about the soil, the climate and the crop nutrient needs before recommending the use
(including timing and quantity) of any fertiliser.
Example: If wheat is grown in rotation with a fallow, then nitrogen will accumulate in
the soil during the fallow year. The yield of wheat grown in the same place the
following year may therefore be limited by the amount of available phosphate instead;
so the wheat will respond to a phosphatic fertiliser.
But if the wheat is grown continuously there is little or no accumulation of nitrogen,
and so the wheat will respond to a nitrogenous fertiliser, until the available phosphate is
used up. But the wheat will not then respond to phosphate by itself, because it is now
nitrogen that is the limiting factor. A compound fertiliser containing both nitrogen and
phosphate would then be appropriate.
Economic Return from Fertiliser
A value:cost ratio of 1 means that the value of the increase in yield equals the cost of the
fertiliser used (including transport, application and other costs).
A value:cost ratio of 2 represents a profit of 100%, and so on.
Thus the term economic response of a crop to a fertiliser means that the value:cost
ratio is greater than 1. In other words it would be advantageous to apply fertiliser in
these conditions—see chart below.
Another commonly used term is the “fertiliser response ratio”, the number of
additional kilograms of additional crop produced per kilogram of additional plant
nutrient applied. In countries with low to middle level yields and in areas which are not
too drought prone, typical ratios are 8–12 for cereals, 4–8 for oilseed crops and 30–50
for roots and tubers. These high ratios do not however always translate into economic
incentives for using fertiliser, especially when the fertiliser is expensive, product prices
are low and marketing opportunities are low.
The chart below shows the results of a fertiliser trial with cabbages in Columbia:
Fertiliser
N–P–Kb Yield
kg/haYield
increase
Kg/ha %Value
$/haNet
return
$/haValue : Cost
ratioControl
45–0–0
45–45–0
45–45–4553,500
75,267
75,367
94,433
—
21,767
21,867
40,933
—
41
41
77
685
946
936
1172
—
261
251
487
—
16.1
9.5
14.2
Nitrogen Flush
In many dry tropical soils the amount of nitrogen that is available to plants can fluctuate
very much during the year. For example, at the start of the rainy season there is often a
flush of available nitrogen, which can be taken up by wild plants and also any crops that
have been planted early enough in the season.
This useful phenomenon is called the Birch effect or Nitrogen flush and is one good
reason why it is often a wise idea to plant as early as possible in the dry, tropical