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CALCIUM IS ONE OF THE MOST important contributors to soil and plant health. When there is a shortage, farmland is not as productive as it could be, as plants’ ability to absorb and utilize other minerals is negatively affected.
According to Research from Makerere University soil science department, as much as 65% of Uganda’s soil is deficient in calcium. According to Neal Kinsey, a soil biologist from Kinsey Agriculture Services in the US at a recent Biofarm information day, fertilizing when the soil’s calcium level is incorrect is a waste of money.” A deficiency in calcium will reduce the nutrient up take efficiency of plants, meaning that more nitrogen, phosphorous, potassium and other nutrients will be needed in the soil to attain the same results as would have been possible if there was sufficient calcium in the soil,” he says.
The reduced availability of these minerals, in turn, has a negative impact on production. A shortage of phosphorous and potassium, for example, reduces water uptake and results in plants requiring more water than they would have if these minerals were freely available.
Calcium not only affects fertilizer efficiency, but also has a huge impact on soil structure and health, binding individual soil particles together to form stable aggregates or soil granules. Pores between soil particles in calcium-efficient soils are accordingly larger, resulting in better water and air movement as well as soil drainage and aeration.
Soil organisms benefit from the improved soil structure because they need water and oxygen to function optimally.
Calcium deficiency? Don’t bother with PHStudying soil PH to determine if a calcium deficiency exists does not work. The PH only indicates the availability of nutrients and doesn’t identify which elements are deficient or in excess. This is because other cations, such as sodium, potassium and magnesium have a greater impact on soil PH. In soil science, a cation is a positively charged ion which has fewer electrons then protons, and is attracted to cathodes. “Soil PH does have an impact on production and can result in many problems when it is too high or too low,” Kinsey says.” It is however much more important to have a nutrient-balanced soil than an ideal PH.”
He explains that he once worked with blueberry farmers in the US who produced some of the best quality blueberries in a land where the soil nutrients were balance even though the soil PH was deemed too alkaline for blueberry production.
Does the soil have enough calcium?So how do you know if there is enough calcium in the soil? Firstly, it’s not so much the amount of calcium in the soil that matters, but rather the balance between calcium and other cations.
The best way to measure that balance is to use the Albrecht extraction method to calculate the cations in the soil and to determine the balance between them. The quantity of cations depends on the cation exchange capacity (CEC) of a specific soil, which in turn is determined by the amount of clay and organic matter. Clay soil can hold more cations than sandy soil can. The Albrecht method presupposes that around 80% of the CEC should be represented by calcium and magnesium. Soil compaction increases when the ratio of calcium to magnesium is higher than 80%.
Magnesium, however, has the opposite effect of tightening soil by breaking down soil aggregates and reducing pore space.
An excess of magnesium therefore results in reduced aeration and water movement.
Striking a balanceThe balance between calcium and magnesium depends on the soil type. Calcium should ideally account for between 60% and 70% of the CEC, while magnesium should account for 10% to 20%. Sandy soils have larger pores between particles than clay soils do and for this reason require a larger percentage of magnesium to reduce the size of soil pores and reduce the water infiltration rate. In clay, larger percentages of calcium would be needed to increase pore sizes and enhance water and air movement. Adding calcium to soil usually results in huge benefits, such as reduced aluminium and manganese toxicity, increased microbial activity and decomposition, increased symbiotic nitrogen fixation by rhizobium bacteria on legumes, and increased availability of phosphorous and molybdenum.
It is best to use calcitic lime (calcium carbonate) when the amount of calcium at base saturation is less than 60%. Calcium carbonate does not have an anionic component does not have anionic component that can strip the soil of cations. In slightly acidic soils the carbonate component of the limestone will react with hydrogen ions to form water and carbon dioxide, which increases the PH. This reaction releases free calcium into the soil solution, which is then available because the soil is less acidic than before.
In slightly acidic to neutral soils, where there is an excess if one or more of the other cations, the calcium carbonate will be broken down by soil microorganisms. Burnt lime (calcium hydrogen) can also be used to increase the calcium level, but it reacts very quickly, is easy to over-apply and is more expensive.
Gypsum (calcium sulphate) should only be used to increase the calcium level if the calcium on the exchange complex exceeds 60%, as the sulphate portion in gypsum strip calcium and other cations out of the soil if used at a lower calcium percentage. Kinsey says it would be asking for trouble to apply more than 2200kg of gypsum per hectare per year because of the amount of sulphur applied with the gypsum.
Calcium nitrate can be used as a soil application or foliar spray to satisfy immediate crop requirements. It should however not be used to build the calcium from the soil.
Calcium at levelsAn excess of calcium is also undesirable even though this situation is quite unusual in South Africa. Calcium at a high levels replaces other cations-magnesium, potassium, sodium and hydrogen-in the soil, causing deficiencies in those nutrients, and ties up the micronutrients iron, zinc, boron and copper. Kinsey says that 90% of phosphates and iron in the soil will be rendered unavailable to plants when the calcium level exceeds 70% and 85% respectively, at base saturation.
What to do about excessive calcium
Addressing an excess in calcium is more difficult than addressing a deficiency. Kinsey said that the best way to rectify a soil that has too much calcium would be to rectify other cation deficiencies on the exchange complex, as this will automatically lead to lower calcium levels.