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Careful, there’s more to fertiliser application than just digging it in

Boniface Kaimenyi (centre) attends to his Snow peas plantation in Meru county. Photo by WILLIS AWANDU

Following last week’s article on ‘how ready are you for the planting season?’ Several farmers asked me to go deeper on soil enrichment.

The best way to determine which nutrients are needed and in what amounts is by testing the soil. Proper selection of fertiliser mix, based on soil test results will ensure proper nutrition which is essential for satisfactory crop growth and production.

Soil-testing will help determine the status of available nutrients and develop fertiliser recommendations to achieve optimum crop-production.

This is important because the profit potential for farmers will depend on producing enough crop per unit area and keeping production costs below the selling price. Efficient application of the correct types and amounts of fertilisers for the supply of the nutrients is an important part of achieving profitable yields.

Selecting the fertiliser

The best guide to selection of fertiliser is by reading the label on the bag. The numbers on the label refer to concentration (percentage) of three major nutrients: nitrogen (N), phosphate (P), and Potassium (K). For example, a 17-17-17 fertiliser contains 17 per cent nitrogen, 17 per cent phosphate and, 17 per cent potash. Fertiliser labels have been standardised so that the concentration of nitrogen, phosphate and potash always appear in the same order on the front of the package.

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Some organic fertilisers can contain other nutrients like sulphur, iron and zinc. If soil test report indicates levels of some nutrients are high or excessive, select products containing lower concentrations of these nutrients. Further application of these nutrients may create an imbalance and reduce plant growth. If soil-test report indicates a need for nitrogen, select high nitrogen materials.

Nitrogenous fertilisers

It is important to have enough nitrogen in the are soil at a time when the crop’s growth is most rapid. Basically, we use two types of nitrogen — ammonium or ammonium formers like Urea. The common Nitrogenous fertilisers on the market NPK, Urea and CAN which are mostly used for top-dressing.

Although at times nitrogen can be used most efficiently by a crop if it is applied either while the crop is growing or just before it starts to grow. Split application of nitrogen will be very helpful to avoid leaching. Since plants would be deprived of nutrients, the plants will be susceptible to pests and diseases. In such cases, the remedy may not be increasing the amount of fertiliser applied but reducing the acidity.

To reduce acidity, liming (application of calcium- and magnesium-rich materials) is recommended. Lime will make the soil environment better for plants and associated microorganisms as well as increase concentration of essential nutrients.

Knowing the level of acidity in your soil will also help determine the amount of lime you need. Acidic soils will reduce your yields in two principal ways: by increasing the presence of toxic substances like aluminum, manganese and hydrogen ions, and by reducing the availability of important plant nutrients like phosphorus, calcium, magnesium, and molybdenum.

This acidity will result in shallow roots with swollen tips, preventing water and nutrients from deeper soil layers to be accessed. Also, legumes such as beans show poor nodulation. Lime is a valuable amendment that helps plants flourish by raising soil pH. A low soil pH, or acidic soil, is often the underlying problem when it comes to many common garden problems.

Liming has many benefits. It improves soil quality thereby improving yields. When the soil has a neutral pH, it allows microbes to increase thereby breaking down the organic matter. Corrected acidic soils enable plants to grow stronger and become resistant to diseases. Lime is also a source of calcium which is an important element for plant growth.

To get maximum benefit from fertilisers, they should not only be applied in proper time and in the right manner but any other aspects should also be considered. Different soils react differently with fertiliser application. Similarly, the Nitrogen, Phosphorus and Potassium requirements of various crops differ and even for a single crop, the nutrient requirements are not the same at different stages of growth.

-The writer is an expert on sustainable agriculture and agricultural innovations. [email protected]

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