vii THE NITROGEN REQUIRED BY PLANTS 61 



dissolving a little of each in a little cold water), and 

 are therefore very easily washed out of the soil. 



For this reason they are often scattered over the 

 soil after the wheat has begun to grow, so that they 

 may be absorbed at once by the roots and not allowed 

 to wash into the drains* 



We can, however, enrich a soil with nitrogen 

 suitable for plant food in another way. 



We have learnt that leguminous plants can take 

 up the nitrogen from the air. 



If, then, we sow a crop of lupins, and after they 

 have grown to maturity plough them in, we shall 

 add to the store of nitrogen in the soil the decaying 

 lupins yielding up to the next crop the nitrogen 

 they obtained from the air. Or we may encourage 

 the growth of clover by the application of lime or slag 

 phosphate powder, and the root residue from the 

 clover will enrich the soil for succeeding crops. 



Conclusion 



Let us now sum up what we know of the food of 

 plants. 



We have learnt of what substances a plant is 

 composed. 



We have found that it contains water, charcoal, 

 nitrogen, and ash. 



