102 ELEMENTS OF AGRICULTURE 



supply of both water and organic matter, and so in- 

 crease its fertility. How they do this is explained in 

 succeeding paragraphs. 



100. How the Water Supply of Soils is Renewed. — 

 The supply of water in soils is renewed from time to 

 time by rains, much of the rain water being held in 

 the deep soil as free or ground water. From the deep 

 soil it rises into the surface soil as film moisture which 

 provides growing plants with a constant supply ._ The 

 more free water a soil holds the greater the store for 

 growing plants. If for any reason the free water of a 

 soil becomes exhausted the plants growing in the soil 

 wilt and die. Growing plants require a constant supply 

 of water, which they can obtain- only from the film 

 moisture of the soil. Bains are of ten weeks apart, and 

 unless water is held by the soil plants suffer. The soil 

 below the water table may be considered as a reservoir 

 which holds rain water, much as a sponge, and sup- 

 plies it as needed to growing plants. If there be a 

 leak in the reservoir the water supply is lost. The 

 depth of the soil and the number and size of the 

 soil particles in a 'measure determine the amount of 

 film moisture a soil is capable of holding. But in 

 order that a soil may contain film moisture water must 

 be first absorbed and held below as free water. If rain 

 water runs off the surface, the soil can contain but 

 little moisture. In bare fields this is often the case; 

 the rain water runs off the surface, cutting great gullies 

 and doing much damage to the land. On land covered 

 with growing plants or the litter of dead plants, rain 

 water is prevented from running off, and gradually 



