152 MISSOURI AGRICULTURAL REPORT. 



food materials. Observations, however, have shown that the plant 

 sap during vigorous growth is, and apparently nuist of necessity be, 

 strong in all of the plant food elements. But in order that this may 

 be so maintained, we have reason to think that the soil solution outside 

 of and in contact with the absorbing roots must he maintained cor- 

 respondingly strong. Certain it is that if we place salt meat in pure 

 water it becomes less salt ; and if we wish to render it more salt we 

 must place it in a brine more concentrated than that present in the 

 meat, and we believe that the same relations hold for plants, although 

 sufficient proof is not at hand and a different view is held by many. 

 If the view here expressed is correct, any highly productive soil must 

 be rich in the readily water-soluble plant foo 1 elements and it must 

 carry through the growing season much more of these than will be 

 required to produce the crop upon the ground, or else it must possess 

 the conditions which permit the soluble food materials to develop faster 

 than they are needed. It need not seem strange, therefore, that soils 

 increase in productive power with the addition of manures and ferti- 

 lizers even where it may be shown there is present in the soil at the 

 time and in readily soluble form more than the crop can remove. 

 If the physical condition of a given soil — its openness, its extent of 

 internal surface and its crumb-structure — is such that it must carry in 

 soluble form more than can be removed by a dozen crops in order to be 

 able to supply the plant food elements as rapidly as the one crop can use 

 them, then such a soil must decrease in productive power whenever any 

 one of the esential food elements cannot be delivered to the crop as rap- 

 idly as needed even though there be present in the soil enough for a 

 thousand crops. 



SOURCE OF THE SOLUBLE PLANT FOOD ELEMENTS. 



The water-dissolved plant food elements, which are the direct 

 source of the plant food derived from the soil, come from two ultimate 

 sources, (i) the mineral portion of the dry soil itself, and (2) nitrogen 

 from the soil-air fixed by micro-organisms. But, indirectly, the im- 

 mediate sources of the plant food elements carried in the soil moisture 

 are the slow solution of the soil grains themselves and the decay of or- 

 ganic matter which overspreads and comes in contact with their more 

 or less extended surface. The amount of internal superficial area given 

 to the root zone of a field by the aggregate surfaces of the soil grains 

 is a very important factor of productive capacity. Tliis is so because 

 it is over this surface the soil moisture is spread to become charged 

 with the essential plant food elements derived from the soil grains 



