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Importance of the Organic Phosphorus Fraction in Extracts of Calcareous Soil¹

ABSTRACT

Laveen clay loam soil samples were collected immediately following the harvest of forage sorghum [Sorghum bicolor (L.) Moench]. Two sets were prepared before growing turnips (Brassica rapa L.) as the test crop in the greenhouse. One set was without treatment and the other, to simulate winter and spring conditions, was incubated for 20 days after addition of N and dextrose. Phosphorus uptake by turnips and changes in soil P forms were positively correlated with corresponding measurements taken during the growth of the sorghum crop. With this as a basis, the effect appears to be similar to that which occurs in the field on the immobilization and mineralization of available P. Inorganic and total soil P were determined in extracts obtained by five procedures. Correlations among P uptake by forage sorghum, turnips, and soil P measurements indicated their value in predicting available P. Prediction values for two of the procedures were improved by including the extractable organic P with the inorganic P for the incubated soil. Results showed that (i) time of sampling, (ii) the crop to be grown, (iii) the method of extraction, and (iv) the means of converting organic P to a measurable inorganic form in the extract are of prime concern in deciding when to include extractable organic P in the P evaluation of a given soil.

¹ Contribution from Dep. of Soils, Water, and Engineering, Arizona Agric. Exp. Stn., Tucson, AZ 85721. Published with the approval of the Director as Journal Article No. 2696.

² Assistant Agricultural Chemist, Cotton Research Center, 4201 E. Broadway, Phoenix, AZ 85040.

Received for publication December 22, 1976. Accepted for publication September 13, 1977.