Opinion piece by Dr Ben Ngwene, AGCO

Each seed we plant in the soil must receive adequate nutrition to reach its full potential. It is up to the farmer to ensure that his field meets basic nu­tritional requirements to achieve opti­mal yields.

If he neglects this important step, he risks weak growth and a crop more susceptible to diseases with a di­minished ability to produce successfully. While we are waiting for the rains, here are some pointers on how to ensure crops absorb enough minerals essential for their survival.

To get a good harvest, a field needs two categories of plant nutrients, name­ly macronutrients and micronutrients. Macronutrients are required in relatively large amounts while micronutrients are required in far lesser amounts. Macronutrients are further divided into primary and secondary groups based on their importance. Nitrogen (N), Phos­phorus (P) and Potassium (K) are some of the main primary nutrients which are readily available in mineral fertiliser. The most commonly used secondary nutrients are magnesium, calcium and sulphur. Now, the reason why most smallholder farmers yield weak har­vests is because they do not have ac­cess to these secondary nutrients.

Micronutrients on the other hand are only required in small amounts, and are less likely to result in crops displaying deficiencies if they are not available in the soil. These include iron, manga­nese, boron, copper, zinc and molybde­num. All these nutrients are absorbed through the roots as dissolved ions in the soil solution. Therefore, soil mois­ture is very important for plant nutrition and we must take moisture conserva­tion seriously.

How to assess crop requirements for nutrients

Crop requirements for nutrients are mainly influenced by crop type and yield expectation. For example, if we are ex­pecting our maize yield at about 6 t/ha, we need about 140 kg/ha of N. This will come in the form of mineral fertiliser, manure and soil supply. A local agri­culture extension service can help with these calculations.

It is also imperative to know what is in our soil. A certified institution can conduct soil testing and provide full analysis on the soil type so the farmer can plan the best nutrient management for his field. The sandier the soil is, the less it can hold water and nutrients and the more the farmer needs to split fertiliser application to minimise loses. The analysis will also reveal the soil pH level. The ideal pH for most arable soils is about 6 to 7,5; however, if a field has acidic soil (pH above 7,5) it can be cor­rected through a process called liming. Liming is the application of calcium and magnesium rich materials such as chalk and limestone to soil which react as a base which neutralises soil acidity. Lim­ing also promotes the activity of earth­worms and the breakdown of organic material that releases nitrogen, phos­phorus, sulphur and micro-elements.

The rate, timing and application is very important in nutrient manage­ment. Plants need various nutrient rates and ratios at different growth stages. The optimum timing for fertiliser ap­plication is therefore determined by the nutrient uptake pattern of the crop and has a significant effect on crop yields. Furthermore, different nutrients must be applied at different timing as well.

Usually Phosphorus (P) and Potassium (K) are applied prior to crop establish­ment because they are less prone to leaching. Nitrogen (N) on the other hand should be split. This varies with differ­ent crops and should match the period of crop uptake during planting (starter), high vegetative growth (sidedress), and flowering (sidedress). For instance, leg­umes only need small amounts of basal N to boost growth prior to N-fixation. And after a legume crop, there is more N in the soil which should be taken into ac­count when calculating fertiliser require­ments for the next crop!

Soil type also affects the timing and frequency of fertiliser application. Split application of fertiliser is recommended mostly for large-scale farmers whereas most smallholder farmers use the broadcasting application method. But this might not be as efficient as banding. Simply put, broadcasting is spreading fertilisers uniformly all over the field while banding, either in rows or hill placement, ensures that the fertiliser is placed close to where it is needed reducing waste.

So in closing, if you take care of the nutrition of your soil, the soil will take care of your crop yield. For proper ferti­liser management start with calculating the fertiliser rate according to crop type and yield expectation, know what is in the soil (soil test), then apply calculated amounts of mineral fertiliser and manure at the proper time to limit waste.

For more information on AGCO’s fine equipment and agricultural courses, send an e-mail to Benard.Ngwene@agcocorp.com