Garden Fertilizers and Feeding

A fertilizer adds one or more essential plant nutrients to the soil. Unlike soil amendments, which add bulk to improve the physical structure of soil, they are both more concentrated and simpler chemically. Manufacturers often confuse gardeners by using general terms such as ‘feeding the soil’ and ‘enrichment’ for both products but their roles are quite separate. Whereas soil amendments are added in generous amounts before, at or after planting and precise quantities are not too critical, fertilizers should be added more carefully. Apply too little fertilizer and the objective will not be achieved; apply too much and this can adversely affect plant growth or even have an environmental impact, for example, by permitting excess nitrogen to leach into water courses. For optimum results, fertilizer should always be added at the rates and at the times recommended on the packaging.

Why plants need feeding

Although plants manufacture organic nutrients within their green tissues by photo-synthesis, they obtain virtually all their mineral nutrient requirements from the soil. In a natural habitat, where native plants grow at natural spacings and aren’t interfered with or removed from the site when they mature or die, a nutritional balance is maintained. Sufficient nutrient is returned to the soil through the decomposition of dead plants to satisfy the needs of those still growing. This is far from the case in gardens, especially in vegetable gardens and annual beds, where we grow largely non-native plants, grow them in unnatural ways (confined in containers for example), grow them much closer together than actually happens in nature, remove them at the end of the season and replant the same site very soon afterwards. Without the use of supplementary feeding, the plants would fail far short of our needs and expectations.

Plant nutrients

roses benefit from being fertilized twice a yearThere are three major elements needed by plants: nitrogen, phosphorus (generally referred to as phosphate) and potassium (generally referred to as potash). Of these, Nitrogen is most likely to be in short supply at any time as it is easily washed out of soils by rain, especially during the winter. In addition, there are minor elements – calcium, magnesium and sulphur — which are also important but needed in smaller quantities. Finally, there are elements needed in extremely small amounts, referred to as trace elements. These are iron, manganese, boron, copper, molybdenum, zinc, sodium and chlorine. Most British soils contain adequate supplies of minor and trace elements, even for the added demands of garden plants, so the majority of garden fertilizers are blended to supply the three major nutrients, particularly nitrogen.

Main types of garden fertilizer

Although it is possible to supply one or two elements at a time by the use of so-called straight fertilizers like potassium sulphate, most gardeners find it easier to use compound mixtures.

Major elements and their role in plant growth

major elements and their role in plant growth

These are balanced blends of different chemicals, formulated principally on the basis of plant’s nitrogen requirements, with the amounts of phosphorus and potassium balanced accordingly. Fertilizer packaging is required by law to state the proportions by weight of the three major plant nutrients in a formula usually expressed as N:P:K. It is important to remember, however, that these are relative proportions, not the absolute amounts. A fertilizer with an N:P:K content of 3:3:5, for instance, is relatively just as potassium rich as one of 6:6:10.

Recommended essential garden fertilizers

1. A balanced general purpose solid fertilizer

This type of fertilizer is particularly valuable for use among vegetables and other plants at or just before the start of the growing season. The commonest artificial blend available in the UK is the granular mixture called Crow more which contains N:P:K at 7:7:7.

The principal organically based compound fertilizer is fish, blood and bone, a blend of dried blood, finely ground bone meal and sulphate of potash (not, of course, organic). Like all organically based fertilisers, its composition is variable but is approximately 5.1:5.:6.5. The nitrogen from the dried blood tends to be more slowly available than that from the ammonium sulphate in artificial mixtures and the phosphorus is released more slowly from the bone meal.

2. A general purpose liquid fertiliser, relatively high in potassium

A fertilizer of this type should be the main-stay of most gardeners fertilizer usage during the height of the season. There are several branded liquid products of this nature, varying in their relative nutrient contents and most containing additional minor and trace elements, since they tend to be used extensively for plants growing in soilless composts. The concentrated liquid tomato fertilizers, for instance, generally with a composition of around 5:5:9 derived from inorganic components, are of this type. Most of the soluble fertilizers purchased in the form of powders or crystals also fall into this category, having nutrient ratios of about 15:5:20.

