (Waldron, 1977; Waldron and Dakessian, 1982).

E X P E R I M E N T A L D E T A I L S

The soil used in this experiment is described as a Regosol (Food and Agri- culture Organization, 1974) and was collected from the Mojosari Experimental Station 112°38'E, 7°30 ' S, ~ 45 km southwest of Surabaya in East Java, In- donesia. The soil can be classified as a loam with 40% sand, 38% silt and 22% clay. Prior to preparation of the pots, the soil was air dried (water content 5.3% ) and ground to pass a 2-mm sieve.

Polythene tubes, 108-mm diameter cut into 150-mm lengths, were used as pots. One kilogram of air-dry soil with 200 ml water was prepared in these pots by packing to a depth of 100 mm, thus giving an equivalent dry bulk density of

EFFECT OF PLANT ROOTS ON SOIL STRENGTH 331

1.04 Mg m -3. As the soil in some of the pots was to be puddled, a compacted plug of soil with a bulk density of 1.8 Mg m -3 was prepared in the bottom of the pot. One kilogram of air-dry soil and 600 ml water were placed in the pots and allowed to stand for 1 day before mixing 20 times with a spatula. Mixing 20 times in this way gave a puddled consistency similar to a field soil with a small but insignificant change in bulk density to 1.03 Mg m -3 and, according to Tranggono (1988), is the equivalent of one complete working with plough and harrow in the field.

As the main treatments were on soils with and without plants, four irrigation treatments were prepared in an attempt to simulate field conditions, i.e. two where the soil was puddled and two where the soil was not. Puddling is stan- dard practice in many areas, however flooding without puddling also occurs (Tranggono, 1988), as does the rain-fed situation. This led to the following four irrigation treatments: I1, puddled soil, kept flooded with water to a depth of 20 mm; I2, puddled soil, irrigated each week with 30 mm of water; I3, non- puddled soil, irrigated each day with 5 mm water; I4, non-puddled soil, irrigated each week with 30 mm water. The watering regimes of the four treatments were such that I1 and I3 received the same amount of water each day, while I2 and I4 received the same amount of water each week. Each treatment was replicated four times.

Rice seeds were planted at 10 per pot. The seeds emerged after 2 days and after 1 week they were thinned to five per pot; at 2 weeks they were thinned again to three and finally at 3 weeks they were thinned to two plants per pot.

Soil strength was measured as bearing capacity and shearing resistance. Bearing capacity was measured with a laboratory penetrometer (electrically driven ) at a constant speed of 0.9 mm s-1 with a 15-mm diameter plate at- tached to the measuring transducer and measured to 20-mm depth in each case. Shearing resistance was measured using an Eijkelkamp I self-recording vane shear tester with the top of the vane 10 mm below the surface. Two sets of readings were taken, on Day 45 and Day 70 after seedling emergence. Water contents were determined at each sampling by weighing the pots; plant weights were ignored as the plant weight (root plus tops) at harvest was on average 4.3 g, this is 0.4% of the total weight of dry soil. At Day 70, the roots were washed and dry weights (70 ° C ) determined and plant heights measured.

Statistical analysis was carried out using a split-plot design; the plant and no-plant pots being the main treatments, and the puddling-irrigation treat- ments being the split plot.

RESULTS

Values of bearing capacity, shearing resistance, root weight and plant height are presented in Tables 1, 2 and 3, respectively. The dry treatment I4 was not

1Use of this equipment does not imply its endorsement.

332

TABLE 1

Bearing capacity of soils with and without plants (kPa)

S.T. WILLATT AND N. SULISTYANINGSIH

Irrigation treatment

I1 12 13 I4

LSD (5%)

With plants 45 days from emergence 70 days from emergence

without plants 45 days from emergence 70 days from emergence

Levels of significance With plant/without plant (P)

45 days P = 0 . 0 5 70 days P = 0 . 0 1

Irrigation treatment (I) 45 days ns 70 days P = 0.001

P X I 45 days P = 0.05 70 days P = 0 . 0 1

59.5 74.8 66.9 - 54.8 73.3 118.6 395.4

44.8 28.6 41.2 - 27.1 15.1 47.9 193.7

7.0 11.3

22.9

6.6 26.4

included at Day 45 as the soil was considered too dry because of the choice of sampling day in relation to irrigation time.

