248 SCIENCE OF GARDENING. Part II. 



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

 when it can be procured. 



1187. The saline comjwunds 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 suffi- 

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



1 188. 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 alkalies is to give solu- 

 bility 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. 



1 189. The mineral alkali or soda is found in the ashes of sea-weed, and may be procured 

 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 oxvgen 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 decomposition 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 sometimes a useful 

 manure, and it is probable that its efficacy depends upon many combined causes. Some 

 persons have argued against the employment of salt ; because when used in large quan- 

 tities, 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 recom- 

 mends 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 exuvias 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 effects 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 ve- 

 getable manure. 



1190. Other compounds. Besides these compounds of the alkaline earths and alkalies, 

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

 are nitre, or the nitrous acid combined with potassa. Sir Kenelm Digby states, that he 



nade barley grow very luxuriantly by watering it with a very weak solution of nitre ; but 

 he is too speculative a writer to awaken confidence in his results. This substance consists 

 of one proportion of azote, six of oxygen, and one of potassium ; and it is not unlikely 

 that it may furnish azote to form albumen or gluten in those plants that contain them ; 

 but the nitrous salts are too valuable for other purposes to be used as manures. Dr. Home 

 states, that sulphate of potassa, which was just now mentioned as found in the ashes of some 

 peats, is a useful manure. But Naismith (Elements of Agriculture, p. 78.) questions his 

 results ; and quotes experiments hostile to his opinion, and, as he conceives, unfavorable 

 to the efficacy of any species of saline manure. Much of the discordance of the evidence 

 relating to the efficacy of saline substances depends upon the circumstance of their having 

 been used in different proportions, and, in general, in quantities much too large. 



1191. Solutions of saline substances were used twice a week, in the quantity of two 

 ounces, on spots of grass and corn, sufficiently remote from each other to prevent any in- 

 terference of results. The substances tried were super-carbonate, sulphate, acetate, nitrate, 

 and muriate of potassa ; sulphate of soda ; sulphate, nitrate, muriate, and carbonate of am- 

 monia. It was found, that in all cases when the quantity of the salt equalled one thirtieth 



