Response of 'Jonagold' apple trees in the first three years after planting to mono-ammonium phosphate fertilization under replant problem conditions

The objective of the experiment was to examine response of immature apple trees to application of mono-ammonium phosphate (MAP) fertilizer on replant problem soil. The study was carried out during 2001-2003 under a greenhouse on 'Jonagold' apple trees/M.9 EMLA planted singly in 50 L polyethylene containers filled with a sandy loam soil with low status of both organic matter and phosphorus (P) in soil solution. This soil originated from an apple orchard unfertilized with P for 23 years. The biological test showed the presence of specific replant disease in the soil. Immediately before apple tree planting, the soil was mixed with MAP at rates of 1, 2, and 3 g L- 1. Trees grown in the soil untreated with MAP served as a control. Each year apple trees were drip-irrigated and supplied with nitrogen (N) at differentiated rates to achieve a level of 50 g N per plant. The results showed that MAP application increased soil solution P status. Simultaneously, MAP supply at rates of 2 and 3 g L- 1 caused a drop in soil pH value in the last two years of the experiment. MAP treatments increased both dry weight and length of fine roots (< 2 mm in diameter), vigor of trees, the number of flower clusters per tree, flower intensity, the number of fruits per tree, and P concentrations in leaf and fruit tissues. Fruits from MAP-supplied trees were firmer than those of the control trees. Mean fruit weight, titratable acidity, and soluble solids concentration of 'Jonagold' apples at harvest were not influenced by MAP treatment. Fruits from MAP-supplied apple trees had increased calcium concentration only in one year. It is concluded that pre-plant application of MAP at a rate of 1g L- 1can be recommended on coarse-textured soils with low P status in soil solution to increase precocity of apple trees. However, MAP-supplied apple trees have to be watered to avoid the risk of osmotic stress.