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Soil acidity: Importance of liming

AS PROMISED last week, I will endeavour to discuss soil acidity in this article. Many of us wonder what soil acidity is. In a layman’s language, soil acidity is a measure of the concentration of hydrogen ions in soil solution. It is sometimes confusing if we want to state it in scientific terms because it is measured from a scale of zero to 14.
The lower the number on the pH scale the higher the concentration of hydrogen ions, hence the acidity. On the other end, it can either be alkaline in worse-off cases; have sodic. A soil that is sodic means that soil pH is high and has high concentrations of sodium ions too. It is very expensive to make a soil that is sodic productive than one which is acidic.
What we all need to understand is that soil acidity is becoming more of a problem in our Zambian agriculture today than it was 50 years ago. This is because our land is becoming scarce and people are now practising more permanent cultivation unlike a couple of years back when one had an opportunity to shift their cultivation. Acidic soils create production problems by limiting the availability of some essential plant nutrients. It increases the soil solution’s toxic elements such as manganese and aluminium.
Many of us think that fertiliser, water and weed management are the only important things in crop production. It is the reason none of us has ever dared to take the soil samples of our fields for analysis at various laboratories in the country. What soil acidity does is that it complexes the soil nutrients; meaning even when you have applied adequate amounts of fertiliser necessary for that particular crop’s production, it will not be available. It is like putting a sweet in your mouth with plastics on; you will not get the sweet taste.
Many of us will wonder as to what causes soil acidity. There are many factors that cause soil acidity but I will mention about five to six factors which are the major causes in Zambia.
Soils become acidic if you have an acidic parent material.  Allow me to presume that soil is formed from weathering of rocks coupled with rotting of organic material. If the soils have acidic bedrock at the bottom of the soil, it may cause soil acidity as it continues to weather or break down.
The other factor which causes it is high rainfall and leaching. As it rains, it leaches certain elements leaving the aluminium and hydrogen ions, hence making the soil acidic. This is the reason you will find that soils that are in region one (this is the region of high rainfall mostly found in the north) are highly acidic. The soil found in Mbala is most likely to be more acidic than that found in Magoye in the south.
Organic matter decay can also cause soil acidity. Thirdly, the harvesting of high yielding crops will mine the soil off the basic nutrients which may lead to soil becoming acidic. The other factor is that the continual application of nitrogenous fertilisers such as urea and ammonium nitrate without remedial effects can lead to acidifying of the soil.
In Zambia, we also have another factor which we do not consider much but it is more detrimental than even application of fertilisers; it is mining. When mines releases sulphur dioxide in the atmosphere and these come into contact with water at high temperatures, they react to form sulphuric acidic, which rains in form of acidic rain and this contributes to soil acidifying.
So, what can we consider to be acidic soil? In agriculture, soils with pH less or below 5.6 may be considered to be acidic. And this may have aluminium concentrated enough to limit or stop root development. As a result, plants cannot absorb water and nutrients leading to exhibition of nutrient deficiency symptoms and stunted growth of crops or plants. I am sure you are sitting on the edge of your seat wanting to ask the question as to what can be done once the soil is acidic. This is simple; you first need to take your soil for testing at Mt Makulu, Misamfu, UNZA or any other research station, including some fertiliser companies. Once you know the level of acidity, then you can apply lime. I know that many farmers, especially the small-scale ones, have difficulty applying lime using their hands.  Indeed, it is difficult if you do not have an applicator and you want to apply about 40 hectares of your field by hand. Of late, there are some liquid lime that can be applied through spraying, making our work a bit easier.
Some companies like MRI and others are selling liquid lime which they are calling Cal-lime. The advantage of this lime is that it works faster in that it neutralises the acidity in the solution.  The most commonly used liming material is agricultural limestone, the most economical and relatively easy-to-manage source. The limestone is not very water-soluble, making it easy to handle. Lime or calcium carbonate’s reaction with acidic soil shows acidity (H) on the surface of the soil particles. As lime dissolves in the soil, calcium (Ca) moves to the surface of soil particles, replacing the acidity. The acidity reacts with the carbonate (CO3), to form carbon dioxide (CO2) and water (H2O). The result is a type of soil that is less acidic (has a higher pH).
What we also need to know is that soil acidity is in two forms; there is active acidity which is the acid in soil solution and the reserve acidity which is the acid on adsorbed on the soil colloid. As we neutralise the active acidity, the reserve acidity will be released in solution. This is the reason that we need to continue liming our soils at least every after three years, if we are using powdered lime. A word of caution is that lime is not a substitute for fertiliser.
Soil is a living thing; let us look after it very well. God bless the farmers.
The author is an agribusiness expert.