Scientific registration n : 765 Symposium n : 20 Presentation: poster The Effect of Tillage on the Rock-Fragment Content of Cultivated Layers in the Mediterranean. Effet du labour sur la piérrosité des horizons en zone méditerranéenne. OOSTWOUD WIJDENES Dirk (1), POESEN Jean (2) VANDEKERCKHOVE Liesbeth (1) DE LUNA Elena (1) (1) Laboratory for Experimental Geomorphology, KU Leuven, Redingenstraat 16, B- 3000 Leuven, Belgium (2) Fund for Scientific Research, Flanders and Laboratory for Experimental Geomorphology, KU Leuven, Redingenstraat 16, B-3000 Leuven, Belgium Introduction A high amount of rock fragments in cultivated topsoils can have both negative and positive effects on the management of soils. Farmers are often concerned about the decline in soil fertility or they fear possible damage to their tillage equipment. However, in dryer areas that are threatened by desertification, such as parts of the Mediterranean, high amounts of rock fragments at the soil surface reduce runoff and increase infiltration (Poesen en Bunte 1996). Moreover, van Wesemael et al. (1996) and Nachtergaele et al. (1997) showed that soil moisture levels were much higher in stony soils than in comparable less stony soils. Several authors have investigated whether the development of a stony topsoil is influenced by the action of tillage because tillage may encourage interparticle percolation (Kouwenhoven and Terpstra 1970, 1979; Oostwoud Wijdenes et al. 1987). When a mixture of particles with different diameters (such as a soil) is disturbed, segregation occurs whereby the smallest particles accumulate at the bottom of the mixture and the largest particles at the top. This process is known under various names, such as interparticle percolation, kinetic filtering and kinetic sieving. It occurs because the smaller particles have more opportunities to move or slide through the differently sized openings that exist during the disturbance than larger particles. In the laboratory Kouwenhoven and Terpstra (1970, 1979) simulated the effect of tillage on particle segregation in the top layer of a soil. They used a tine-like tool that moved through a bed of glass spheres with different diameters. The authors showed that segregation rapidly occurred. In the field Oostwoud Wijdenes et al. (1997) investigated whether the widespread occurrence of stony surface layers in Southeast Spain could be the result of frequent chiselling by the farmers. Field experiments were set-up in an almond grove whereby soil pits were filled with four layers of rock fragments, each layer with a different mean particle diameter. Oostwoud Wijdenes et al. (1997) found that when initially the finest layer was at the top and the coarsest at the bottom of a pit, already after two tillage passes the upper four centimetres of the soil was dominated by the largest particles. These results have several implications. Removal of stones by farmers, will never be very successful in top soils that contain many rock fragments. In some cases, however, where a stony topsoil could be advantageous, moderate chiselling 1
may be a good management strategy to conserve soil moisture and reduce soil erosion. The field experiments by Oostwoud Wijdenes et al. (1997) were conducted under dry conditions in rather loose soil material. However, in practice rock fragments occur in a variety of soil types and farmers often till their fields when the soil is moist. There is little information on the effect of the fine earth texture and soil moisture on the rate of particle segregation by tillage. Therefore, a series of laboratory experiments was set up to determine the effect of soil moisture and cohesion on the vertical movement of rock fragments by using two types of fine earth as a soil matrix in which the rock fragments are embedded. Methodology and experimental design The field experiments were conducted to determine the change in vertical rock-fragment size distribution after a certain number of tillage passes, until steady state (no more change) was reached. Soil pits were prepared and filled with four layers of rock fragments. Each 4-cm-thick layer contained a known distribution of rock-fragment sizes. One series of pits contained the coarsest layer initially at the bottom and the finest on top, while a second series showed an even distribution of coarse and fine particles throughout the profile. The pits were subjected to different tillage frequencies (max. of 8 passes, all to a depth of 16 cm) by a caterpillar tractor pulling a chisel with a duckfoot. After the tillage operations, rock-fragment content (by mass) of each layer was determined by sieving and weighing (Oostwoud Wijdenes et al. 1997) The experimental set up in the laboratory was very similar to that of the field experiments conducted in Southeast Spain, however, at a smaller scale. An experimental flume, 120 cm long, 60 cm wide and 20 cm deep, was filled with three, four-cm-thick layers, containing rock fragments and fine earth. Each layer consisted of a predetermined rock-fragment/fine-earth mixture. The layers were perturbed by the movement of a hand-held cultivator to simulate tillage action. The device carried three chisels with duckfoots. The two outward chisels were twelve cm apart and lay in one row, while the middle one was placed six cm in front. A maximum depth of twelve cm could be reached with the device. To avoid clogging of the tines with rock fragments a maximum rock fragment diameter (b-axis) of 4 cm was used (Oostwoud Wijdenes and Poesen 1998) Three size classes were used instead of four (field