The nutrients needed by plants are taken from the air and from the soil. This
publication deals only with the nutrients taken from the soil. If the supply of
nutrients in the soil is ample, crops will be more likely to grow well and
produce high yields. If, however, even only one of the nutrients needed is in
short supply, plant growth is limited and crop yields are reduced. Therefore,
in order to obtain high yields, fertilizers are needed to supply the crops with
the nutrients the soil is lacking. With fertilizers, crop yields can often be
doubled or even tripled. The results of many thousands of demonstrations
and trials carried out on farmers’ fields under the former FAO Fertilizer
Programme over a period of 25 years in 40 countries showed that the weighted
average increase from the best fertilizer treatment for wheat tested was about
60 percent. The yield increase varied, of course, according to region (for
example due to lack of moisture), crop and country.
The efficiency of fertilizers and the yield response on a particular soil
can easily be tested by adding different amounts of fertilizer to adjacent
plots, and then measuring and comparing the crop yields (see Chapter 12).
Such tests will also show another very important effect of fertilizer use,
namely that fertilizers ensure the most effective use of land, and particularly
of water. These are very important considerations where rainfall is low or
crops have to be irrigated, in which case the yield per unit of water used may
be more than doubled. Rooting depth of the crop may be increased (Figure 2).