SOIL CHEMICAL AND MINERALOGICAL CHANGES DUE TO ACIDIFICATION IN OKANAGAN APPLE ORCHARDS

Abstract

Five Okanagan Valley soil series that had been in orchard for from 12 to 40 yr were studied to determine the effect of orchard practices on chemical and mineralogical soil properties. The soils were coarse-textured with relatively low organic C contents. Acidification produced by fertilization, irrigation and herbicide use had lowered soil pH values from between 5.0 and 6.0 in the alleys between the trees to about 3.0–3.5 near the trees and had greatly decreased permanent-charge cation exchange capacities, base saturation percentages, and ratios of exchangeable Ca/Mg, Ca/K, Mg/K. The pH-dependent or variable charge increased with organic C content and thus originated mainly from the organic matter of these coarse-textured soils. The proportional losses of exchangeable bases during acidification followed the order Ca > Mg > > K. Acidification had also released large quantities of plant-available Al and Mn. The soil clays from both the higher and lower pH sites were relatively poorly crystalline with X-ray amorphous constituents and mica as major components. Acidification appeared to have decreased mineral crystallinity as well, as indicated by consistently higher levels of extractable oxides of Si, Al and Fe in the clays from the acidified soils. These effects of orchard practices on chemical and mineralogical properties developed rapidly and were evident in one soil within 12 yr. Practical implications of these results for improving the management and fertility of orchard soils are discussed. Key words: Acidification, chemical and mineralogical changes, Okanagan soils, apple orchards