Nutrient Management Research Database

http://ucanr.edu/sites/Nutrient_Management_Solutions/files/212755.pdf

We examined interrelationships among crop load, P and K uptake, and root
growth in mature pistachio trees that characteristically bear heavy (on-year) nut crops in alternate years. Uptake and partitioning of P and K among tree parts were determined during nut fill (late May to early September). Although root growth was reduced during nut fill in on-year trees compared with off-year trees, there was no relationship between root growth and the uptake of P or K from the soil. Our data support the hypothesis that sink
demand rather than root growth regulates the uptake of P and K in pistachio trees.

Experiments were conducted to assess the effects of alternate bearing on root growth and P and K nutrition of pistachios in the San Joaquin Valley.  Root observation boxes were used to observe actively growing, white roots in on-year and off-year trees.  On-year and off-year trees were also excavated following spring growth flush and nut fill, separated into biomass fractions, weighed and analyzed for P and K.   

On-year trees started root growth earlier in the spring than off-year trees and produced more roots during this period.  During nut growth, on-year trees produced fewer white roots and had significantly lower root growth rates.  

Heavy cropping in on-year trees greatly increased K uptake and removal compared to off-year trees and slightly increase P uptake and removal.