From the Ground Up 8:

Nutrition and Fertilising


by Derek Burch

Research through the years has found that plants need 16 or 17 chemical elements to grow normally. Three of these come from the air or water, six more are needed in relatively large amounts and the rest only in minute quantities, but those minute amounts are still essential.

These chemicals are used to build the plant body step by step and to develop the complicated substances that control this sequence of steps that takes place.

The essential elements come from the soil solution, either from naturally occurring materials in the soil or from nutrients added by the gardener or farmer. Although there may be other very good reasons to try to use organic materials to give these essential elements, the plant roots take in the chemicals in solution, and do not seem to distinguish the source.

Even though these essential elements are vital, they do not give the plant energy to grow, and cannot take the place of the simple sugars formed in the process called photosynthesis in which light energy is converted to a chemical, storeable form using the three essential elements from the air and water. Put in another way, a plant in a place without enough light will eventually die, no matter how much it is fertilised - it is important to realise this.

Another point that must be made is that it is possible to have too much of an essential substance - this can be as bad as being deficient. The ideal way to set up a fertiliser program begins with a soil analysis - if you know where you stand, and know where you need to be, it is easier to lay out the steps to get there.

Benefits of organic fertiliser

Note: to a chemist, an organic compound is anything which includes carbon. Organic growers use the word to mean that the fertiliser comes from recently living sources, or from a natural rock.

1. There is less chance of burning plant roots
2. There is less chance of harming the beneficial microorganisms that live around the plant roots
3. Microorganism activity in the soil is increased (often "good" types will then outcompete disease organisms)

4. Soil structure and nutrient- and water-holding capacity is improved


There is a bewildering array of fertilisers offered in garden centers and nurseries. In many countries each bag must carry a guaranteed analysis: sometimes including only the major nutrients, but often with others, including the so-called micronutrients, listed. The major nutrients are usually given in the order nitrogen(N): phosphorus(P): potassium(K).

What should you look for? Bearing in mind that your fertiliser program should be based on a soil analysis and a plan for applications, there are still some generalities that can be made. The ratio of the major elements is good clue to what the fertiliser will do for the plant. A high nitrogen level compared with the other two will stimulate the growth of the leafy parts of the plant, so might be useful when a plant needs encouragement in the early stages of its growth, or when leafy growth is the major need. Think of why we have a lawn, but don't ignore the need for a healthy root system by overdoing the nitrogen level.

Phosphorus is often cited as the element to encourage roots, but the research results on this are not clearcut, and many soils need only one application a season. However, while you are wasting money by buying this element in every bag of fertiliser that you use, it will often cost more to get a low phosphorus analysis than to accept the more commonly available types. (Don't ask!)

A high ratio of potassium to nitrogen will often stimulate flowering. Potassium nitrate is very useful in this regard. It is readily soluble, and with an analysis of 13-0-44 does a great job. One warning of the times, however: it is a prime bomb-making material, so don't go buying truckloads of it in the United States, unless you want one of supersleuth's men on your doorstep.

There is a so much more to say about fertilisation - slow release or regular, liquid application, time of year and stage of growth, not to mention the form in which the element is present in the fertiliser. More of all that next time.

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