Development of Soilless Composts
The development of soilless composts
The first pioneering work on soilless composts to make a big impact on the horticultural world was that done at the University of California and published in 1957. In fact, soilless composts are frequently referred to as UC composts or mixes (the UC standing for the University of California), even if they are not in fact following any UC formulae. Work was begun in the Department of Plant Pathology at the University of California, L.A., in 1941 to find a better compost for growing plants in containers. Because of the difficulty of finding turf for composting and problems of toxicity after steaming, the John Innes method was not satisfactory and a substitute had to be found.
The UC composts
Eventually five basic compost mixes were evolved, ranging from all peat to all sand, with varying proportions of each in between. To these basic composts could be added one of six fertilizer formulae, three containing readily available nitrogen in the form of potassium nitrate and the others reserve, or longer-availability nitrogen.
When the UC system was first tried in Britain it was not very successful. It must be remembered that these composts were developed for conditions in California where the climate is distinctly Mediterranean. In Britain the lower temperature and light intensity resulted in nitrogen starvation in plants in UC composts as the breakdown rate of organic nitrogen was too slow. The fine structure of the compost, designed to retain moisture in the high transpiration conditions of California, caused waterlogging in the cooler, more humid climate of Great Britain. The result was to make many people deeply suspicious of the whole idea of soilless composts; others however realized that it was just a matter of adjusting the formulations to the British climate, altering the water-holding capacity and the proportions of nitrogen to potash in the fertilizers.
Nitrogen supply in-based and soilless composts
The key to success in the nutrition of soilless compost is the supply of nitrogen, which must be provided in both readily available and “slow release” forms. When slow release nitrogen is added to soil or compost, it undergoes a number of changes into other compounds before it is available to the plants. Some of these compounds become toxic to plants in comparatively moderate concentrations and all of them are toxic in excess.
When an organic material such as hoof and horn is applied to the soil, the nitrogen it contains is made up of complicated protein-like compounds which are converted by the soil micro-organisms into inorganic ammonium form which is in turn converted into nitrate form — the form in which it can be taken up by the plants. The first stage is performed by what may be called the ammonifying bacteria and the second stage is performed by what may be called the nitrifying bacteria.
Obviously any condition which affects the population or the activity of these organisms will affect the rate of breakdown of the organic nitrogen into ammonium and nitrate compounds. The soil contains a very large number of micro-organisms including bacteria, fungi, actinomycetes, algae, nematodes and protozoa. All these forms of microscopic life exist in the soil in a state of dynamic equilibrium or ever-changing balance among themselves. Many of these organisms are harmful to plant life, therefore soil to be used in compost is sterilized to try to remove the harmful micro-organisms while leaving the beneficial micro-organisms as unaffected as possible.
In a soilless compost, the situation is rather different. The peat and sand are naturally almost sterile without sterilization and hence the compost when made up will contain few, if any, of the micro-organisms that can convert organic nitrogen into the readily available nitrogen. Because of this it can be seen that readily available nitrogen must be provided in the compost in the form of nitrate until such time as the population of micro-organisms has been built up so that they in turn can provide the nitrates that the plants require. This is an important difference between soil-based and soilless composts and it led to the development of the ‘starter’ solution for the UC composts. The use of calcium nitrate provided the readily available nitrogen until the hoof and horn began breaking down. The grade of hoof and horn would influence the speed with which nitrogen was released. Finer particles present a larger surface area for the bacteria to work on and hence the nitrogen release would be more rapid than from a coarser grade of hoof and horn.
Two other factors concerning the release of nitrogen must be pointed out. Firstly, the bacteria that convert the organic nitrogen to ammonium nitrogen are able to carry out the conversion even in the absence of oxygen. By contrast, the bacteria that convert the ammonium nitrogen into nitrate nitrogen must have an adequate supply of oxygen so that the ammonia may be converted to nitrate.
Secondly, the rate of nitrification is dependent on the pH of the compost and it occurs in two distinct stages:
Ammonium —> Nitrite —> Nitrate
The conversion of ammonium to nitrate goes through an intermediate stage known as nitrite. The first stage to nitrite will proceed up to a pH of 7.5 but the second stage from nitrite to nitrate will only proceed satisfactorily up to a pH of 7.0. It is obvious that between a pH of 7.0 and 7.5 the ammonium nitrogen will be converted as far as the nitrite but conversion of the nitrite to nitrate will be much slower, leading to a build-up of nitrite in the compost. Excess nitrite is damaging to all plants, but one of the most sensitive is antirrhinum. The first sign of damage is the yellowing of the younger leaves, turning to white in the presence of nitrite. On examination, the roots of the plants will be found to be brown and scorched.
Below a pH of about 5.5, the ammonium to nitrate conversion is slowed down but the breaking down of organic nitrogen to ammonium nitrogen continues. This time, at too acid a pH, ammonia will accumulate which is also damaging to plants.
From the above, it can be seen that the rate and type of nitrogen applied to a compost must be carefully controlled. Composts containing organic sources such as the John Innes composts and the UC mixes with hoof and horn must not be stored for periods longer than one or two weeks before use, as ammonia will build up from the breakdown but nitrification of it cannot proceed as oxygen will not be able to penetrate the compost. The pH should be kept in the range of 5.5 to 7.0 because, above this pH, nitrite will accumulate while below this pH range ammonia will build up.