Einfluß von Form und Plazierung der N‐ und P‐Düngung auf Wurzelentwicklung, Phosphat‐Aufnahme und Wachstum von Sonnenblumen (Helianthus annuus) und Zuckerhirsen (Sorghum bicolor)

Influence of form and placement of N‐ and P‐fertilisation on the root development, the P‐uptake, and the growth of sunflower ( Helianthus annum ) and sweet sorghum ( Sorghum bicolor ). In pot‐ and rhizotrone‐experiments with Sorghum bicolor and Helianthus annuus , a higher P‐uptake was achieved by placement of diammonphosphate (DAP) than by either placement of triplesuperphosphate (TSP) or by broadcasting DAP or TSP, respectively. Dual placement of TSP and ammonia increased the P uptake but it was less effective than placement of DAP for sunflowers, but not for sweet sorghum. Placement of nitrogen resulted in a local root accumulation, where ammonia had a greater effect than nitrate for both of the species. Single placement of TSP had no effect on either P uptake or root development. Thus, the higher P uptake at DAP placement could be attributed to three main factors: the local root accumulation caused by the placement of nitrogen, a specific physiological ammonia‐effect, and the lower binding intensity of the phosphate in the DAP‐band compared with placed TSP (W erner and S trasser 1993). The improvement in spatial availability caused by the NH 4 + ‐induced root accumulation was of a greater importance for the P‐uptake than was the improvement of chemical availability in the DAP‐band. The better response of sunflowers in comparison with sweet sorghum was attributed to better acquisition ability in sweet sorghum.