Methods for predicting the optimum and the range of soil water contents for tillage based on the water retention curve
Introduction
The optimum water content for tillage (OPT) can be defined as “the water content at which tillage produces the greatest proportion of small aggregates”. If soil is tilled when it is wetter than this optimum water content, then large clods can be produced and soil structural damage can occur. If the soil is drier than the optimum water content, then tillage requires excessive energy and can also produce large clods.
Some experiments have been reported in which tillage has been done in the field when the soil has been at different water contents, and the resulting aggregate size distributions have been obtained by sieving (for example). An optimum water content was identified by Bhushan and Ghildyal (1972) on a lateritic sandy loam. They found that more small aggregates were produced when tillage was done at 0.77θPL than when it was done at either 0.60θPL or 0.99θPL. Here, θPL is the lower plastic (or lower Atterberg) limit of the soil and is the gravimetric water content at which the consistency of freshly moulded soil changes from plastic to brittle. Similarly, Ojeniyi and Dexter (1979) found maximum production of small aggregates when tillage of a sandy loam was done at 0.9θPL. This soil contained 0.17 kg kg−1 clay and was a Chromic Luvisol under the FAO soil classification system. Their results were incorporated into a model for the prediction of soil structures produced by tillage (Dexter, 1979). A similar relationship between the OPT and the plastic limit was found by Arndt (1964) for soil at Katherine in the Northern Territory of Australia. Allmaras et al. (1969) found that the surface roughness (which is related to the presence of clods) of tilled soil was minimum when tillage was done at θPL.
Research into soil friability, which can be defined as “the tendency of a mass of soil to crumble under the action of an applied force” has also shown that friability of soil is maximum at a water content close to θPL (Utomo and Dexter, 1981).
For several soils therefore, it has been found that the optimum water content occurs in the vicinity of θOPT=0.9θPL. However, this has the limitations that θPL is a property of moulded soil, and not of undisturbed soil in the field, and also that many sandy soils are not plastic and do not have a plastic limit. However, tillage of sandy soils can also cause damage to the structure.
Increasing cloddiness when soil is tilled at water contents greater than θPL has been reported by several researchers including Patterson et al. (1980), Adem et al. (1984), and Tisdall and Adem (1986).
The upper (wet) tillage limit, θUTL, of soils is not at a constant water potential. Dexter (1988) made a synthesis of the results of Heinonen and Pohjanheimo (1962), Koenigs (1976), Boekel (1979), and Buitendijk (1985), and showed that the upper tillage limit (UTL) occurs at more negative matric water potentials with increasing clay content of the soil. However, the results from these authors could not be used to produce a regression equation for the matric water potential at the UTL because not all the papers gave the soil compositions.
This effect of clay content is consistent with the finding of Dexter (1990) that the matric water potential (ΨPL) of six freshly moulded soils with clay contents ranging from 0.185 to 0.668 kg kg−1 at the lower plastic limit (θPL) is also more negative with increasing clay contentThe data of Terzaghi et al. (1988) for 13 Uruguayan soils with clay (clay) contents ranging from 0.18 to 0.41 kg kg−1 and with organic matter (OM) contents ranging from 0.019 to 0.073 kg kg−1 were used to givewhich is very similar to the relation found by Koenigs (1976) for 20 Dutch soils
Tillage at water contents greater than θPL may also destabilize and damage the soil structure. The application of energy to soil wetter than θPL has been found to increase the content of readily dispersible clay, whereas the application of energy to soil drier than θPL had no effect on the content of readily dispersible clay (Watts et al., 1996). This is important because clay dispersibility is indicative of soil instability.
The relationship between the lower plastic limit (θPL) and the field capacity (θFC) has been considered by Boekel, 1959, Boekel, 1965. Here, field capacity is defined as the water content to which a field will drain within a few days after heavy rain or irrigation. θFC has been found experimentally to correspond to a matric water potential of about −100 hPa. When soil is tilled when it is wetter than θPL, it will deform plastically with consequent destruction of the structure. It has been noted that when θFC/θPL<1, the soil will drain to a water content at which no excessive structural damage will occur on tillage. On the other hand, if θFC/θPL>1, then the soil will never drain to a water content which is ideal for tillage. Unfortunately, most clay soils drain extremely slowly, and in the field are usually wetter than θPL unless they are dried by water extraction by plant roots.
