AGRICULTUKAL CHEMISTRY. 189 



stance as to its fertilizing effect. Can we, by the study of the com- 

 position of a crop, decide what manure is most likely to benefit it ? 

 or can we determine, from the composition of a manure, what crop it 

 is best adapted for ? The answer to these questions is, in many 

 cases, No ! In laying down the general principles which are to be 

 regarded in a rational theory of manuring, we have had frequent 

 occasion to make the truth of a proposition depend upon "other 

 things being equal." Now it happens, unfortunately for the sim- 

 plicity of our science, that "other things" are often in the highest 

 degree unequal and unlike, so that we must busy ourselves with the 

 slow work of induction from facts mostly yet to be extricated by toil- 

 some experiment from their present confusion, rather than incumber 

 theory and disgust practice by generalizing deductions that cannot 

 fail to be premature and erroneous. There are many cases in which 

 the effect of a fertilizer can be immediately connected with its com- 

 position. It not ^infrequently happens that pasture lands from which 

 the only matters agriculturally removed are the ingredients of cheese, 

 after long use, deteriorate, refuse to nourish dairy animals, and be- 

 come nearly worthless. The' use of bones or phosphatic manures 

 restores such fields to perfect pastusage; and the explanation afforded 

 by chemistry — viz: that all the phosphate of lime put in the milk as 

 a provision for the formation of the bones of a young animal is 

 permanently alienated from the soil in the exports of cheese, so 

 that exhaustion of this substance is caused, unless phosphates be ap- 

 plied — is entirely satisfactory. 



The leguminous plants, though the richest in nitrogen of all our 

 crops, do not by any means require nitrogenous manure to the extent 

 demanded by wheat, which removes from the soil but one-half as 

 much, or less, of this substance. The difference here is obviously 

 due to the fact that the leguminous plants have deeper roots, more 

 foliage, and a longer period of growth. 



Leguminous plants are rich in lime and sulphur, and hence are 

 often remarkably grateful for applications of gypsum. Fruit and 

 shade trees yield an ash largely consisting of carbonate of lime, and 

 their growth, especially on meager sandy soils, is often wonderfully 

 enhanced by the accident of some oyster shells or old mortar being 

 thrown on the ground over their roots. 



The grasses and grains contain a large amount of silica in their 

 stems and leaves; but the artificial use of soluble silicates of potash 

 and soda has rarely been attended with more benefit than that of the 

 corresponding chlorids, and for the reason that silica is so universally 

 distributed. 



Mr. Lawes, of England, found that on his farm wheat might be 

 grown for a dozen years or more in succession on the same field, and 

 give an average crop of 17 bushels per acre, without manure; while 

 a contiguous field, planted in turnips, in three years came to yield 

 scarcely anything. Mr. Lawes then found that, by the use of nitro- 

 genous manures, the wheat crop was at once doubled, while the 

 turnip crop was hardly affected; and, on the other hand, a mixture 

 of sulphate and soluble phosphate of lime (super-phosphate of lime) 



