Soilless container root media have little capacity to retain P, and preplant amendments of triple superphosphate (TSP) and water-soluble fertilizer (WSF) P applications are readily leached from them. A soilless medium modified with Al-2(SO4)(3) was tested to reduce such P losses. Aluminum sulfate solutions were applied to a 70 sphagnum peat : 30 perlite (v/v) medium to result in 0.32, 0.96, and 1.92 kg Al/m(3) and dried at 70C. Adsorption isotherms (25C, 0 to 500 mg P/liter) showed that P retention increased as the rate of Al addition increased. In a greenhouse study, plants of Dendranthema x grandiflorum (Ramat.) Kitamura 'Sunny Mandalay' were grown in Al-modified media and an unmodified medium in factorial combination with P from preplant amendment of 0.1 kg TSP-P/m(3), or P applied at each watering as WSF at rates of 5.5 or 21.8 mg P/liter, The two highest rates of Al were excessive and resulted in low pH and excessive soluble Al levels in the root medium solution early in the cropping cycle, which were detrimental to plant growth. When the root medium was modified with 0.32 kg Al/m(3), soluble Al levels in medium solution were not significantly different than in the unmodified control. TSP-P that leached was substantially reduced by the addition of Al, yet sufficient P was released throughout the cropping cycle for adequate plant growth, Plants grown in Al-modified medium with 0.1 kg TSP-P/m(3) did not differ from control plants in unmodified medium + 0.27 kg TSP-P/m(3) and were larger than plants grown in unmodified medium + 0.1 kg TSP-P/m(3), Aluminum modification of the root medium substantially reduced P leaching when used,vith WSF containing P, In addition, growth of plants in unmodified medium fertilized with 5.5 vs, 21.8 mg P/liter was similar.
