136 



Physics of the Soil. 



just at the under limit for most vigorous growth while the 

 upper limit is given in the next table. 



Showing upper and lower limits of best amount of soil moist- 

 ure for one type of soil. 



It will be seen from this table that, to bring the surface 

 four feet of soil from the lower limit of the best productive 

 stage of water content to the upper limit, requires an ap- 

 plication of 23.54 pounds per square foot, or a depth of 

 rainfall equal to 4.527 inches. This therefore represents 

 the available moisture in this type of soil and is about one- 

 third of its 'full capillary capacity. 



162. Kinds of Soil Which Yield Their Moisture to Crops 

 Most Completely. When the roots of plants come to draw 

 upon the supply of soil-water those soils yield their mois- 

 ture to the plant most completely whose grains have the 

 largest diameter or, more precisely, which have the smallest 

 internal surface to which the moisture may adhere and over 

 which it is spread. 



Referring to the table in (161), giving the per cents, of 

 moisture which were too low to permit the plants to supply 

 their needs, it will be seen that under the corn the water in 

 the sand fyad been drawn down to about 4 per cent. ; in the 

 surface loam to 7 and 8 per cent. ; while in the intervening 

 more clayey portion only to 11 and 12 per cent. The fun- 

 damental truth which should be grasped here is that all 

 these soils are equally dry so far as the needs of the corn 

 crop are concerned, and one of the reasons why they are 