experiments): rock fragments from 2.7 to 4.0 cm and from 1.2 to 2.2 cm and fine earth <0.2 cm. The fine earth consisted of sand or silt-loam. The silt-loam was sieved down to aggregates <0.5 cm in order to simulate a very fine seedbed. The two different fine earths, sand and silt-loam, were used to study the effect of cohesion of the soil matrix on particle segregation. The siltloam originates from top soils in the loess area from Central-Belgium (70-80% silt and 10-20% clay) and the sand (Brusselian sand) is a Tertiary deposit which underlies the same loess formation. Since it has been shown that the fine fraction migrates downwards and not upwards, all experiments were carried out with an initial distribution of the finest fraction on top and the coarsest material at the bottom. Each layer contained a high percentage of one fraction. However, in order to study the cohesive effect of the soil matrix, the rock fragment to fine earth ratio was kept at about half. Four moisture regimes were simulated to resemble wet (field capacity), intermediate and dry soil moisture conditions. Moisture was added by sprinkling each layer after it was spread out into the trough. The water could then infiltrate in the layers for half an hour before the 2
experiment started. This resulted in a fairly equal distribution of moisture thoughout the entire soil depth. A water depth of 20 mm was added to simulate approximately field capacity (equivalent to 0.17 cm³/cm³ moisture by volume). Two intermediate levels were simulated by adding 14 and 7 mm of water in the same manner (equivalent to 0.12 cm³/cm³ and 0.06 cm³/cm³ moisture by volume). The fourth regime was an air dry soil. Results and Discussion Field experiments The field experiments showed that after two tillage passes the coarsest fraction already dominated the top layer and the rate of sorting proceeded much slower thereafter. Fig. 1 shows this change in grain-size and rock-fragment size distribution in the top 4 cm (coarsest layer was initially at the bottom and the finest on top). Fig. 1. Effect of the number of tillage passes on particle and rock fragment size distribution in the 0-4 cm layer. (Rm is rock-fragment content by mass). 100 Rm (kg/kg) 90 80 70 60 50 40 < 0.6 cm 0.6-2.5 cm 2.5-5.0 cm > 5.0 cm 30 20 10 0 0 1 2 3 4 5 6 7 8 number of tillage passes Laboratory experiments For all four moisture levels (air-dry, 0.06, 0.12 and 0.17 cm³/cm³) the percentages sand decrease and the percentages rock fragments > 2.7 cm increase in the upper 4 cm after two tillage passes and even more so after six tillage passes. Even at the 0.17 cm³/cm³ moisture level the rock fragments and fine earth show a vertical mobility, however, the rate of movement is slower for all moist experiments compared to that for the dry ones. In addition, the upper layer was never dominated by the coarsest fraction as was the case during the field experiments after 2 tillage passes (Oostwoud Wijdenes et al. 1997). 3
The increase of intermediately sized rock fragments (1.2 to 2.2 cm) was always larger than that of the coarsest (2.7 to 4.0 cm) rock fragments in the upper layer. The results for the loamy mixture show a faster rate of change in the layers than that in the sandy mixture. In the upper layer, the increase of the intermediate fraction was of the same magnitude as that occurring in the sandy mixture. However, the coarsest fraction (2.7-4.0 cm) showed a bigger increase despite its longer travel distance. It was also observed that significant lateral transport of the loamy material occurred at the highest moisture level (0.17 cm³/cm³). This happened because the material became very sticky and blocked the openings between the tines of the device. As a consequence, the experimental soil mixture became thinner and only two layers (4 cm each) could be sampled after six tillage passes. Movement of largest fraction Fig. 2 shows the changes in content of rock fragments larger than 2.7 cm in the upper 4 cm of the profile for the four moisture levels in the sandy mixture. The graphs show that the percentage of coarse rock fragments steadily increases from 2 to 6 tillage passes. Fig. 2. Changes in content of rock fragments (Rm) larger than 2.7 cm in the upper 4 cm of the profile for the four moisture contents in a sandy matrix. Rm(kg/kg) 60 50 40 30 20 sandy matrix dry 0.06cm³/cm³ 0.12cm³/cm³ 0.17cm³/cm³ 10 0 0 1 2 3 4 5 6 tillage passes The results for rock-fragment increase (> 2.7 cm) in a loamy soil matrix are shown in fig. 3. As was the case for the sandy matrix, the curves continue to rise after 2 tillage events. However, the increase in large rock fragments is much larger than it was for the sandy matrix. The increase in rock fragments for the highest soil moisture level (0.17 cm³/cm³) lagged clearly behind that of the other experiments. However, at this moisture level considerable lateral transport of the soil mixture occurred because its cohesion 4
