C. W. Watts

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.

The effect of soil strength on the yield of wheat

Although it is well-known that high soil strength is a constraint to root and shoot growth, it is not clear to what extent soil strength is the main physical stress that limits crop growth and yield. This is partly because it is difficult to separate the effects of soil drying and high soil strength, which tend to occur together. The aim of this paper is to test the hypothesis that for two different soil types, yield is closely related to soil strength irrespective of difference in soil water status and soil structure. Winter (Triticum aestivum L., cv. Hereward) and spring wheat (cv. Paragon) were grown in the field on two soils, which had very different physical characteristics. One was loamy sand and the other sandy clay loam; compaction and loosening treatments were applied in a fully factorial design to both. Crop growth and yield, carbon isotope discrimination, soil strength, water status, soil structure and hydraulic properties were measured. The results showed that irrespective of differences in soil type, structure and water status, soil strength gave a good prediction of crop yield. Comparison with previous data led to the conclusion that, irrespective of whether it was due to drying or compaction (poor soil management), soil strength appeared to be an important stress that limits crop productivity.

An assessment of the vulnerability of soil structure to destabilisation during tillage. Part I. A laboratory test

Abstract A simple laboratory apparatus, consisting of a falling weight, was developed to impose different specific mechanical energy inputs to soil aggregates. Disruption or damage to the soil aggregates was quantified in terms of the amount of mechanically-dispersible clay (particle size 0.7 μm or less), measured by turbidimetry. Natural aggregates, collected from the field were used to determine the sensitivity of three soils to damage as a function of specific energy input and soil water content. Results show that the amount of mechanically-dispersible clay is a function of both soil water content and specific energy input. Below a certain soil water content threshold, approximating to the plastic limit, even relatively large specific energy inputs (in tillage terms) have little effect on clay dispersion. However, as soil water content increases above the plastic limit, the soil becomes increasingly sensitive to mechanical disruption. For soil water contents between the plastic and liquid limits, the amount of clay dispersed is proportional to mechanical energy input. As soil becomes wetter, the rate of increase in dispersion becomes greater. A simple empirical model was developed to predict the amount of dispersible clay from soil water content and mechanical energy input. The laboratory method provides a convenient technique for rapid assessment of soil to mechanical damage, particularly that caused during tillage.

Continuity of air-filled pores and invasion thresholds for a soil-borne fungal plant pathogen, Rhizoctonia solani

Abstract Fundamental knowledge of the way fungi explore the pore volume within a soil is crucial if we are to understand how soil physical conditions affect population dynamics and invasion of many important fungal parasites and saprophytes within soil. In this study, spread of the fungus Rhizoctonia solani Kuhn (AG4), an economically important soil-borne plant pathogen and saprophyte, was quantified in relation to tortuosity and continuity of the air-filled pore space. Samples containing fine or coarse sand were equilibrated at matric potentials ranging from −1 to −7 kPa and inoculated with R. solani . We quantified the colony extent and biomass of R. solani spreading from a localized source of inoculum using microscopy and a monoclonal antibody-based immunosorbent assay. Air permeability rapidly increased when the air-filled pore space became continuous below a threshold matric potential of −2.0 kPa for the fine sand and −1.0 kPa for the coarse sand. Fungal spread was limited at high matric potentials with a colony extent of 6.6 and 4.6 mm for the fine and coarse sand, respectively, but increased to 32.6 and 28.5 mm, with a sharp transition below a threshold matric potential of −2.2 and −1.3 kPa. The average colony biomass dropped markedly at the same threshold matric potential. We conclude that the ability of the fungus to invade soil depends on the connectivity and tortuosity of the air-filled pore volume. At near-saturated conditions, the spread was spatially constrained to a relatively small volume, forming small dense colonies. In a well connected air-filled pore volume, larger colonies with low biomass density were formed. The broader implications for invasion of soil by a fungus and for transmission of fungal diseases are discussed.

An assessment of the vulnerability of soil structure to destabilisation during tillage. Part II. Field trials

Abstract Tillage operations were conducted on a Calcareous Pelosol over a range of soil water contents and at differing tillage intensities, using both a mouldboard plough and a rotary cultivator. Measurements were made of the tillage specific energy requirements and tests were made to assess subsequent aggregate disruption by measuring the quantity of mechanically-dispersible clay. The rotary cultivator had a specific energy requirement between three and four times that recorded for the plough and both implements showed a consistent increase in energy requirement as soil water contents declined towards the soil plastic limit. Aggregates collected following tillage yielded greater amounts of dispersed clay than those collected immediately prior to tillage, with differences increasing with soil water content, but differences were not significant at and below the plastic limit. Aggregates collected following rotary cultivation yielded larger amounts of dispersed clay than those collected following ploughing, at a given soil water content. The results of the field experiments were in good agreement with predictions of laboratory rests reported in Part I, in which the quantity of clay dispersed was related to soil water content and the specific energy of disruption. The three soils cultivated showed large differences in their sensitivity to disruption measured in terms of the quantity of mechanically-dispersible clay. Aggregates collected and dried following tillage had a greater tensile strength than aggregates collected immediately prior to tillage. The water content at the time of collection was also shown to influence aggregate strength. Aggregate strength was found to correlate with the quantity of clay dispersed.

