Many a times farmers ask the question,
what amount of fertilizer should I put to get the maximum yield of maize, soya
or wheat? This is a brilliant question that a business minded Farmer should ask
because farming is a business with input and output. When output outweighs
input then its profit and vice versa it’s a loss.
Inputs are what we put into the soil
and this includes fertilizer, seed, chemicals, water, labour and management. In
this article we will start looking at fertiliser input and the question “How
much fertiliser should one put per unit area at any given time to a particular
crop to realise maximum crop yields?” Before answering this question lets
remind farmers that each crop has a specific nutrient requirement hence forth a
special fertilizer. With this in mind we have variety of fertilizers to address
these requirements. Some examples of specific Fertiliser for some crops are
indicated in the table below. We commonly talk about fertilizer in terms of
Nitrogen, Phosphorous and Potassium since they are the main food source of the
plant. The Nitrogen: Phosphorous: Potassium (N:P:K) ratio is different for each
crop for example tobacco does not require lots of nitrogen hence its
formulation is low in nitrogen. The same with other crops like shrubs, trees
and flowers which requires less of phosphates hence its formulation (15:5:20).
It is imperative to note that while
other crops have a close degree of resemblance like soya bean and sugar bean
they however have different nutritional requirements. With soya bean we require
lots of boron for flowering purposes and because of that there is a special
formulated fertilizer high in Boron Compound “L”. This is different with sugar
beans which does not tolerate boron and hence have got a special formulated
fertiliser for it Compound “D”, or Double “D”. Last example is of Potato which
has a high affinity for potash hence use high analysis formulated fertilizer
Compound “C” (6:24:20). So before embarking into crop production know the right
fertilizer for that crop. While the Nitrogen: Phosphorous: Potassium is written
the same on all fertiliser bags, it is important to know that it is the
potassium source that differs with each fertiliser. They are two sources of
potassium used to manufacture fertiliser and thus we have MOP which is Murate
of Potash with 60% potash and SOP Sulphate of Potash with 50% Potash and 18%
sulphur. MOP is also simply known as chemically as potassium chloride (KCI2).
These two sources of Potassium differ greatly in their salt index. What is salt
index? Salt index in its simple terms measures the salt concentration that a
given fertilizer induce to a soil.
The salt index of fertilisers is an
index of the extent to which a given amount of various fertilizers increase the
osmotic pressure of a soil solution.
With this simple definition in mind, it
is interesting to know that potassium chloride has a salt index of 114.3 while
SOP has an index of 46. So MOP induces a great deal of salt concentration in
the soil and hence greatly affects the osmotic balance of the soil solution and
compared to SOP. Crops vary in their degree to tolerate salt concentration
caused by fertiliser. Cereal crops are least sensitive thus wheat and maize
while soya bean is moderate and vegetables and tobacco most sensitive. MOP is a
cheaper source of Potassium as compared to SOP and also because cereals
tolerate to a great extend salt concentrates MOP is used in cereal fertiliser
as a potassium source and this includes compound D and double D contains 100% MOP.
Hence if we use such a fertiliser to salt intolerant crops like tobacco and
vegetable yield and quality are compromised. However for salt intolerant crops
like tobacco and vegetable a special formulated fertiliser has been formulated
which contain SOP as a potassium source. These fertilisers include Compound B,
C and S. Compound B and S contains 100% SOP and compound C contains 75% SOP and
25% MOP.
These variations also explain the
difference in prices of these fertilisers. It is important to note that
compound S (7:21:7) has a salt index of 27.8 and hence safe for seed bed
operations where moisture availability is important. Thus we recommend compound
S in seedbed operations of vegetables and tobacco. So far we have looked at
different fertilisers and crop requirements as a factor affecting crop yield.
Thus wrong fertilizer means reduced yield and quality. Now back to our question
‘What amount of fertiliser one needs to put into a given crop area to get
maximum yield?” Before knowing the amount of fertiliser to put into a crop, one
needs to know the level of nutrients currently in the soil first and secondly
what nutrients amounts that crop remove per given harvest. Also what the roots,
stem, leaves, tassels, and other plant parts remove from the soil. This is for
the Agronomist and other scientist to know. So to address this question
adequately we need to test the soil first to ascertain the nutrient value of
the soil includes soil pH. Eight essential parameters are tested which includes
pH, conductivity, phosphate, potassium, calcium,
Soil Sampling, Testing and Analysis is
the Way
Magnesium, Sodium and nitrogen. With
these results the Windmill Agronomist can analyse these results and recommend
you the right fertiliser combinations and quantity for the desired yield,
liming material, any soil additives like MOP, SOP, SSP and Gypsum. So the basis of knowing the right amount of
fertiliser to put in the soil is a Soil testing and analysis then
Recommendation. The next question is when and how to take soil samples.
Well this is simple, soil samples are usually taken for analysis from April or
after harvest. This is fundamental in that when liming is required to correct soil pH it has to be applied at least three month before
intended planting dates for effective results or otherwise the effect is
realised the following season. Also the test results will indicate which liming
material to use either calcitic or dolomitic lime.
So why wait, have your soil samples
tested, so you can maximize your yield at low cost.
by
K. Bhepe
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