The ill-effects usually attributed to acid phosphate are not due in any great degree directly to the sulphuric acid used in its making, but to the bad farming methods that so often attend its use. When the need of commercial fertilizers is first recognized, acid phosphate seems to meet the need. The soil's store of available phosphoric acid gives out first, and this fertilizer brings a new supply. If the available potash is in scant amount, the acid phosphate helps in this direction by freeing some potash. The phosphoric acid has peculiar ability in giving impetus to the growth of a young plant, and that enables it to send its roots out and obtain more nitrogen than it otherwise would do. The farmer thus may come to regard it as a means of securing a crop, and there is neglect of manure and clover. If a field is thin and fails to make a sod, there is no immediate compulsion to use manure or to grow a catch crop to get organic matter, but the field is cropped again with grain. Soon the supply of humus is exhausted, the soil lies lifeless, and the stores of available nitrogen and potash are in a worse depleted state than formerly.
The fault lies with the method. The phosphoric acid in the acid phosphate was needed. Profit from its use was legitimate, but the necessity of supplying organic matter became even greater than it would have been otherwise. Tens of thousands of our most successful farmers use heavy applications of acid phosphate, but they keep their soils in good physical condition by the use of manure or clover, and they apply potash and nitrogen when needed. The clover is assured by using lime wherever it is in too limited supply, and that is the case in most instances, regardless of the use of any kind of commercial fertilizer.
Basic Slag.—When iron ores contain much phosphorus, its extraction by use of lime gives a by-product in the making of steel that has agricultural value. The ores of the United States usually do not give a slag sufficiently rich in phosphorus to be valuable. Nearly all the basic slag used as a fertilizer is imported from Germany, and usually contains 17 to 18 per cent of phosphoric acid. The availability of the plant-food in this fertilizer has been the subject of much discussion. The chemist's test which is fair for acid phosphate is admittedly not fair when used for basic slag. Field tests, at experiment stations and on farms, are our best sources of knowledge. When the soil is slightly acid, each 1 per cent of phosphoric acid in the slag appears to be about as valuable as each 1 per cent of the available phosphoric acid in an acid phosphate. Some of the effectiveness may be due to the lime, although very little of it is in forms regarded as valuable for the correction of soil acidity. There is evidence that basic slag favors clover. It has not been found feasible to ship this material many hundreds of miles inland from the seaboard to compete with acid phosphate, but it is an excellent source of phosphoric acid for soils that are not rich in lime.
Muriate of Potash.—The mines of Stassfurt, Germany, contain an inexhaustible supply of potash in various compounds. Muriate of potash is prepared from the crude salts, and the commercial product on our markets has the appearance of a coarse and discolored salt. It is handled in large bags, and inclines to become moist by absorption of water from the air. It contains some common salt. The content of actual potash is about 50 per cent. The potash is readily available, but the loss from leaching out of the soil is very small. Muriate of potash is our cheapest source of potash, and should be used for all staple crops except tobacco, sugar beets, and, possibly, the potato. Tests even on heavy soils fail to show any injury to the quality of the potato, and on light soil the muriate may always be used.
Sulphate of Potash.—Some sulphate of potash is imported into this country. Its content of potash may vary 1 or 2 per cent below or above 50. Its physical condition favors mixing more than does the muriate. It usually costs several dollars a ton more than the muriate, and the fact that it is known to favor quality in tobacco, and is popularly supposed to do so in the potato, creates demand at the higher price. It is soluble in water, and quickly available. As a rule, it has no higher agricultural value than the muriate.
Kainit.—Unlike muriate and sulphate of potash, kainit is a crude product of the German mines, having received no treatment to remove impurities. It contains 12 to 13 per cent of potash, and is rated as a sulphate, but one third of it is common salt, and in effect upon quality it should be classed with muriate and not sulphate. Its low content of plant-food should confine its use to regions relatively near the seaboard. When shipped far inland, the price becomes too high to give a reasonably cheap pound of potash.
Wood-ashes.—Wood-ashes contain lime and potash, with a small percentage of phosphoric acid. The market price is above agricultural value, and any needed potash should be obtained from the German potash salts.
Other Fertilizers.—Manufacturers of commercial fertilizer make use of other materials, some of which, like manufactured nitrogen, are excellent, and others are low in quality and slow in action. The sources of plant-food that have been described form the great bulk of all fertilizers on the market, and from them may be selected all the materials a farmer needs to use on his land, either singly or home-mixed. In most instances the selection will embrace only four or five of these fertilizing materials.
Salt.—Salt is not a direct fertilizer, and its use is not to be advised unless it can be secured at a very low price per ton. Some soils have been made more productive by the application of 200 to 300 pounds per acre, and chiefly in case the salt was mixed well with the soil when the seed-bed was made. The practice of using salt as a top-dressing on wheat in the spring gives less effectiveness it is believed. Salt frees potash in the soil, and may have some practical effect upon soil moisture. As a soil amendment, salt has had more reputation than its performance justifies. If land is infertile, it is better, as a rule, to apply actual plant-food.
Coal-ashes.—There is no plant-food of value in coal-ashes. The physical condition of heavy soils is improved by an application, and their use may be quite profitable in this way if cost of application is small. When used as a mulch, ashes conserve moisture.