PHOSPHATE-FERTILIZER-INDUCED SALT TOXICITY OF NEWLY PLANTED APPLE-TREES

Monoammonium phosphate (MAP) or triple superphosphate (TSP) added to planting holes often stimulates early growth of apple trees (Malus domestica Borkh.); however, high rates may stunt or kill trees. A greenhouse study was conducted on a Quincy sand (a mixed, mesic Xeric Torripsamment) and a Cowiche silt loam (a fine-loamy, mixed mesic Aridic Argixeroll) to examine effects of soil type, P source (MAP or TSP), and P rate (0-2.75 kg P/m3) on soil salinity, soil acidity, and apple tree growth in nonleaching soil systems during a 56-d period after planting. Rootstock and old scion masses were not affected by soil type or P treatments. In the Quincy sand, new scion mass and root mass were inversely related to P rate in both MAP- and TSP-amended soil; tree death occurred at P rates above 1.65 kg/m3. In the Cowiche soil, root mass was independent of and new scion mass was inversely related to P rate. At equivalent P rates, TSP produced lower soil salinity and greater soil acidity than MAP. For a given P source, soil salinity and acidity were positively related to P rate. Soil salinity decreased and soil pH increased over time. Relative new scion mass was inversely related to time-integrated soil salinity; however, the relationships were soil-specific. Total exchangeable soil acidity was positively related to P rate and was soil-dependent. Exchangeable soil Al was detected only at the two highest TSP rates in the Quincy soil. Indirect evidence suggested that Mn, NH3, and NO2 phytotoxicities and any nutrient deficiencies were absent. The results suggest that transient soil salinization is the primary mechanism for MAP- and TSP-induced apple tree phytotoxicity.