From Table 1 it is clear that plant roots have a significant effect on the bearing capacity of the soil, the plants always increasing the bearing capacity. The maximum value between treatments without plants and with plants was 4.8 times that in the same soil treatment I2 at Day 70. For the other treatments, on Day 70 this ratio was ~ 2.2. The differences were smaller on Day 45, al- though I2 still had the largest ratio (2.6), while for the other treatments this averaged 1.4. At Day 70, when the roots were washed and weighed, there were more roots in treatment I2 than in the other three treatments (Table 3), and significantly more than in I3 and I4.

Irrigation treatment has no effect at Day 45, however I4 was not measured on that day. At the later sampling, the irrigation treatment (simulating field treatments) was significant, I4 having a much larger bearing capacity than the other treatments. The interaction between plants and irrigation also has a significant effect on bearing capacity.

The values of water contents were not different in the I1 and I2 treatments with and without plants. For I3, the difference was 0.5% (volumetric water content) on both sampling days and for I4 the difference was 2.4% (see Table 4 for water content values).

The shearing resistance of the soil was greater with plants than without

EFFECT OF PLANT ROOTS ON SOIL STRENGTH

TABLE 2

Shearing resistance measured by shear vane of soils with and without plants (kPa)

333

Irrigation treatment

I1 I2 I3 I4

LSD (5%)

With plants 45 days from emergence 70 days from emergence

Without plants 45 days from emergence 70 days from emergence

Levels of significance With plant/without plant (P)

45 days P = 0.05 70 days P = 0.05

Irrigation treatment (I) 45 days ns 70 days P=0.001

P × I 45 days ns 70 days P=0.05

5.0 5.6 4.6 2.7 2.6 3.6 10.6

3.8 3.5 3.0 2.0 1.5 2.3 5.1

0.40 0.68

0.47

1.16

TABLE 3

Weight of roots (g per pot), plant height (cm) and AS (the difference in shearing resistance between soil with and without plants) 45 and 70 days after emergence

Irrigation treatment Significance LSD level (5% 0.05)

I1 I2 I3 I4

Root weight (g) 1.76 2.07 0.92 1 .01 P=0.001 Plant height (cm) 24.4 23.9 21.4 1 9 . 8 P=0.001 AS (kPa)

Day 70 0.7 1.1 1.3 5.5 - Day 45 1.2 2.1 1.7 -

0.38 1.40

plants (Table 2 ) for all treatments. In Treatments I1 and I2, where there was no difference in water content between pots with and without plants, this effect was then a result of plant roots only. In Treatment I3, small differences in water content may make a difference in shearing resistance, but this is not likely to be significant. The ratio of shearing resistance for plant to no plant averaged 1.5, except for the driest treatment where the ratio was 2.1. In I4, the difference in shearing resistance is a combined water-root effect. The differ-

334 S.T. WILLATT AND N. SULISTYANINGSIH

TABLE 4

Water content (m 3 m -3 ) a t sampling for pots in all t r ea tments

Day Irrigation t r ea tmen t

I1 I2 I3 I4

45 Plants 0.465 0.440 0.365 N M No plants 0.465 0.440 0.370 NM

70 Plants 0.465 0.440 0.355 0.316 No plants 0.465 0.440 0.355 0.340

NM = not measured.

ences are significant at the 5% level for treatments with and without plants on both Day 45 and Day 70, while for irrigation treatment and plant-irrigation interaction this only applies on Day 70.

In the wetter soils (I1 and I2), root weight and plant height were greater than in the dry soils (I3 and I4) (see Table 3, and in each case significantly so.

DISCUSSION

The irrigation treatments created in this experiment included puddled and non-puddled soils, although the latter treatments were given plenty of water. Puddling has been shown to lower shearing resistance (Kularatna, 1981 ) and this also lowers bearing capacity because of the relationship between bearing capacity and shearing resistance, shown in Fig. 1.

The linearity of the vane shearing resistance-bearing capacity relationship is similar to that obtained by Taylor et al. (1966) where the ratio of penetration resistance (bearing capacity) and shearing resistance was 28:1. In Fig. 1, the lines are drawn separately for the two sampling days and the r 2 values are 0.79 for Day 45 and 0.99 for Day 70. As the shearing resistance is related to the basic properties of adhesion and cohesion between the soil and water, the linear relationship between these two variables indicates that bearing capacity is also related to these two properties. The length of time from preparation appears to have an overriding effect, even on the irrigation treatments, in determining the bearing capacity-shearing resistance relationship.

