Addition of organic matter

It is normal in horticulture to return plant residues to cultivated areas where possible. Whether or not the plant remains are worked into the soil in which they have been grown depends upon their nature. The residue of some crops, such as tomatoes in the greenhouse, is removed to reduce disease carry over and because it cannot easily be incorporated into the soil. Other crops, such as hops, are removed for harvesting and some of the processed remains, spent hops, can be returned or used elsewhere. Wherever organic matter is removed, whether it is just the marketed part, such as top fruit from the orchard, or virtually the whole crop, such as cucumbers from a greenhouse, the nutrients removed must be replaced to maintain fertility (see also fertilizers).

Open, easily worked soils are created by the addition of large quantities of bulky organic matter. Clay soils are made easier to manage and their working range increased by the addition of organic matter. Stable, wellstructured sands and silts are only possible under intensive cultivation if high humus levels are maintained by the addition of large quantities of bulky organic matter.

Bulky organic matter, such as compost, straw, farmyard manure and peat, is an important means of maintaining organic matter and humus levels. It also ‘opens up’ the soil, i.e. improves porosity. The main problem is finding cheap enough sources because their bulk makes transport and handling a major part of the cost. They can be evaluated on the basis of their effect on the physical properties of soil and their small, nutrient content.

Straw
Straw is an agricultural crop residue readily available in many parts of the country, but care should be taken to avoid straw with harmful herbicide residues. It is ploughed in or composted and then worked in. There appears to be no advantage in composting if allowance is made for the demand on nitrogen by soil bacteria. About 6 kg of nitrogen fertilizer needs to be added for each tonne of straw for composting to preventing soil robbing. Chopping the straw facilitates its incorporation and while not decomposed it can open up soils. On decomposition it yields very little nutrient for plant use, but makes an important contribution to maintaining soil humus levels. Straw bales suitably composted on site are the basis of producing an open growing medium for cucumbers.

Farmyard manure (FYM)
This is the traditional material used to maintain and improve soil fertility. It consists of straw or other bedding, mixed with animal faeces and urine. The exact value of this material depends upon the proportions of the ingredients, the degree of decomposition and the method of storage. Samples vary considerably. Much of the FYM is rotted down in the first growing season, but almost half survives for another year and half of that goes on to a third season and so on. A full range of nutrients is released into the soil and the addition of major nutrients should be allowed for when calculating fertilizer requirements. The continued release of large quantities of nitrogen can be a problem, especially on unplanted ground in the autumn, when the nitrates formed are leached deep into the soil over the winter and can pollute waterways.

FYM is most valued for its ability to provide organic matter and humus for maintaining or improving soil structure. As with any bulky organic matter, FYM must be worked into soils where conditions are favourable for continued decomposition to occur. Where fresh organic matter is worked into wet and compacted soils, the need for oxygen outstrips supply and anaerobic conditions develop to the detriment of any plants present. Where this occurs a foul smell (see sulphur) and grey colourings occur. FYM should not be worked in deep, especially on heavy soil.

Horticultural peats
Sphagnum moss peats have a fibrous texture, high porosity, high water retention and a low pH. They are used extensively in horticulture as a source of bulky organic matter and are particularly valued as an ingredient of potting composts because, with their stability, excellent porosity and high water retention, they can be used to create an almost ideal root environment.

Sedge peats tend to contain more plant nutrients than sphagnum moss. They are darker, more decomposed and have a higher pH level, but also have a slightly lower water-holding capacity. They tend to be used for making peat blocks. Considerable efforts are being made to find alternative materials to replace peat in order to avoid destroying valued wetland habitats from which they are harvested.

Leaves
Leaf mould is made from rotted leaves of deciduous trees. It is low in nutrients because nitrogen and phosphate are withdrawn from the leaves before they fall and potassium is readily leached from the ageing leaf. They are often composted separately from other organic matter and much valued in ornamental horticulture for a variety of uses, such as an attractive mulch, or when well rotted down, as a compost ingredient. They are commonly composted in mesh cages, but many achieve success by putting them in polythene bags well punched with holes. The leaves alone have a high C:N ratio so decomposition is slow and it is not usually until the second year that the dark-brown crumbly material is produced, although the process can be speeded up by shredding the leaves first.

Unless they are from trees growing in very acid conditions, the leaves are rich in calcium and the leaf mould made from them should not be used with calcifuge plants. Pine needles are covered with a protective layer that slows down decomposition. They are low in calcium and the resins present are converted to acids. This extremely acid litter is almost resistant to decomposition. It is valued in the propagation and growing of calcifuge plants, such as rhododendrons and heathers, and as a material for constructing decorative pathways.

Air-dried digested sludge
This consists of sewage sludge that has been fermented in sealed tanks, drained and stacked to dry. The harmful organisms and the objectionable smells of raw sewage are eliminated in this process. It provides a useful source of organic matter, but is low in potash. Advice should be taken before using sewage sludges because in some regions they contain high quantities of heavy metals, such as zinc, nickel and cadmium that can accumulate in the soil to levels toxic to plants.

Leys
The practice of ley farming involves grassing down areas and is common where arable crop production can be closely integrated with livestock. At the end of the ley period the grass or grass and clover sward is ploughed in. The root action of the grasses and the increased organic matter levels can improve the structure and workability of problem soils. There are some pest problems peculiar to cropping after grass that should be borne in mind (see wireworms), and generally the ley enterprise has to be profitable in its own right to justify its place in a horticultural rotation. It is practiced in some vegetable production and nursery stock areas.