IX.] GREEN MANURES 273 



Continent, and cowpeas in America being among the 

 plants most commonly employed. The practice has 

 three objects : — 



(i) The improvement of the texture of the land by 

 increasing the store of humus ; this is particularly 

 valuable on heavy clays and on the light sandy soils 

 at the other end of the scale. 



(2) The saving of the store of nitrates, which on 

 light warm soils form with great rapidity after harvest, 

 and which may then easily be washed away. If some 

 catch crop like mustard is sown immediately the stubbles 

 are clear, it will grow with great rapidity after the first 

 rain and will gather up these nitrates, converting them 

 into proteins, which become more slowly available on 

 the decay of the plant material. 



(3) For cleaning purposes ; when the land is in very 

 foul condition a good many weeds can be got rid of by 

 growing a smothering crop. 



On many soils green manuring may be extremely 

 valuable, especially where there is any shortage of 

 farmyard manure ; a green crop of mustard turned in, 

 especially if it had been previously manured with some 

 mixture of artificials, will have all the lasting beneficial 

 effects of a coat of dung. Of course the "seeds" crop 

 in the rotation has much the same effect, because of the 

 roots and stubble left behind, but it does not always 

 come round often enough in the rotation to keep the 

 land in condition. 



When vetches, lupins, or other leguminous crops are 

 grown, the land is also enriched by the nitrogen gathered 

 from the atmosphere by the bacteria living in the root 

 nodules, and large areas of land in Pomerania and East 

 Prussia have been brought under cultivation from the 

 state of barren sandy heath, by ploughing in lupins 



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