Daniel I. Anion 315 



soil treatment on the plant. In one case an element added to the soil 

 is of direct benefit by virtue of its being absorbed by the plant; in 

 the other case the benefit to the plant accrues from secondary changes 

 in the soil brought about by the treatment. For instance, it was shown 

 by Horner et al. (25) that vanadium has a beneficial effect on nitro- 

 gen fixation by Azotobacter, and Jensen (27) reported a similar effect 

 on nitrogen fixation by Clostridium. It is thus conceivable that addi- 

 tions of vanadium to a soil low in nitrogen might be reflected in im- 

 proved plant growth, but this cannot be taken as evidence that vana- 

 dium is essential for the growth of higher plants. Other instances of 

 indirectly beneficial effects of soil treatments on plants are due to the 

 ion exchange status of a soil where an application of one ion may, 

 through exchange reactions, render another ion available for the needs 

 of the plant (7). The exchange of one ion for another, as for example, 

 calcium for sodium, may also favorably alter the physical condition of 

 a soil and improve plant growth through better aeration and water 

 penetration. It is likely that most of the "beneficial" growth responses 

 from adding to soils inorganic elements not recognized as essential may 

 be due to such indirect effects on the soil. (In some cases, however, a 

 partial substitution within the plant of a nonessential element for an 

 essential one, may be involved. This point will be treated later in the 

 discussion). 



Even when failure of plants to grow is caused by a single nutrient 

 deficiency in the soil, it may be difficult to discover the missing element 

 by soil treatment alone. A deficiency of an element in the soil may 

 often be due not to the low concentration but to the unavailability of 

 the nutrient to the plant. There are cases where the fixing power of 

 the soil is such that relatively huge applications of an element are re- 

 quired to give a plant response even though the quantity needed by 

 plants is very small. Treatment of the soil only, in the absence of in- 

 formation derived from other techniques, could therefore easily lead to 

 erroneous conclusions. A case in point is afforded by a study of zinc 

 deficiency in California (17). In a peach orchard showing zinc defi- 

 ciency, it was computed from results of plant analysis that the trees and 

 the fruit removed about 8 ounces of zinc in seven years, yet an analysis 

 of the soil showed 3000 pounds of zinc to the acre within the root zone. 



