How Plants Feed

How Plants Feed

The bulk of every green plant comes from water and air. The air supplies mainly carbon; the water, oxygen and hydrogen. The green colouring matter in the leaf (it may be masked by other colourants but this does not matter) is able to synthesize these, using sunlight for energy, into complex substances which become sugars, starches and proteins. But to perform these marvels the plant also needs very small quantities of a whole range of chemicals.

Soil Enrichment

There are a number of ways in which the gardener can enrich the soil. Dead plants still contain most of the chemicals they have taken from the soil and as they decay these chemicals will be made available once again. At the same time, the decaying vegetable refuse will improve the texture of the soil and enable it to hold more moisture.

Animal manure behaves in the same way. As it decays chemicals are liberated from it and the texture of the soil is improved.

If the animal manure is already well de-composed when it is applied, the chemicals will be more immediately available. Precisely the same applies to vegetable refuse, which is one reason for building it up into heaps, known as compost heaps, and leaving it for a few months to rot before it is used, cither as a surface mulch or dug in.

Animal droppings and vegetable refuse are known as bulky manures because they contain comparatively small quantities of chemicals and so should be applied in considerable quantity. A hundredweight of stable or farmyard manure or of well-made garden compost will be adequate for 5sq. Yd (5sq. m) of land, though as a rule gardeners cannot afford to be so generous and must be content to spread it over 10 or I2sq. Yd (10 or I2sq. m).

So far as the chemicals in the manure are concerned, a similar effect might be produced by spreading one pound of well-chosen concentrated fertilizer, but this would be unlikely to have any beneficial effect on the texture of the soil, that is. Its ability to hold plenty of water and still retain enough air to keep itself and the roots growing in it in good health.

The aim in manuring the soil is, therefore, twofold: to improve texture and to improve chemical content. As a rule this can be done most efficiently by using chemical fertilizers in conjunction with bulky manures or composts, or with peat, which is a kind of natural compost which has the merit of being readily available at reasonable prices and in well-defined grades.


The Most Important Chemicals Plants need a considerable number of different chemicals but only a few are likely to be deficient in most soils. The most important of these are nitrogen, phosphorus and potassium, followed by magnesium, iron and manganese.

There are a great many ready-mixed fer-tilizers available, often referred to as compound fertilizers, and these usually provide nitrogen, phosphorus and potash. The makers are obliged by law to state how much of each of these elements their fertilizers contain, the figures being quoted as percentages of nitrogen, phosphoric acid and potash. When a fertilizer contains approximately the same amount of these three items, it is referred to as well balanced. The National Growmore fertilizer is of this character, its formula being 7 per cent nitrogen, 7 per cent phosphoric acid and 7 per cent potash. Often these figures are quoted without the percentage sign, so that National Growmore may be referred to as a 7:7:7 fertilizer. Such a quotation is always in the order nitrogen, phosphoric acid and potash.

Action of Specific Chemicals

Well-balanced fertilizers are excellent for maintaining the general fertility of the soil, but for special purposes one may require a fertilizer that is richer in one chemical than in another. Nitrogen tends to promote growth, so that for a leaf crop such as lettuce, cabbage or grass it may be desirable to use a fertilizer that has a lot of nitrogen in proportion to phosphoric acid and potash. Phosphorus is particularly necessary in the early stages of growth when seedlings or young plants are making a lot of roots, so fertilizers for seedlings and root crops usually have a high phosphoric acid ratio. Potash has a marked bearing on fruitfulness and ripening, so potash tends to be the highest percentage in fertilizer specially prepared for fruit trees or for tomatoes, once they have started to crop.

Ingredients Most Commonly Used

This kind of analysis shows quite accurately the kind of fertilizer one is buying but it tells us nothing about its ingredients. National Growmore, for example, might be prepared in several different ways provided these gave the desired 7:7:7 percentage of nitrogen. Phosphoric acid and potash.

The three ingredients most commonly used are: sulphate of ammonia for nitrogen. Superphosphate of lime for phosphorus. And sulphate of potash for potassium. These are good ingredients and they can be mixed together without troublesome chemical interactions.

Other popular sources of nitrogen are dried blood and hoof and horn meal. As they are animal residues they are much more complex than the fairly simple chemicals just named. Plants cannot make use of these complex chemicals, which must first be broken down into simple chemicals by normal processes of decomposition in the soil. This takes time and so such organic fertilizers have a longer life in the soil than some readily available inorganic chemicals. They are sometimes referred to as slow acting, though this is a purely relative term. Certainly both dried blood and finely ground hoof and horn meals can have an effect on growth within a week or so in warm damp weather, but their effect will be slower in cold, dry conditions because these slow down the rate of decay. Very’ coarse hoof and horn meal also takes longer to decay and liberate its chemical plant food than fine hoof and horn meal.


Bonemeal is an animal source of phosphorus and exactly the same applies to it as to hoof and horn meal. Its availability is directly related to the fineness of grinding, for bone flour will decompose quickly in the soil whereas coarsely crushed bones may take years to decay.

Wood Ashes

Wood ashes are a natural source of potash and seven pounds of fresh wood ash may contain as much potash as one pound of sulphate of potash. It is always worth while to preserve wood ashes in a dry place and to scatter them over the soil as a fertilizer where potash is required.

Mixed Fertilizers

Occasionally the gardener may need to use single chemicals but as a rule mixtures will suit his purpose better and there are plenty of these available for almost every conceivable purpose – general fertilizers, rose fertilizers, chrysanthemum fertilizers, vegetable fertilizers, fruit fertilizers, tomato fertilizers, lawn fertilizers and so on. The important thing with all these is to use them at the right rate and at the right time. As a rule, manufacturers give full instructions but failing this some general observations may be useful.

The greater part of these fertilizers is usually readily soluble, which is a good thing since plants can only take in chemicals from the soil in solution. But it does also mean that, if they are applied a long time before the plants need them, they may be largely washed out of the soil uselessly. They are, therefore, primarily for spring and summer use.

Most are highly concentrated and so 40Z (1 log) per sq. yd, or sq. metre, of a compound fertilizer such as National Growmore is usually the maximum safe application at any one time, and something like 8oz (225g) per sq. yd. Or metre, the maximum safe application throughout any one year. A normal treatment for vegetables, for example, would be to give the soil 40Z (nog) per sq. yd, or metre, of a well-balanced compound fertilizer in early spring and then two more applications, each of 2 0z (55g) per sq. yd, or metre, one in late spring, the other in mid-summer. To give too much at any one time would be to risk scorching roots and leaves by raising to too high a level the concentration of chemicals in the soil water. If by some mischance too much is given, the best way to get rid of it is to soak the soil heavily with water and so wash the chemicals out.

Since they are soluble and easily washed into the soil by rain, these fertilizers may be scattered over the surface of the soil and either be left there or be raked or watered in. Bulky manures and composts, by contrast. Are more effective if mixed with the soil when digging or forking, though they can also be spread on the surface, a process known as mulching.

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