SOIL-PLANT RELATIONSHIPS AND PLANT NUTRITION 369 



because the content of nutrient elements in plants is related to the supply 

 perhaps more than to the minimum needed. However, these figures are 

 sufficient to indicate that types of land use which involve annual removal 

 of much of the vegetation involve also depletion of the nutrient resources 

 at a substantial rate. 



Plant requirements for water can be expressed either on an area basis or 

 in terms of the amount necessary to produce a unit of dry weight. That the 

 growth of plants in great areas of the world is limited by the inadequacy of 

 the rainfall is well known, but the amount required by plants in humid areas 

 is far greater than usually realized. Moreover, relatively short periods of 

 inadequate supply are now recognized to bring about substantial depression 

 in the total seasonal growth if they occur at a critical stage in the growth 

 cycle of the plant. As with nutrients, the quantity of water used to maturity 

 by plants is not necessarily the minimum requirement and is influenced by 

 such factors as air temperatures and humidities. On an area basis the amounts 

 are large. Not infrequently the demand in a summer month may exceed the 

 total rainfall less evaporation for that month, which means that, unless the 

 storage capacity of the soil for water is considerable, there may be a period 

 of inadequate supply. Expressed as weight of water required to produce a 

 unit of dry plant tissue, the figures are also substantial. Some species are 

 more efficient than others in water use, but in general 150-600 pounds of 

 water must be taken up and transpired for each pound of dry matter syn- 

 thesized. Water is by far the most important plant nutrient quantitatively, 

 and the ability of different soils to supply water greatly influences the 

 array of native vegetation and the selection of crops and crop varieties that 

 can be profitably grown when the land is farmed. 



The requirements of plant roots for oxygen cannot be expressed in the 

 same way as the requirements for nutrients or water, though for most species 

 there is no question as to essentiality. Oxygen is needed to maintain aerobic 

 cellular processes in root tissues, and although small amounts of oxygen can 

 reach the roots of some species from the tops through internal air spaces, the 

 major source is from the soil atmosphere. Many attempts have been made 

 to determine the composition of the soil atmosphere, and some objections 

 have been raised to most of the procedures used, but even so it appears 

 probable that ordinarily the soil atmosphere does not differ greatly from the 

 air above in amounts of the major components. The salient difference is 

 that the oxygen content may be a little lower, and the carbon dioxide content 

 appreciably higher, than in normal air. That the differences are not greater 

 is due to the rapidity of gaseous diffusion and the continuous process of 

 equilibration between the soil atmosphere and the infinitely large reserve in 

 the air above. This means that the porosity of a soil really determines the 

 oxygen supply to the roots and that oxygen deficiencies are only likely in 

 soils of low porosity or where the pores are filled with water to such an extent 



