| Our objective was to determine critical soil (P/[Al+Fe))M-III (Mehlich-III) molar ratios for acid sandy cranberry soils, and to assess P requirements based on soil and tissue testing. Exploratory experiments were started in 2000. Fertilizer trials were conducted during the 2001--2003 period with yearly P applications of 0, 13, 26, and 39 kg P ha-1 to permanent plots. The main effect of P additions was larger on foliar P concentration compared to cranberry yield. Critical (P/[Al+Fe]) M-III molar ratios were found to be <0.03 for below optimum and >0.06 for above. The P requirements were 13 to 39 kg P ha-1 in the below optimum (<0.03), 10 to 28 kg P ha-1 in the optimum (0.03--0.06), 0 to 13 kg P ha-1 the above optimum (0.06 to 0.12). No P should be added above 0.12 (based on a critical water-soluble P value (PW) of 9.7 mg P W kg-1). The selection of a critical foliar concentration of 1.1 mg P kg-1 may triple P requirements compared to 1.0 mg P kg-1. |
