Nutrition and Fertilising
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.
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.