192 



SOILS. 



mineral matters, including even those which, like quartz, appear 

 to be most insoluble and refractory (see chapt. 3). The water 

 of the soil is thus enabled to convey to the roots of plants, in 

 solution, all kinds of plant food contained in the soil. It 

 should be noted that distilled (hence also rain-) water is a 

 more powerful solvent, e. g., of glass, than ordinary waters 

 containing mineral matter, and even free acids. 



Practically, plants take up all their water supply from the soil 

 in the liquid form ; and hence the soil-conditions with respect to 

 this supply are of the most vital importance to plant growth. 

 The most abundant supply of mineral plant food may be wholly 

 useless, unless the physical conditions of adequate soil-mois- 

 ture, access of air, and warmth, are fulfilled at the same time. 

 On the other hand, comparatively few plants are adapted to 

 healthy growth in soils saturated with water, or in water 

 itself; and but few among these are of special interest from 

 the agricultural standpoint. 



Water-requirements of Gro-^-in^ Plants. The amount of 

 water contained in any plant at one time, however large, is hut 

 a small proportion of what is necessary to carry it through its 

 full development. When we measure the amount of water 

 actually evaporated through the plant in the course of its nor- 

 mal growth, we find it to be several hundred times the quantity 

 of dry vegetable substance produced; varying according to the 

 extent and structure of the leaf-surface, the number and size 

 of the breathing pores (stomata) of the leaves, and the 

 climatic conditions (including specially the duration of active 

 vegetation, and temperature during the same), from 22^ to 

 as much as <>!_> times the weight of the mature, dry plant. 



The following are extreme figures f. >r \\ater consumption of 

 different plants as reported by different observers, viz., Lawes 

 and Gilbert in Kngland. llellriegel in northern (iermany, 

 \Vollny in Southern (iermany (Munich), and King in Wis- 

 consin: Wheat, 225 to 350,; barley. jf>j to 774; oats, 402 to 

 665; red clover. 24*) to 453; peas. 235 to 447; mustard and 

 rape, 845 to <ji2 rcspccti\ ely ; the latter figure being the 

 maximum thus far reported. The highest figures given are 

 throughout very nearly those of Wollny, working in the very 

 rainy climate of Munich. 



from I'lants in nifTcrcnt Cli unite*. Tt might 



