PRINCIPLES OF TRACER METHODOLOGY 



13 



soil. Actually the plant under the specific growing conditions is the best 

 judge of nutrient availability. Fried and Dean (55) used the principle of 

 isotope dilution to evaluate experimentally the availability of soil phos- 

 phorus to the plant. The method was based on the assumption that a 

 plant confronted with two sources of phosphorus would utilize them in 

 direct proportion to the availability of the element in each. The above 

 equation may then be expressed as 



Sb 



B 



A -\- B 



or 



B[l - (Sp/Sb)] 

 op/Sb 



P-5) 



where A = amount of nutrient available in soil 



B = amount of labeled nutrient added as a standard 

 Sp = specific activity in plant 

 Sb = specific activity in standard labeled nutrient 

 It must be remembered that in these studies the soil-nutrient availabil- 

 ity is expressed in terms of the added standard nutrient. In pi'actice, a 

 P^--labeled phosphate source, usually monocalcium phosphate, is thor- 

 oughly mixed with the soil at some arbitrary rate, and plants are then 

 grown and harvested after a given period. The specific activities of the 

 phosphorus in the harvested plants and in the standard source are meas- 

 ured. Typical A values are shown for three soils in Table 1-1, as quoted 

 by Hendricks (56) . It is noted, as expected theoretically, that the yield 

 and rate of fertilizer phosphate addition did not affect the values of 



Table 1-1. The Phosphate-supplying Capacity, A Value, of 

 Three Soils. Test Crop: Millet 



[Results of M. P>ied, quoted b.y Sterling B. Hendricks, Rudioi.sotopes in Fertilizer 

 Usage, Soil Fertility and Plant Nutrition, in "Use of Isotopes in l^lant and Animal 

 Research" (Conference Sponsored by Kansas State College, Argonne National 

 Laboratory, and Isotopes Division, U.S. Atomic Energy Commission, June 12-14, 

 1952), TID-5098, pp. 41-47, April, 1953.] 



