Dr William Albrecht prescribed that calcium should represent 60% to 70% of the exchange capacity of the soil, with magnesium between 10% and 20%. These levels vary according to soil texture and the exchange capacity of the soil. (See “Biological farming: putting more fuel in your soil’s tank” in Farmer’s Weekly 18 April 2008). Together, these elements should total no more than 80% or the soil will become hard and increasingly unmanageable.
Addressing imbalances
A farmer must first determine whether there is an excess or deficiency of each element by means of a soil test. This test should be comprehensive and measure all the cations and micronutrients in the soil. International soil expert Dr Neal Kinsey advises that the same lab that analyses the farmer’s soil test should also interpret the results, as different labs use different measurement techniques and interpretation models. As an example, Dr Kinsey refers to a incident with one of his clients. A neighbour who had beautiful maize fields told the client that soil analysts said his calcium content should be around 78%. “This was totally in contrast with our advice to maintain calcium between 60% and 70%,” says Dr Kinsey. “On further investigation, we found that the maize lands measured in the desired range on the Albrecht test, while the other lab indicated a calcium content of 78%.”
pH, which then affects the availability of all the other nutrients in the soil. Deficiencies in a cation should always be corrected first, as this often restores other cations to their desired percentages. If there’s an excess of a specific cation, then certain anions such as nitrates and sulphates, must be used to strip it from the soil. Calcium has the most important role of all cations because it helps open up the soil so that element in excess can be removed. Dr Kinsey uses the following illustration: take a calcium shortage of 53%, with an excess of magnesium at 40% of exchange capacity. By adding a pure calcium source such as calcitic lime, which usually contains between 28% and 38% calcium and very little magnesium, you would be able to raise the calcium level to 60%. This would lower the magnesium to 33%. As a rule, magnesium levels will drop 1% for every 1% increase in calcium, and vice versa.
Hen manure is calcium-rich
Using hen manure is another good source of calcium to build up levels in the soil. Most manure is low in calcium, but hen manure contains high levels, especially if the hens were given calcium to strengthen the egg shells. “Farmers must nevertheless always analyse manure to determine its level of nutrients, as too much of any nutrient can be detrimental to the soil. Analysing manure should not be a once-off – it should be done as the feed of the manure source is adjusted according to season,” Dr Kinsey advises. four major cations – calcium, magnesium, potassium or sodium – renders that element the weakest link in the chain when sulphur is applied, according to Dr Kinsey. This means that sulphur will only reduce the excess of an element. Once calcium, magnesium, potassium and sodium are in the desired ratio for that exchange capacity, sulphur will start to reduce the levels of all the cations proportionally.
This is also why gypsum, which contains about 20% calcium and 17% sulphur, should only be used when calcium is above 60%. But because gypsum contains sulphur, the excess sulphur and nitrogen will strip the soil if calcium is below 60%. Once the calcium level is above 60%, the sulphur in gypsum will strip the soil of other cations that are in excess. It’s therefore a good product to use where there is an excess of magnesium, but the calcium level is in the desirable range.Dolomitic lime, which contains about a third less calcium than calcitic lime, but between 8% and 12% magnesium, should only be used to build calcium levels in the soil if there is a magnesium and calcium shortage. Dolomitic lime usually contains more than 4,3% magnesium, whereas calcitic lime contains less than that.
Managing magnesium If there is only a magnesium shortage, its level can also be increased with magnesium sulphate, which is the cheapest and most available form of magnesium, or chelated magnesium, which increases the magnesium level only. Increasing the magnesium without adding calcium reduces the percentage of calcium in the soil by the same amount.Alternative magnesium sources should therefore not be used unless there is sufficient or too much calcium in the soil.
Nitrogen can have the unwelcome effect of increasing magnesium levels by stripping the soil of calcium. Dr Kinsey says that the magnesium level can’t be reduced unless enough calcium and sulphur are supplied to remove the excess magnesium. Another important consideration is that both an excess and shortage of magnesium makes it unavailable to plants. Dr Kinsey recalls that he once had a client who farmed with cattle in Arkansas. The cattle were deficient in magnesium even though a soil analysis indicated that the soil had a 35% magnesium content. The farmer had to give the animals magnesium oxide to prevent them from getting staggers until he could get the magnesium of the soil into the desired range.
Liming facts Many people believe that you can’t apply too much lime. But this is not true: an excess of calcium ties up other cations like magnesium, potassium, sodium and hydrogen, causing deficiencies and making pH-rendering nutrients unavailable. It also ties up many of the micronutrients such as iron, zinc, boron and copper. This compromises production and means more of these nutrients will be needed to achieve optimal plant production, which in turn increases input costs. It’s much harder to address an excess of calcium than a deficiency. Dr Kinsey recommends the best way is to rectify other cation deficiencies in the exchange complex. This would automatically help lower the calcium levels. He says only then should sulphur be used to strip the soil of excess calcium. Contact SA Biofarm on (012) 333 4222 or visit www.sabiofarm.co.za. |fw
