Summary
Maintenance of good soil structure and avoidance of soil compaction have always been important considerations for farmers and growers. The moves toward more intensive farming practices (eg., higher stocking rates), the use of increasingly heavy machinery (eg. larger tractors) and guidance of the Resource Management Act toward preserving soil condition has refocussed attention on these aspects of soil management.
In days-gone-by, fallowing fields was a reasonably common practice which was thought to be restorative in terms of both soil structure and fertility. In relation to soil structure, fallowing was not a particularly restorative practice particularly if the soil was cultivated for weed control. The more organic material that is added to the soil during the season the more beneficial will be the effect on soil organic matter content and structure. Thus, any crop is better than no crop and a good high yielding crop is better than a poor one. A high yielding crop will have greater root growth and return more organic material to the soil than a poor crop.
Some confusion exists over the term "restorative" as it can refer to both soil fertility and soil structure. These "restorative" effects are, however, not interchangeable. For example, clover seed crops, or other legume crops, may have beneficial effects on soil N fertility since N2 is fixed in their root nodules. However, legumes have a characteristically poor effect on soil structure (unless they are used as green crops) because they have a small, sparse root system and return only small amounts of organic material to the soil. By contrast, grass seed crops don’t add N to the system themselves (N is added as fertiliser) but they have a large, dense ramified root system and have a very beneficial effect on soil structure.
Controversy surrounds the practice of incorporating cereal straw back into the soil as an additional input of organic material. A number of farmers have tried it, found it impracticable or unsuccessful and returned to burning the straw. Where rotation of arable crops and pasture is practised, the benefits of straw incorporation are unlikely to be evident. However, where continuous arable crop production is practised, it is advisable to return as much organic matter as possible. In such cases, straw incorporation will be beneficial in the long-term. It is important to chop the straw reasonably finely prior to incorporation. Straw incorporation is now common practice in England, where stubble burning has been banned, and many new harvesters have straw-chopping equipment incorporated so that harvesting and chopping are carried out in one pass.
Another controversial issue is the place of direct drilling (zero tillage). Whilst a minority of cropping farmers use direct drilling, many have tried it and returned to conventional cultivation. Research has generally shown that crop yields are similar under direct drilling and conventional tillage. The major benefit of direct drilling is that the soil surface is essentially undisturbed and often covered with mulch of decomposing plant residues. This protects the paddock from soil loss through water and wind erosion. In localities where wind erosion is a problem the formation of very fine seed beds using conventional heavy rollers or crumbler bars can result in large soil losses. It is, however, important to realise that although direct drilling may preserve the condition of a well-structured soil for longer than conventional tillage, it is not a practice that will restore the condition of a poorly structured, compacted soil.
In some situations, soil structural breakdown and compaction occur as a result of long-term continued arable production. However, in many cases, a specific paddock becomes compacted because it was worked or stocked heavily when it was wet. Such events can be unavoidable, for example, when a crop is ready to be harvested just after heavy rains or wet conditions predominate during a period when cultivation is required. In such situations ameliorative action may need to be taken (e.g. subsoiling) to restore soil physical conditions.
Structural breakdown and compaction are not problems restricted to arable cropping systems. Structural breakdown is particularly prevalent on intensively managed market garden soils whilst compaction below the wheel tracks in orchards is a recognised problem. In pastoral production treading damage, especially where heavy stocking rates occur during wet periods, can result in soil compaction and subsequently reduced pasture yields. Although natural recovery does occur under pastoral conditions where cumulative treading effects have occurred over a number of years, aeration (subsoiling) can be required to restore pasture production.
Reduced crop or pasture yields as a result of structural breakdown and/or compaction represent an economic loss to the farmer. While such a loss is difficult to estimate, some attempt to quantify this aspect has been made in the report.
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