Literature Review
2.3 Surface soil structure
2.3.2 Tillage-induced surface soil structure and crop production
Any significant change in soil structure due to tillage affects soil water, soil aeration, soil heat and soil mechanical resistance properties, as well as soil chemical and biological properties, in both the short and in the long-term.
2.3.2.1 Soil water
a) Infiltration and evaporation of water
The number and geometrical properties of water-conducting soil pores determine the effect of tillage on infiltration. Large, continuous, vertical soil pores, which open to the soli surface, enhance infiltration. The blocking of water conducting pores, as could occur with compaction or where there are unstable surface clods, decreases the amount of stored water and Increases the risks of runoff and water-Induced soli erosion (Unger and McCalla, 1980). Changes In soil water Infiltration rate in response to tillage were considered by Ehlers (1975), Edwards (1982), Tisdall and Adem (1986) and others.
Evaporation can be reduced by a coarse soil structure on top of the tilled layer (Hillel and Hadas, 1972). A thick mulch layer has also been shown to be effective (Bond and Willis, 1969). Hillel and Hadas (1972) observed that field studies of the possible effects of tillage practices on water loss by evaporation were giving conflicting results.
Ojeniyi and Dexter (1984) reported that low soil water content in a tilled soil, on a . seasonal basis, could be attributed to the presence of 4-8 mm and 8-16 mm diameter voids and increased mean aggregate size and macro-porosity in the top layer of tilled soil. This could be due to reduced penetration of turbulent air currents into·
Inter-aggregate cavities where voids are less than 4 mm diameter (Hillel and Hadas, 1972). In tilled soil with a coarse structure, daylight evaporative water loss was reduced compared to finer structured soli.· However, during the night this trend was apparently reversed (Ojenlyl and Dexter, 1984). Soil surface roughness affects soil thermal properties and the energy balance (Allmaras et aI., 1972; 1977; Cruse et aI., 1980), solar radiation reflection (Bowers and Hanks, 1965; Allmaras et aI., 1972; Cruse et aI., 1980) and, therefore, evaporation (Allmaras et aI., 1977; Linden, 1982).
The identification of the most important structural features determining water loss from a tilled soil would enable modification of tillage methods to conserve water for the survival of the seedling. It was noted by Wingate-Hill (1978) that relationships between soil structure and water supply had not progressed to the stage where it was possible to define, in any quantitative manner, tillage requirements for cereal crop production.
b) Water storage capacity
The amount of plant-available water (I.e. the volumetric water content between field capacity (soil matric potential of -33 kPa) and wilting pOint (-1500 kPa)) can be strongly affected by tillage and traffic (Boone, 1988). Severe deformation of wet soil (Boone et al., 1984), or extreme crumbling (Kuipers, 1961), could cause large increases In the water content at field capacity, possibly leading to reduced plant growth due to lack of aeration. A large water storage_~city, or total porosity, Is desirable because it helps prevent temporary saturation after heavy rain thereby reducing surface aggregate slaking, runoff and water erosion.
c) Soli water movement
In order for water to be supplied to plant roots the soil must be readily able to transmit water to the root surfaces In response to potential gradients. Minimum hydraulic conductivity of the bulk soil must be around 10-4 to 10-5 mm day-1 if water supply is not to restrict plant development (Taylor and. Klepper, 1975; Reicosky and Ritchie, 1976). The number, continuity and size of the largest soil pores or cracks, determines the saturated hydraulic conductivity (Kg) of the soil. Compaction will
therefore decrease Kg. Pore discontinuities, caused by soil deformation when wet, can reduce Ks' Rapid water transport might enhance drainage to greater depths.
Investigations into changes in soil hydraulic properties in response to tillage include those of Ehlers (1976); Douglas et al. (1980); Klute (1982) and Mielke et al. (1986).
2.3.2.2 Soli aeration
Slaking or puddling of a soil surface reduces gas diffusion. Rathore et al. (1982) reported a 50% reduction In the rate of oxygen diffusion in a soil within 24 hours of the formation of a wet soil crust. Surface sealing, in terms of gas diffusion, only occurs when all pores at the soil surface are water-filled. Oxygen diffuses through water at a rate approximately 10000 times slower than through air. Continuous air-filled pores are required in the soil down to the optimum depth of rooting of the plants (Boone, 1988).
