1 12. 



its wmt< r f o the plants; the clay holds on to its hygroscopic 

 water and gives but a small amount to the plfntr. 



The hygroscopic capacity of a soil depends on the texture of tie 

 particles find, the content of or^nnic matter. tince hygroscppic moisture 

 is a. function of the surface exposed, it results th" c the larger the 

 surface nrea exposed by Uie soil particles, the greater the hygroscopic 

 capacity of the soil. Reference to the t??ble on pa-.;e 110 shows fine- 

 textured soils or clay ^oils to have the greatest surface . . , nd 

 these hold the -root hygroscopic i/ioiature. Sand soil:*., with a relatively 

 small eurf.-ctt -G-, hold a sir^all -mount of tiiis forr;,: of water. Vhis fact 

 is illustrated bj te foliovvi.x; w^ble: 



hygrosc. water 

 at 21 U C. 



Very fine sand 



-.ck 



7-3 

 16.^' 



46.0 



the 



The *bove soils were pure separates derived by mechanical analy- 

 sis. Vhe, uras s :rve to show the direct relation between; 



1) The surface area exbibitc-:! by Uie soil particles and 

 j;rosco'.;aO i.oioture retained. 



2) Jly^roecopic moisture a/id temperature 



3) Hygroscopic moisture and humidity of th-? atmosphere. 



roscopic ..oisture decreases with increnye in temperature. 

 It varies directly P.S the relnivu liuinidity of t)ie acr.iOBp}ior -vith which 

 soil is in contact. Consequently, in the fiir-dried condition, while 



ys r oor;.<: moifcture, it suiuom exliilite its maximum 



hygroacu . pacity. Under average conditions of humidity, a light sand 



may retain fro 0.5 -o 1> , a silt loam 2-4/T and a clay u-1^/ . Tiiis is 

 course uiiJ-vailablo I'or plant a. 



Capillary water is used for plant food. It ia Uic film or capil- 

 lary noifctu supports planty. The roots of ordinary crops are 



\-.c tiit; moisture needed by thros cling their way between the 

 soil particleo, v:here Diey may come in intimate contact with t,heoe 

 moisture fil s and r>;sorb the needed uppl^ of wciter, witli bwin^ exclud- 

 ed fro.u tlie r-.ir aupply wliicii promo tee tiicir growth. Fox, in the capil- 

 lari/ly moiut yoil, tlie water i; ret.uivicti chiefly in the very tjnaall spa- 

 ce a, -- . U large spaces occupied by air. 



.axinui:. diauunt of capill;iry completely fill the pore spaces. 

 T}ie pore/ : .ipace may range from y/^ in a clean s? nd to 60-70>. in a well 

 granulated clay, and 80-90/ in a muck aoii . The following table gives 



