Going underground

Checking for problems below ground level at an early stage can pay dividends for growers, Peter Groeneveld advises.

A soil pit should be typical of your field - image: HW
A soil pit should be typical of your field - image: HW

Before planting or sowing a new crop, it pays to consider a soil profile. The exercise will assess your soil texture and structure and help you to decide whether improvements need to be made to increase productivity.

Growers tend to focus on problems above ground level and only when plants begin to show symptoms of deficiencies is attention given to the soil-plant relationships. Many growers could face plant growth problems because of the combination of heavy rains and a deteriorating soil structure. Soil compaction has increased significantly over the past 10 years because of the use of heavy equipment. The intensification of horticulture and agriculture have contributed to this problem.

The effects of soil compaction can be found down to 1m in depth. Many issues can be prevented and remedied by learning more about your soil. Soil structure is greatly improved by good drainage, an active soil life, ample organic matter content, and correct tillage at the right time, an optimum pH, and balanced nutrient levels.

Micro-organisms decompost organic matter and release nutrients to plants. The water-holding capacity of the soil is improved, which allows better root penetration, giving plants more oxygen, nutrients and water.

The organic matter content of soils is falling to dangerously low levels due to intensive farming systems and extensive use of artificial fertilisers and chemicals. There is a positive correlation between the organic matter content and soil diversity. In general, a minimum of a two per cent organic matter content is required. Unfortunately, most soil tests do not analyse the organic matter content.

Soil diversity decreases sharply when organic matter declines, or when soil is compacted. A decrease in biodiversity will threaten agricultural and horticultural production, ground and surface water quality and climate regulation.

How to make a soil profile

Dig a square pit approximately 800x800x800mm. It is easiest to dig when the soil is moist, especially when dealing with clay soils. Choose an area that is representative of your field, not at the edge of the crop.

The first thing to check is the soil structure. This is mainly determined by the range of soil particle sizes available (sand, silt and clay). Coarse sand particles have a diameter (in micrometers) of 200-2,000, fine sand 20-200, silt two-20 and clay less than two.

Plant growth is related to the particle-size composition of soils. One of the most important effects is the influence of texture on water supply. By rubbing and kneading a sample of wet soil, one can determine the texture of your soil.

Look at the different soil layers (horizons) and the different colours in the profile and check whether there are any impermeable layers that have to be broken up later to avoid waterlogging. On sandy soils the top layer is mostly darker in colour because of the built up organic matter. Also look at the root development of the present vegetation. If the roots suddenly stop at a certain level, there could be a hard layer (clay) preventing further growth.

Earthworms are absolutely essential for the improvement of the soil structure. They feed on organic matter and soil particles, some coming on the surface to collect it and then dragging it down into their burrows. Their excreta, in the form of worm casts, contain a high proportion of soil and bacteria.

Earthworms can only thrive in soils under certain specific conditions. They are intolerant of drought and frost and therefore dry, sandy soils and thin soils overlying rock are not usually favourable environments for them.

Measures for successful profiling

Having evaluated your soil profile, you can consider taking the following measures: Only work the soil when conditions are optimum (not wet), avoiding soil compaction, but do not over work the soil disturbing the soil life activity. Break up any impermeable layers by sub soiling and apply plenty of organic matter (compost or farm manure).

Dressings of farm yard manure or compost, particularly if generous applications are given regularly, can have very beneficial effects on the soil structure and fertility. Encourage the development of soil micro- organisms and if necessary introduce earthworms and beneficial bacteria for the decomposition of organic matter and the formation of stable humus.

The rate of decomposition of the organic matter depends on soil aeration, calcium supply and temperature. Check the pH of your soil regularly. Most plants grow best with a soil pH of 6.4. At this level, most nutrients are freely available to plants.

If the pH drops below four to 4.5, soil life activity will drop sharply. Also check the levels of the main nutrient levels N, P and K and if possible the organic matter content level and apply preferably organic fertilisers to remedy any shortages.

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