Irrigation Strategy and Crop Load Affect Nutrition Mangement of Drip-Fertigated Ambrosia/M.9 Apple Trees.

Unreliable irrigation water supply in western North America has stimulated interest in various deficit irrigation strategies and their potential role in affecting crop quality. Ambrosia/M.9 apple trees planted in 2003 on a sandy loam soil at 1 x 3 m received daily drip-irrigation through two 4 L hr^-1 emitters per tree, automatically scheduled using an electronic atmometer.  Trees were fertigated daily for 6 weeks after full bloom to supply 75 g N and 30 g K tree^-1 yr^-1 and at full bloom to supply 20 g P tree^-1 yr^-1 Irrigation treatments included twice daily at 1) 100% and 2) 50% ET replacement; 3) twice daily at 50% ET replacement one side of the tree; 4) 50% (2007), 25% (2008) and 18% (2009) ET replacement every two days. Crop subplots were adjusted to low, medium and high rate of fruit cm^-2 trunk cross sectional area (TCA), with crop load increasing over time from 2.5, 4.5, 9 fruit cm^-2 TCA in 2007 to 3.75, 7.5, 15.0 fruit cm^-2 in 2009. The nutrient implications of adopting deficit irrigation strategies under different crop management scenarios were assessed. Leaf and fruit Ca concentration was affected more by crop load than irrigation regime. Ca concentration and mass uptake per fruit were consistently highest with highest crop load. In contrast the least stressful irrigation treatment (I1) produced largest fruit of low Ca concentration and Ca uptake per fruit that did not differ from I4-fruit. Leaf and fruit P and K had consistently decreased concentrations as crop load increased from low to high rates. Severe deficit irrigation (I4) usually reduced leaf P and fruit K concentrations. N nutrition was least affected by treatments with high crop load associated with increased fruit N in 2 of 3 yrs. but there were few consistent effects of irrigation on leaf and fruit N.