The Importance of Feeding Plants

The feeding of plants has always been a subject that every gardener knows about and appreciates but which very few actually carry out, and certainly not to the extent that they should. It’s a bit like children cleaning their teeth; they all know it must be done and what happens if they don’t, but most still think it’s all too much bother and not worth the trouble; tomorrow’s another day.

Can you believe that many of the plants in our gardens aren’t what they should be? This is particularly the case with vegetables because, in this part of the garden above all others, we want good results that are going to please us and save us money.

feeding plants There is a slight divergence of opinions when it comes to feeding plants, especially vegetables, because this is the point at which the ‘no chemical’ gardener would begin to break away from what might be called mainstream or orthodox gardening. Anyone who does not intend to use chemicals will almost certainly frown on the use of what are referred to as artificial fertilisers.

‘Artificials’ are man-made plant foods, as opposed to those which occur naturally, but there is no clear dividing line between the two and it is left largely to one’s conscience and preference as to which ones are considered ethical and which are not.

For example there can be very few fertilisers more natural in origin than hoof and horn, but where does basic slag fit into the picture? Slag is a by-product of the steel industry and used to be widely used as a phosphatic fertiliser. The phosphorus comes from the iron ore during the smelting process. It certainly is not an organic material and it isn’t naturally occurring, but it seems to be acceptable in a way that, say, super-phosphate isn’t. We live in a strange world full of strange thoughts and ideas!

There are two equally important sides to the subject of feeding plants; organic matter and plant nutrients. They are completely separate but operate together and woe betide any gardener who thinks he can get away with ignoring either; both are essential for strong and healthy plant growth. 


Because organic matter is at the very heart of gardening, you’d do well to be aware of the different sorts that you are likely to find available and use. Broadly speaking, they fall into two groups; those that are bulky and which are used mainly for improving the structure of the soil, and those which are applied in small quantities as a source of plant foods. 

Farmyard Manure

In years gone by, the most common form of bulky organic matter was farmyard manure. Most of the gardening went on in the country where the supply was virtually limitless — what would be described nowadays as a ‘renewable asset’. There’s still no shortage of farmyard manure but the number of gardens has increased so dramatically that there just isn’t enough to go round. Added to that, there are now many more town gardens than there used to be so the problem of transport crops up.

Farmyard manure is still thought by many to be the elixir of plant life but science has taught us that this is far from the case and that there are perfectly good alternatives. The quality of farmyard manure can vary greatly and this is due to a number of things. The proportion of straw or other litter will have an effect, as will the type of animal that produced the dung. The degree of decomposition will also influence its value.

By and large, though, an average sample of farmyard manure (if there is such a thing) will contain approximately 76% water, 0.64% nitrogen, 0.23% phosphates and 0.6% potash. This may not sound very much as against the 21% nitrogen contained in sulphate of ammonia, and indeed it is not, but it does represent a useful amount, and anyway farmyard manure should be thought of more as a bulky supplier of organic matter rather than a nutrient source. It isn’t the nutrient value of bulky organic manures but their physical effect on the soil that is important.

One thing that can be said for most bulky forms of organic matter is that they contain the trace elements. 

Domestic Waste

Many local authorities are now collecting and composting garden and other suitable waste materials to form a very useful source of bulky organic matter for digging into the garden. It is, if you like, garden compost produced on a massive scale and, in many cases, better than you can produce at home. A word with your local council should quickly put you in touch with a nearby source. 


Sewage sludge used to be a common material for spreading on the garden but we tend to take rather a dim view of it now — and yet we are quite happy to use animal manures. Raw, but dried, sewage sludge on its own is of low value as a soil improver, but when mixed and composted with straw it provides a valuable manure. A typical sample would contain as much as 60 per cent more nitrogen than farmyard manure, about the same amount of phosphates, but rather less potash. It can be regarded mainly as a source of nitrogen and organic matter.

However, the supply and use of sewage sludge is now so tied up with rules and regulations that not only could it be difficult to find, but it may also be illegal to use it. In any event, there are plenty of alternatives, both as effective and easier to find, that don’t carry the same problems.

