NATIONAL COUNCIL OF HORTICULTURE 13 



be no more than three to four bushels of wheat per acre, for this is the 

 rate at which rock weathering and erosion are supplying phosphorus 

 to the soil from below. 



* 



STRUCTURE. 



From the standpoint of structure soils differ very widely, both in 

 the extent of their internal surface and in the character and extent of 

 Ihe segregation of their particles. These differences are fundamental 

 and very important in determining the relative productive capacity and 

 in directing agricultural practice. An acre-four-feet of one foot gran- 

 ite blocks would possess an internal surface of 24 acres to which 

 water might adhere, upon which plant food might develop and where 

 it might be stored, over which the roots of plants might spread and 

 feed, and where soil organisms might dwell. To reduce the diameters 

 of these cubes from one to one-thousandth of a foot would increase 

 the internal surface one thousandfold, making it aggregate 24,000 

 acres per acre of field. But even this surface is too small to maintain 

 a high productive capacity. Our coarsest sandy soils possess an in- 

 ternal surface per acre-four-feet exceeding 45 square miles per acre 

 of field; our loams, 270 square miles, while our finest clay types 

 possess an internal surface exceeding 1,300 square miles per acre of 

 field. It is clear, therefore, that there must be wide differences in 

 the productive capacity of soils due to differences of internal surface 

 alone, even when their chemical natures may be identical. This must 

 be so because where there is more surface more water can be retained, 

 plant food may form more rapidly and more may be stored and held 

 in reserve and even accumulated during intervals of small demand as 

 well as retained against loss by leaching. 



The innermost portion of water films investing soil grain sur- 

 faces, in our judgment, is held there with so much force as to be 

 little subject to change, by either drainage or capillary movement, and 

 also becomes highly charged with plant food which likewise is strongly 

 retained, escaping only by the slow process of diffusion when the roots 

 of plants are placed in contact with the soil grain surfaces or when 

 the excess of hydrostatic and capillary portions of water are moving 

 by. We have found, for example, that when a chemically cleaned 

 sand was charged with a solution of potassium nitrate ten repeated 

 washings in twice its weight of distilled water left in the films of 

 moisture retained by the sand grains enough of the nitrate to repre- 

 sent 244 pounds per acre-four-feet. Plant food so retained by soils 

 may still be available to crops for their root hairs are similarly invested 

 with water films and when placed in apposition with the soil grains 

 the water films become common to the two and simple diffusion per- 

 mits the root to feed upon the plant food so retained. 



This brings me to consider a principle underlying proper land 

 drainage. It is very important that when rain falls upon a field the 

 excess water remain only just long enough on its way through the 

 open water passages to' saturate the soil; anything longer than this 



