Relative Values. The relative strengths of the various materials containing lime may be known and yet doubt continue respecting the choice to be made. The conflicting claims of dealers, and inaccurate deductions from a single test made by some individual, aid the confusion. If there were always the single purpose of correcting soil acidity, and if there were the same ease of application in case of all the materials, the choice would present much less difficulty. Notwithstanding this, most land now has a lime requirement, or will have one as leaching, crop removal and chemical change within the soil continue, and the puzzle is no worse than a score of others that present themselves continuously in farming.

Destroying Acids. The cost of liming to improve the physical condition of land is prohibitive for most farms remote from supplies of stone that can be burned and put upon the land at a low price per ton. Where stone is at hand, and soils are intractable, lime burned on the farm should be used. Some slight benefit to a stiff soil may be obtained from the light application that is deemed practicable where all forms are costly, but this benefit is not usually marked in case of an application of a ton or less of burned lime. It is a safe statement that most buyers of lime in some form or other will profit chiefly through the correction of soil acidity and promotion of bacterial life. This renders the situation more simple as any carbonate, hydrate or oxide of lime will accomplish these purposes.

Composition. The first consideration is the actual content of calcium and magnesium. A guaranteed analysis is the only safe basis of purchase. The unstable nature of fresh burned and hydrated forms makes an exact statement of percentages impossible for goods not wholly fresh, but at least the purity of the original limestone can be judged.

Equivalents. One ton of fresh burned lime, made from pure stone, is equivalent to 2640 pounds of the hydrate, and to 3570 pounds of pulverized limestone or of air-slaked lime. It is easy to carry in mind the proportions expressed by 1, 1-1/3 and 1-3/4. If there were no other considerations, such as convenience in handling, evenness of distribution, etc., to take into account, one ton of fresh burned lime, one and a third tons hydrated and one and three-quarters tons finely pulverized limestone would have the same value when delivered in the field. Lime fully air-slaked, high-grade marl, and finely pulverized limestone would have the same value, ton for ton.

Even Distribution. The value of even distribution is not easily overestimated. If lime in proper amount does not go into each square foot of an acid soil, some of the soil will remain sour unless mixing is done by implements of tillage. Lime is diffused laterally through the soil in a very slight degree. If a strip of sour land is protected by canvas, so that no dust from lime applied to uncovered land can blow upon it, a seeding to clover will show that plants a few inches from the edge of the limed area will fail to start thriftily and may die before their roots reach the lime. Full effectiveness of an application is possible only through even distribution.

Using Lump Lime. Lump lime, slaked on the farm, is difficult to apply satisfactorily. Spreading with a shovel from small heaps is bad practice, and when the lime is slaked in a large heap, it cannot be handled as well as pulverized stone or commercial hydrated lime. The latter two are in condition for application by means of a lime distributor, or even a fertilizer attachment of a grain drill. The farm-slaked lime contains impurities that interfere with distribution.

An Estimate. It is always hazardous to attempt an estimate of cost of labor without knowing the particular farm conditions, but the expense and discomfort attending the slaking and use of lime bought in lump state justify a willingness to pay as much for a ton of hydrated lime as lump lime would cost, although the former has only three-fourths as much strength as the latter. Some farmers pay nearly twice as much for the hydrated, partly to escape the inconvenience and partly because they hope that the extraordinary claims for superiority made by some dealers may prove true. They should know that it is only fresh burned lime slaked, but incline to credit a claim that special treatment enhances value in some mysterious way.

Comparing lump lime with finely pulverized limestone, the factors of expense and discomfort and final lack of perfect distribution of the former remain important. The stone is relatively easy to handle, being slightly granular and passing through a distributor without trouble. The fact that it is not caustic, like the hydrated, is in its favor. When everything is taken into account, one is justified in using limestone or air-slaked lime at a cost per ton three-fourths as great as that of lump lime. It is to be borne in mind that in these estimates the cost per ton is not that at the factory or at one’s own railway station, but on the farm. The freight and cartage to the farm are based on weight of material, and more material per acre is required when the worthless portion has not been driven off by burning. If one must use one and three-quarters tons of limestone to have the equivalent of one ton of fresh burned lime, it is evident that the cost of freight and cartage of the worthless portion might make cost prohibitive if distances were very great. Farms lying a long distance from a railway station may easily find that fresh burned lime is the only form of lime they can afford. The basis for correct estimate is cost delivered in the field.