Wool Shoddy

A typical analysis of this material would be something like 0.8 per cent nitrogen, 0.54 per cent phosphates and 0.43% potash plus, of course, trace elements. Wool shoddy, the waste from the Yorkshire woollen mills, used to be a splendid form of bulky organic matter. Unfortunately, man-made fibres have reduced the amount of pure wool shoddy considerably, but it can still be had. The mixed grade of shoddy now being produced is by no means useless, however. The nitrogen content is reduced, but its physical effect on the soil is just as good, if not better, as the man-made fibres take much longer to rot than the wool.

The highest grade of pure wool shoddy would have a nitrogen content of 12-15%. Lower grades would be 5-10% and contain 75-80% organic matter.


Whilst farmyard manure or stable manure is not always easy to come by, something that most of us con find is straw. The benefit of this is not just that there’s a lot of it about but that it is also far easier to take home and a much less anti-social material.

Straw by itself is of little value to gardeners, but once composted it provides a good alternative to farmyard manure. Nutritionally it compares quite favourably with it as well, having some 0.5% nitrogen, 0.2% phosphates and 1% potash.


Like sewage sludge and wool shoddy, peat is becoming something of a ‘product of a bygone age’.

When one thinks of peat, it is normally as a constituent of seed and potting composts and as a material to use for improving the soil when planting trees and shrubs. It is not in the same class as farmyard manure, straw or garden compost.

The trouble with peat is that the particles are very small so the physical effect on the soil is less than that of coarser materials. The nitrogen content varies from 1.5 to 2%, phosphate about 0.03% and potash 0.1%, so you can see that its nutritional value is almost nothing. Peat’s real worth lies in its acidifying action when applied to alkaline soils, and its help in the creation of a fine tilth.

The pH of peat may vary from 3.0 to 6.0, depending on its type (moss peat is the most acidic, sedge the least). The most valuable would be in the range pH4-5.

Mushroom Compost

Spent mushroom compost is readily available in some areas and it makes a useful soil improver. It consists mainly of straw composted with horse manure and with ground chalk added to raise the pH (and so reduce the acidity). Once again, the plant food value is low but it is valuable for its soil improving qualities.

One thing to remember about spent mushroom compost is that the chalk content usually makes it unsuitable for use amongst ericaceous plants. This will not matter when used with vegetables. 

Woody Materials

Sawdust, wood shavings and chippings are all quite easily found in some areas as waste products from timber and furniture works. The worst thing about any woody materials is that they will rob the ground of nitrogen if applied ‘raw’ and uncomposted. This means that they should either be composted with other waste vegetation and an activator before use, or they should be applied in conjunction with a fertiliser containing nitrogen. This is because, during decomposition by micro-organisms, a lot of nitrogen is used up. 


One of the most valuable sources of bulky organic matter is leaf-mould. In rural areas this is often quite easy to find but, before collecting it yourself out of woods, do make sure that you are allowed to.

In most woods, the best leaf-mould will be found a few inches below the surface as it will have lain there for several years and be well rotted. Freshly fallen leaves are perfectly all right but they should be composted for upwards of a year before they are fit for use.

Raw leaves will not only need nitrogen to rot down, like sawdust, but they will also blow about all over the garden if used as a mulch. Once again, the nutrient content is low but their value to the soil is high. 


Near the coast, there has always been a strong leaning towards seaweed and it certainly does do a lot to improve the soil. Fresh seaweed has a similar nitrogen content to farmyard manure but much less phosphate and about twice as much potash. It has the same organic matter value but rots down a lot quicker because of its physical nature and nitrogen content. In fact, it is excellent for adding to straw to rot it down quicker.

Seaweed has a useful amount of trace elements, principally iron, manganese and zinc. However the real benefit of seaweed lies in its alginate content. These extraordinary chemicals even have a number of uses in industry, but their particular interest to gardeners lies in their ability to improve the soil structure by opening up clay soils and binding together sandy ones. What seaweed therefore lacks in fibre, it more than makes up for in alginates. 

