498 STATE BOARD OF AGRICULTURE. 



THE SOIL SOLUTION OBTAINED BY THE OIL PRESSURE 



METHOD. 



J. FRANKLIN MORGAN, RESEARCH ASSISTANT IN BACTERIOLOGY. 



INTRODUCTION. 



The study of the soil, the medium in which the plant grows, has been 

 approached from many angles. The sum total of these efforts has thrown 

 some light on what the soil is, what it lacks for good plant growth, and 

 thus what it requires to make up this deficiency. The one phase which 

 has been studied to the least extent is that of the actual medium from 

 which the plant takes part of its nourishment, — that is, the soil solution. 



"A solution^ is a mixture of substances chemically and physically 

 homogeneous throughout, the proportions of whose components may be 

 altered continuously without i)roducing an abrupt change in any prop- 

 erty of the solution." It may be more briefly stated : "A solution- is a 

 mixture of substances so intimate that they cannot be mechanically 

 separated; as, for example, by filtration." 



Soil solution, then, is a homogeneous mixture of the water (the sol- 

 vent) and the soluble ingredients of the soil (the solutes), consisting 

 of gases, liquids and solids. These solutes may influence the solubility 

 of other substances. This is more especially true of the solutes which 

 makes the solution acid or alkaline in reaction. For example, carbon 

 dioxide dissolved in water forms a weak acid and acids as oxalic, etc., 

 are formed from the decomposing organic substances. Nevertheless the 

 final solvent that holds all in solution is the water. 



The somewhat confusing term, soil extract, should not be understood 

 to be the same as the soil solution. The latter is a solution produced by 

 the forces operative under natural conditions. The former is one pro- 

 duced artificially by the use of solvents such as acids or water for the 

 purpose of bringing into solution some particular element or all of the 

 elements in the soil. 



THE SOIL SOLUTION IN THE SOIL. 



The solid phase of the soil which holds the liquid phase as a medium 

 for nourishing the plants and microorganisms is formed from weath- 

 ered rock of various kinds and from the decaying organic matter of 

 plants and animals. The amount, size, and composition of the different 

 particles together with the amount of organic matter present determine 

 the type of soil. The maximum amount of soil solution that the soil 

 can hold depends upon the physical character of the soil, — the finer the 

 soil and also the more organic matter, humus, present, the more the par- 

 ticular soil will hold on account of the larger surface given for adsorp- 

 tion. The capillary water present is more firmly held by the force of 

 surface tension. 



In order that the plant may take up through its roots the mineral con- 

 stituents in the soil, the latter must be in solution. As the plant re- 



^Bigelow's Theoretical and Physical Chemistry (1912 Ed.), P. 36. 

 ^Coulter, Barnes and Cowles Text Book of Botany, (1910 Ed.), p. 303. 



