C. D. Grant

Physical assessment of a soil with respect to rooting potential

Abstract The potential suitability of a soil to accommodate growing roots was assessed on the basis of: (1) the existing pore space available for unobstructed root growth; and (2) the obstruction offered to a growing root by the soil matrix. The former was evaluated by measuring the air permeability of undisturbed soil cores, equilibrated at a chosen water tension. The latter was evaluated by measuring the spectrum of the tip resistance encountered by a slowly penetrating fine probe. The influence of cropping history on this potential suitability was examined by comparing the same soil with two different cropping sequences (corn grown continuously for five years and forages grown for three years followed by two years of corn). Measurements were started shortly before the forages were plowed under. Air permeabilities were higher for the plots on which forages had been grown, indicating greater potential suitability for unobstructed root growth. The penetration experiments showed that the average resistance to deformation of root-sized pores was equal for the two cropping patterns. The average bulk density was higher in the continuous corn plots than in the plots on which forages had been grown. After analysis of our data we postulated that the effect of the higher bulk density was offset by lower aggregate strength in the continuous corn plots. Implications of this hypothesis are discussed.

A new procedure to determine soil water availability

The integral water capacity is first introduced as a flexible method to quantify various soil physical limitations when calculating available water in non-swelling soils. ‘Weighting’ functions that account for hydraulic conductivity, aeration, and soil resistance to penetration are applied to the wet and dry ends of the differential water capacity, and then integration is performed. The concept is extended to swelling soils by applying the theory of Groenevelt and Bolt (1972), which enables overburden pressures to be taken into account. A set of shrinkage lines measured by Talsma (1977) is analysed using this theory, which enables precise values of overburden potentials to be calculated as a function of the moisture ratio for different load pressures. The addition of the overburden potential to the unloaded matric potential causes minor shifts in the classical limits of plant-available water (viz. –1/3 bar and –15 bar). However, when other soil physical restrictions are taken into account (such as in the concept of the least limiting water range), the consequence for available water deeper in the root-zone (due to an overburden pressure) is far more serious. This is primarily because the matric potential at which aeration begins to be satisfied shifts to a considerably lower value, making a large quantity of water at the wet end no longer available. Examples of weighting functions derived from the literature are applied and their implications for available water in swelling soils are discussed.

Assessing effects of aerobic and anaerobic conditions on phosphorus sorption and retention capacity of water treatment residuals.

Water treatment residuals (WTRs) are the by-products of drinking water clarification processes, whereby chemical flocculants such as alum or ferric chloride are added to raw water to remove suspended clay particles, organic matter and other materials and impurities. Previous studies have identified a strong phosphorus (P) fixing capacity of WTRs which has led to experimentation with their use as P-sorbing materials for controlling P discharges from agricultural and forestry land. However, the P-fixing capacity of WTRs and its capacity to retain sorbed P under anaerobic conditions have yet to be fully demonstrated, which is an issue that must be addressed for WTR field applications. This study therefore examined the capacity of WTRs to retain sorbed P and sorb further additional P from aqueous solution under both aerobic and anaerobic conditions. An innovative, low cost apparatus was constructed and successfully used to rapidly establish anoxic conditions in anaerobic treatments. The results showed that even in treatments with initial solution P concentrations set at 100 mg l(-1), soluble reactive P concentrations rapidly fell to negligible levels (due to sorption by WTRs), while total P (i.e. dissolved + particulate and colloidal P) was less than 3 mg l(-1). This equated to an added P retention rate of >98% regardless of anaerobic or aerobic status, indicating that WTRs are able to sorb and retain P in both aerobic and anaerobic conditions.

Curvature of shrinkage lines in relation to the consistency and structure of a Norwegian clay soil

Abstract Two sets of shrinkage data for a Norwegian clay Topsoil and Subsoil provided in the literature by Olsen and Haugen [Geoderma 83 (1998) 67] were re-evaluated by fitting them to the Groenevelt and Bolt [Soil Science 113 (1972) 238] constitutive shrinkage equation. The shrinkage curves were differentiated to obtain the slopes, and the slope curves were subsequently differentiated to obtain the curvatures of the shrinkage lines. The point of maximum curvature (at the dry end) of the shrinkage line was used to unambiguously separate the region of “residual” shrinkage from the region of “zero” shrinkage, and this point identified the shrinkage limit. The first inflection point on the wet side of the maximum curvature was found to correspond well with the water content at the Atterberg lower plastic limit, provided that the fitted curve matched the measured data closely at the wet end. Another point on the curvature line is suggested to correspond with the end-point of ‘structural’ shrinkage but this could not be confirmed with the limited data available.

