134 Mineral Nutrition of Plants 



are subject to rapid change with time or treatment. There appears to 

 be little appreciation of this fact. This is indicated in part by the very 

 common tendency to depend upon fertilizers alone with little thought 

 given to physical factors when it is desired to increase the productive 

 capacity of a soil. 



In recent years there has been an increasing appreciation on the part 

 of some workers of the importance of the physical properties of the 

 soil in crop production. The reasons for this are probably threefold: 



(a) with general adoption of high yielding varieties of crops and heavy 

 fertilization rates, frequent failures to obtain the expected yield in- 

 creases have shown that factors other than fertility often limit yields; 



(b) with continued cultivation many of our soils have lost or are losing 

 much of their organic matter with a corresponding deterioration of 

 favorable soil structure, so that physical properties are becoming poor 

 enough to seriously limit plant growth, even though fertilizers may 

 be liberally supplied; and (c) under semiarid and arid conditions, where 

 initial content of soil organic content was low, a change from native 

 vegetation growing on undisturbed soil under low rainfall to intro- 

 duced field and orchard crops growing on frequently tilled, irrigated 

 soil, has been observed to be associated with distinct structural changes. 



Recent well-publicized attempts to make record-breaking corn yields 

 have shown very strikingly that for high yields to be obtained, the 

 physical, as well as the chemical factors must be at their optimum. It 

 has also become quite apparent that unfavorable physical properties 

 have profound effects upon the chemistry of the soil and upon availa- 

 bility of plant nutrients, both native and added. Little direct work has 

 been done on this subject, so that there is need for a general appreciation 

 of this lack in our knowledge of the soil-plant system, and for a con- 

 certed attack on the problems by both soil and plant scientists. It is 

 the purpose of this paper to point out the importance of the physical 

 properties in crop production, the ways in which physical properties 

 change or affect nutrient availability, and the need for a general reali- 

 zation of the inadequacy of the strictly chemical approach and for the 

 study of the soil as a system. 