Storage. One advantage possessed by the limestone is ease of storage. There is no inconvenience or loss. The stone may be ordered at any time of the year when teams are least busy upon other work, and it can be held till wanted. In this way the cost of cartage to the farm may be kept relatively low, and the material is at hand when wanted, regardless of rush of work or delays of railroads. This advantage is partial counterbalance to the cost of freight on the worthless portion of unburned stone.

Valuing Limestone. The estimates, so far as labor and convenience are concerned, are merely suggestive, and rest upon the presumption that the stone is satisfactorily fine. It has been urged in another chapter that immediate effectiveness is determined by fineness, but as a working basis we assumed that when all the stone would pass through a screen having sixty wires to the inch it would give the desired results. The coarsest portion would not be available at once, but when an application is heavy enough to serve for a year or more, we have enough very fine material in such a grade of stone to meet immediate need. When estimating values of such a grade and coarser grades, the amount per acre to be used is a factor. The coarse is unsatisfactory if the price is not low enough to permit an application sufficient for a considerable term of years, so that it will contain all the fine material needed at once. In that case the coarser material may be expected to meet later need, and may be even more desirable for such purpose, as it would not be subject to leaching.

Coarse grinding costs much less than fine grinding, and it is the resulting low price that permits the heavy application. As stone varies in hardness and ability of the small particles to withstand disintegrating forces in the soil, an estimate of the difference in price between a 60-mesh limestone and a 10-mesh one could not serve as a safe guide. The buyer should know the percentages of a limestone passing through screens of various sizes before he makes a purchase, and should demand part of the saving in cost of production that attends coarse grinding.

Oyster Shell. Ground oyster shell should be given about the same valuation as limestone. It is a lime carbonate, and the percentage of worthless material in it varies somewhat It is coarsely ground, but the large pieces disintegrate in the soil much more rapidly than limestone would do. It contains a little nitrogen and phosphoric acid, partially available, as an offset to coarseness and some lack of purity, as compared with the highest grade of fine stone. It is profitable to buy oyster shell at limestone prices if used liberally enough to furnish a supply for a term of years. The oxide, or burned shell lime, would be nearly the equivalent of burned stone if it did not change to hydrate and air-slaked forms so rapidly that it rarely is on the market in the full strength of fresh burned lime.

Hardwood Ashes. As a source of lime, ashes have become far too expensive. The composition of ashes on the market is widely variable, dirt and moisture often accounting for much of the weight. The lime in fresh burned ashes is peculiarly effective, being finely divided and in oxide form, but the ashes on the market have much of the lime water-slaked and air-slaked. Unless analysis is made at time of purchase, a buyer should not estimate the content of lime in a ton at a value greater than assigned to one-half of a ton of limestone. The additional value of the ashes, due to the potash content, is wholly another consideration.

Marl. No more should be paid for a ton of good chalk marl than an equal weight of fine limestone would cost. Each is a good carbonate of lime, with the same capacity for destruction of acids.

Agricultural Lime. This variable product should not be bought unless actual composition is known, or the cost is as low as that of pulverized limestone, and even then it may be a bad purchase, the methods of the manufacturer being the determining factor. If such lime is chiefly a dumping place for low-grade stone and forkings, it has small agricultural value.

Land Plaster. The soil wants lime in carbonate form. The oxide and hydrate change to carbonate, and therefore are good. Land plaster is a sulphate, and its tendency is to make a soil sour. It should not be considered as a means of correcting soil acidity.

Basic Slag. The amount of effective lime in basic slag, as made by modern methods, is so small that its value is nearly negligible. Basic slag is a good source of phosphorus, and in addition has a tendency toward correction of soil acidity, but such tendency has little cash value for land that requires a considerable dressing of lime to furnish a base with which soil acids may combine.

An expression of opinion was obtained recently from some leading soil chemists of this country, and upon such expression we base the estimate that when pulverized limestone costs three dollars a ton, the value of the lime in a ton of basic slag should not be placed higher than 50 cents, and some chemists believe that the lime content is entirely negligible as an agent in soil amendment.

Lime in Other Fertilizers. The demand for lime is leading some men to state a lime content for their goods that is designed to mislead. Such lime is not in a form to combine with soil acids, and is as valueless as the very large amount of lime in acid soils that is in compounds having no power to affect free acids.