Using Fertilizers and Fertilizer Types
Plants, like people, thrive on a balanced diet – thethey grow in must be enriched with all the foods needed for healthy growth.
Plants manufacture their own body-building substances from raw materials in the soil — weathered rock and humus. Several of the essential chemical elements for plants are readily obtainable from the air and the soil and do not need supplementing under normal conditions — these are carbon, hydrogen and oxygen.
Other essential nutrients, dissolved in water, are taken up as a ‘drink’ by hairs growing near the tips of the roots. The three principal elements on which plant life depends — nitrogen (abbreviated to N), phosphorus (P) and potassium (K) — are absorbed in this way. A well-tended soil will be rich in all these, together with the so-called ‘trace elements’. These are required in very small quantities only — or a trace — but are just as essential as the big three, N, P and K. Nitrogen is ‘fixed’ in the soil — and so rendered available to plants — by the action of various soil-borne bacteria.
In time, a plant will use up most of the available nutrients, so it is essential to supply more if the plant is to thrive. Nitrogen is leached rapidly from the soil bywater following rainfall
All the complex needs of a plant could be met by enriching the soil with farmyard manure — animal droppings mixed with straw. The highly productive kitchen gardens of Victorian times received huge quantities of stable manure, and the natural processes of the soil, assisted by enough lime to keep it ‘sweet’, made it unnecessary for the gardener to worry about details of plant nutrition.
This simple method of maintaining fertility is no longer practicable for most gardeners, who must now rely on whatever organic material is available and on supplementing it with organic or inorganic chemical fertilizer types. Manures do a combined job of feeding the soil and improving its structure, but are slow-acting in building up fertility.
Chemical fertilizer types feed plants more directly, supplying nutrients that can be absorbed immediately, but they do nothing tostructure and may even deplete its reserves of organic material.
Aim to strike a balance, adding as much organic material as is possible, and using fertilizers to remedy known deficiencies.
Each of the essential nutrients — nitrogen, phosphorus (in phosphate) and potassium (in potash) —can be bought separately as so-called ‘straight’ fertilizers, or in balanced mixtures known as ‘compound’ or ‘general’ fertilizers. The latter type generally include the most important trace elements.
Some fertilizers derived from dead animals, including dried blood and hoof and horn mix, are rich in nitrogen. But they are expensive and are normally used only in special circumstances, such as in sowing and potting composts.
Less expensive sources of nitrogen are the inorganic fertilizers —the cheapest being sulphate of ammonia. More expensive are nitro-chalk, nitrate of ammonia and nitrate of soda. These work faster than sulphate of ammonia because they are already in the nitrate form which roots absorb and use.
When plants need a quick-acting stimulant, it is usually best supplied by nitrogen alone. For example, spring cabbages respond quickly to a top-dressing of a nitrate fertilizer at 15g (½ oz) per sq m/yd once growth starts.
Other overwintered vegetables also need added nitrogen in spring to replace that lost from the soil by winter rains or not yet made available by the action of bacteria because of low soil temperature.
Most fruit crops appreciate a dressing of sulphate of ammonia in spring. A more active source of nitrogen is urea, sprayed on to the foliage of starved fruit trees and bushes in the spring. Urea spray can be bought from a nurseryman, but be sure to follow the manufacturer’s instructions carefully. Phosphorus sources Phosphorus, in the form of soluble phosphates, is particularly important to plants in the seedling stage and in the formation of roots. When the soil has too little phosphate the leaves turn dull purple, they are smaller and the growth of the plant slows down. Too much phosphate will cause a premature ripening of the plant.
Phosphatic fertilizers include slow-release bonemeal, which is popular because it is organic and readily available, and superphosphate of lime.
When seeds are sown out of doors, a dressing of superphosphate at two tablespoons per sq m/yd should be applied before sowing, and lightly raked in.
Potassium, in the form of potash, increases the intensity of flower colour and is sometimes given as a top-dressing to improve the formation ofand the ripening of fruits. It also increases a plant’s resistance to pests and diseases and hardens the tissues. Potash is especially required for fruit crops, and potatoes.
Light soils in particular are likely to need added potash, and the most suitable form for the gardener is sulphate of potash.
Grow-more is a very popular and well-tried compound fertilizer, containing 7% nitrogen, 7% phosphorus and 7% potassium. The ingredients are sulphate of ammonia, superphosphate of lime and sulphate of potash.
Another compound formula is John Innes base fertilizer containing hoof and horn mixture, superphosphate and sulphate of potash. The N, P and K content is more carefully balanced and, since the nitrogen is of organic (natural) source, this fertilizer is usually more expensive.
Fertilizers are also available in formulations for special purposes — there are quick-absorption liquid feeds for potash-loving plants such asand carnations, for instance.
Concentrated compound fertilizers, sold in both solid and liquid form, must be diluted before use. They are easy to apply as a top-dressing and are quickly absorbed by plants.
Some liquid fertilizers are derived from seaweed and humus extracts; others are made solely from chemical elements. They are mixed in various proportions to suit the needs of various plants and soils.
Plants take several days to make use of nutrients absorbed through their roots. However, if their leaves are sprayed with dilute solutions of fertilizers the process is accelerated. Special fertilizers for foliar feeding are available, some being based on soluble, inorganic fertilizers while others are all-organic preparations with a seaweed base. All types are quickly absorbed.
Foliar feeds should be regarded as a supplement to manures or fertilizers rather than as the sole means of feeding the crop. They are particularly useful, however, if the plants have a poor root system or during dry spells when the plants have difficulty in drawing nutrients from the soil.
Never apply more fertilizer than recommended. Measure roughly the area to be fed, then weigh the required amount of fertilizer on a kitchen scale — line the scale with paper and wash it out thoroughly afterwards, or preferably keep an old scale solely for this purpose. Halve the amount of fertilizer when farmyard manure or has been dug in.
A fortnight before sowing seeds, spread a compound fertilizer evenly over the soil and hoe or rake it into the surface. Do not dig it in or it will soon wash down out of reach of the roots.
Do not scatter fertilizers along a seed drill as they may injure the germinating seedlings. Apply top-dressings along the sides of the rows or around plant roots and lightly hoe them in. Do not allow inorganic fertilizers — except the foliar feeds — to touch the plant’s foliage or it will be scorched.
In dry weather follow the application of fertilizers with a good watering because they cannot be aborbed by the plants until they are dissolved. The condition of the soil and the weather also dictates the types of fertilizers to use. A light, sandy soil, for example, needs more potash than heavier soils, especially in gardens where soft fruits are grown. In districts with a heavy rainfall, nitrogenous fertilizers wash out quickly and should be replaced by regular top-dressings of sulphate of ammonia or nitrate of soda.
Store fertilizers in a dry place as they quickly absorb moisture. If you transfer a chemical to a new container, label it clearly. Store out of reach of children — some fertilizers are poisonous.