50 Practical Farming 



ences in the pressure of the heated and cooling air can be 

 easily traced in a shallow well. 



We have seen that the capacity of a soil 

 Proportion of to retain moisture varies generally in pro- 

 Rainfall portion to the percentage of clay it contains. 

 Soil Few soils do retain in the upper five feet of 

 the soil as much as twenty inches of water, 

 while some authorities claim that for the best results in 

 the crop the water content should be held up to fifty 

 inches or more. While the water-fall of a section may 

 annually amount to this much in inches, the percolation 

 and evaporation very largely reduce the percentage avail- 

 able to the crops. In practical agriculture, then, the con- 

 servation of the soil moisture becomes of the greatest 

 importance especially where the annual amount is below 

 the optimum. Even in regions of heavy rainfalls as in our 

 South Atlantic states a large part is carried off by surface 

 washing in the torrential rains of summer, another part 

 evaporates, and still another passes down into the subsoil 

 in the drainage, so that even with fifty inches of annual 

 rainfall it is one of the most difficult of problems to retain 

 for the use of plants as much as one-third of the rain- 

 fall. The importance, then, of using every means possible 

 to prevent the loss of water will become apparent to any 

 one. The practical means for accompHshing this will be 

 more fully treated in another lesson. 



We have seen that water standing too near the sur- 

 face is damaging to our cultivated crops since it makes 

 the soil cold and shuts out the oxygen of the air which 

 is so essential to the welfare of the plant roots. We have 

 also noted that the point in the soil where water lies per- 



