112 THE COMPOSITION OF CROPS IX 



business of the farmer. Give him these in sufficient abundance 

 and at a sufficiently cheap rate, and the rest would be easily 

 found. ]Slearly one-half (45 per cent.) of the ash of wheat, 

 and one-tenth of the ash of turnips, consist of phosphoric acid. 

 Potash, which may be placed next in importance, forms nearly 

 one-third of the ash of wheat, and one-third or upwards of the 

 ash of turnips. Lime, which indeed is in many respects excep- 

 tional in its importance, would rank next ; but, on account of its 

 abundance in nature, its pecuniary value is less than that of 

 magnesia and sulphuric acid. This last appears largely, — to the 

 extent of one-tenth or more in the ash of turnips, and to con- 

 siderable amount in that of potatoes. 



Xow, if the reader were hastily to suppose or infer from what 

 has just been said with respect to the relative composition of 

 the ash of wheat, and of the ash of turnips, that a " crop " of 

 wheat requires more phosphoric acid than a " crop" of turnips, 

 and that each requires from the soil about the same amount 

 of potash, he would make an enormous mistake. Yet this 

 absurd mistake is often made, and made too sometimes by those 

 from whom more accuracy of thought might be expected. The 

 truth is that a crop of turnips requires from an acre of land 

 nearly twice as much phosphoric acid, and five times as much 

 potash as a crop of wheat. 



When the chemist proceeds to find by analysis the composi- 

 tion of a plant or vegetable substance, one part of the process 

 consists in burning a given weight of it ; and, having done so, 

 he finds that the ash (not to speak of the water or the combus- 

 tible matter, amounts to only 1, 2, or 3 per cent, as the case 

 may be, of the whole weight ; or, if the substance contains much 

 water, as in the case of turnips, the ash may be rather less 

 than 1 per cent., say 0'6 to 0*8 per cent. But whatever the 

 relative weight of the ash may be, he takes say 100 parts — call 

 them grains, grammes, or ounces — of it, and ascertains how 

 much, how many parts in 100, of each of the nine elementary 

 substances above referred to, the ash contains. He states the 

 results in percentages in the form of a column of figures — 

 intes^ers and decimals, the sum of which is of course 100 or 

 nearly so. ISTow, by looking at this column of figures without 

 considerinsj the relation which the whole ash bore to the whole 

 weight or 100 parts of the normal substance analysed; and 

 without considering, it may be, the relative value and importance 

 of the several elementary substances named in it, many farmers, 

 and not a few others, are liable to be greatly misled, and to form, 

 and carry away, ideas as to the composition of crops which are 

 quite erroneous. They are liable to be still further misled, and to 

 form opinions still more erroneous if, without such consideration, 

 they proceed to compare the ash of one plant or crop with that 



