1922.] 



( i KKEN M aM HI.NC. 



(1) Effect on the Si(}>pl<j of Minerals. — Farmyard manure 

 adds potash and some phosphates to the soil, and these being 

 derived partly from feeding stuffs imported from outside the 

 farm, are a gain to the soil. A green manure, on the other 

 hand, only returns to the soil those mineral substances which 

 it first took from it, so that before it can be equal in its effects 

 to farmyard manure in this respect, it must be supplemented 

 by mineral manures; there is, however, no difficulty about this, 

 and indeed, it always pays to grow the green crops with 

 mineral manures in order to get as large a bulk of green stuff 

 as possible. Further, although the green crop only returns to 

 the soil those minerals it took from it, a deep-rooted green 

 manure crop, by opening up the subsoil, will not only bring up 

 from the subsoil mineral substances which on its decomposition 

 will be added to the surface soil, but also, the ensuing main 

 crop will itself have a better chance of penetrating into the 

 subsoil with its roots, and tapping the mineral resources there. 

 There is also some evidence that a green manure used in con- 

 junction with raw mineral phosphate renders the phosphoric 

 acid of the latter more readily available to the succeeding crop. 



(2) Effect on the Supply of Nitrogen. — Farmyard manure 

 similarly adds to the soil large quantities of nitrogen. Much 

 of this nitrogen has been purchased, either in the form of 

 cake, or as manures used for the growth of roots or forage crops. 

 The nitrogen in green manures, on the other hand, may be 

 wholly, or largely a clear gain. Thus a leguminous crop collects 

 from the atmosphere large amounts of nitrogen, which are 

 added to the soil, when the crop is turned in. An average crop 

 of vetches may easily add to the soil as much nitrogen as 

 10 tons of stable manure to the acre. Even a non-leguminous 

 crop, though incapable of fixing atmospheric nitrogen, saves 

 nitrogen for use by the subsequent main crop, by absorbing 

 from the soil nitrates which would otherwise be lost in the 

 drainage water. Green manures therefore may be regarded 

 as comparing not unfavourably with farmyard manure as a 

 means of adding nitrogen to the soil. The relative advantages 

 of green manures and of fallowing on stiff soils are not definitely 

 known. Fallowing is known to have a very beneficial effect on 

 the biological processes of nitrification and nitrogen fixation, 

 both of which are depressed by a growing crop, hut on the other 

 hand, the accumulated nitrate of a bare fallow may be lost by 

 leaching in the autumn. Probably the best plan on soils which 

 arc known to benefit by a bare summer fallow is that already 



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