Fertilizer basics

The decision to use a particular type of fertiliser needs to be supported by some understanding of the preferred soil conditions and other requirements of the plants being raised. To grow properly, plants require a broad selection of nutrients. Ideally, these occur naturally and are directly available from soil or compost. The three macro or major nutrients: Nitrogen (N), Phosphorus (P), and Potassium (K), are required in significant quantities. They work together and in combination with other nutrients but each of them has a dominant role in certain aspects of growth and plant development. Nitrogen, for example, is required for shoot and leaf growth. Phosphorus is the most crucial nutrient for developing functional root systems, while the main role of potassium is supporting fruit and flower formation. Around a dozen additional elements are required in relatively small quantities. These include Calcium, Magnesium, Sulphur, Zinc, Iron, and Selenium; also know as the trace or minor nutrients. They make significant contributions to overall growth and development, and seem to be particularly important for building general immunity and resistance against disease.

Each species of plant has an optimal requirement for quantity and balance of the essential nutrients. When plants fail due to a specific nutrient deficiency, it can be challenging to first identify then correct the problem. When affected, most plants will exhibit some general symptoms like stunted or deformed growth, leaf discolouration, mottling, and root shrinkage. Often, these signs first become apparent at the outer branches and leaf tips since these are furthest from the source of nutrients. In some instances, a soil will contain nutrients that are locked and unavailable to the plant. Excessively high concentrations of certain elements can create toxic effects or block the availability of other essential nutrients. They are both risks that should be considered prior to soil treatments, particularly those based upon a synthetic chemical product.

Chemical fertilizers

Synthetic chemical fertilizers are water soluble and capable of releasing their nutrients quickly. They are usually manufactured from mineral salts like nitrates of potassium, calcium, and ammonium. High concentrations of these are not good for plants in the long term and their effects have been compared to those of highly refined sugar in human physiology. To improve the appeal of their products, most manufacturers of synthetic fertilizer have developed coatings that enable them to break down at a much slower rate and release correspondingly smaller quantities of water soluble nutrient at a time. While this represents some improvement, it cannot prevent the problems and wastage associated with their eventual leaching from soils into rivers, drains and reservoirs. The repeated use of fertiliser also neglects the basic principle of assisting plants by improving their soil conditions.

During their manufacture, the concentration and quantity of various compounds within synthetic fertilizers can be precisely controlled according to standard formulae. This enables the products to be expressed as a series of three numbers. The first is the percentage by weight of nitrogen, followed by phosphorus then potassium. In addition, the percentages of each contributing trace element can be clearly defined. By contrast, constituent compounds in natural organic fertilizers can only be estimated and will vary according to the ingredients used for each batch.

An organic approach

Within the organics movement, fertilizers are generally afforded a minor role compared to soil conditioners. Traditionally, the emphasis has been placed upon building the soils capacity to deliver nutrients rather than feeding these directly as supplements to plants. When organic fertilizers are required, they are made from natural materials such as bone meal, fish, seaweed, nettle, comfrey, and other plant extracts. Strategic applications of organic fertilizer can provide nutrients and trace elements that are absent from a soil. They are released slowly to plants because they contain compounds that need to be broken down by soil bacteria. This process can be accelerated under warm, humid conditions or retarded in cold and dry.

Liquid preparations of organic fertilizer are easy to use, readily absorbed by most plants and less likely to alter soil chemistry than granular fertilizers. It is quite easy and rewarding to make liquid fertiliser at home using organic materials. Nettle leaf extract is a good general fertilizer which contains a balance of macro nutrients and trace elements. Nettles can be collected in spring then placed inside a hessian sack that is loosely tied then suspended in a large bucket containing approximately ten litres of water for each kilogram of leaves. After one month the liquid should be strained, diluted with five parts water then bottled in suitable containers. Liquid fertiliser can also be made from comfrey, kelp, clover, and other crops used for green manure.