Organic matter

A high level of organic matter helps to maintain a stable soil structure and improves the water-holding capacity of the soil. In the past, growers used to steam sterilize greenhouse soils and then add well-rotted manure after sterilization. This procedure reduced the release of ammonia and other toxic substances, and it also helped to reinoculate the soil with beneficial organisms. However, the danger of introducing disease organisms and weed seeds always remained. An additional complication in the use of manure or muck soil as a source of organic matter is the inherent potential for contaminating the greenhouse soil with herbicide residues. The recommended amounts ofmanure varied from 45 to 225 t/ha, depending on the kind of manure and the soil conditions. For example, spent mushroom compost has a high nutrient content and can cause soil conductivity problems, whereas uncomposed straw may induce nitrogen deficiency.

In recent years the addition of organic matter has increasingly been viewed as a means of improving the soil condition (structure) and not so much as a means of increasing the nutrient content of the soil. In fact, the nutrient content of most manures and other nonstandardized sources of organic matter is considered a liability rather than an asset because of its extreme variability and the unpredictable effects the various nutrients can have on the crop to follow. At present, coarse peat is the most satisfactory material as a source of organic matter. This type of peat is acid, with a pH of about 4, and therefore has the added benefit of reducing the pH of calcareous soils; where the soil is noncalcareous, add ground limestone to loose peat at an approximate rate of 5 kg/m3 to neutralize the peat's acidity When used to improve the soil conditions, e.g., on new sites, apply peat generously at rates of up to 500 m3/ha. When the soil reaches the desired condition, reduce the rate; the need for an annual dressing remains because soil organic matter decomposes under glass rapidly. Apply loose peat to soil at a yearly rate of 100 m /ha.

Broadcast peat and lime before the main cultivation and incorporate them into the top 30 cm of the soil.

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