32 THE FOOD OF PLANTS CHAP. 



growing there grass, trees, bracken, gorse, and 

 numerous weeds will suck up through their roots 

 the mineral food they require, and which exists along 

 with the sand and clay of the soil. And when they 

 die, they will decay upon the soil, and replace the 

 mineral food they have taken from it, ready to be 

 used again by fresh plants. The soil will therefore 

 remain rich in the food the plant requires, whether 

 it be mineral or decaying vegetable matter. 



Let us now suppose this land to be brought under 

 cultivation by the farmer. 



He grows crops upon it of wheat, grass, turnips, 

 and other things. When these are ripe, he does not 

 leave them to decay on the land, but takes a large 

 part of them away. In the case of wheat he takes 

 away the straw and ear, but leaves the roots. In 

 the case of potatoes he takes away the tubers, but 

 burns the leaves and stems on the field and re- 

 turns the ashes left by them to the soil. If he is 

 using land for grazing, the cattle eat the grass and 

 return' a great portion of the food to the soil as 

 dung, but some they keep for building up their own 

 flesh and bone. Thus we find that when soil is 

 being cultivated, a certain amount of the plant food 

 it contains is removed every year, and consequently 

 the soil may get temporarily exhausted. It no longer 

 contains sufficient food to grow good crops. 