Weather, cropping practices and sampling depth effects on tensile strength and aggregate stability

Abstract The stability in water and the tensile strength of aggregates reflect the ease of fragmentation of soil by different processes. The objectives of this study were to determine if these two parameters respond to weather and soil management in a similar manner and how this response changes with depth in the Ap horizon. Studies were conducted on plots of a permanent rotation trial established in 1925 on a red-brown earth (a calcic Rhodoxeralf) in South Australia. The stability and tensile strength of aggregates declined in a similar manner with the number of wetting events prior to sampling and the variation in these characteristics due to weather was often much greater than the variation due to management practices. The influence of sampling depth and management practices on the two parameters was, however, very different. Tensile strength increased with depth in the Ap horizon whereas aggregate stability decreased. The increase in tensile strength appeared to be due to the less severe wetting events experienced at depth whereas the decrease in stability was correlated with the decrease in organic carbon content with depth. The tensile strength of this soil was less sensitive to management than was aggregate stability. It is concluded that there are only limited circumstances under which changes in tensile strength can be predicted from changes in aggregate stability.

Spatial variability of soil penetrometer measurements at the mesoscopic scale

Abstract Laboratory penetrometer measurements are reported for 10 undisturbed cores from two soil types: a Typic Eutrocrept and an Aeric Ochraqualf. Resistance to penetration was measured at numerous points within each core, which permitted an analysis of spatial variation at the mesoscopic scale (10 −3 –10 0 m). The objectives were to examine the extent of autocorrelation in the data, determine the optimum number of penetrations per sample, and identify physical properties influencing variability. Two different penetrometers were used: a constant-load needle penetrometer and a constant-rate micro-penetrometer. Data were expressed as a percentage of the maximum depth attainable with the needle penetrometer (PMP), and as the slope ( m ) and intercept ( n ) of linearized percent penetrability vs. tip pressure relationships for the micro-penetrometer. No correlation was found between the m and n parameters, suggesting they represent independent physical properties contributing to soil strength. The PMP and m measurements were positively skewed, whereas the n measurements were normally distributed. None of the parameters exhibited any spatial autocorrelation. The average fractal dimension for all three parameters was 2.89 ± 0.09, indicating Gaussian random variation. For a given bulk density, the mean PMP decreased exponentially with decreasing moisture content. Conversely, at constant water potential, mean values of m and n decreased linearly with increasing bulk density. Coefficients of variation (CV) were relatively uniform for the n parameter (11.5–20.1%), but depended upon soil moisture and bulk density in the case of PMP and m , respectively. The CV for PMP increased exponentially with decreasing soil moisture, while a curvilinear relationship was found between the variation in m and bulk density. This was modelled with a second order polynomial, which predicted a maximum CV of 36.1% at 1.14 Mg m −3 bulk density. The amount of variability encountered was greater than previously reported for laboratory penetrometer measurements. The fact that this variation was not independent for two of the three parameters investigated, has important implications for sampling at the mesoscopic scale. Before a study is undertaken, preliminary measurements should be made to determine the optimum number of samples for the parameter under consideration. Sampling requirements for soil penetrability appear to increase with decreasing moisture content and bulk density.

On water availability in saline soils

We present a formulation for the effect of the osmotic pressure of the soil solution on the availability of soil water for plant uptake in the extreme case that the reflection coefficient of root-cell walls is always unity. We also present a new equation to fit the water retention curve, which allows for an inflection point and is solidly anchored at both the wet end (saturated water content) and the dry end (water content at 150 m head, the permanent wilting point). By differentiating the fitting-equation one finds the differential water capacity, which is subsequently multiplied by a weighting function to account for the impediment caused by soluble salts. The weighted differential water capacity is then integrated over the entire range of the matric head from zero to infinity. This produces the integral water capacity and constitutes the total amount of water the soil can hold and release to a hypothetical plant that behaves like a perfect osmometer. We illustrate the approach using data found in the literature for a wide range of soil textures. In this paper the lower boundary of water availability in the presence of soluble salts is defined and calculated as would be registered by a perfect osmometer (reflection coefficient of unity). The upper boundary of water availability is found by setting the weighting coefficient at unity at all times, which implies a reflection coefficient of zero, and in turn that the salts in the soil solution have no influence on the availability of water (as would be registered by a tensiometer). The upper and the lower boundaries constitute the envelope within which the actual availability of water to real plants occurs, and implies a variable reflection coefficient plus the occurrence of active plant osmo-regulation. This establishes a framework within which water availability to real plants experiencing real osmo-matric conditions can be evaluated.

