A.R. Dexter

Advances in characterization of soil structure

Soil structure is defined as “the spatial heterogeneity of the different components or properties of soil”. Aspects of soil structure which are important for plant development, soil water balance and soil workability are reviewed briefly. The different types of soil structure which occur on different size scales are placed in a hierarchical order. Different mechanisms give rise to the different hierarchical orders. Similarly, different physical/chemical/biological processes are involved in the stabilization of the different hierarchical orders. A number of methods for measuring soil structure are described. Preference is given to methods involving direct observation of structural features by scanning electron microscopy and by optical scanning of impregnated sections and fracture surfaces. These need to be supported by assessments of the stabilities of compound particles in water and of the mechanical strengths of compound particles as a function of water content. “Good” soil structure is described as one where all the hierarchical orders are well-developed and stable. The greatest lack of knowledge appears to be in the 2–100 μm size range which is too large to have been studied by colloid chemists and too small to be visible to the naked eye. It is suggested that more observations of soil structure should be made in this size range, as it may hold many important clues on how to manage soil structure in the field.

Methodology for determination of tensile strength of soil aggregates

Abstract Tensile strength of soil aggregates can be calculated from their crushing forces provided that the aggregate diameters are known. Three simple types of apparatus are described with which it is possible to measure the crushing forces of soil aggregates of a wide range of sizes and strengths. Five different methods for estimation of aggregate diameter are compared. Methods for the preparation of natural and artificial soil aggregates for testing are described and advice is given concerning the levels of replication required.

Penetration of very strong soils by seedling roots of different plant species

The abilities of seedling roots of twenty-two plant species to penetrate a strong growth medium were compared under controlled conditions. Seedlings were grown for 10 days in compression chambers filled with siliceous sandy soil at 0.2 kg kg−1 water content and mean penetrometer resistance of 4.2 MPa. Root elongation and thickening were measured after growth. The results show that soil strength reduced the elongation of roots of all plant species by over 90% and caused the diameters of the roots to increase compared with control plants grown in vermiculite (0 MPa resistance).Differences in both root elongation and root diameter were observed among plant species. Generally, the roots of dicotyledons (with large diameters) penetrated the strong medium more than graminaceous monocotyledons (with smaller diameters). There was a significant positive correlation (r=0.78, p<0.05) between root diameter and elongation over all the species in the stressed plants. The species were ranked according to the relative root elongation and relative root thickening. Based on this ranking, lupin (Lupinus angustifolius), medic (Medicago scutelata) and faba bean (Vicia faba) were the species with the greatest thickening and elongation while wheat (Triticum aestivum), rhodesgrass (Chloris gayana) and barley (Hordeum vulgare) had the least. The weight of the seeds did not seem to influence either the thickening or elongation of the roots.

Influence of root diameter on the penetration of seminal roots into a compacted subsoil

A field experiment was conducted to evaluate the influence of root diameter on the ability of roots of eight plant species to penetrate a compacted subsoil below a tilled layer. The soil was a fine sandy loam red-brown earth with a soil strength of about 3.0 MPa (at water content of 0.13 kg kg-1, corresponding to 0.81 plastic limit) at the base of a tilled layer. Relative root diameter (RRD), which was calculated as the ratio of the mean diameters of roots of plants grown in compacted soil to the mean diameters of those from uncompacted soil, was used to compare the sensitivity of roots to thicken under mechanical stress.Diameters of root tips of plants grown in soil with a compacted layer were consistently larger than those from uncompacted soil. Tap-rooted species generally had bigger diameters and RRDs than fibrous-rooted species. A higher proportion of thicker roots penetrated the strong layer at the interface than thinner roots. There were differences between plant species in the extent to which root diameter increased in response to the compaction. The roots which had larger RRD also tended to have higher penetration percentage.The results suggest that the size of a root has a significant influence on its ability to penetrate strong soil layers. It is suggested that this could be related to the effects which root diameter may have on root growth pressure and on the mode of soil deformation during penetration.

Methods for predicting the optimum and the range of soil water contents for tillage based on the water retention curve

Abstract Information is needed on the range of soil water contents for tillage. The objective of the work was to develop methods for the prediction of the soil water contents at which tillage may be done satisfactorily. Three water contents are considered: the lower (dry) limit, the optimum water content, and the upper (wet) limit. This paper makes a synthesis of published results from tillage and soil physics experiments and also includes some new experimental results. The effects of tillage are considered in relation to some “fixed points” including the lower plastic limit, field capacity and a new fixed point “the inflection point”. These considerations lead to methods for prediction of the lower (dry) tillage limit, the optimum water content, and the upper (wet) tillage limit in terms of the parameters of the van Genuchten equation for soil water retention. Predictions can be made in terms of soil composition through the use of pedotransfer functions for the parameters of the van Genuchten equation. The new methods will enable the effects of soil degradation and climate change on tillage work days to be estimated. The results are potentially mappable using geographic information systems.

Mechanics of root growth

SummaryA model is developed for the rate of elongation of a root tip in terms of the balance of pressures acting on the root. Differentials of this equation give expressions for the changes in root elongation rate with respect to soil water potential and soil mechanical resistance. The model predicts that root cells osmoregulate against both water stress and soil mechanical resistance with predicts that root cells osmoregulate against both water stress and soil mechanical resistance with similar efficiencies which are less than 100%. Analysis of published data leads to the conclusion that root tips of pea osmoregulate with 70% efficiency. A working equation is developed for the elongation rate of roots in conditions of combined water stress and mechanical resistance.

