Fertiliser in the Dry Tropics
With irrigation, spectacular responses to fertilisers are often obtained, producing large
increases in yield. But under rainfed conditions alone, response to fertiliser is highly
variable and often uneconomic, for several reasons: there may be insufficient soil
moisture in the root zone at the time when the fertiliser is present, the nutrients supplied
may be unavailable, and there may be other damaging checks to plant growth caused by
drought which impede the uptake of nutrients.
Apart from Phosphorus, which is often either absent or present only in very small
amounts in “bagged” fertilisers, the use of fertilisers in the dry tropics is not
recommended unless there is sufficient soil moisture. In other words in dry areas it is
often moisture and not nutrients which is the limiting factor to plant growth.
Plant Food Ratios
A compound fertiliser which contains 22% N, 11% P 2 O 5 and 11% K 2 O has a plant
Another fertiliser containing, for example, 13% N, 12% P 2 O 5 and 20% K 2 O has a
ratio of approximately 1:1:1.5.
These ratios are normally printed on the outside of the fertiliser container.
Recommendations for applying fertiliser are usually expressed in terms of kilograms
(kg) of plant food required by a particular crop per hectare (ha).
The number of kilograms of a particular nutrient in 100 kg of fertiliser is the same as
the percentage of nutrient in the fertiliser, so a 50 kg bag of “straight” ammonium nitrate
(34%N) contains 17 kg of nitrogen. A 50 kg bag of 12:12:18 “compound” fertiliser
contains 6 kg of N, 6 kg of P 2 O 5 and 9 kg of K 2 O.
If the fertiliser also contains other elements in addition to N, P and K this fact should
also be written on the bag, so 12:12:18 + 6S means that there are 3 kg of sulphur present
per 50 kg bag in addition to the N, P and K. In UK agriculture there is some confusion
regarding “kilograms” versus “units”. Whilst the level of elemental N is quoted, the
levels of P 2 O 5 and K 2 O are normally quoted—rather than elemental P and K. Sulphur is
quoted both ways.
Metric/Imperial Conversion
See also Section 3. “Conversion Tables and Statistics” pages 313–316.
1 lb = 0.454 kg 1 kg = 2.205 lb
1 acre = 0.405 ha 1 ha = 2.471 acres
1 lb/acre = 1.121 kg/ha 1 kg/ha = 0.892 lb/acre
Fertiliser Response
If the yield of a crop is increased due to the application of a fertiliser, that crop is said to
change, such as the rainfall pattern, the timing of the fertiliser application, the previous
crop grown and so on, then that crop may not respond at all, or it may respond in a
different way, to the same application of fertiliser.
An example is quoted below to illustrate how difficult it can be to predict the
response of a crop to an application of fertiliser, and the need to understand something
32 TONY WINCH
respond to that fertiliser under those particular conditions. However if those conditions
nutrient ( food“ ”) ratio of 2:1:1.