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A Comparison of Ammonium Metaphosphate and Ammonium Orthophosphate with Superphosphate on the Yield and Chemical Composition of Crops Grown Under Field Conditions¹

ABSTRACT

Sweet Sudan and Iron-Clay cowpeas were grown under irrigation in the field on a Lufkin fine sandy loam, an acid Planosol, and on a San Saba clay, a highly calcareous Grumusol (Rendzina). The plots were fertilized with 50-30-60 per acre, with the check receiving 50-0-60. The treatments consisted of ammonium sulfate, ammonium orthophosphate, ammonium metaphosphate, 20% superphosphate, muriate of potash, and calcium sulfate, applied in such a way as to give a valid comparison between the ammonium phosphates and superphosphate. The forage from one-half the area of each plot was harvested 4 weeks after planting and the remainder at the end of 8 weeks. Forage yields and chemical composition were determined.

Crops at both locations responded to all P treatments, with the Sudan yields being increased about 90% over the check in contrast to only about a 10 to 15% increase by the cowpeas. The greatest yield response was by Sudan on the San Saba clay; the least response was by the cowpeas, also on the San Saba clay. All sources of P were about equal in their separate effect on crop yields at each location.

The P content of the separate crops was increased about the same by each source of P, but the P content was highest in the crops grown on the acid Lufkin fine sandy loam. The P content of the cowpeas grown on the San Saba clay was not appreciably affected by P additions.

The N, S and Ca levels in the forage were not affected by the P treatments, but were affected by location and species, and were greatly affected by the stage of maturity at harvest, the less mature forage being higher in these nutrients.

¹ Journal Paper No. 2805, Texas Agr. Exp. Sta., College Station. Based on research done under Project 68, Texas A. & M. Research Foundation, sponsored by the Lion Oil Co. (now part of Monsanto Chemical Co.), El Dorado, Ark. Presented before Div. IV, Soil Science Society of America, Nov. 18, 1957, at Atlanta, Ga.

² Associate Professor, Instructor, and Associate Agronomist, respectively, Texas Agr. Exp. Sta. The writers wish to express appreciation to the Lion Oil Co. (now a branch of Monsanto Chemical Co.), El Dorado, Ark. for the sponsorship of this research through the Texas A. & M. Research Foundation. Grateful acknowledgement is also due various staff members of the State Chemist and Soil Testing Laboratories, both parts of the Texas A. & M. College System, for suppling analytical data on soil and plant samples.

Received for publication January 15, 1958. Accepted for publication October 13, 1958.