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Availability of Rock Phosphate as Measured by an Acid Tolerant Pasture Grass and Extractable Phosphorus¹

As agricultural development intensifies in tropical countries and as costs of superphosphate production increase, the need for further evaluation of rock phosphates has become apparent. An acid tolerant pasture grass (Brachiaria decumbens Stapf.) was used to evaluate the initial and residual (3 years) availability of P from several phosphate materials. A field study was conducted on a Typic Haplustox of the Cerrado of Central Brazil. Initial availability of P supplied as normal superphosphate, Hyperphosphate, Thermalphosphate, and North Carolina rock phosphate was similar when the soils were not limed (pH = 4.3, Al = 1.4 meq/100g). Where lime had been added and pH was increased to 5.4 the P in Hyperphosphate and N.C. rock phosphate was initially less available than P added as superphosphate and Thermalphosphate. After 10 months of contact, lime no longer significantly depressed P availability in these relatively soluble rock phosphates as measured by forage dry weight. The P in the other phosphate rock of low citric acid solubility, Araxá, was virtually unavailable during the 3 months after application. However, within 13 months, maximum forage yields were attained where this material had been applied to unlimed soil and at 25 months maximum yields were obtained on both limed and unlimed soil. Higher rates of Araxá material were required to obtain maximum yields than with normal superphosphate. Forage yields were 2 to 4 times greater where superphosphate was surface applied each year than where similar amounts were incorporated prior to planting. A comparison of extractant methods for soil P indicated that highly acidic extractants (e.g., 0.05 N HC1 + 0.025 N H₂SO₄) overestimated available P where rock phosphate had been applied. Where superphosphate or rock phosphate had been applied, P availability was better assessed by extractants such as 0.5 M NaHCO₃ and Bray 1. The results emphasize the need to supplement laboratory and short-term studies with longterm evaluations of P materials with acid tolerant perennial crop species available in tropical countries.

Key Words: Oxisol • Pasture fertilization • Acid tolerant species • Rock phosphate • Mitscherlich equations • Cerrado • Brazil • Tropics • Fertilizer evaluation

² Former graduate research assistant, North Carolina State Univ. (now assistant soil scientist, Dep. of Agronomy and Soil Science, Univ. of Hawaii); former research associate, Cornell Univ. (now associate professor, Dep. of Soil Science, North Carolina State Univ.); professor, Dep. of Soil Science, North Carolina State Univ.; and coordinator, Project of Resource Utilization, CPAC, Brasilia, Brazil.

Received for publication February 27, 1981.