14 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



phosphate-supplying capacity. The A values have been used to evaluate 

 the phosphorus fertility of more than 100 soils, and good correlation has 

 been obtained with estimations by yield methods in the field (35). The 

 fertility of other elements in the soil can also be estimated by this proce- 

 dure. Obviously this same method can also be used to measure the effi- 

 ciency of fertilizer sources added to a particular soil. Some of the factors 

 which must be taken into account and which may require investigation 

 for specific conditions include the character of the standard compound, 

 the method of placement, the degree of mixing, the characteristics of the 

 test plant, and the soil acidity. 



As stated by Hendricks (56), almost 100 field experiments were in prog- 

 ress during 1952, employing 18 crops and with the following objectives: 



1. Comparative availability of fertilizer materials 



2. Effect of granulation of fertilizers on availability 



3. Effects of time and placement of fertilizer application 



4. Effectiveness of surface applications to sod crops 



5. Availability of materials applied as foliar sprays 



6. Effect of lime, nitrogen, and potassium supply on phosphorus 

 availability 



7. Effect of irrigation on phosphorus availability 



8. Comparison of crop species in utilization of phosphatic fertilizers 



9. The residual value from phosphates in rotation and permanent 

 fertility experiments 



10. Availability of soil phosphates 



An important application in the field of animal nutrition involving 

 estimation of endogenous fecal excretion was proposed by Hevesy (11) 

 and extended to cattle by Kleiber et al. (57), who worked with P^'-, and by 

 Visek et al. (58) and Comar et al. (59), who used Ca*^. It has always 

 been difficult to estimate the true digestibility or utilization of inorganic 

 materials, primarily because they can be recycled and reused in the body 

 processes by secretion into the tract at one level and reabsorption at 

 another. In this respect they are unlike many organic nutrients which 

 undergo a one-way catabolism and whose utilization may be determined 

 from excreted catabolic end products. 



The basis of this application may be described from Fig. 1-1, which for 

 convenience has been based on the ingestion of 100 units of dietary cal- 

 cium. If 100 units of calcium is ingested by an animal and A units is 

 absorbed from the tract, then 100 — A units will appear in the feces as 

 unabsorbed calcium. If E represents the endogenous calcium for every 

 100 units of the element ingested, then 



^ Total fecal calcium = 100 - A + E (1-6) 



