Mechanics of Tillage and Traction 3(2+1)

LESSON 30. MEASURES FOR OPTIMIZING CROP GROWTH BY AVOIDING EXCESSIVE SOIL COMPACTION

1. Avoid high machinery contact pressures, especially during repeated passes on fields. For a cultivation program which requires between five and ten passes of machines on the field per year, it is recommended that the tire contact pressure of the vehicles involved be limited to less than 70 kPa.

2. Avoid if possible, travelling on fields with machines when the top soil is moist, close to the “optimum” moisture content for compaction. Densification of soil can be up to five times as severe at the optimum water content under a given compacting pressure as when the soil is quite dry.

3. Avoid excessive slipping of tractor tires during field operations, which could double soil density changes under the same weight. Undue rates of wheel slip also lead to premature wear and costly replacement of tires. A maximum slip rate of 16% is recommended.

4. Attempt to manage cultural programs such that a healthy system of strong roots, and sufficient organic matter remain in the top soil. Compaction studies on a vigorous cereal stubble have shown that an extensive root system near the soil surface can reduce compaction damage under machinery loads by about two thirds compared to bare soil with low organic matter content (Chasse et al., 1975).

30.2. TILLAGE OF COMPACTED SOIL

Compaction increases the strength of a soil, and the energy which is required in order to cut or till it. Even when sufficient extra energy is expended to cut and loosen a compacted soil structure, the resulting structure will probably not be the same as that of the original uncompacted state. Studies of possible methods by which to alleviate the effects on soil structure caused by heavy construction machinery, for instance, have indicated that the cutting and lifting of a severely compressed clayey or silty soil results in a rather blocky structure. Large clods of 10 to 20 cm sizes are separated by the tillage action, but these structural units are relatively compact, hard and impervious in themselves. Furthermore, it is very difficult to refine the top soil texture subsequently, by means of conventional secondary tillage tools such as discs, cultivators and the like.

Cutting and loosening is about the only practice which can begin to improve a compacted and damaged top soil structure. With the years, the open spaces between clods will allow the passage of roots, moisture and air. And their action will eventually enter into the compacted soil blocks, to gradually open larger pores through root penetration, wetting and drying, and the overall soil tilth will experience improvement. What cannot be expected is that a single loosening tillage action will immediately restore a healthy compacted soil profile to its former uncompacted structural quality.