478 EATING OFF COMPARED WITH GREEN MANURING. 



rape-dust, which in ordinary seasons is not so Ukely to suffer loss 1 

 1 offer you the following -explanation: — 



If you raise your turnip crop by the aid of the bones or rape-dust eJone 

 you add to the soil what, in most cases, may be sufficient to supply near- 

 ly all the wants of that crop^ut you do not add ail which the succeed- 

 ing crops of corn and clover require. Hence if these crops are to be 

 grown continuously, and for a length of time, some other kind of ma- 

 nure must be added — in which those necessary substances or kinds 

 of food are present which the bones and rape-dust cannot supply. 

 Farm-yard manure contains them all. This is within the reach of 

 every farmer. It is, in fact, his natural resource in every such diffi- 

 culty. He has tried it upon his clover crop in the circumstances we 

 are considering, and has necessarily found it to answer. 



Thus to explain the results at which he has arrived in this special 

 case, chemical theory only refers the practical man to the general prin- 

 cipls upon which all scientific manuring depends — that he must add to 

 the soil sufficient supplies of every thing he carries off in his crops — and, 

 therefore, without some such dressing as he actually applies to his 

 clover crop, he could not long continue to grow good crops of any kind 

 upon his land, if he raise his turnips with bones or rape-dust only. 



Ii might, I think, be worthy of trial, whether the use of the fer- 

 mented dung for the turnips, and of the rape-dust for top-dressing the 

 after-crops, would not, in the entire rotation, yield a larger and more 

 remunerating return. 



§ 19. Of eating off witn, sheep. 



The practical advantages derived from eating off turnips and clover 

 crops with sheep are mainly of two kinds. Light lands are trodden 

 down and solidified, and they are at the same time equably and more 

 or less richly manured. With this latter effect, that of manuring, 

 some interesting practical facts and theoretical considerations are 

 connected. Thus — 



1°. In the preceding lecture (p. 419) I mentioned to you that in some 

 parts of Germany, spurry, among other plants, is extensively grown, 

 and with much profit, for ploughing in as a green manure. Now it is 

 mentioned that the crops of rye which follow a crop of spuriy are some- 

 times quite as great when it has been eaten off with sheep or cattle as 

 when it has been ploughed in (Von Voght, Uber Manche Vortheile 

 der gruner dungung.) 



2°. In accordance with this statement is the opinion of many skil- 

 ful practical men among ourselves, that a crop of clover or of tares 

 will cause a larger after-growth of corn, if it be eaten off with sheep, 

 than if it be ploughed in in the green state. 



The correctness of these practical observations appears from a 

 brief consideration of one of those interesting theoretical questions 

 we have recently been discussing. 



When a crop is eaten off by full-growm animals, it returns again to 

 the soil, deprived of a portion of its carbon only (p. 473.) The manure 

 contains all the nitrogen and saline matter of the green vegetables, and 

 in a state in which they are more immediately available to the uses of 

 the young plant. Thus far, then, we can understand that in certain 



