THE SOIL 313 



Fine clay-loam soils contain more space for air than 

 coarse sandy soils because they do not pack so closely, 

 the particles being light. But the air spaces in sandy soils 

 are larger than in finer soils, and this allows the air to 

 move about so freely in coarse, sandy soils that too much 

 water is lost by evaporation. Thus we see that too 

 much air in the soil is not good. Coarse soil is usually 

 too well aerated or aired and also too well drained. 



There are some marsh plants that can grow in standing 

 water. Rice is one of them, but the common farm crops 

 could not thrive under such conditions. Even rice re- 

 quires some air in the soil, the same as water lilies and 

 submerged seaweeds, but these plants are able to get air 

 from the water. Ordinary water has a large quantity 

 of air dissolved in it. 



217. Humus. Humus is the decaying plant and 

 animal matter of the soil. It is composed of the roots 

 and stems of dead plants and of animals which died in 

 and on the soil. Humus is organic matter and gives most 

 soils a dark color. It is necessary for the growth of good 

 crops, but the common seed-plants cannot live directly on 

 humus. Plants can take food from the soil only in soluble 

 form; they cannot absorb humus, but they can take up 

 the soluble compounds which come from decaying humus. 

 In order to have plant food in sufficient quantity in or- 

 dinary field conditions, it is necessary to have humus de- 

 caying continuously. If the humus decays too rapidly, 

 the plants cannot use all of it and part will be wasted. 

 If the humus decays too slowly, there will not be sufficient 

 food for the growing plants and the crop will be small. 

 Plants may be grown in moist sand if all of the food 

 elements are supplied, but this is a difficult process. 



Humus serves several other good purposes besides 



