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What happens when you lime?Watch out for some common misconceptions about what happens when lime is added to the soil. These can prevent you getting the most out of your investment in lime.by KEITH REIDAcid soil, or low soil pH, limits crop production on a small but significant part of Ontario's crop land. In a recent summary of soil test results from across Ontario, compiled by the Potash and Phosphate Institute Canada, 10 per cent of the samples had soil pH levels below six.While a pH in this range will not harm all crops, it will certainly reduce the nitrogen fixation by legumes, and it could indicate that pockets within the field are acid enough to hurt production of even relatively tolerant crops like corn. The correction of soil acidity is relatively straightforward -- add agricultural lime. There are, however, some common misconceptions about what happens when lime is added to the soil, and these can prevent you getting the most out of your investment in lime.
The chemistry of limingSoil acidity is a result of excess hydrogen ions (H+), both in the soil solution and adsorbed onto clay and organic matter in the soil. In addition, these excess hydrogen ions start to chew on the clay minerals in the soil, releasing aluminum and manganese ions that can damage growing crops. For liming to be effective, it must neutralize the excess hydrogen ions and take them out of the system. This can be done with compounds like calcium oxide (quicklime) or potassium hydroxide, where the hydrogen ions react directly with the oxide (O=) or hydroxide (OH-) to form water (H2O). These compounds, however, are expensive and difficult to work with. It is much more common to use a carbonate of calcium or magnesium, otherwise known as calcitic or dolomitic lime, as the liming material.The reaction of hydrogen ions with carbonate is slightly more complex than with oxides or hydroxides. In this case, the carbonate ion (CO3=) combines with two hydrogen ions to form carbonic acid (H2CO3). This, in turn, breaks down to carbon dioxide (CO2) and water (H2O), removing the hydrogen ions from the system. The calcium and magnesium added with the lime take up the spaces vacated on the clay and organic matter by the hydrogen ions, keeping the electrical charges in balance, but they do not increase the soil pH on their own.
The effectiveness of a liming material depends on how much carbonate is present (the neutralizing value), and on how reactive the material is. The neutralizing value can be reduced by the presence of other materials in the limestone, like clay or gypsum, but it is also affected by the chemical make-up of the liming material. Magnesium atoms weigh less than calcium atoms, so dolomitic limestone, which has half calcium and half magnesium atoms, has more carbonate in the same weight of material than calcitic limestone. This means that the neutralizing value is about four per cent greater. The reactivity of the material depends mostly on its particle size. Finely-ground materials have a large surface area where the neutralizing reaction can take place. Coarse materials will react so slowly that they are practically useless for changing soil pH.
First, the hydrogen ions need to get together with the carbonates in the soil solution. Both the lime and the hydrogen exist in balance with the solid forms, with only a small part of each in the solution at any time. As the liming reaction occurs, and the reactants are removed from the solution, more will diffuse out from the solid reserves, but this process takes time. Adding to the time is the indirect path that the ions must take around soil particles and air spaces so they can actually react. As the soil dries out, the length of this path increases as the short-cuts through larger soil pores are blocked by air spaces, and so the speed of the reaction slows down. Similarly, frozen soils block the pores with ice, so diffusion can't take place. In any case, the less mixing of the lime with the soil, the harder it is for the hydrogen ions to get together with the carbonates and the slower the process. The net result is that soil pH change can start to happen immediately, but can take up to 18 months to reach completion. This is particularly true for lime applied during very dry or very cold periods.
How long will the increased pH last?The natural tendency is for soil pH to decline over time, as acids are deposited in the soil with rainfall, or released from plant roots or during organic matter decay, or during the conversion of ammonia to nitrate. The addition of limestone helps to prevent this acidification, but a bigger factor is the buffering capacity of the soil itself.Since this buffering occurs on both directions, a soil that needs more lime to raise the pH will need more acid to cause it to decline. This means that a very coarse-textured soil, which tends to be very poorly buffered, might need to be limed again in three or four years, while a clay soil may not need liming again for a decade or more.
Regular soil testing is the best way to monitor changes in soil pH. BF
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