Fertilizers: What plants require?

You cannot go on taking things out of the soil without putting anything back. In nature plants return the nutrients they have taken from the soil when they die. In the garden the vegetables are removed and eaten, and the chain is broken. Compost and other organic materials help to redress the balance, but there may not be enough available to do the job properly and then fertilizers are needed.

The most natural way of adding nutrients to the soil is to rot down old plant material in a compost bin, and then return it to the soil.

What plants require

The main foods required by plants are nitrogen (N), phosphorus (P) and potassium (K), with smaller quantities of magnesium (Mg), calcium (Ca) and sulphur (S). They also require small amounts of what are known as trace elements, including iron (Fe) and manganese (Mn).

Each of the main nutrients tends to be used by the plant for one specific function. Thus nitrogen is concerned with plant growth and is used for promoting the rapid growth of the green parts of the plant. You should, therefore, add nitrogen to help leafy plants such as cabbage but cut back on it with plants such as runner beans, because you do not want to promote lush leaves at the expense of flowers and beans. Phosphorus, usually in the form of phosphates, is used to create good root growth as well as helping with the ripening of fruits, while potassium, in the form of potash, which is used to promote flowering and formation of good fruit, is, for example, the main ingredient in tomato feed.

The natural way

The most natural way to add nutrients to the soil is to use compost and other organic matter. As we have already seen, such materials are important to the general structure of the soil, but they also feed it. Well-rotted farmyard manure and garden compost have been the main way that gardeners have traditionally fed their gardens. However, some of today’s gardeners are unhappy with this method because they claim that you cannot know which fertilizer you are adding and in what quantity, because the quality of organic materials varies so much. Although they concede that organic material is useful for adding bulk, they prefer to use bought fertilizers to feed the soil.

Organic material normally contains less of the main nutrients than concentrated fertilizers, but it is often strong in trace elements, and although they may not contain such a strong concentration of nitrogen, they do release it over a longer period which is of great benefit. Because of its other benefits, farmyard manure and garden compost are still the best way of treating the soil.

Organic fertilizers

Synthetic, concentrated fertilizers are broken down into two groups: organic and inorganic. Organic fertilizers are made up of chemicals derived from naturally occurring organic materials. So bonemeal (ground-up bones) is quite strong in nitrogen and phosphates, making it a good fertilizer to promote growth, especially at the start of a plant’s life.

Bonemeal also has the advantage in that it breaks down slowly, gradually releasing the fertilizer over a long period. When you apply bonemeal, you may want to wear gloves. Other organic fertilizers include fish, blood and bone; hoof and horn; and seaweed meal. Because they are derived from natural products without any modification, they are deemed “safe” by organic growers.

Inorganic fertilizers

These are fertilizers that have been made artificially, although they are frequently derived from natural rocks and minerals and the process may just involve crushing. They are concentrated and are usually soluble in water. This means that they are instantly available for the plant and are useful for giving a plant a push when it is required.

They do, however, tend to wash out of the soil quickly and need to be replaced. Some are general fertilizers, and might contain equal proportions of nitrogen, phosphorus and potassium, for example. Others are much more specific. Superphosphate, for example, is entirely used for supplying phosphorus, while potassium sulphate is added to the soil when potassium is required.

Increasing numbers of gardeners are turning against inorganic fertilizers, unaware that they are not as artificial as is generally believed. Many are not classified as organic simply because they are not derived from living things. Nevertheless, it is their concentrated form and the fact that they can be readily washed from the soil that lead many gardeners to object to their use.

This kitchen garden is planted with a delightful mixture of herbs, vegetables and flowers, growing in well-fed beds.

Slow-release fertilizers

A modern trend is to coat the fertilizers so that they are released slowly into the soil. These are expensive in the short term, but because they do not leach away and do not need to be replaced as frequently, they can be considered more economic in the longer term. They are particularly useful for container planting, where constant watering is necessary (with its attendant rapid nutrient leaching).

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