Phosphorus efficiency of ornamental plants in peat substrates

A pot experiment was conducted to investigate factors contributing to phosphorous (P) efficiency of ornamental plants. Marigold (Tagetes patula) and poinsettia (Euphorbia pulcherima) were cultivated in a peat substrate (black peat 80% + mineral component 20% on a volume basis), treated with P rates of 0, 10, 35, 100, and 170 mg (L substrate)(-1). During the cultivation period, plants were fertigated with a complete nutrient solution (including 18 mg P L-1) every 2 d. Both poinsettia and marigold attained their optimum yield at the rate of 35 mg P (L substrate)(-1) and the critical level of P in shoot dry matter of both crops was 5-6 mg g(-1). After planting, plant-available P increased at lower P rates to a higher level for poinsettia than for marigold, but no significant change was observed at higher P rates. Balance sheet calculations indicated that at lower P rates more P was fertigated than was taken up by the plants. Root-length density, root-to-shoot ratio, and root-hair length of marigold were doubled compared to that of poinsettia. Root-length density increased with crop growth, and 10 d after planting the mean half distance between roots exceeded the P-depletion zone around roots by a factor of 3 and 1.5 for poinsettia and marigold, respectively. Thus, at this early stage poinsettia exploited only 10% of the substrate volume whereas marigold utilized 43%. Later in the cultivation period, the depletion zones around roots overlapped for both crops. Taking into account P uptake via root hairs, the simulation revealed that this was more important for marigold compared to poinsettia especially at low P-supply levels. However, increase of P uptake due to root hairs was only 10%-20% at optimum P supply. For the two lower P levels, the P-depletion profile around roots calculated for 10 d after planting showed that after 2 d of depletion the concentration at the root surface was below the assumed K-m value (5 mu W) and the concentration gradient was insufficient to fit the demand. A higher content of plant-available P in the substrate was observed for poinsettia compared to marigold in the treatment with P application adequate for optimum growth, because more fertigated P was accumulated during early stages of cultivation due to lower root-length density of poinsettia. The observed difference of root morphological parameters did not contribute significantly to P-uptake efficiency, since P mobility in the peat substrate was high.