b. Manufactured Manures
Dissolved Bones.—These are bones treated with oil of vitriol, as in superphosphate manufacture. By this treatment bones become much more readily available, and are used to a considerable extent, more especially for root crops. Their composition varies with the method of manufacture and the extent to which they are dissolved. Speaking generally, they will have from 11 to 19% of soluble phosphate, with 20 to 24% of insoluble phosphates, and if pure should contain 3% of nitrogen. When mixed with superphosphate in varying amount, or if made with steamed and not raw bone, they are generally known under the indefinite name of “bone manure.”
Compound Manures.—To this class belong the manures of every description which it is the aim of the artificial manure manufacturer to compound for particular purposes or to suit particular soils or crops. The base of all these is, as a rule, mineral superphosphate or else dissolved bones, or the two together, and with these are mixed numerous different manurial substances calculated to supply definite amounts of nitrogen, potash, &c. Such manures, the trade in which is a very large one, are variously known as “corn manure,” “turnip manure,” “grass manure” and the like, and much care is bestowed on their compounding and on their preparation in good condition to allow of their ready distribution over the land.
IV.—Potash Manures
These, with few exceptions, are natural products from the potash mines of Stassfurt (Prussia). Until the discovery of these deposits, in 1861, the use of potash as a fertilizing constituent was very limited, being confined practically to the employment of wood ashes. At the present time a small quantity of potash salts—principally carbonate of potash—is obtained from sugar refinery and other manufacturing processes, but the great bulk of the potash supply comes from the German mines. In these the different natural salts occur in different layers and in conjunction with layers of rock-salt, carbonate of lime and other minerals, from which they have to be separated out and undergo subsequently a partial purification by re-crystallization.
The principal potash salts used in agriculture are—(1) sulphate of potash, which is about 90% pure; (2) kainit, an impure form of sulphate of potash, and containing much common salt and magnesia salts, and giving about 12% of potash (K2O); (3) muriate of potash, which is used to a great extent in agriculture, and contains 75 to 90% of muriate of potash; and (4) potash manure salts, a mixture of different salts and containing from 20 to 30% of potash.
Potash is much esteemed in agriculture, more especially on light land (which is frequently deficient in it) and on peaty soils, and for use with root crops and potatoes in particular. For fruit and vegetable growing and for flowers potash manures are in constant request. Clay land, as a rule, is not benefited by their use, these soils containing generally an abundance of potash. Along with basic slag, potash salts have been frequently used for grass on light land with advantage.
V.—Miscellaneous Manures
There are, in addition to the foregoing, certain materials which in a limited sense only can be called “manures,” but the influences of which are mostly seen in the mechanical and physical improvements which they effect in soil. Such are salt, and also lime in its different forms.
Salt.—The action of salt in liberating potash from the soil has been explained. As a manure it is sometimes used along with nitrate of soda as a top-dressing for corn crops, in the belief that it stiffens the straw. For root crops also, and mangels in particular, it is employed; also for cabbage and other vegetables.