Time after preparation (or age) reduces the soil shearing resistance ad bear- ing capacity of Treatments I1 and I2, i.e. the puddled soil both with and with- out plants. However, for Treatment I3 bearing capacity increased with age while shearing resistance decreased. Tranggono (1988) showed that the bear- ing capacity of puddled soil increases with time after puddling in laboratory experiments for up to 28 days, and for up to 70 days in the field. However with the soil (Regosol) used in this experiment, the change did not indicate an increase between Days 45 and 70.

EFFECT OF P L A N T ROOTS ON SOIL S T R E N G T H 335

12

11- 10"

~ 9. 8. c

¢~ 6.

e- 4.

~ 3.

e" 1.

0 0

7 / -" f o / / + p f

o/¢ i . ' / + o + i f

f

, t I " f f f j -

100 200 300 400 ;00

Bearing Capacity (kPa) Fig. 1. The relat ionship between bearing capacity (X) and shear vane resistance (Y) O, Day 45 after emergence, Y= 1.61 + 0.051X; r 2 = 0.79. ÷ , Day 70 after emergence, Y= 1.09 + 0.23X; r 2 = 0.99.

The bearing capacity of soil with roots was constant with time, while in soil without roots the bearing capacity decreased with time, which shows that a rice plants grow in a paddy field the soil is better able to support traffic. How- ever, even after Day 70 when the soil is in a wet condition, the bearing capacity was less than 95 kPa, the pressure usually applied by human traffic (Willatt, 1984). The average Asian farmer is physically smaller than the average West- ern person, fr whom the 95 kPa was determined, and even if the Asian farmer has smaller feet the load is still ~ 95 kPa. Hence there is good reason to expect that in the latter part of the season some support by the soil is possible. The average pressure applied by cattle is given as between 160 and 190 kPa. These data indicate that it would only be possible to support animals in the dry treat- ment for operations after planting.

Finally, values of AS are given in Table 3 for both Day 45 and Day 70, where AS is the difference in shearing resistance due to plant roots (Waldron and Dakessian, 1982). Values of AS are larger for I2 than I1 where the soils are saturated or near saturation, and 12 has 17% more root than I1 which is not i

significantly different. However, when studying individual pots, the regression between root weight and S for the pots with plants in these two treatments is linear and significant at 5%, i.e. more roots leads to greater shearing resistance.

To conclude, roots do increase soil strength and thus, as a rice season pro- gresses, the soil is better able to support the traffic. During those periods when the soil is not flooded (I2), which is often the case prior to weeding, strength

336 S.T. WILLATT AND N. SULISTYANINGSIH

is greater. Fur ther , in the per iod while the soil is drained, root growth may also be promoted .

ACKNOWLEDGEMENTS

The project was carr ied out while the au thors were at Brawijaya Univers i ty , Malang, Indonesia .

REFERENCES

Food and Agriculture Organization, 1974. FAO/UNESCO Soil Map of the World. 1:500,000. Vol. 1. UNESCO-FAO, Paris.

Kularatna, R.M., 1981. Shear strength-moisture content relations for puddled rice soils. M.Sc. (minor thesis), Agricultural University, Wageningen (unpublished).

Taylor, H.M., Roberson, G.M. and Parker, Jr., J.J., 1966. Soil strength-root penetration relations for medium- to coarse-textured soil materials. Soil Sci., 102: 18-22.

Tisdall, J.M. and Oades, J.M., 1980. The management of ryegrass to stabilise aggregation of red- brown earth. Aust. J. Soil Res., 18: 415-422.

Tranggono, R., 1988. Puddling in a "sawah" and its effect on the physical properties of soil. Ph.D. Thesis, La Trobe University, Bundoora, Australia.

Utomo, W.H. and Dexter, A.R., 1981. Age hardening of agricultural topsoils. J. Soil Sci., 32: 335- 350.

Waldron, L.J., 1977. The shear resistance of root-permeated homogeneous stratified soil. Soil Sci. Soc. Am. J., 41: 843-849.

Waldron, L.J. and Dakessian, S., 1982. Effect of grass, legume and tree roots on soil shearing resistance. Soil Sci. Soc. Am. J., 46: 894-899.

Willatt, S.T., 1984. Soil compaction. Dep. Agric., Victoria, Agric. Note Set. No. 136. Willatt, S.T. and Tranggono, R., 1989. Optimising tillage for wetland soil. 2nd ACIAR Workshop

on Draft Animal Power, Cipanas, Indonesia, July, 1989.