On the nature of soil aggregate coalescence in an irrigated swelling clay

Aggregate coalescence in irrigated cracking clays constrains crop yields, yet little is known about it or how it can be managed. A measure of coalescence is introduced to separate the effects of natural aggregate-bed densification from those of age-hardening; this measure, χ, comprises a ratio of the net change in (tensile or penetrometer) strength, Y, that occurs in relation to the corresponding net change in dry bulk density, ρb, as follows: χ = ΔY/Δρb. A laboratory study was conducted to illustrate the variation in χ for a virgin and cultivated cracking clay exposed to 16 weekly cycles of wetting and draining. Penetrometer resistance and tensile strength at –100 kPa, plus bulk density and other physical and chemical properties, were measured throughout the experiment. The cultivated soil rapidly became denser and stronger, it developed larger aggregates, and its water-uptake rate in the air-dry state was significantly greater than that for the virgin soil. The χ values suggested that age-hardening processes constituted a greater component of coalescence in the cultivated soil than it did in the virgin one, and this was thought to be mediated by the large differences in the content and composition of organic matter in the two soils.

An analysis of the fragmentation of remoulded soils with regard to self-mulching behaviour

A power-law relation was used to analyse the (mass-derived) number-size distributions of fragments generated by wetting and drying remoulded soils. Various soils from Europe and Australia produced a range of values for the two fragmentation coefficients, d and k, generated by the power-law function. Both coefficients had physical significance with respect to self-mulching behaviour. Likened to a fractal dimension, the d coefficient varied directly with the tendency of the remoulded soil to fragment during wetting and drying. Assessment of the number of generated fragments >1 cm was made with the k coefficient. Consideration of both coefficients together in a plot of k v. d enabled similar soils to be grouped and falsely large values of d to be identified; k values were small for limited fragmentations even if the size distribution of the fragments that were produced gave large values of d. Most strongly self-mulching soils produced d values >1 . 5 after three wet/dry cycles, and k values that increased sharply after one cycle and declined with subsequent wetting and drying. Other soils with lesser abilities to self-mulch generally produced smaller d values and more variable k values. Reasonable correlations were found between these two coefficients and other measurements of self-mulching behaviour, particularly after three cycles of wetting/drying. Examination of the aggregate size distributions produced from remoulded soils in this way offers the potential to understand more clearly the dynamics of structure regeneration in soils exhibiting various degrees of self-mulching behaviour.

Application of the Groenevelt–Grant soil water retention model to predict the hydraulic conductivity

We outline several formulations of the Groenevelt–Grant water retention model of 2004 to show how it can be anchored at different points. The model is highly flexible and easy to perform multiple differentiations and integrations on. Among many possible formulations of the model we choose one anchored solely at the saturated water content, θs, to facilitate comparison with the van Genuchten model of 1980 and to obtain a hydraulic conductivity function through analytical integration: where, k0, k1, and n are fitting parameters. We divided this formulation by θs to obtain the relative water content, θr(h), and inverted the function to produce a form required for integration, namely: in which the parameter β is introduced to accommodate both the ‘Burdine’ and ‘Mualem’ models. The integrals are identified as incomplete gamma functions and are distinctly different from the incomplete beta functions embodied in the van Genuchten–Mualem models. Rijtema’s data from 1969 for 20 Dutch soils are used to demonstrate the procedures involved. The water retention curves produced by our Groenevelt–Grant model are virtually indistinguishable from those produced by the van Genuchten model. Relative hydraulic conductivities produced by our Mualem and Burdine models produced closer estimates of Rijtema’s measured values than those produced by the van Genuchten–Mualem model for 19 of his 20 soils. This work provides an alternative to the widely used van Genuchten–Mualem approach and represents a preamble for the, as yet unsatisfactory, treatment of the tortuosity component of the unsaturated hydraulic conductivity function.