rhs2 course xtract 1

2.3 The Physical Properties of Soil

As we have seen earlier soil is made up of clay, silt and sand. The majority of the soils found in Britain are termed loams, they have a wide variety of particle sizes from clay to sand and usually a quite high proportion of silt. If such soils are to be usefully cultivated for agriculture or horticulture the maintenance of the structure is of very great importance. This is usually achieved by way of a process called soil aggregation. Clay plays a vital role here (a content of at least 10% being required), as well as a good supply of organic residues. Lime is needed in varying quantities to maintain the appropriate acidity/alkalinity balance (pH level) and an active microbial population is essential; needed to break down fresh organic material and help maintain soil structure.

Cultivation is the activity which probably has the most significant impact on the soil. Cultivations are carried out with the intention of (a) producing a suitable tilth or soil structure, (b) controlling moisture, aeration and temperature, (c) destroying weeds and pests, (d) burying or clearing crop residues or incorporating manures (FYM or green) and (e) attempting to remedy compaction problems caused by all or any of the above.

Methods of cultivation vary and obviously depend a great deal on the area that is to be worked, its cultivation history, the intended crop, the previous crop and, of course, in market gardens, the tackle available. If the area is accessible, large enough and the appropriate equipment is available, then ploughing may be considered, but, unless the conditions are just right serious problems can result. If the land is heavy and wet the soil may become compacted or, if it is too dry, large unmanageable clods may result which need further working to eliminate, once again inviting possible compaction.

In general and all things being equal (they never are of course) the best course to follow is that which employs the least number of cultivations using the lightest possible equipment consistent with the desired end result. If compaction becomes a real problem and causes waterlogging the only effective remedy is to break up the soil pan, usually to be found at the limit of habitual plough or cultivator depth, and improve the drainage which, in this case means subsoiling or drawing mole drains. Ideally, of course, we should never reach this stage, however.....

The diagram, overleaf, illustrates the difference between poorly and well structured soils and indicates the importance attached to the employment of sound cultivation and husbandry techniques aimed at maintaining good soil structure.

The overall impact of cultivation methods on soil structure is very much dependent on soil type. Sandy loams over limestone for example, can be subjected to repeated shallow cultivations without causing damage to the soil structure, but sandy loams without the benefit of free lime will suffer and so need to be ploughed every few years to restore the damaged soil structure. Light sandy and silty soils will compact anyway and need annual ploughing if the structure and friability of the topsoil is to be maintained.

Cultivations play a very important role in weed control, especially on holdings (or, of course, in gardens) farmed organically. Shallow cultivations encourage weed seed germination which, as well as making the control of annual weeds much easier, also means that the soil surface is more quickly covered with the growing crop plants that both protects the soil crumb structure from damage during heavy rain and inhibits the growth of emerging weed seedlings.

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