| Summary: | Radioactive and stable isotopes have been widely used in studies of biological problems in the last two decades. These labelled atoms or molecules allow undertaking experiments where ordinary chemical and instrumental measurements are not practical. They can be readily detected, traced or determined quantitatively, even at extremely low concentration (32). Radioactive phosphorus labelled fertilizers have made possible the determination of the quantity of phosphorus in the plant derived from applied fertilizer, hence the quantity derived from the soil can be calculated. Such information has indicated that the relative proportion of soil to fertilizer phosphorus absorbed by plants is related to the amounts of available phosphorus in soil. Hence, there exists the possibility of using fertilizers labelled with radioactive phosphorus to evaluate the available phosphorus levels of soils. This bas been done by many workers with a wide range of crops grown in various soils. It was found that the measured availability of a nutrient in the soil was inf1uenced by the various factors that affect absorption by plants. Therefore, the amount of a nutrient available in a soil depends not only upon the soil itself but also upon the specific condition under Which a given crop is grown. There is fairly general agreement that availability is a property of the soil-plant system and not of the soil alone. [...]
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