AVAILABILITY OF PLANT NUTBIENT8 343 



of free water is comparatively rapid and that of capil- 

 lary water relatively slow, the soil moisture supply is 

 usually somewhere between the point of lento-capillarity 

 and free water. In this condition each particle or aggre- 

 gation of particles is enveloped in a thin moisture film^ 

 and this film water is constantly in motion although the 

 movement is rather slow. 



Soils are more or less soluble in pure water ; and in soil 

 water, charged as it always is with carbon dioxide, they 

 are still more readily soluble. Consequently the moisture 

 films constantly tend to approach a state of equilibrium 

 with respect to the water-soluble matter in the soil parti- 

 cles. If plants are entirely dependent for their mineral 

 nutrients on the supply in the soil-water solution, the 

 strength of this solution becomes an important matter. 

 The supply of mineral nutrients for higher plants will be 

 discussed later (par. 339). Even if the plant itself 

 has no influence on the supply of mineral nutrients that go 

 into solution, the quantity of food that it finds in the 

 soil solution already prepared for its use must constitute 

 an important factor in its growth. 



Unfortunately there is no adequate method of ascer- 

 taining the strength of the solution. Attempts have 

 been made to remove this solution from the soil, but it 

 is altogether unlikely that the analyses of the liquid 

 obtained represent the composition of the soil solution, 

 because of the very small quantity of the liquid available 

 for analysis and also because of the uncertainty that the 

 sample obtained was representative of the soil solution. 



247. Devices for obtaining a soil solution. — An at- 

 tempt by Briggs and McLane ^ to sample the soil solution 



^ Briggs, Lyman J., and McLane, John W. The Moisture 

 Equivalent of Soils. U. S. D. A., Bur. Soils, BuL 45, pp. 6-8. 1907, 



