Different forms of soil potassium as affected by the age of pistachio ( Pistacia vera L.) trees in Rafsanjan, Iran

Abstract

Pistachio is a major crop that accounts for a considerable portion of Iranian agricultural exports. Despite the importance of potassium in pistachio production, no information is currently available on the changes of different forms of K in soil with pistachio tree age. The objective of the present study is to evaluate the status of different forms of soil K in pistachio orchards with trees of different ages. Pistachio orchards of three different ages of 10, 20, and 40 years old were selected in three different regions in Rafsanjan area, Iran. A piece of virgin land adjacent to each pistachio orchard was also selected as control. Three soil profiles were excavated in each orchard as well as three from the control virgin plots to sample their genetic horizons. Water soluble, exchangeable, nonexchangeable, and structural K were extracted and determined. Results showed that the amounts of soluble K in the virgin land were higher than those in the pistachio orchards with different tree ages. In soils initially rich in exchangeable K, growth of pistachio trees over 10, 20, or 40 years significantly reduced exchangeable K by 8.4, 46.3 and 43.6%, respectively. For the other two regions where soil exchangeable K was primarily lower, average reduction in exchangeable K was 2 to 20%. The nonexchangeable K fraction was significantly lower in the 10, 20, and 40-year-old pistachio orchards as compared to that in the uncultivated control soil, especially in regions where soil exchangeable K was primarily low. The highest reduction in nonexchangeable K was 50% for the 40-year-old pistachio orchards in the region where exchangeable K was lowest in the soil. Moreover, the reduction in nonexchangeable K was highest in the root zone as compared to the other soil layers. These findings suggest that uptake of potassium by plants, pistachio tree age, and primary status of exchangeable K are important factors affecting K depletion from K bearing minerals in soils under pistachio orchards. This indicates the possible need to apply K fertilizers to old pistachio orchards where exchangeable K might be lower than 200 mg kg − 1 .