soil analysis report gives the levels of the various nutrients found
in the soil sample. However, the numbers can be confusing. Different
laboratories use different methods and will give different numbers
on the same sample. They may even use the same methods, but express
the results in different units. What should you do?
the numbers. In addition to giving numbers, a good laboratory MUST
give a description of the soil nutrients. Some will give a written
description, saying for example phosphorus is very low
or calcium is high. Others, such as Agro Services
International give an easy to understand chart showing the status of
each nutrient. Most people prefer the charts because they gives a
good overall picture of the soil properties at a glance,
but either method can be used.
We now have a
description of our soil, phosphorus is low, so how much should we
apply? This depends on the crop to be grown. For example, lettuce
needs more phosphorus than banana grown on the same soil. The
laboratory usually gives a GUIDE as to how much should be applied
for the selected crop, but this should not be taken as a
commandment, it has to be modified to suit your actual
major factor to consider is your yield goal. If you have good
growing conditions, high yielding varieties, proper drainage,
irrigation, pest and disease control, you can achieve high yields.
This requires a good supply of nutrients and therefore fertilizer
application rates should be high. However, if your field has
problems that you cannot solve (like no irrigation in a low rainfall
area) and you know that you will not get high yields, the fertilizer
rate should be reduced as the crop will not make use of large
amounts of nutrients. The laboratory does not know your field
conditions and cannot make these decisions for you. In some
cases, the lab will ask for your yield goal and adjust the
recommendations to suit, in other cases, they may give a generalized
recommendation that you must adjust yourself.
The economics of
fertilizer use is another factor that the laboratory cannot decide
for you. Some growers apply high rates of phosphorus when the
fertilizer price is low. When the price is high, they reduce their
rates and rely on the residual nutrients from the previous
applications. This works with nutrients that do not move in the
soil, but cannot be used for nitrogen application as the excess
nutrient does not remain in the soil for the next crop.