By Prof Lenah Nakhone, PhD, Crop Horticulture & Soils Department, Egerton University
It is that time of the year when farmers are preparing their farms and are ready to plant taking advantage of the rains come next planting season. Every farmer normally hopes to get the best harvest at the end of the season. However, the successes of the crops depend on proper soil fertility management through the use of the right fertiliser.
Why add fertiliser to the soil?
All the nutrients in our food originally come from the soil. Soils naturally contain many nutrients like nitrogen, phosphorous and potassium. But when the nutrients are missing or are in short supply, plants suffer from deficiency and do not grow well. Once crops are harvested, the natural supply of nutrients in the soil must be re-filled, which is why it is important to add nutrients to the soils through the use of fertilisers.
Fertilisers contain nutrients such as nitrogen, phosphorus, and potassium and they are applied to supplement the elements for healthy crops. Genuine fertiliser when applied to your farm will lead to improvement of the soil health, your crops will grow vigorously and you will get the best harvest for higher profits.
After getting the genuine fertiliser, how and when do you apply it?
Before adding fertiliser, you should send a soil sample to a laboratory for baseline testing. By testing soil, farmers know which nutrients are missing and how much to apply to the soil. If too little is added, crops will not produce as much as they should. If too much is added, excess nutrients will run off the fields and pollute streams and groundwater.
So, while fertilisers serve an important purpose, farmers must be careful to use the right amount, at the right time. When it comes to applying fertiliser, more does not mean better. Too much fertiliser can damage and maybe even kill your plants. Be sure to check the label for the N-P-K ratio, as you may be able to use a general fertiliser with close to the same nutrient percentages but at a lower price.Mwihaki Sample report
A sample soil analysis report complete with fertiliser recommendations on page 2, including the type of fertiliser, the time and rate to apply
Knowing when to fertilise is as important as using the right fertiliser. If you don’t apply the fertiliser at a time when the plant can use it, then all the fertiliser will go to waste. For example, the most important time to apply super phosphates (TSP or SSP) or DAP is before planting. This is because phosphorus is critical for seedling root development and early growth.
Apply very soon before planting to ensure that fixation does not render the phosphate unusable by the time it is needed by seedlings. Repeat the application of fertiliser as needed to maintain good plant growth. Using smaller amounts of fertiliser at frequent intervals may be more beneficial than applying at once at a higher rate.
How to spot fake fertiliser
Many farmers have claimed losses due to fake fertilisers sold to them. When you are buying a fertiliser, make sure the contents are listed on the bag with their percentages clearly labelled, for example, N:P:K 23:23:23. Also check the packaging of the production batch number and seal. Also check the purity of the fertiliser you are buying. One way of doing this is to look at the physical and some chemical properties. For example, DAP is hard, granular, brown or black in colour and does not break easily by nails. If some granules of DAP is heated slowly on a plate, it swells.
If it is super phosphate like TSP or SSP and it is hard, brown or black in colour and hard to crack by nails, there is a possibility that the fertiliser has been adulterated with DAP and NP fertilisers. If it is heated, its granules do not swell whereas granules of DAP and other complexes swell. Urea is white, shinning and has consistent particle size and is round in shape. It is completely soluble in water and one feels cold when the solution is touched. It also melts when kept on a hot plate. If the particle surface colour is too dark or reflective, then it was mixed with impurities. A laboratory fertiliser quality analysis may help if you want to be 99.99% certain of the quality of your fertiliser before making that big purchase.
Hey, don’t apply fertiliser before you test that soil
Not many farmers consider taking their soil for testing before venturing into farming. Usually, there is a general perception that soil contains everything the crop needs with probably the exception of few major nutrients that can be obtained from fertiliser. The conditions that contribute to soil health include; the right proportion of nutrients and organic matter; adequate amounts of soil air and soil water and most importantly soil pH.
Soil pH generally refers to the degree of soil acidity or alkalinity. Chemically, pH is the concentration of hydrogen ions in the soil. Soil pH is measured in a scale of 0 to 14. if pH values are greater than seven, a solution is said to be basic or alkaline, while if the pH is seven, the solution is acidic. Soil pH affects its physical, chemical and biological properties and processes as well as plant growth.
The nutrition, growth, and yields of most crops decrease when pH is low and increase as pH rises to an optimum level. Indiscriminate use of nitrogenous fertiliser, especially DAP (Diammonium Phosphate) and CAN (Calcium Ammonium Nitrate) is a major contributing factor to elevated pH in most soils. In addition, many nutrient deficiencies, decline of microbial activity and crop yield and deterioration of environmental conditions are due to poor soil pH levels.
Soils can also be acidic or alkaline due to the mineral composition as well as external factors, including plant root exudates, acid rain, mine spoils and decomposition of organic matter in the soil. Acidic soils are common in areas with abundant rainfall. In the soils, nutrients dissolve slowly or not at all. Critical plant nutrition is locked up in insoluble mineral compounds hence plants cannot utilise them.
Fertiliser is of little use in acidic soils because nutrients cannot be absorbed. Most field crops including beans, sugarcane, barley and vegetables like cabbage, asparagus, broccoli, melons and spinach will grow best if pH is close to neutral (pH 6 to 7.5). A few crops will tolerate relatively acidic soils (pH 5 to 6) such as oats, corn, tobacco, wheat, carrot, cucumber and potato. In acid soils, calcium and magnesium, nitrogen, phosphorus and boron are deficient, whereas aluminium and manganese are abundant. In alkaline conditions, phosphorus, iron, copper, zinc and boron may be deficient
Diseases Will Thrive
Some diseases will thrive when the soil is either alkaline or acidic. For example, the fungus Gaeumannomyces graminis is favoured by alkaline pH and infects wheat, barley, rye and several grasses. Plants grown in acid soils can experience a variety of problems, including aluminium, hydrogen and manganese toxicity as well as nutrient deficiencies of calcium and magnesium.
Nitrification and nitrogen fixation are also inhibited by low pH. Additionally, mobility and breakdown of herbicides and insecticides in the soil are affected by soil pH. Also, the solubility of heavy metals is increased in solutions of low pH. Failure of degradation of these compounds can lead to environmental pollution when they eventually find their way to water ecosystem.
Other processes that depend on soil pH include organic matter mineralisation, which is conversion of organic matter into utilisable nutrients. To manage acidic soils, farmers need to add limestone or gypsum to increase soil pH. In addition, crop rotation especially by replacement of leguminous crops with non-legumes can help interrupt the acidifying effect of leguminous crops.
Application of basic fertiliser such as NPK instead of DAP help curb acidic soils. In areas with alkaline soils, application of ammonium fertiliser, urea and ferrous sulphate can decrease the soil pH. Farmers should always consider taking soil samples for testing at least every growing season.