158 CHILDREN'S GARDENS 



makes a home and feeding ground for the countless 

 bacteria necessary in a fertile soil. It has some of 

 the purifying action of charcoal to keep the earth 

 sweet. The finer it is, and the more thoroughly it 

 is mixed, the more rapidly and completely does the 

 rock give up its minerals to the soil water. This 

 thorough mixture of humus gives the soil that crumbly 

 texture so much desired. 



Air of the soil supplies oxygen and nitrogen to 

 assist in the chemical changes, and for microscopic 

 life. As it moves about it ventilates the soil. 



Water of the soil keeps the humus decaying slowly, 

 and in a way that gives the best result for a fertile 

 soil. When this water falls as rain, it washes from 

 the air above, the carbon dioxide, ammonia and other 

 gases. There is more carbon dioxide in the soil-air 

 and the water takes it up. Pure water dissolves the 

 rock very slowly, but with the carbon dioxide and 

 humic acids, it dissolves much faster. The bacteria 

 are always busy making new compounds which the 

 water dissolves, when the plant roots can take them 

 in. The water moving about in the soil also carries 

 oxygen to be used, and moves air in and out of the 

 soil to help the ventilation. 



So it is important to have these all present and so 

 arranged that they can work constantly and freely. 



How does a soil get worn out? By constant taking 

 away of crops without replacing humus. As the 

 amount of humus lessens the soil holds less water, the 

 surface is washed away more easily, the number of 



