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Other Forms Of Lime

Air-Slaked Lime. A pure limestone is a carbonate, and the chemical formula is CaCO3. When it is burned, the carbon dioxide (CO2) is driven off, leaving CaO, which is calcium oxide, called fresh burned lime. In this process 44 pounds of a stone weighing 100 pounds passes into the air, and there remain 56 pounds of lime. When it air-slakes, it takes back the carbon dioxide from the air, and the new product becomes CaCO3, or carbonate of lime, and regains its original weight of 100 pounds. This is what would happen if the process were complete, and it is nearly so when the exposure to the air is as perfect as possible. Fifty-six pounds of valuable material are in the 100 pounds of air-slaked lime, just as is the case with limestone, and there is no difference in effectiveness except in so far as the air-slaked material is absolutely fine and available, while most pulverized limestone is less so. In making purchase for use of land the buyer cannot afford to make any appreciable difference in price in favor of air-slaked lime, as compared with a fine stone. Air-Slaking a Slow Process. Lime changes to an air-slaked condition slowly unless it has full exposure. Old heaps will remain in hydrate form for many years, excepting the outside coat, which excludes the air. Complete air-slaking would not reduce ability to correct soil acidity, the total amount of calcium and magnesium remaining constant, but weight would be added in the slaking, and therefore the value per ton would be reduced. The slowness with which air-slaking proceeds gives reason to expect that any bulk of old lime may contain a considerable percentage of the hydrate, and therefore have greater strength than a true carbonate like limestone. This is a consideration of value to a buyer. Agricultural Lime. Some manufacturers have found in the demand for lime by farmers an opportunity of disposing of much material that would not be satisfactory to manufacturers and builders. In some cases this so-called agricultural lime is sold at a price that is not beyond value, but it varies much in its content of pure lime. If the unburned cores of kilns are ground up, the material simply retains the value of unburned stone. Any air-slaked material put into it has like value. Forkings, ground up, have less value, and sometimes no value at all. Some better material may go into this mixture that is given the name "agricultural lime," and the product cannot be standardized or have a valuation given it that would be true for another lot. Some manufacturers are marketing limes of fair values under this designation, but the values change as the material changes. There are other manufacturers who are putting poor stuff on the market. Unless one knows the manufacturer and his processes, he should not pay a great deal for "agricultural lime." It is much better to buy a high-grade lime or limestone that is more nearly constant in composition. When the word "agricultural" is part of the brand, there is assurance that the percentage of waste stuff in it is relatively high. Unless one knows to the contrary, he should assume that a ton of finely pulverized limestone is worth more per ton than "agricultural lime." Marl. Marls vary in composition, as limestones do, but there are beds of chalky marl that contain very little clay and sand and are nearly a pure carbonate. It is only marls of high degree of purity that can be put on the market with profit, but beds of less pure marl furnish dressings for farms of the locality in many sections of the country. Some of these inferior marls have had so much clay and sand mixed with the lime carbonate that dressings must be heavy. The best lime marls provide excellent material for the correction of soil acidity, the actual value per ton being practically the same as that of the finest pulverized limestone. Some dealers in marl make extravagant claims for their goods, but any farmer may easily put these claims to the test and learn that he should not expect more than a fairly good carbonate of lime can do. Marl improves the physical condition of stiff soils only when used in large amount per acre, and this is true of any carbonate form, such as limestone. Little effect upon physical condition should be expected from the light application usually given when marl is purchased and transported some distance to the farm. The chalk marl on the market is used to correct soil acidity, and at the best it is worth only what good lime carbonate is worth. It has no hidden virtues, and cannot take the place of fertilizers. It is an excellent means of meeting the lime-requirement of land when bought right, and its fine division makes it distinctly superior to coarse stone. There should be no confusion of a lime marl with the so-called "green sand" marl. The latter is low in lime, and may be acid, the value of the marl being in a considerable percentage of plant food contained. Oyster Shell. Ground oyster shell is a good source of carbonate of lime. The percentage falls below that of limestone, but in addition there is a little nitrogen and phosphoric acid. An analysis of a good quality of oyster shell, as found on the market, will show 90% carbonate of lime. Burned oyster shell has something near the same composition as lime made from stone, but it goes back to hydrate and air-slaked forms rapidly. There is no large amount of burned shell lime on the market, the material known as shell lime being the ground shell, or lime carbonate. Wood Ashes. A large supply of lime in excellent form was afforded by hardwood ashes, but this product has ceased to have any important value to our agriculture. The chief supply on the market is low in quality, containing moisture and dirt in considerable amount, the form of lime being changed from an oxide to the hydrate and carbonate. Gas Lime. Prof. E. B. Voorhees, in "First Principles of Agriculture," says: "Gas lime is also frequently used as manure; in gas works, quicklime is used for removing the impurities from the gas. Gas lime, therefore, varies considerably in composition, and consists really of a mixture of slaked lime, or calcium hydrate, and carbonate of lime, together with sulfites and sulfides of lime. These last are injurious to young plant life, and gas lime should be applied long before the crop is planted, or at least exposed to the air some time before its application. The action of air converts the poisonous substances in it into non-injurious products. Gas lime contains on an average 40% of calcium oxide, and usually a small percentage of nitrogen." Lime After Magnesium Removal. A by-product in the removal of magnesium from a magnesian limestone is an excellent material for correction of soil acidity, on account of its physical condition. Its exposure to the air causes much of the hydrate to change to an air-slaked form, and its value per ton lies somewhere between that of very finely pulverized limestone and hydrated lime.

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