increased dramatically. As a consequence, after 6 tillage passes the soil depth was reduced to two third of its original depth. The measurement at this moisture level is therefore not comparable with that of the sandy experiments and not shown in the diagram. Fig. 3. Changes in content of rock fragments (Rm) larger than 2.7 cm in the upper 4 cm of the profile for the four moisture contents in a loamy matrix. Rm (kg/kg) 60 50 40 30 20 loamy matrix air-dry 0.06cm³/cm³ 0.12cm³/cm³ 10 0 0 1 2 3 4 5 6 tillage passes Thus the loamy matrix responds different from the sandy matrix. The higher level of particle segregation in the loamy matrix may indicate that the aggregates are partly destroyed during the tillage action due to the crushing effect of the moving rock fragments. Hence, the size distribution of the particles changes towards an increase in small particles and the ratio between the particle diameters of the largest and smallest particles in the mixture increases. Despite the increase of cohesion due to the water, which hampers particle percolation, there are relatively more small particles or aggregates that can move through the openings between the rock fragments than in the case of the moist sand. Also, at the lower and intermediate moisture contents (0.06 and 0.12 cm³/cm³) the wetting might not have been completely uniform. Some aggregates may still have interiors that are more dry or less wet than their surfaces. When the aggregates are broken due to the tillage action, these dryer inner particles are released and are more mobile. Another factor is that the water holding capacity of silt loam is greater than that of sand. Hence, the greater total surface area of the particles in the silt loam result in thinner water films at the same moisture content than in the sand and therefore create less cohesion. However, when moisture content increases further the cohesion increases strongly and the loams get very sticky. Interparticle percolation is therefore lower at the higher moisture content (0.17 cm³/cm³). Compared to the field experiments (Oostwoud Wijdenes et al. 1997) the particle 5
segregation at air-dry moisture levels occurred mostly slower in the laboratory. This may be due to plot preparation, to the difference in equipment or to the speed of the movement of the tillage tool. The aggregated silt loam in the laboratory does not behave as a cohesionless silt loam matrix which was present in the finest fraction in the field. Also, a more intense mixing (and therefore segregation) was achieved in the field with the chisel and duck-foot than with the hand held cultivator in the laboratory which moved at a speed of about 0.25 to 0.3 m s -1 compared to a speed of 0.6 to 0.7 m s -1 in the field. In practical terms the experiments showed that tillage also leads to an increase in rock fragments in topsoils when the soil is moist. Only when the volumetric water content of the soil exceeds 0.12 cm³/cm³, silt-loam soils become very sticky which restricts vertical segregation. For a sandy matrix vertical segregation of particles still occurs at volumetric moisture contents of 0.17 cm³/cm³. The implication of these results is that farmers can reduce the risk of water erosion and conserve moisture by tilling dry stony soils at least one or two times but no more than three to four times. This practice is relatively simple and is already applied by many farmers, however, not always with the intention of soil and water conservation. The experiments also showed that in order to increase rock-fragment content in the surface layers significantly, excessive tillage is not necessary. This is fortunate since frequent ploughing, particularly on hillslope convexities, can cause serious tillage erosion (e.g. Poesen et al. 1997). Farmers also plough the soil to destroy the capillary structure in order to reduce evaporation. However, recent research results obtained by van Wesemael et al. (1996) indicate that a rock-fragment cover reduces evaporation under wet conditions, which should reduce the need to plough just after the rains. Acknowledgements The research for this study was carried out as part of the MEDALUS collaborative research project. MEDALUS was funded by the European Commission Environment and Climate Research Programme (contract: ENV4-CT95-0118, Climatology and Natural Hazards) and this support is gratefully acknowledged. Ludo Cleeren is thanked for his help with the preparation of the aggregated silt-loams. Laurent Beuselinck kindly provided the data on the particle-size distribution of the silt-loam. References Kouwenhoven, J.K., Terpstra, R., 1970. Mixing and sorting of granules by tines. J. Agric. Engng. Res., 22: 153-163. Kouwenhoven, J.K., Terpstra, R., 1979. Sorting action of tines and tine-like tools in the field. J. Agric. Engng. Res., 24: 95-113. Nachtergaele, J., Poesen, J., van Wesemael, B., 1997. Application and efficiency of gravel mulches in southern Switzerland. Soil Technology 22p.(in press). Oostwoud Wijdenes, D., Poesen, J., Vandekerckhove, L., de Luna, E., 1997. Chiselling effects on the vertical distribution of rock fragments in the tilled layer in a Mediterranean soil. Soil and Tillage Research Oostwoud Wijdenes, D., Poesen, J., 1998. Laboratory tests to determine the effect of moisture and cohesion on the vertical segregation of rock fragments by tillage (submitted to Geoderma) Poesen, J., Bunte, K. 1996. Effects of rock fragments on desertification processes in Mediterranean environments, In: Brandt, J. and Thornes J. (eds.) Mediterranean 6
Desertification and Land Use, J. Wiley & Sons, Chichester, U.K., 257-269. Poesen, J., van Wesemael, B., Govers. G., Martinez-Fernandez, J., Desmet, P., Vandaele, K., Quine, T. and Degraer, G., 1997. Patterns of rock fragment cover generated by tillage erosion. Geomorphology, 18: 183-197. van Wesemael, B., Poesen, J., Kosmas, C.S., Danalatos, N.G., Nachtergaele, J., 1996. Evaporation from cultivated soils containing rock fragments. Journal of Hydrology 182 (1-4), 65-82. Keywords : rock-fragment content, rock-fragment sorting, tillage, soil and water conservation. Mots clés : teneur en fragments de roche, taille des fragments de roche, labour, conservation du sol et de l'eau 7