Amelioration of soil by natural processes

Abstract A number of “natural” processes and their roles in soil amelioration are discussed and some new results are presented. Drying of soil produces cracks which can provide paths of low resistance for root growth and also can produce fracture of aggregates as the clay matrix shrinks around rigid inclusions such as sand grains. Rapid wetting of soil can induce micro-cracks which can make the soil weaker and more friable and can also increase the penetrability of the soil by roots. Ageing of soil after disturbance increases its water stability and its ability to resist mechanical stresses. The binding together of soil by roots and fungal hyphae can also induce stability as can the exudates from roots and other soil organisms. Roots are particularly important because of the biopores which they leave when they decay. Biopores (comprising root channels and earthworm tunnels) can provide important pathways for root penetration of subsequent crops, and it is shown that they can result in significantly yield increases. It is suggested that these “natural” processes can be developed and exploited as “modified natural” processes, and that a considerable potential exists for using them to improve our soils at a modest cost.

The influence of organic matter in reducing the destabilization of soil by simulated tillage

Abstract Different agricultural practices can result in a decline in soil organic carbon (SOC) and a consequent reduction in soil structural stability. Experiments were conducted on soils with a range of SOC values, to quantify the destabilizing effects of increased tillage intensity. Different tillage intensity was simulated with the use of a falling weight, where specific energy levels, similar to those experienced during tillage, were reproduced. The level of destabilization was assessed by the quantity of mechanically dispersed clay (using a turbidimetric technique) and the quantity of water-stable aggregates (WSA) > 0.25 mm remaining after being shaken in water. The quantity of clay dispersed increased with increasing water content, in the absence of any mechanical pretreatment, the rate of increase rising sharply with declining SOC. Following simulated tillage, and at water contents above the plastic limit, clay dispersion increased in proportion to the energy of disruption, and also increased with decreasing SOC levels. Below the plastic limit all the soils were relatively insensitive to mechanical disruption. A simple empirical model was derived to link clay dispersion to SOC, water content and energy of disruption. The proportion of WSA declined sharply with decreasing SOC, and to a lesser extent following tillage. The quantity of WSA following simulated intensive tillage (300 J kg −1 ) of grassland (SOC, 2.8–3.2 g (100 g) −1 ) was greater than that present, prior to tillage from fallow, arable and arable/ley rotation treatments (SOC 1.1–2.5 g (100 g) −1 ). Aggregate tensile strength was found to be relatively insensitive to differences in SOC. However, variations of strength within treatments, an indicator of soil friability, increased in proportion with SOC. A turbidity index was derived in which the turbidity of natural and remoulded aggregates was compared. Variation of this index with increasing mechanical energy is used as an indicator of the sensitivity of soils to damage during tillage. A visual representation is constructed to link the sensitivity of soils to damage during tillage with both SOC and water potential. These experiments illustrate that management practices, which lead to a long term reduction in SOC, are responsible for an increase in aggregate strength and reduction in stability plus an increase in sensitivity of soils to structural decline following subsequent tillage.

Maximum axial and radial growth pressures of plant roots

SummaryThe axial root growth force exerted by seedlings of pea (Pisum sativum cv. Greenfeast), cotton (Gossypium hirsutum cv. Sicot 3) and sunflower (Helianthus annuus cv. Hysun) was measured. Effects of different seedling age and different batches of seeds on axial root growth pressure were investigated.Mean values of the maximum axial root growth pressure (Pa) estimated from the maximum axial root growth force (Fmax) and root diameter were 497, 289, and 238 kPa respectively for pea, cotton and sunflower seedlings of same size. Pa and Fmax were significantly influenced by seedling age and for pea seedlings of same age they varied with the seed batch.A new technique was developed for estimating radial root growth pressure and was tested on pea seedlings. Each pea root was confined both in the axial and radial directions in a cylindrical chalk sample at a constant water potential. The roots exerted radial stress which caused tensile failure in a proportion of the chalks. The measurement of tensile strength of duplicate chalks enabled estimation of the maximum radial pressures exerted by the roots. The maximum axial and radial root growth pressures were of comparable magnitude.

Dynamics of soil aggregation in an irrigated desert loess

Abstract Experiments were made on an irrigated loess in the Negev Desert, Israel. Lysimeters were filled with disturbed, homogenized soil, and a single almond tree was planted in each of them. The soil in these lysimeters was sampled 1.5 and 2.5 years after the start of the experiment. Additionally, some samples of older, undisturbed soil were examined. Aggregate tensile strength was measured by an indirect tension (crushing) test, and the dry bulk density of the aggregates was determined. It was found that aggregate tensile strength increased progressively with time after disturbance such that the old undisturbed soil was approximately three times stronger than the soil 1.5 years after homogenization. Higher levels of root density and more intensive drying increased aggregate strength. Aggregate density first increased with time after homogenization, but then appeared to decrease steadily towards a low equilibrium value. Mechanisms are proposed to explain these observations.