122 THE COMPOSITION OF CEOPS IN 



more r-dmarkable demand for sulphuric acid. We at once see 

 the reason why we cannot obtain what we regard as a normal 

 produce of these root crops without the direct and immediate 

 application of manure. On glancing at the amount of fertilising 

 matter afforded by ten tons of well-rotten dung, we find that the 

 quantities of the several ingredients correspond in a certain 

 degree with the requirements of the root crops, phosphoric acid 

 and magnesia being, however, considerably in excess. Now, we 

 find that in well-rotten dung nearly one-half of the nitrogen — 

 something less than one-half of the phosphoric acid, the sul- 

 phuric acid, and the magnesia, and nearly all the potash — are in 

 a soluble condition — that is, ready for the immediate use on the 

 crop to which the dung is applied. Twenty tons of dung per 

 acre would then supply the immediate wants of the root crop, 

 and leave, roughly speaking, something more than one-half of its 

 fertilising matter for the subsequent crops of the rotation. 



But, without noticing any of the numerous other points which 

 these figures suggest, we conclude with a few remarks on the 

 main question that arises, namely, Does the composition of a 

 particular crop indicate the special manure which that crop 

 requires ? Not necessarily in so far as nitrogen is concerned. 

 But then there are means by which a certain amount of that 

 element of manure (in its combined form) can be carried to plants 

 ctb exira, so to speak, that is, beyond what may exist in the soil 

 or in the manure supplied to it. Of these means it seems 

 evident that some species of plants have much greater powers 

 of availing themselves than others. Such power of gathering 

 compounds of nitrogen seem to belong to plants having a large 

 extent of leaf surface, such as turnips and clover, and having 

 consequently large powers of transpiration — that is, of passing a 

 great amount of water through their substance in a given time. 

 A sunflower having thirty-nine square feet of leaf surface has been 

 found to exhale twenty ounces of water in a day. Every one must 

 have observed the immense power which the turnip plant has in 

 this respect. The cereal plants have comparatively a limited 

 extent of foliage. A turnip plant will probably, in a given time, 

 pass ten times as much water through its tissues as several plants 

 of wheat. The water, having performed its office of carrying 

 food into the plant, is given off. Every gallon of water so 

 passing through the tissues will carry some amount of combined 

 nitrogen. That nitrogen may in part have existed in, and come 

 from, the soil or manure; but may also have come partly from 

 the atmosphere in the shape of ammonia or nitrites, either 

 brought down by rain, or absorbed from the moist air in its con- 

 tinual passage through the heated particles of the loose, porous 

 soil around the plant. Its growth is greatly promoted by having 

 the soil around it in that open and porous condition. Its large 



