For a number of years, experiments have been conducted at the John Innes Horticultural Institution to determine the best composts for raising and growing pot plants. Throughout, the object has been to produce a compost giving optimum rather than maximum growth from germination until maturity. The possibility of producing standard composts for raising and growing pot plants has also been studied.
The most important results of these experiments are, first, that better germination and seedling growth are obtained in a standard seed compost to which fertilisers have been added; secondly, that almost all plants can be grown to greater perfection in a standard potting compost than in the wide variety of mixtures commonly used.
Both these results are contrary to usual practice and are of extreme importance to the gardener.
Two such standard composts have been in use at Merton for the past two years for a great variety of plants and have proved highly successful. It is proposed in a series of short articles to describe some of the experiments which have led to the adoption of these composts.
The value of phosphatic fertilisers to the seedling is well-known in farming and general gardening practice out-of-doors, but this knowledge has seldom been applied to seedlings raised under glass. We have previously shown (Scientific Horticulture, 1936) that most horticultural soils are markedly deficient in phosphates, but usually contain enough of other plant foods for seedling growth.
PHOSPHATIC FERTILISERS AND LIME
The experiment described below was made to get information as to the relative value of three phosphatic fertilisers, superphosphate of lime, bone-meal and bone-flour when added to composts for seed sowing.
The compost consisted of two parts good loam (sterilised), one part moss-peat, and one part of coarse sand (sterilised). This compost was divided into two heaps, and to one chalk was added at the rate of one-and-a-half ounce per bushel. Each of these heaps was again divided into four equal parts, giving four heaps with chalk and four without chalk. To three of these composts in each group fertilisers were added in the form of superphosphate, steamed bone-flour and bone-meal, at a rate equivalent to one-and-a-half of superphosphate (sixteen per cent, P2O5). In each of the eight composts, seeds of tomato, celery and Chinese primula were sown and raised. The tomatoes were potted into three-and-a-half-inch pots when they were large enough. The celery and Primulas were left in their seed-pans. The growth of the primula and tomato in these composts is shown in both figures. Celery gave almost identical results.
The points to be noted from the experiments are as follow:
1. The poorer growth obtained when chalk is added in the absence of phosphate.
2. The better growth obtained when any of the three phosphates is added in the absence of chalk.
3. The poorer growth obtained when bone-flour or bone-meal are used in the presence of chalk.
4. The greater availability of the phosphate in superphosphate compared with bone-flour and bone-meal.
5. The best results are obtained when superphosphate is added, with chalk.
In commenting upon these results, it should be said that the rate of application of chalk is roughly equal to half-a-pound per square yard. The loam, moss-peat and sand used contain no appreciable amount of chalk or lime. Thus the amount of chalk added was comparable to a good outdoor dressing. It appears therefore that superphosphate is the best phosphatic fertiliser to add to seed soils, especially if chalk or lime is present.
W.J.C. Lawrence and J. Newell, John Innes Horticultural Institution, Merton
? Taken from The Gardeners’ Chronicle, 1 May 1937