Phosphorus nutrition of Caustis blakei grown with two phosphorus sources of different solubility in two soils of differing phosphorus adsorption capacity

A barrier to the domestication of the phosphorus (P) sensitive Australian species Caustis blakei (Cyperaceae) is the standard production systems used commercially which invariably result in problems associated either with P deficiency or P toxicity. This paper reports on the growth responses of Caustis blakei cv. M63 to applications of fertiliser P as either monocalcium phosphate (MCP) or granulated Guano Gold (R) rock phosphate (RP) in two soils with different capacities to adsorb P. The Caustis M63 plants grown in the two soils did not show P toxicity symptoms when fertilised with RP, but shoot dry weight was 30-60% lower than the control in both soils at the highest rate of MCP-P application (156 kg ha(-1), 184 g m(-3)) and this was associated with visible symptoms of drying of the tips of the ultimate branchlets, in the Mt Cotton soil only. The greatest shoot and root dry weights were achieved by plants grown in the higher P adsorbing Palmwoods soil fertilised with RP at P rates of 30-184 g m(-3). Caustis plants grown in the Palmwoods soil had 2.3 times greater root dry weights than plants grown in the Mt Cotton soil irrespective of the P fertiliser type used. Caustis plants growing in Mt Cotton soil which did not receive P showed significantly lower shoot and root dry weight when compared to plants in the Palmwoods soil, probably due to the low initial bicarbonate-extractable P and the high buffering capacity of the Mt Cotton soil. The P concentration in shoots of Caustis fertilised with MCP at 184 g m(-3) was higher when grown in Mt Cotton soil (0.22%) than in the Palmwoods soil (0.15%). The P concentration was lower in the terminal ultimate branchlets (TUB); 0.15% for the Mt Cotton soil and 0.10% for the Palmwoods soil, suggesting that shoots would provide a more useful indicator of P toxicity than the TUB. It is interesting to speculate as to why plants in the Palmwoods soil showed greater root growth and fewer symptoms of P toxicity. This could be because the Palmwoods soil had the greater P adsorption capacity. These results indicate in ground production of Caustis cut foliage will require careful management of P nutrition and understanding of the complex soil/plant interactions associated with the acquisition of P. (c) 2006 Elsevier B.V. All rights reserved.

Phosphorus uptake from green manures and phosphate fertilizers applied in an acid tropical soil

Quantifying the relative contribution of different phosphorus (P) sources to P uptake can lead to greater understanding of the mechanisms that increase available P in integrated P management systems. The P-32-P-33 double isotope labeling technique was used to determine the relative contribution of green manures (GMs) and P fertilizers to P uptake by Setaria grass (Setaria sphacelata) grown in an amended tropical acid soil (Bungor series) in a glasshouse study. The amendments were factorial combinations of GMs (Calopogonium caeruleum , Gliricidia sepium and Imperata cylindrica) and P fertilizers [phosphate rocks (PRs) from North Carolina (NCPR), China (CPR) and Algeria (APR), and triple superphosphate (TSP)]. Dry matter yield, P uptake, and P utilization from the amendments were monitored at 4, 8, and 15 weeks after establishment (WAE). The GMs alone or in combination with P fertilizers contributed less than 5% to total P uptake in this soil, but total P uptake into Setaria plants in the GM treatments was three to four times that of the P fertilizers because the GMs mobilized more soil P. Also, the GMs markedly increased fertilizer P utilization in the combined treatments, from 3% to 39% with CPR, from 6-9% to 19-48% with reactive PRs, and from 6% to 37% with TSP in this soil. Both P GM and the other decomposition products were probably involved in reducing soil P-retention capacity. Mobilization of soil P was most likely the result of the action of the other decomposition products. These results demonstrate the high potential of integrating GMs and PRs for managing P in tropical soils and the importance of the soil P mobilization capacity of the organic components. Even the low-quality Imperata GM enhanced the effectiveness of the reactive APR more than fourfold.