45 



whils th© rate of diffusion o« the less diffusible saj.t is 

 di«tinetly diminished. The relative diffuaibiiity is axso 

 sometimes affected by the strength of th» eolutxon employed."} 



While phosphoric acid is not shown in th«» tabJla, it 

 probably has a low rat© of diffUEibixity. 



Tho abundance of the wat€»r greatly aff(»tt8 diffuelon. 

 In water cultures, proportions of piant food nutriante 

 occur far below the amounts required, if they were only 

 taken up by the plant in the proportions occurring in the 

 solution. In sandy soils, therefore, the ^.iffusioa is 

 better than in clayey soils on account of the greater 

 thickness of the water film. The hindering effect of 

 adhesion would also be relatively less in sandy soils. 

 But it is found th&t in heavy sOils, evan, the nitrates are 

 nearly completely takcfn up. Kins (The Soil, 117) states 

 that at the time the erops are growing, the roots remove the 

 nitrates, whetl€ th'ss© roots are found, so thoroughly that 

 the measured amounts at any on© place is small, the plants 

 tending to pick it up as rapidly as it may be forMg«'< In 

 the results of the same author (Bu, Soij.s 26), previously 

 quoted, where weeds were growing on two types of soil the 

 nitrates were almost eompietely removed. There is eon- 

 siderable water retained in field soils in all cases, hence. 



