FERTILE SOILS 



91 



particles, prying open their crevices and pushing the 

 pieces apart so that the agents of disintegration can more 

 readily attack them. By their decay plants provide the 

 humus so necessary for making soil fertility. 



Animals like moles and gophers plow their holes through 

 the soil, mixing up the particles and making the soil porous, 

 so that the water can 

 readily get in to aid in 

 breaking up and decom- 

 posing the soil particles. 

 These holes also provi'de 

 openings through which 

 plant roots and soil or- 

 ganisms can obtain the 

 oxygen and dissolved 

 food they need. Ants 

 each year move vast 

 quantities of fine mate- 

 rial to the surface, and 

 in some places change the surface soil in a few years. 



Angleworms, the most important animal soil builders, 

 channel the soil with their burrows, thus providing ready- 

 made openings for the growing roots and by increasing 

 the porosity of the soil aid in its ventilation and drainage. 

 They swallow the soil as they make their burrows, in 

 order to get the decaying vegetable matter for food, and 

 they grind it fine as it passes through their bodies. Every 

 year they bring to the surface great quantities of this 

 finely ground soil mixed with the undigested vegetable 

 matter. Darwin estimated that the angleworms in Eng- 

 lish soil deposited one fifth of an inch of these castings 

 each year over some parts of the surface. This is the 

 finest kind of fertilizer. It is a common saying that the 

 more angleworms the better the soil. 



ANTHILL. 



This soil has been brought from below and 

 piled up by the ants. 



