Limestone, or calcium carbonate, CaCO3, makes up about 10 per cent of the sedimentary rocks on earth. Sometimes limestone contains appreciable amounts of magnesium. It’s then called dolomitic limestone, CaMg (CO3)2. Under pressure, limestone rock becomes metamorphosed into marble.
Limestone rock is quarried in Alberta, British Columbia, Manitoba, Ontario, Nova Scotia and Newfoundland. Ontario and Quebec produce over 70 per cent of Canada’s lime, the vast amount of which goes to form cement for concrete construction and roadways. Limestone production nationwide is close to a 100 million tons and worth about a billion dollars.
Crushed limestone, powder-like, is generally sold under the name of lime in its natural or unheated state.
Limestone rock is converted by high temperatures (900 C plus) into quicklime (CaO) — carbon dioxide (CO2) is driven off. Heating a 100 pounds of clean limestone rock will give about 56 pounds of quicklime, and drive off 44 pounds of carbon dioxide.
Quicklime, or calcium oxide, is the product used in the cement industry. When limestone is used for making cement, the CaO and other products are mixed with water to form hydrated lime (Ca(OH)2), which is mixed with sand or gravel to form cement or concrete. Over time, the concrete hardens into a rocklike structure by reabsorbing CO2 to reform the original calcium carbonate. It’s a sort of recycling — the carbon dioxide driven off through the heating process is reabsorbed over time. It’s sustainable.
Hydrated lime is formed when quicklime (CaO) absorbs water to form Ca(OH)2. This product is the lime used in the beet industry and for water purification by municipalities.
Limestone in agriculture
In agriculture, when we say lime what we really mean is just plain crushed limestone rock to form a coarse or fine powder. On the Canadian Prairie, limestone or its heated component quicklime or hydrated lime is primarily intended for making concrete, repairing roadways, purifying municipal water, or purifying sugar beet pulp. Relatively small amounts are used for agriculture and horticulture, particularly commercial soil mixes.
In tropical and semitropical countries like Brazil, vast amounts of limestone are used to neutralize acidic soils, often with pH levels that are 4.5 and less, for growing soybeans, corn and other crops.
In Eastern Canada limestone is used agriculturally in Nova Scotia and PEI to raise the cultivated soil levels from a pH of four or five up to six or more. Most of PEI’s red clay soils are quite acidic.
On the Prairies, Alberta has the most acidic soils with pH ranging from 4.5 to 5.5 on around 35 per cent of arable cropland. Saskatchewan and Manitoba have far less in the way of acidic soils, ranging from an estimated 10 to 15 per cent of total cropland.
Cropland soils with a pH of 5.5 or less are unsuited for alfalfa and most field crops suffer significantly reduced yields. Acidic croplands with a pH of five or even 5.5 may have large areas where the soil is 4.5 or even lower. Alberta has some land in the Peace Region where the pH is only 3.5.
When alfalfa is grown at a pH between 5.5 and six it can generate only 50 per cent of its potential yield. At a pH of five, most wheat varieties will lose up to 20 to 30 per cent of expected yield.
Farmers wishing to lime acidic cropland to raise the pH can look to spent sugar beet lime which is about 30 to 50 per cent lime, municipal spent lime used in water purification and wood ash which contains lime as well as potash, phosphate and micronutrients.
A problem that has developed recently is stratified soil acidity caused by zero or minimum tillage. Acidifying nitrogen fertilizers are generally applied no deeper than three inches, and crop residue containing acidifying nutrients is left on the soil surface. In acidic soils, especially those with a pH below six, the top four to five inches of soil under zero till can become quite acidic — the pH in the top three inches could fall below 4.5.
This creates a problem with the availability of molybdenum (Mo). This soil mineral required at only ounces per acre becomes unavailable when the pH level is below five. Although this trace mineral is needed in very small amounts, the nitrogen metabolism in crop plants comes to a halt in its absence. Field crops cannot pick up nitrates or transfer them to the ammonia form. Molybdenum absence even stops sulphur metabolism. In such acidic soils, crops refuse to grow. Sometimes the cropland itself is low or deficient in molybdenum, as is now well documented in the state of Idaho. Liming the soil raises the soil pH and releases the molybdenum into its soluble form, allowing plants to take up nitrogen or legumes to fix nitrogen. Such a miniscule amount of micronutrient with a major crop production role. Liming acidic soils, especially those that have been under zero till for 15 or 20 years, could result in major yield increases.
When I grew up in Wales, on our poor acidic moorlands we often wondered why grazing horses and sheep always crowded the roadside banks and ditches along the highways. Highways all over the world are just ribbons of gravel mixed with crude or distillate bitumen or asphalt — giant oil spills with high melting points. Highways are frequently repaired with limestone chips. These limestone chips are scattered by high-speed traffic and find their way into the ditches. It was finally realized that these scattered limestone chips were raising the pH of the adjacent acidic soils (sweetening the soils), favouring vigorous grass and clover growth that was far more nutritious to the sheep and horses than the adjacent relatively unproductive acidic moorlands. A huge tribute indeed to the value of liming acidic cropland.