Amelioration of Agriculturally Induced Acidity

R. J. Zasoski

R. J. Southard

R. D. Meyer

J. P. Edstrom

R. G. Burau

Introduction

Application of ammonium-based fertilizers can reduce soil pH. When soil microbes oxidize ammonium to nitrate they generate acidity. Soils have the capacity to buffer some of this acidity however, in the limited volume wetted by drip irrigation systems, rapid decreases in pH have been demonstrated at the Nickels Trust Orchard. Not only have these pH decreases been rapid, but also the soil pH levels were quite low. Soil pH values of 3.9 or less have been found below emitters in several locations at the Nickels Trust Orchard. This project is investigating methods to ameliorate low pH in the orchard. Two separate studies are examining soil acidification and its amelioration. The first study compares the effectiveness of mixing lime in basin to surface applications and the second study is evaluating calcium acetate as an alternative liming material to increase pH.

Comparison of Lime in the Surface and Mixed with the Basin Soil

Sampling. In 1995, two different lime (CaCO ₃ ) treatments were added to soil in the emitter basins. During the subsequent period, the untreated control and lime treatments continued to receive urea fertilization in the basins. Lime treatments were applied to the surface (1.2 lbs. lime /basin) or the lime was mixed with the basin soil to a depth of 18 inches ( 3.6 lbs. lime/ basin). Three replications of these treatments and a control (urea only) were sampled in the fall of 1998. Samples were taken at four locations spaced 9 inches apart at a radial distance of 27 inches from the tree trunk. The first sample location was centered in the drip basin and each subsequent sampling location was an additional 9 inches along the circumference toward the row middle. Each sampling location was cored to a depth of 30 inches in 6- inch increments with a 2-inch diameter soil auger. Sampling at equal distances from the tree trunk was designed to minimize differences in root density while moving away from the emitter location.

Soil Preparation. After air drying at 90° F for one week, the samples were ground with a mortar and pestle and passed through a 0.1-inch mesh sieve. Soil pH reaction was measured in a 1:1 water-to-soil and in 0.01 M calcium chloride, soil-to-solution suspensions after equilibration for 1 hour.

pH Profile of Agriculturally Acidified Soil

Evaluated over all depths, lime mixed with the soil resulted in significantly greater pH values (P< 0.001) compared to the urea treatment and surface lime applications. The effects of lime applications were quite localized. At a distance of 9 inches away from the basin, there were no significant differences between the treatments. A higher rate of lime application (3.6 lbs./basin) mixed with the upper 18 inches of soil effectively increased pH in the mixed area and in soil below the mixed zone. This effect was not evident 9 inches away from the basin The reason for this greater affect of lime in the mixed treatment is not immediately evident. The rate of lime application was higher, and mixing lime with the soil allows for greater contact between the soil and the lime.

A surface lime application increased pH in the soil surface, but the soil remained acidic in close proximity to and below the applied lime. This indicates that ability to ameliorate acidity is very localized in this treatment.

Interestingly, soil pH is lower at depth than at the surface. The reason that soil pH is greater in the surface is not clear. The irrigation water is basic (pH 8.3) and this helps to explain the higher surface pH. The decreased pH at depth indicates that nitrification is not taking place in the surface soils (0-15 cm) or that denitrification is counteracting the acidification from ammonium oxidation. Urea may leach downward before it is hydrolyzed and subsequently nitrified. The details of this aspect are interesting but beyond the scope of this report.

Studies of Calcium Acetate Effects on Soil Acidity

Field Studies of calcium acetate effects on Soil pH. We established a field trial to examine the effects of calcium acetate on soil pH. An untreated control, calcium acetate, and lime were added to cylinders of soil located beneath emitters in the orchard. A 20-inch diameter circle was excavated to a depth of 26 inches. The soil from two or three sample locations was sieved through a ½ inch screen, mixed, subsampled and replaced in the original excavation hole. The soil was separated into an upper 6-inch segment (A horizon) and a 20-inch segment (B horizon). Treatments were mixed with the upper 6-inch segment and placed on top of the mixed and replaced B-horizon material. Screening and mixing the samples will serve to homogenize the soil for subsequent samplings. The rate and magnitude of pH change in these samples will be followed by sampling over time.

Send mail to jlschmierer@ucdavis.edu with questions or comments about this web site.
Copyright © 1999 Colusa County Cooperative Extension, Univ. of California
Last modified: September 01, 2000