Part 3 (1/2)
LIME HYDRATE
_Slaking Lime._ The usual means of reducing fresh burned stone lime to a condition that makes even distribution upon land possible is by slaking.
A few years ago considerable effort was made to create a market for lime pulverized by machinery, but the difficulty in excluding the moisture of the air so that packages would not burst has been in the way of developing a market. Slaking, by the addition of water to the fresh burned lime, is the common method of getting the required physical condition. When the slaking is done on the farm, the custom has been to distribute the lime in small piles in the field, placing the piles at such convenient distance apart that the lime, after slaking, could be spread easily with a shovel.
The water for slaking comes from rains, or from moisture in the air and earth. The method is wasteful and can be justified, if ever, only where farm-burned lime costs little per ton, and the nature of the soil is such that a relatively heavy application can be safely made. The distribution is necessarily uneven, and if the required amount goes upon all the surface, a great excess is sure to go upon a portion of it. Very often an excess of water puddles much of the lime in the pile, and lumps may be seen lying in ineffective form in the soil for years. The practice is responsible for much of the excessive application that brought the use of caustic lime into disrepute.
_Slaking in Large Heaps._ A preferable method is to put the lime in flat heaps of large size and about four feet deep, so that water may be applied or advantage be taken of rainfall. The value of the lime is so great that one can well afford to draw water and apply with a hose so that the quant.i.ty can be controlled with exactness. When fresh burned lime is perfectly slaked, each 56 pounds of pure lime becomes 74 pounds of hydrated lime, water furnis.h.i.+ng the added weight.
_Hydrated Lime on the Market._ A popular form of lime on the market is the hydrate. Manufacturers first burn the stone, and in the case of a pure limestone they drive off 44 pounds of each 100 pounds of the weight in burning. Then, they combine enough water with the lime to change it to hydrate form, and that adds 18 pounds weight. It is run through a sieve to remove any coa.r.s.e material, and then packed in bags which help to exclude the air. The small packages in which it comes upon the market make handling easy, and this helps to bring it into demand.
Its good physical condition makes even distribution possible, and thus permits maximum effectiveness to be obtained. It is only slaked lime, identical in composition and value with lime of the same purity slaked on the farm, but some dealers have been able to create the impression that it has some added quality and peculiar power. This does no credit to the public intelligence, but the hunger of soils for lime is so great that investment at a price wholly out of proportion to the price of farm-slaked lime has rarely failed to yield some profit.
_Degree of Purity._ It is always a reasonable a.s.sumption that hydrated lime has been made from stone of a good degree of purity. A local stone, burned on the farm, may be of low grade, but no man of business judgment would erect a costly plant for burning and hydrating lime where the purity of the stone would not afford a good advertis.e.m.e.nt in itself.
On the other hand, we find very little hydrated lime on the market that has not had sufficient exposure to the air to become changed in some part to an air-slaked condition, or has had refuse mixed with it.
Air-slaked lime is not worth as much per ton as the hydrate because it cannot correct as much soil acidity, and the percentage of the former cannot be determined by the buyer. Its presence may not be due to any wrong-doing of the manufacturer, and, on the other hand, the increase in weight that attends air-slaking may be welcomed in some degree by a dishonest manufacturer before the goods are s.h.i.+pped. The difficulty in preventing hydrated lime from adding to its weight by becoming air-slaked is a point to be taken into consideration.
The percentages of air-slaked material in hydrated limes are widely variable, and no manufacturer can standardize his product on the market surely for the benefit of the farmer. In some instances the product is adulterated with refuse material in finely pulverized condition.
CHAPTER XIII
OTHER FORMS OF LIME
_Air-Slaked Lime._ A pure limestone is a carbonate, and the chemical formula is CaCO_3. When it is burned, the carbon dioxide (CO_2) is driven off, leaving CaO, which is calcium oxide, called fresh burned lime. In this process 44 pounds of a stone weighing 100 pounds pa.s.ses 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 a.s.surance that the percentage of waste stuff in it is relatively high. Unless one knows to the contrary, he should a.s.sume 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 coa.r.s.e 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.