Increasing cloddiness when soil is tilled at water contents below the optimum has been reported by Lyles and Woodruff (1962), Allmaras et al. (1969), Bhushan and Ghildyal (1972), Ojeniyi and Dexter (1979) and Watts and Dexter (1994). However, the lower (dry) tillage limit, θLTL, is not as well defined as the UTL. There is no water content at which the tillage response changes suddenly. This point is considered later in the paper.
The finding of Adem et al. (1984) and Tisdall and Adem (1986) that tillage produced larger aggregates with increasing water content (but below θPL) was for the rather special case of soil which was initially finely divided in the form of micro-aggregates. Tillage of such soil cannot produce further aggregate breakdown and can only push micro-aggregates together to form larger aggregates. This case is not discussed further in this paper.
Very little work has been reported on the range of water contents for tillage. Hoogmoed (1985) pointed out that the range is usually narrow for soils with high clay content and becomes wider for soils with lower clay contents. Soil degradation usually reduces the range and therefore the opportunities for tillage.
It is not easy to take tillage machinery to fields with different soil types on many different dates to obtain graphs showing the resulting tilth as a function of water content. Therefore, there is a need for simpler, quicker tests and prediction methods which can enable the optimum water content (θOPT) to be determined readily.
The objective of the work reported here was to develop methods for predicting not only the OPT, but also the range of water contents for tillage. This is done in terms of the soil composition and also from the water retention characteristics of the soil as described by the van Genuchten (1980) equation. The parameters of this equation have been determined and published for enormous numbers of soils and have been incorporated into digital soil maps using geographic information systems (GISs) (e.g., Wösten et al., 1999).
Section snippets
The water retention curve and the inflection point
A typical soil water retention curve is shown in Fig. 1. This shows how, when a soil is dried from saturation, the water content, θ, decreases as the matric water potential, Ψ, becomes more negative. In Fig. 1, values of Ψ are shown as their modulus, h, for ease of plotting. Water retention measurements were fitted to the van Genuchten (1980) equationHere θSAT and θRES are the water content at saturation and the residual water content, respectively, α a scaling
Soils
Results are presented for five soils which are all from the Rothamsted modern long-term experiment at Highfield at IACR, Rothamsted in England. The experiment was established in 1949 on land with a previous history of permanent pasture. It was set up to investigate a range of crop rotation practices and is described in detail by Johnston (1972). Five treatments were sampled to obtain a wide range of contents of soil OM. The soil designations used in this paper and corresponding treatments are
OPT
The optimum water content for tillage, θOPT, was firstly estimated as 0.9θPL. It can be seen from the data in Table 3 that this is very close to the water content at the inflection point, θINFL, of the water retention curve as shown in Fig. 1. Therefore, it was decided to adopt the relationshipValues of θOPT for the Highfield soils are given in Table 4.
Dry limit for tillage
The lower (dry) tillage limit, θLTL, is not a sharply defined point as can be inferred from the references given in Section 1, and
Summary and conclusions
It is clear that properties of disturbed (e.g., moulded) soil are not appropriate for the prediction of the behaviour of undisturbed soil in the field. The water retention curve represents the state of undisturbed soil and therefore provides a better basis for the prediction of other water-related properties of undisturbed soil.
Methods for predicting the lower (dry) limit, the optimum water content, and the upper (wet) limit for tillage have been developed and presented. These are presented in
Acknowledgements
Prof. D.S. Powlson and Dr. P.R. Poulton of IACR, Rothamsted are thanked for giving permission for the collection of soil samples from the Highfield experiment. The work was funded in part by the European Commission INCO-Copernicus project number ERBIC15-CT98-0106.
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