Literature Review
2.3 Surface soil structure
2.3.3 Tillage and soil structure
2.3.3.3 Tillage and soli water content
Tillage at different soil water contents produces seedbeds with different aggregate size distributions (Lyles and Woodruff, 1962; Hoyle et aJ., 1972; Ojeniyi and Dexter, 1979a,b; Adem et aI., 1984; Tisdall and Adem, 1986). Lyles and Woodruff (1962) reported that for a silty-clay-loam soil more erodible particles «0.84 mm diameter) and fewer large clods were created by primary tillage at intermediate soil water contents (15- 23%, no lower plastic limit reported). Wet-sieving analysis however, showed no
measurable effect of soil water content at time of primary tillage on the proportion of water-stable aggregates less than 0.84 mm in diamter. The differences in aggregate size distribution which occurred due to soil water content at primary tillage were quickly obliterated by weathering, especially with high rainfall, and by secondary tillage
operations during which no soil water content treatments were imposed. Lyles and Woodruff (1962) found that tillage differences on aggregate size and stability due to type of Implement were greater than, and persisted longer, than did differences due to soil water content at time of tillage. Clods formed at low soil water content had three to four times more resistance to crushing than those formed at high water contents (> 16%
w/w).
Hoyle et al. (1972) reported that when a wet soil is tilled with a rotary cultivator, aggregates larger than 12 mm In diameter were broken down and aggregates less than 0.5 mm in diameter were bound together Into aggregates no larger than 12 mm In diameter. Bhushan and Ghildyal (1972) examined the mean weight diameters of aggregates produced by moldboard ploughing a lateritic sandy-loam soil. Tillage was done at water contents corresponding to 0.60, 0.77 and 0.99 times the lower plastic limit. They found that a more cloddy seedbed was more often produced by tillage at 0.60 and 0.99 times the lower plastic limit than at 0.77 times the lower plastic limit although there were differences between ploughs with different moldboard design. With some' ploughs, the cloddiness appeared to be still decreasing at water contents of 0.99 of the lower plastic limit.
Ojeniyi and Dexter (1979a) ~howed large effects of tillage management and cropping history on the structure of a loam soil (hard-setting phase of a red-brown earth). The effect of water content on the aggregate size distributions produced from tilling to a depth of 8 to 10 cm with a tined Implement, as observed by Ojeniyi and Dexter (1979a), is shown in Table 2.3. They found that a chisel plough produced a maximum number of small aggregates and a minimum number of large voids at a moisture content of approximately 90% of the lower plastic limit. Ojeniyi and Dexter (1979b) showed that consecutive passes with tillage implements reduced the aggregate size, with a second Implement pass having more effect on soil structure when soil water content was at 1.3 times the lower plastic limit, than when it was at 0.65 times the lower plastic limit. Ojeniyi and Dexter (1979a,b) quantified the structure of a tilled soil by using samples impregnated with paraffin wax (Dexter, 1976) which were cut into sections before aggregate and void size distributions were calculated (Dexter and Hewitt, 1978).
Johnson et al. (1979) observed that ploughing a wet silt-loam to silty-clay-loam soil increased soil surface roughness and that clods resulting from ploughing wet soil showed lower wet-stability than clods formed from ploughing nearer the lower plastic
limit. They also reported a decrease in pore space in soils ploughed at water contents above the lower plastic limit, as would be expected if compaction was occurring.
Table 2.3 Aggregate size distributions following tillage of a loam soil at different water contents with a tined implement (after Ojeniyi and Dexter, 1979a).
PROPORTION OF AGGREGATES LARGER THAN SIZE INDICATED AFTER TILLAGE AT GIVEN WATER CONTENT AGGREGATE
SIZE WATER CC;NTENT AS PROPORTION OF PLASTIC LIMIT
(mm) 0.55 0.65 0.81 0.87 0.94 1.3
1 0.97 0.98 0.92 0.91 0.96 0.89
2 0.88 0.82 0.75 0.67 0.90 0.75
4 0.78 0.63 0.57 0.46 0.66 0.58
8 0.61 0.44 0.42 0.30 0.45 0.41
16 0.39 0.22 0.22 0.13 0.21 0.21
32 0.15 0.06 0.06 0.03 0.04 0.07
64 0.02 0.01 0.01 0.00 0.00 0.01
Adem et at (1984) described a system in which the secondary tillage operations are undertaken when the soil is wet and friable using a specialised tined implement, so allowing the seed to be sown into wet soil after a small number of implement passes.
They reported that on a fine sandy-loam soil there was a relationship between water content at tillage and the percentage of aggregates less than 0.5 mm diameter and 10-20 mm diameter, but little or no relationship with intermediate size fractions. As the water content of the soli increased from 13% (w/w) to 22% (w/w) (lower plastic limit was 19.7% w/w), the percentage of aggregates less than 0.5 mm diameter decreased by 44% and the percentage of aggregates 10-20 mm diameter increased by 34%.
Therefore,
as
the water content increased, tillage either (a) bound up more aggregates less than 0.5 mm diameter into larger aggregates, or (b) broke up fewer aggregates 10-20 mm diameter into finer aggregates.In most of the investigations on soil structural changes due to tillage and on the effects of soil water content at tillage, soil structure has been described using aggregate size distributions from dry-sieving. Sieving, although especially useful in wind erosion
studies, gives no information about the soil pore size distribution. Data on the relationships between water content, matric potential and hydraulic conductivity In freshly-tilled soli are practically non-existent (Linden, 1982).
2.3.3.4 Conservation tillage
Conventional tillage might not be suitable for soils with a high clay content, or for solis with poor structural condition. Conventional tillage of heavy soils could result in a compacted zone of soli being developed below the depth of ploughing. This plough pan can restrict the growth of plant roots and limit the movement of water-and air through the soil (Batey, 1988). Very dry soils with high sand contents might also be poorly suited to conventional cultivation. In this circumstance, tillage might pulverlse the surface soil, producing on drying, a fine tilth readily susceptible to erosion by wind.
To overcome these adverse affects, tillage operations can be restricted by reducing their number or by carrying out as many operations as possible in one pass (minimum-tillage), by tilling only the rows where the plants grow and leaving the remaining area untilled (strip-zone tillage), by leaving a large percentage of residual plant material on or near the surface as a protective mulch (mulch-tillage), or by not tilling and drilling the seed directly Into the stubble of the previous crop thereby relying on herbicides for weed control (no-tillage). Collectively these tillage systems are termed conservation tillage systems. A conservation tillage system is one in which either crop residues are retained on or near the surface, or soil roughness is maintained, or both, to control soil erosion and to achieve good soil-water relations (Allmaras et aI., 1985).
Conservation tillage was considered by the USDA in 1980 as 'the most economical and effective means of soil erosion control' (Allmaras and Dowdy, 1985).
The success of different conservation tillage systems is highly soil-specific and also dependent on how well weeds, pests and diseases have been controlled. The results from conservation tillage systems have been variable (Davies and Cannell, 1975;
Cannell et aI., 1980; Unger and McCalla, 1980; Chaney et al., 1985) and this has probably contributed to a reluctance to include such techniques in New Zealand arable farming systems. Economic analysis of conservation tillage systems indicate that there are significant short-term economic penalties and risks associated with conservation tillage systems (Jolly et aI., 1983; Ladewig and Garibay, 1983; Napier et aI., 1984).
Farmers are reluctant to adopt conservation tillage systems because of uncertain and complex technology inputs and increased risk. Unless yield gains are substantial, there Is Insufficient Incentive to assume the risk.