THE SOIL AND THE CROP 41 



Potash salts may be bought on the market, but like 

 other commercial fertilizers they have not yet come 

 into general use. 



Chemical Effect of Commercial Fertilisers on Soil. 

 There is still another use to which commercial fer- 

 tilizers, like lime and land plaster, are put. They are 

 used not so much because they are themselves plant 

 foods, but because of the chemical effect which they 

 have upon the soil. Your attention has already been 

 called to the fact that plants sometimes starve with an 

 abundance of food near at hand, but in a form in 

 which they cannot use it for food locked up, as it 

 were, like bread and butter in a pantry. If a boy were 

 starving because his food was "locked up" he would 

 want the key. No boy will die of starvation with a 

 well-filled cupboard, unlocked, in the house. Neither 

 will plants starve when suitable food is obtainable. 

 Now, lime and land plaster are two keys that unlock 

 other plant foods in the soil and change them into a 

 form in which the plants can use them. It is, princi- 

 pally, for this reason that they are used. 



Summary. To sum up what has already been said : 



THE BOY'S CHOKES. 



