Growing Food: A Guide to Food Production

phosphates, bone meal, flour etc.

The main content of these are chlorides, also known as muriates.
Examples: muriate of potash, sulphate of potash, nitrate of potash (saltpetre), kainite etc.

The Purpose of Fertiliser
Fertiliser is used to replace elements in the soil that have been lost in a number of ways,
including:

• leaching, where nutrients are carried down below the reach of plant roots;


• supporting the growth of crops and weeds;


• removed from the field within the plant material, either as the crop harvest or when

The Effect of Soil Type
In general, soils with a high organic matter content require less fertiliser than light,
sandy soils with low organic matter content. The main reason for this is normally that
more nutrients are leached from soils with a low organic matter content.
Badly structured clay soils also need more fertiliser than loams or clay loams.

Excessive Use of Fertiliser
Sometimes too much fertiliser is applied to crops, perhaps in the hope of a bumper
harvest, or as a result of bad advice or poorly adjusted application equipment. This is
not only a waste of time, money and fertiliser but there can also be some adverse effects

Another example is when excessive nitrogenous fertilisers are applied to root crops
such as manioc/cassava and sweet potato, when vegetative growth of the aerial parts—
the leaves and stems—is increased, but at the expense of tuber growth which is reduced.

Organic Versus Inorganic Fertiliser
Inorganic, chemical fertilisers can and do very often produce lush green plants that give
high yields. These fertilisers have increased productivity per hectare enormously over
huge areas of the world, and they make a major and mainly useful contribution to
agriculture.
However, excessive reliance on this form of plant nutrition can lead to some serious
problems. The quality of crops which are grown in soils continuously fertilised with
non-organic fertilisers tends to decline, water courses may become polluted by fertiliser
runoff, the soil becomes gradually poorer in organic matter, and the plants which it
supports begin to lose their capacity to tolerate attack from diseases and insects.
At this point farmers are liable to become more and more dependent on using certain
fertilisers. They have to find the cash or credit to buy them—if they are available—and
they have to master the sometimes difficult application techniques; often quite

Examples: ammonium phosphate, basic slag, (triple) superphosphate, mineral rock

30 TONY WINCH

3. Potash Fertilisers

remove about 110 kg of N, 40 kg of P, 130 kg of K and 15 kg of Calcium/ha.

grazed by animals—eg, a 4 MT/ha grain & 6 MT/ha straw crop of maize will

on the crop, some examples of which are set out in 1Cd. “Trace Elements”, page 23.