Correcting soil pH marks first land preparation step

GOVERNMENT has since revealed that more than 70 percent of the country’s soils are acidic and need to be limed to bring them to the right pH levels.

Technically, this meant the soils had a pH value of less than 5,5 for most of the year.

Soil pH or soil reaction is an indication of the acidity or alkalinity of soil and is measured in pH units.

It is defined as the negative logarithm of the hydrogen ion concentration.

The pH scale goes from 0 to 14 with pH 7 as the neutral point. As the amount of hydrogen ions in the soil increases the soil pH decreases, thus becoming more acidic.

From pH 7 to 0 the soil is increasingly more acidic and from pH 7 to 14 the soil is increasingly more alkaline or basic (Environmental Information Series – University of New York).

Acidic soils, particularly in the sub-surface, are known to restrict plant roots’ access to water and nutrients.

In very acid soils, all the major plant nutrients — nitrogen, phosphorus, potassium, sulphur, calcium, manganese and also the trace element molybdenum may be unavailable, or only available in insufficient quantities with plants showing deficiency symptoms despite adequate fertiliser application.

Most plants and other soil processes thrive in soils with a pH range of 5,5 to 6,5.

Obviously, Government’s announcement of such a soil status should under normal circumstances get farmers worried and starting to look for solutions out of the problem.

Most of the farmers that I have talked to about the matter gave me the impression that they expected Government to act on the situation while they watched from the sidelines.

To me, such a reaction did not project an image of farmers who have clearly understood that the idea of farming as a business would only be successful if their soils were healthy.

It will not require rocket science for anyone to realise that Government can only do so much and the farmer does the rest.

In recent seasons, Government has availed lime to many farmers, particularly those doing the Pfumvudza/Intwasa programme, as a way of addressing the soil acidity problem.

This does not, however, mean that the lime those farmers received, courtesy of Government’s

assistance, was enough for the soil to regain the required pH levels.

This means farmers have to be proactive and do periodic soil testing.

The most opportune time to do soil sampling is usually in winter or just after harvesting a summer crop.

With this in mind, those farmers who did not react to Government’s revelations on the problem of acidic soils, must now correct their errors and seek help from experts, in this case their extension officers, to start collecting soil samples for testing once they finish harvesting.

The good thing is that soil sampling can be done any time of the year, but farmers should make sure they do soil testing at approximately the same time of the year.

Late summer or just the beginning of autumn is usually a good time to do sampling, testing and lime application to allow time for the recommended lime to act and change the pH before the next crop is planted.

Taking soil samples is not just a random exercise, but requires the farmer to take samples every two to four hectares.

The field is divided based on soil type and crop performance history.

The farmer will need 250 grammes of soil for a normal test, while the soil quantity for a major test may depend on the parameter of the test and number of its repeating.

Generally, one to one and a half cups of soil may be required. The quantity is also dependent on the soil type.

The soil will need to be dried, ground and sieved before it is sent for analysis to ensure a standardised mixture for the process.

Soil samples are dried at 50 degrees Celsius in cardboard boxes with the grinding done mechanically using a mortar and pestle before being passed through a 12-mesh (approximately 2 mm) screen.

What the farmer must always remember is that soil testing should be done regularly because factors that cause soils to turn acidic are always at work.

Rainwater is easily one of the biggest culprits, as it causes leaching away of basic ions (calcium, magnesium, potassium and sodium) while carbon dioxide from decomposing organic matter and root respiration can form a weak organic acid after dissolving in soil water.

The formation of strong organic and inorganic acids, such as nitric and sulphuric acid from decaying organic matter and oxidation of ammonium and sulphur fertilisers also contribute significantly to the problem.

This makes it critical for the farmer to be always suspicious of the presence of acids in his soil to frustrate his efforts to produce crops competitively.

Lime application to the soil will help raise pH levels thereby eliminating most major problems associated with acid soils.

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