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.

John Nganga and Prof Lenah Nakhone, Crop Horticulture and Soils Department, Egerton University