Book III. SPECIES OF MINERAL MANURES. 341 



2234. Vitriolic impregnations in soils where there is no calcareous matter are injurious ; 

 but it is probably in consequence of their supplying an excess of ferruginous matter to 

 the sap. Oxide of iron, in small quantities, forms a useful part of soils ; it is found 

 in the ashes of plants, and probably is hurtful only in its acid combinations. The ashes 

 of all peats do not afford gypsum. In general, when a recent peat-ash emits a strong 

 smell, resembling that of rotten eggs when acted upon by vinegar, it will furnish 

 gypsum. 



2235. Phosphate of lime is a combination of phosphoric acid and lime, one proportion 

 of each. It is a compound insoluble in pure water, but soluble in water containing any 

 acid matter. It forms the greatest part of calcined bones. It exists in most excremen- 

 titious substances, and is found both in the straw and grain of wheat, barley, oats, and 

 rye, and likewise in beans, peas, and tares. It exists in some places in these islands 

 native, but only in very small quantities. Phosphate of lime is generally conveyed to 

 the land in the composition of other manure, and it is probably necessary to corn crops 

 and other white crops. 



2236. Bone-ashes calcined and ground to powder will probably be found useful on 

 arable lands containing much vegetable matter, and may perhaps enable soft peats to 

 produce wheat ; but the powdered bone in an uncalcined state is much to be preferred in 

 all cases when it can be procured. 



2237. The saline compounds of magnesia will require very little discussion as to their 

 uses as manures. In combination with sulphuric acid, magnesia forms a soluble salt. 

 This substance, it is stated by some enquirers, has been found of use as a manure ; but it 

 is not found in nature in sufficient abundance, nor is it capable of being made artificially 

 sufficiently cheap to be of useful application in the common course of husbandry. 



2238. Wood-ashes consist principally of the vegetable alkali united to carbonic acid; 

 and as this alkali is found in almost all plants, it is not difficult to conceive that it may 

 form an essential part of their organs. The general tendency of the alkalines is to give 

 solubility to vegetable matters ; and in this way they may render carbonaceous and other 

 substances capable of being taken up by the tubes in the radical fibres of plants. The 

 vegetable alkali likewise has a strong attraction for water, and even in small quantities 

 may tend to give a due degree of moisture to the soil, or to other manures ; though this 

 operation, from the small quantities used or existing in the soil, can be only of a second- 

 ary kind. 



2239. The mineral alkali or soda is found in the ashes of sea-weed, and may be pro- 

 cured by certain chemical agencies from common salt. Common salt consists of the 

 metal named sodium, combined with chlorine ; and pure soda consists of the same metal 

 united to oxygen. When water is present, which can afford oxygen to the sodium, soda 

 may be obtained in several modes from salt. The same reasoning will apply to the 

 operation of the pure mineral alkali, or the carbonated alkali, as to that of the vegetable 

 alkali ; and when common salt acts as a manure, it is probably by entering into the 

 composition of the plant in the same manner as gypsum, phosphate of lime, and the 

 alkalies. Sir John Pringle has stated, that salt in small quantities assists the decomposi- 

 tion of animal and vegetable matter. This circumstance may render, it useful in certain 

 soils. Common salt, likewise, is offensive to insects. In small quantities it is some- 

 times a useful manure, and it is probable that its efficacy depends upon many com- 

 bined causes. Some persons have argued against the employment of salt ; because when 

 used in large quantities, it either does no good, or renders the ground sterile ; but this is 

 a very unfair mode of reasoning. That salt in large quantities rendered land barren, 

 was known long before any records of agricultural science existed. We read in the 

 Scriptures, that Abimelech took the city of Shechem, " and beat down the city, and 

 sowed it with salt;" that the soil might be for ever unfruitful. Virgil reprobates a salt 

 soil ; and Pliny, though he recommends giving salt to cattle, yet affirms, that when 

 strewed over land it renders it barren. But these are not arguments against a proper 

 application of it. Refuse salt in Cornwall, which, however, likewise contains some of 

 the oil and exuvise of fish, has long been known as an admirable manure. And the 

 Cheshire farmers contend for the benefit of the peculiar produce of their county. It is 

 not unlikely, that the same causes influence the efl'ects of salt, as those which act in 

 modifying the operation of gypsum. Most lands in this island, particularly those near 

 the sea, probably contain a sufficient quantity of salt for all the purposes of vegetation ; 

 and in such cases the supply of it to the soil will not only be useless, but may be 

 injurious. In great storms the spray of the sea has been carried more than fifty miles 

 from the shore ; so that from this source salt must be often supplied to the soil. Salt is 

 found in almost all sandstone rocks, and it must exist in the soil derived from these 

 rocks. It is a constituent likewise of almost every kind of animal and vegetable 

 manure. 



2240. Other compounds. Besides these compounds of the alkaline earths and alkalies, 

 many others have been recommended for the purposes of increasing vegetation ; such 



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