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

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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.

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