134 THE CULTURE OF FARM CROPS. 



overplus either runs off from the surface or sinks into the 

 subsoil or escapes through the drains. In drained land this 

 power of retaining or holding water in the largest quantity 

 is of the highest advantage, for, while the injurious excess 

 is carried off and removed a large supply remains for. the 

 sustenance of the crops. The difference between soils in this 

 respect is quite large. Thus 100 Ibs. of the following named 

 soils will begin to part with water, if it be 



A pure sand when it has absorbed 25 pounds. 



A limestone sand when it has absorbed 29 



A sandy loam soil when it has absorbed 40 



A limestone clay loam when it has absorbed... 45 



A pure clay loam when it has absorbed 50 



A pure clay when it has absorbed 70 



A dry peat when it has absorbed 180 



The best arable soils are therefore able to hold within 

 their interstices from 40 to 70 per cent, of their weight of 

 water; while the best grass lands will easily hold even 

 more than their own weight. As grass thrives all the 

 better, the larger the supply of water may be, the most 

 retentive soils are therefore better used for meadows than 

 for grain crops. 



In the climate of America this ability to hold water dur- 

 ing the frequent long drouths of the growing season, gives 

 a high value to those soils which possess it in the highest 

 degree ; and also has a noteworthy bearing upon the ques- 

 tion of drainage ; for where a soil is able to, and will, retain 

 more water in its pores, without parting with it by percola- 

 tion, it is all the more necessary and the least injurious to 

 supply the land with an escape for the surplus. For the 

 more water that is held by the soil, the less air can be con- 

 tained in it and air is quite as useful for the growth of plants 

 as water is, for while water is the vehicle by which nutri- 

 ment is conveyed into plants, it has been shown that a large 

 quantity of food is derived by plants either directly or in- 

 directly from the atmosphere, which requires this vehicle 

 for its conveyance. 



A fact of much interest in this connection is that those 

 soils which absorb the most water resist evaporation for the 

 longest period. The power of absorption is due to the sur- 