Assessment of a wide span vehicle (gantry), and soil and cereal crop responses to its use in a zero traffic regime

Abstract The adverse effect of soil over-compaction on crop production efficiency was the basis for a programme to assess soil and crop responses to a zero traffic regime based on a 12 m gantry. The vehicle and its operating system, together with tasks ranging from fertilizer and spray application to draught and powered cultivations and cereals harvesting, are described. Results indicated that the gantry was a practical means of separating the cropped and wheeled (zero traffic) areas of a field. Cultivation draught and energy savings of up to 50% and 70%, respectively, were identified on a clay soil where traffic was eliminated from the cropped area. There was also evidence that this regime resulted in significant improvements to soil structure and crop establishment. The average yield of wheat from the zero traffic plots in 1989 was 6.8 t ha −1 , compared with 5.7 t ha −1 from the conventionally managed soil. In the dry season of 1989–1990, the yield of oats was not differentially affected by treatment.

Biological And Physical Processes That Mediate Micro-aggregation Of Clays

The aggregation of clays after the addition of organic materials is described. Clays were incubated with or without added organic matter in the form of grass, straw, or charcoal and needed to be dried to a water potential of −1.5 MPa or less to aggregate. It was the fine fraction of the clay (<0.5 μm) that aggregated after organic additions, and this was apparent even in clays or clay mixtures that had a broad particle size distribution. Grass was more effective than straw at aggregating the clay. When chloroform was added to the samples, there was little aggregation, suggesting that micro-aggregation after the addition of organic material to clay is mainly microbially mediated. The aggregation of clay in the presence of added substrate was temperature dependent, with an optimum between 20 and 30 °C, but in the absence of substrate aggregation increased with temperature without a maximum. This supports the hypothesis that biological mechanisms are playing an important role in aggregation.

A novel grass hybrid to reduce flood generation in temperate regions

We report on the evaluation of a novel grass hybrid that provides efficient forage production and could help mitigate flooding. Perennial ryegrass (Lolium perenne) is the grass species of choice for most farmers, but lacks resilience against extremes of climate. We hybridised L. perenne onto a closely related and more stress-resistant grass species, meadow fescue Festuca pratensis. We demonstrate that the L. perenne × F. pratensis cultivar can reduce runoff during the events by 51% compared to a leading UK nationally recommended L. perenne cultivar and by 43% compared to F. pratensis over a two year field experiment. We present evidence that the reduced runoff from this Festulolium cultivar was due to intense initial root growth followed by rapid senescence, especially at depth. Hybrid grasses of this type show potential for reducing the likelihood of flooding, whilst providing food production under conditions of changing climate.

Structural stability of two Romanian soils as influenced by management practices

The effects of non-arable vs. arable land use as well as of wheeled traffic, crop rotation, manure applications and inorganic fertiliser application on soil stability parameters were investigated. The stability of a soils structure was assessed by the dispersibility of clay in water, by measurement of the proportions of water-stable aggregates, and by the tensile strength of dry soil aggregates. An index of soil structural stability is defined and used. In general, there were trends towards increasing soil structural stability with non-arable agriculture, less wheeled traffic, crop rotations rather than monoculture, and fertiliser application.

Estimating soil strength in the rooting zone of wheat

When roots abstract water thus drying the soil, crop growth may be reduced by increasing strength of soil as well as the lack of water. Strong soil impedes root growth, restricting access to deeper water. As a result, there is a need to estimate soil strength in order to model crop response to dry soil correctly. The strength of soil can be routinely assessed with a penetrometer but measurements are time consuming and hard work to acquire at the frequency required to understand soil-water-plant relations. To make progress, a published relationship that derives penetrometer pressure from both water relations in soil and density was improved to take account of the effects of depth including the friction that results from the increasing hydrostatic pressure. These relationships were then incorporated into an agroecosystem model so that the dynamics of strong soil and its effect on wheat could be simulated. The combined model requires the moisture release curve (but this can be derived from other commonly-measured soil properties), daily rainfall, temperature, and potential evaporation and the agronomy of the crop. Modelled values of penetrometer pressure were simulated well compared with measured values in artificially strengthened (compacted) and weakened (irrigated) soils. Simulations of the strength of soil and the matric potential before anthesis are compared with measured total dry-matter yields of winter wheat in experimental fields. The results lend weight to the hypothesis that wheat yield is limited by the strength of soil in the field and that soil strength, rather than soil matric potential, better explains differences between soils.