LOOKING AFTER THE SOIL – 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. Soil sampling, results analysis and fertiliser recommendation is done by:
The major fertilizer companies e.g. Windmill, ZFC Limited.
Agriculture faculties of local Universities e.g. Africa University.
So why wait, have your soil samples tested, so you can maximize your yield at low cost. Acknowledgements – K.Bhepe