The experiment has two treatments and each treatment has two levels: foliar K application and fertigation as shown in Table 1 and explained below. Control trees will receive a standard 100 lbs K applied to the soil in early April. All treatments will be imposed upon trees previously receiving 100 lbs K per acre as described in Zeng et al. (2000). These trees were shown to respond to the provision of 100 lbs. in each year of the experiment and will e expected to respond in subsequent years. These trees also showed localized K deficiencies within the tree suggesting that demand had exceeded supply during the critical period of nut fill. This system will allow us to contrast the effectiveness of liquid K supplementation aimed at a specific stage of crop growth.
Foliar K applications at either 0 of 0.5% K (equivalent to 1% KNO3) using liquid K fertilizer (KNO3) will occur at 20 day interval during the period of primary nut fill, i.e. from early June to middle August. A total of 5 foliar sprays will be made annually (about 17 pounds K per acre per year). Fertigation using liquid K fertilizer with either 0 or 100 pounds of K equivalent per acre will be applied annually. The application will be made from early June to middle August with the fertilizer equally split at each time. The application will also be made at 20-day intervals coincident with the foliar applications.
The five treatment combinations will be replicated five times and designed as a Latin square of 5x5, so that soil differences can be reduced. Each of the 25 plots consists of five adjacent trees on the same row. To prevent inter-treatment nutrient uptake by the penetrating roots, border trees and rows are established to separate the treatment plots from each other.
Leaf and fruit samples will be collected each time before foliar K application and analyzed for K, Ca, and Mg by inductively coupled plasma-atomic emission spectrometry. Leaf N will be determined by micro-Kjeldhal following the procedures of Weinbaum and Neumann (1977). A correlation of fruit dry weight accumulation with these elements will be established to determine the treatment effects.
The effect of K foliar fertilization or fertigation will also be evaluated as the increase in final yield and the improvement of quality by analyzing the percentage of split nuts, blank nuts and stained nuts and 100-nut weight, the major quality criteria to grade pistachio nuts in California pistachio industry (Ferguson et al., 1995). At the time of harvest, total fresh weight of fruits will be recorded by plot. Random samples of five pounds of nuts will e collected from each plot. These samples will be hulled using a mechanical huller within 12 h after harvest. After drying, the hulled nut will be divided into in-shell, marketable nuts (filled split nuts) and non-marketable nuts (nuts with aborted embryos or stained pericarps) and re-weighed to calculate the dry weight to fresh weight ratio of marketable nuts. Total marketable nut yield will be determined based on the total fresh weight per plot, the dry weight to fresh weight ratio of marketable nuts. The nut quality parameters will be determined by counting 500 hulled, in-shelled nuts randomly collected from each of the dried samples.