CHAPTER XIV 



Humus 



In the last chapter we studied how nature takes care that 

 the soil shall remain fertile, if only man will not interfere. 

 She restores chemicals to the ground by means of the rot- 

 ting of the fallen plants. But at the same time she is 

 keeping up the supply of what we have already mentioned 

 as humus, which is decaying animal or vegetable matter. 

 Every animal, insect, or plant, that falls and lies in the field, 

 adds humus to the soil. 



Humus is important for several reasons. We know already 



that it enables the soil to hold moisture, and that it improves 



clay soils by making them lighter and 



warmer. We can easily see that it 



contains chemicals on which plants 



Fig. 57. — The finest can grow. But beyond this it im- 



neediepoint, magnified, proves the soil by bringing about two 



with a speck of dust above , . . . . , . , „ 



and bacteria in front, kinds of changes in the soil. One IS 

 These tiny bacteria are our a chemical action, bv which a carbon 



helpers in all humous soils. , , , , .' . , , , , , 



compound (carbon dioxid, the one that 

 supplies carbon from the air) is set free to act in its turn on 

 the chemicals in the soil. The other is a bacterial action, by 

 which bacteria (those tiniest of living things, of which there 

 are at least thirty million in every ounce of soil) are en- 

 couraged to live on the decaying humus. These bacteria 

 take the nitrogen in the soil, and make it ready for plant use. 

 Thus for good gardening humus is needed. It is true that 



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