Typically, for normal plant development, at least 10% of the soil volume at field capacity Is required to be gas-filled pores where at least 10% of the gas in these pores is oxygen (Dexter, 1988). In a soil with large, continuous pores, water infiltrates quickly and hence
the surface is sealed for a shorter period. Soli surface roughness affects soil air exchange (Allmaras et al., 1977).
2.3.2.3 Soli temperature
Surface radiation reflection, absorption and emission characteristics and
aerodynamic roughness are all tillage-related factors affecting heat flux into and out of the soli. The processes Involved in the interaction between micro-climate and the soil are complex, dynamic and not well documented for tillage effects (Cruse et al., 1982).
Modification of the soli surface structure might allow earlier sowing by extending the period during which the seedbed is above a critical minimum temperature. Examples where tillage effects on soil temperature and heat balance have been measured include Allmaras et al. (1977); Cruse et al. (1982), and Gupta et al. (1984).
2.3.2.4 Soil mechanical impedance
a) Plant establishment
Tillage-induced soil structure directly influences the surface penetration of growing plant shoots and their subsequent success in establishment. Changes in soil
mechanical impedance were discussed by Cassel (1982) and Cassel and Nelson (1985). Rainfall intensity and other climatic conditions interact with soil properties in determining the extent to which the soli surface is slaked and the mechanical strength of the surface crust at time of emergence (Rawitz et al., 1985). The uptake of water and oxygen by the establishing plant Is determined by the properties of the soil in contact with, and in the immediate vicinity of, the emerging seed (Hadas and Russo, 1974a,b).
b) Root growth
Roots growing through the soil take the path of least mechanical resistance, often growing through continuous cracks, large pores and along planes of weakness.
Stratification of soil structure by the creation of a seedbed or of a ploughpan (Ehlers et al., 1980), or abrupt changes in soil texture with depth, modifies this root-mass
distribution. In a dense soil, the artificial modification of the number, dimension and
distribution of large pores and cracks oould greatly change the rooting pattern and the root density (Boone, 1976).
2.3.2.5 Soil nutrient factors
The availability of nutrients with depth is affected by the degree of soli inversion by tillage. Generally, when a soil is inverted deeper than the arable layer, less fertile soil Is brought to the surface. In addition to slower early plant growth due to a lower nutrient concentration, a lower surface organic matter content could increase the risk of surface slaking or wind erosion. Changes to the soil water balance or to the plant root
distribution which might occur as a result of tillage will have indirect effects on the plant availability of nutrients.
2.3.2.6 The Ideal seedbed from an agronomic perspective
Authors disagree on which range of aggregate size provides the ideal seedbed (Adem et aI., 1984), but most suggest low amounts of particles less than 0.5 mm . diameter and clods larger than ,20 mm diameter. Desirable aggregate sizes quoted vary from 1 to 5 mm, 2 to 3 mm, 1 to 10 mm diameter, 50% aggregates 3 to 6 mm diameter and the rest smaller, and 75% aggregates 1 to 12 mm diameter (Russell, 1973; Taylor, 1974). It is Important that there Is always less than 15% of fine material «250 "m) which can block the larger pores (Dexter, 1988). The seedbed should provide adequate soil-seed contact for water supply to enable swelling and germination, and also
adequate aeration. A broad requirement is for 10% of the soil volume to be in pores larger than 30 "m for aeration, and a rnaximum volume of pores between 30 and 0.2 "m for water storage (Dexter, 1988). The optimum environment for germination and early growth Is needed only In the vicinity of the seeds. The seedbed might require larger aggregates nearer to the surface for the prevention of water and wind erosion. Uniform depth of seeding is required for uniform crop development (Boone, 1988). The definition of the soil condition needed for agronomic objectives and the development of soil
dynamics for prescribing the soil manipulation which will produce the desired soil condition, is an important priority in agricultural research (Schafer and Johnson, 1982).