3. Bone meal (organic) or superphosphate (artificial)

These slow-release sources of phosphorus should be used routinely in the planting of trees, shrubs, perennials and bulbs.

4. Two lawn fertilizers

Lawns should normally be fed twice a year, in spring and in autumn, but the nutrient requirements are different at these times. I discuss lawn feeding and lawn management elsewhere but you should have two powder-formulated lawn fertilizers; one with a relatively high nitrogen content for spring and summer use and one relatively lower in nitrogen for autumn and winter application. The latter is the fertilizer I recommend for use before seeding or turfing,

5. Rose fertilizer

Fertilizers formulated for roses contain a blend or the major nutrients but with special emphasis on potassium to encourage flowering. Most branded products also contain additional magnesium because roses are especially prone to deficiency of this element. I apply rose fertilizer following the spring pruning and again after the first flush of early-summer flowers. Although formulated specifically for roses, these fertilizers also provide an ideal balanced feed for other flowering shrubs and I feed them all at the same time.

6. Sequestered iron

Although most soils have enough trace elements for plant growth, and fertilizers often contain them as impurities, one minor nutrient that some plants can’t easily absorb from an alkaline soil is iron. Additional iron should be supplied in a form known as sequestered iron, which can be taken up readily.

When and how to feed plants

Rapidity of action is important in choosing a fertilizer. Slow-release products will break down in the soil over a long period to release nutrients, so products such as coarse-grade hone meal are often recommended when planting permanent plants such as trees, shrubs or perennials. For a quicker-acting feed at the start of a growing season, fish, blood and bone or Growmore are useful.

For the quickest results during the height of the summer when fast-growing crops are putting on new foliage, flowers and fruit, you need a liquid fertilizer. At these times it can be usefully applied as a foliar spray using a hose-end dilutor. The leaves absorb the nutrients directly rather than via the normal pathway of the roots and this can be conveniently quicker in the summer months.

Nutrient deficiencies and their recognition

Although genuine deficiencies are rarely serious in British gardens, alkaline soils can make many nutrients unavailable to plants and where heavy cropping takes place, such as in a fruit or vegetable garden, deficiency effects can be seen. Plants growing in soilless composts can also be at risk from nutrient deficiencies unless supplementary feeding takes place after about six weeks.

Fertilizer application

Whether fertilizers are applied as liquids, granules or powders, it is very important that they are spread as uniformly as possible in the area where they are needed. Powders and granules applied to individual plants or to rows of vegetables are almost always spread by hand and, with practice, it is fairly easy to obtain even coverage. Nonetheless, dosage should be judged carefully and handfuls of fertilizer not just thrown around indiscriminately. Modern fertilizer packet labels generally bear dosing instructions in g/square metre (and sometimes in oz/square yard too). With a set of scales, therefore, determine the weight of one of your own handfuls of each fertilizer you use and then write this either on the packet or on a card pinned up in your garden shed. And always remember to wear gloves and wash your hands after handling any fertilizer.

For lawn fertilizers, use a small wheeled spreader for uniform dosing. It is wise to buy a spreader that can be calibrated variously so that it may be used with products from different manufacturers that may need slightly differing dose rates.

fertilizers and their uses


On a small scale, liquid fertilizers can be applied by sprayer or, especially on a law n, by watering can, but for large areas of garden, use a hose-end dilutor, which is a container that can be tilted to the delivery end of a hose-pipe. The container is filled with a concentrated fertilizer solution and the flow of water through the hose draws out concentrate to deliver diluted liquid reed.

30. July 2013 by Dave Pinkney
Categories: Garden Management, Manures and Fertilisers | Tags: , | Comments Off on Garden Fertilizers and Feeding


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