Subclover Early Growth Responses to Levels and Placements of Superphosphate and Ammonium Nitrate

Abstract

Single superphosophate (SSP) commonly has been used to correct soil deficiencies of P and S in rangeland seedings of annual legumes. This study evaluated rates and soil placement of P and S as SSP and N as ammonium nitrate on germination (emergence), seedling growth, and nodulation of subterranean clover (Trifolium subterraneum L.). The soil was a Sobrante‐Las Posas association (fineloamy or fine mixed thermic family of mollic haploxeralfs). Plants were grown from seed in three separate greenhouse experiments. The SSP levels varied from 31 to 4000 kg ha−1 (equivalent to 2.4 and 313 kg ha−1 P and 3.7 and 480 kg ha−1 S). Nitrogen varied from 40 to 160 kg ha−1. Placements were: 5 cm below the seed, 1.25 cm side‐banded, broadcast, and seed placed. Responses measured included emergence, growth rate, shoot weight, nodulation, and P toxicity symptoms. Negative effects of SSP mainly reduced emergence and increased plant toxicity symptoms and occurred at levels ≥1000 to 4000 kg ha−1. Negative effects of N (at 160 kg ha−1) were mainly on nodulation. Positive effects on most growth parameters and nodulation were largely confined to SSP levels ≤500 kg ha−1 and were best expressed by an asymptotic regression function. Placement effects generally were small except for seed‐placed reduction of emergence above 1000 kg ha−1 SSP. Soil‐banded placement treatments usually were more beneficial to growth than either broadcast or seed placed.