The application of lime is often the most neglected soil maintenance practice in farming. We tend to overlook this crucial aspect of maximising the yield potential of our soil. But, it is not only about short term yields and profit. Soil is the only consistent, natural resource available to a farmer and it must be preserved. It can never be replaced. When abused, it can be very costly to repair. In severe cases, it may be too late and could lead to irreversible conditions – soil erosion and desertification.
The purpose of this article is to provide food for thought, in lay-man’s terms. The finer technical details and scientific formulas can be identified in consultation with the experts – soil analysis and fertiliser recommendations. Consult with them!
The level of acidity/alkalinity in the soil is reflected by the scientific term ph. We often hear the word when we discuss soil samples etc. A high ph has less acid than a low ph (the higher the better). A ph of 5,5 or higher for topsoil, and 4,8 for the subsoil, is desirable for most crops. (Soil with a ph of 4 has 100 times more acid than soil with a ph of 6).
At some stage in our lives, we have all suffered from heart-burn or indigestion. This is usually as a result of excessive stomach acids, created by the type, or combination of food that we have eaten. When this happens, we feel uncomfortable. We lose our appetite and our energy. We are unable to function effectively. So what do we do? We drink an antacid or suck a Rennie, to neutralise the acid. Only when the discomfort subsides, are we able to function at our best again.
Soil is almost like our stomachs – it uses water to break down (digest), all the organic and other fertiliser material available. This enables the plant to absorb the nutrients. If there is too much acid in the soil the plant is unable or unwilling, to extract the nutrients (phosphates).
Over time, the application of chemical fertilisers, together with the extraction of various soil nutrients by the plant, causes the acidity to rise. Technical example: Nitrogen is converted to nitrates and hydrogen ions in the soil. When the plant roots are unable to extract the nitrates as a result of acidity, to keep it in the root zone, the nitrates eventually leach away. This leaves only the hydrogen ions – further increasing acidity.
Tillage practices also play a role in ph levels. When the soil is turned, as in ploughing, the natural processes of the organic elements in the soil are disturbed. This affects natural decomposition, which in turn can have an effect on the acidity. Other factors that could have an effect on ph are: high rainfall, high yields, soil types, insufficient/excessive or incorrect fertiliser, and the type of crop planted.
So, just like our stomachs, when necessary, we need to remedy the situation and apply an antacid to the soil – lime.
There are two different types of lime – agricultural lime, which is more generally used, and dolomitic lime applied to soil with a magnesium deficiency. The experts doing the soil analysis will be able to advise you of what and how much to use on each land. This can vary from 500 kg/ha to 2,5 tons/ha or more.
Lime reacts much slower than fertiliser and should be applied before tillage – worked into the ground. Lime can be applied any time of the year but preferably it should happen long before planting where the lime can be given a chance to react in the soil. However, the optimum benefits are long term and usually only seen in the following seasons.
A light textured soil with an effective cation exchange capacity (CEC) of 5 centi-mole charge per kg (milli-equivalents per 100 gram soil) and with an acid saturation percentage of 20%, will have one milli-equivalent of acid to neutralise in each 100 g of soil.
In a hectare of light textured soil at a depth of 30 cm deep, there are 4,5 million kg of soil. This means that
45 million milli-equivalents (milligram) of acid need to be neutralised. This translates to 45 kg of acid per hectare. As calculated above, 50 g pure calcitic lime is needed to neutralise one mole or equivalent or gram of acid. So 45 kg of acid will require 2 250 kg pure calcite to neutralise. A soil with a CEC of 10 and the same acid saturation, will need double the amount of pure calcite.
If a 100% calcium oxide nano suspension (absolutely pure and reactive) is used, one will still need 2,25 ton/1,78 (higher efficiency) = 1,26 tons lime per ha or 1 260 kg/1,3 kg/litre (reported density) = 969 litres per ha.
Even if pure magnesium oxide is used, one would still need 0,9 tons or 692 litres. Usually, the suspension products contain low concentrations of actual lime and that will elevate the mass requirement dramatically. A 15% calcium oxide suspension will require 100/15 x 969 litres = 6 460 litres per hectare using the same reported density.
The fact remains, that no matter how pure or reactive (fine) one can get any natural liming material, one cannot exceed an efficiency of 2,5 times that of pure calcitic lime. Furthermore, there simply is a huge mass of pure acid in a hectare of acidic soil and one accordingly needs a pro-rata high mass of lime material to neutralise it.
The price of lime, in itself is not that expensive, however transport costs from the mines to the farm are very high. Transport costs vary depending on the farm’s proximity to the mines. Many farmers do not have their own spreaders and have to resort to contractors. Many farmers are inclined to see these expenses, and the additional work, as unnecessary and problematic.
However, the correct soil ph, MUST be the point of departure for any farming operation – it is the FOUNDATION on which we build crops. A weak foundation is a recipe for long term disaster. Without a solid foundation we are throwing our money away – most, if not all of the fertiliser applied, becomes ineffective and wasted.
Without water a plant can’t survive, but nothing survives on water alone – with an acidic foundation the plant is unable to absorb the nutrients, no matter how much water!
The only way to determine the acidity and the remedy required is with soil analysis. If the lime requirement is excessive for a single application, it may be necessary to apply lime over a two year period. Once the required ph is achieved it is essential to ensure that optimum levels are maintained, allowing the farmer to test the soil every alternate year, or after an unusual yield or rainfall season.
Remember, the application of lime is not a quick fix. The benefits are only evident over a long term, provided that the ph levels are maintained at an optimum level each season.
In closing – here is some food for thought – treat the soil with the same respect as you would your own stomach. Be careful that you put the correct food into it, and please, make sure it NEVER suffers from heartburn!
