Simply adding potassium is not the answer. In order to increase the levels of fruit potassium it may be best to apply foliar sprays in the late season, or by using some other method. We know from observation that a monitored fertility program such as described in this trial has produced improved color and maturity indices for most of the growers using it in western Washington. In some fields with adequate fertility, early season stress can be an adverse factor. Stress can result from a number of causes, such as lack of sufficient water, mite infestation, or anything that adversely influences accumulation of carbohydrates and the overall health of the tree.
Stress can also affect early nutrient uptake and the resulting fruit quality.
Fruit that looks bad in mid July usually looks bad at harvest. Maintaining good plant health in the early season through regular monitoring, and rarely using additional input of nitrogen, is important. In this short-term study, there has been no significant difference in fruit quality, fruit color, or range of harvest indices to differentiate the control plot, which received water only, from the plots that were fertigated. It is an important fact, however, that these plots were established on soils which had been well maintained nutritionally before the trial was initiated and the fruit quality is good in all plots. A longer term study might resolve some of the questions raised by this trial.