Pine Bark

A material that is increasing in popularity is pulverised pine bark. It has much the same uses as peat but is far longer lasting in the soil due to its hardness. This hardness also means that its physical effect on the soil structure is much greater Not only does it provide organic matter but its physical condition is, in some respects, akin to gravel. When used as a mulch, there is little risk of it blowing about.

The plant food value is virtually nil but, with a natural pH of 4-5, it is another valuable, and longer lasting, acidifying agent. Beware of some brands if you are looking for the completely natural product as they may have additives to increase the nutrient value (and price) and reduce the acidity. 

Garden Compost

The last source of bulky organic matter that I will mention is the cheapest and most readily available of all. If you have a garden, then you are growing the raw material and it is sitting there in front of you — garden compost. We shall see later how easy it is to make and how to go about it, but here we are concerned with its value as a soil improver and supplier of plant foods.

garden compost Contrary to what most people believe, garden compost normally has a higher, plant food value than farmyard manure: 2 – 2.5% nitrogen, 0.5 — 1% phosphates and 0.5 – 2% potash. This makes it a very valuable manure. Obviously the figures are extremely variable because they depend entirely on what the compost is made of, but these are the average readings.

Because the amount of fibrous material also varies greatly, the soil-improving value of garden compost is rather unpredictable but the average sized garden will have enough raw material to make composting worthwhile — and it costs virtually nothing.

That more or less covers the various sources of bulky organic matter. As we have seen, all the various forms do contain plant nutrients to a greater or lesser extent but none have enough to dispense with the need to add specific nutrients to the soil. The problem is that nature is a complete cycle with everything returning to the ground and being used by the next generation of plants. These, in turn, die and are returned to the soil to be broken down; and so on.

In gardens this doesn’t happen. The moment we start to grow crops for eating, we inevitably break the cycle because we are not putting back what we are taking out. We therefore have to return it in some other form and, in the case of plant nutrients, the conventional and convenient way is with fertilisers.

It is quite impracticable (and impossible) for most of us to rely solely on compost or farmyard manure. For the ‘organically’ minded, the choice of naturally occurring materials that contain sufficient plant foods to be called fertilisers is, fortunately, large so there is plenty to choose from.

However, that’s rather jumping the gun, because we should firstly look a bit closer at just what plant nutrients are. There are twelve elements that are absolutely essential to a plant if it is to be strong and healthy. These can be divided neatly into six major elements and six trace elements. The major elements are nitrogen (N), phosphorus (P), potassium/potash (K), magnesium (Mg), calcium (Ca) and sulphur (S). The trace elements are manganese (Mn), iron (Fe), copper (Cu),zinc (Zn), boron (B) and molybdenum (Mo). The word ‘minor’ is sometimes used instead of trace to describe the latter but this can be misleading as it could imply that they are of minor importance. Nothing could be further from the truth; all twelve elements are vital. The only distinction is in the quantities required.

Major elements are needed by the plants in relatively large amounts whereas trace elements are, as might be guessed, only required in minute quantities; a few parts per million in fact. On the whole, fertilisers that are derived from organic sources, such as hoof and horn, fish meal, seaweed meal and treated town refuse, contain trace elements as well as major elements.

Despite the fact that most truly organic materials contain all the essential elements in varying proportions, there is nothing consistent about them as a group or even as individuals. For example, hoof and horn contains 12-14% nitrogen and 1-3% phosphates. Bone meal, on the other hand, contains only about 4% nitrogen but 22-25% phosphates. This illustrates very clearly that no single natural fertiliser contains sufficient quantities of all the elements required by the plants. It is often necessary, therefore, to use two or more of them to supply a plant with its needs. This has to be considered alongside the fact that farmyard manure or garden compost are also being given and these contain small amounts of plant foods as well.

11. August 2013 by admin
Categories: Compost Making | Tags: | Comments Off on The Importance of Feeding Plants


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