Yield Losses Resulting from Soil Acidity

Chapter 6 – Fertility, No. 8, Winter 1987

Roger Veseth

Reductions in crop yield resulting from soil acidity have been well documented in the high precipitation areas of the United States and other countries around the world. Now, research is showing that increasing soil acidity is also becoming a crop production limitation in northern Idaho and other Northwest cropland areas receiving intermediate precipitation levels.

Originally, cropland soils in this region were slightly acid (pH 6.5) to slightly alkaline (pH 7.2) when the sod was broken before the turn of the century. In 1984, Robert Mahler, University of Idaho soil scientist, and Robert McDole, Extension soil specialist, estimated that over 45 percent of agricultural soils in northern Idaho now have pH values less than 5.6. This acidification, which is present mainly in the surface 8 to 12 inches, is attributed primarily to high application rates of ammonium-based nitrogen fertilizer over the past 40 years.

Field Research Trials

From 39 field studies, conducted between 1980 and 1986, Mahler and McDole have developed pH-yield relationships for four major crops in northern Idaho: lentils, spring peas, winter wheat and spring barley. All field sites had initial soil pH values (1:1 water to soil saturated paste) between 5.0 and 5.3. Experimental plots then received varying rates of lime (to increase soil pH) and elemental sulfur (to decrease pH) to provide a range of pH values. Soil pH values were determined in late June of each year.

Yields were based on percent maximum yield, where maximum yield is the highest yielding plot within each study. The use of maximum yield minimizes environmental effects on crop yields between years and field sites. Minimum acceptable pH values were determined for each crop and crop variety, if differences were apparent (Table 1). These values are an estimate of the lowest soil pH level that did not adversely affect yield.

Mahler and McDole point out that when comparing relative tolerance of different crops to low soil pH, rooting depth and pH distribution with soil depth are two important factors.

Table 1. Minimum acceptable soil pH values for maximum yield for four crops grown in northern Idaho (Mahler and McDole, Ul).

Cereals have an effective rooting depth of 3 to 6 feet or more while legumes, such as lentils or peas, produce a majority of their root mass in the upper 20 inches of soil. Since low soil pH values are usually only observed in the upper 8 to 12 inches in northern Idaho soils, these shallow rooted legumes are less tolerant of soil acidity than cereals. However, they stress that soil acidity can adversely affect cereals when: (1) the plant is young with a small root system; (2) plant nutrients applied as fertilizer are immobile in the soil and acidity is adversely affecting root uptake and/or availability of the applied nutrients; or (3) acidity of the surface soil layer is increasing the incidence of plant disease (such as Cephalosporium stripe), shifts in weed populations, and other factors affecting production.

Legumes

Lentils were most sensitive to low soil pH. Mahler and McDole estimate that 52 percent of the agricultural soils in northern Idaho have pH values less than the minimum acceptable value of 5.65. At soil pH values of 5.4, 5.2 and 5.0, their research indicates that lentil yields could be expected to decline to only 79, 65 and 50 percent of maximum yield, respectively. No significant differences were found between the Chilean and Tekoa varieties of lentils evaluated in the study.

The minimum acceptable pH value of 5.52 for spring peas was similar for the Alaska and Columbia varieties. An estimated 42 percent of northern Idaho cropland has pH values less than the minimum acceptable value for maximum pea production. At soil pH values of 5.3, 5.1 and 4.9, they estimated that spring pea yield could decline to only 77, 59 and 42 percent of maximum yield, respectively.

Mahler and McDole point out that the effect of low soil pH on lentil and pea yields may be indirect in that soil pH values less than 5.65 inhibit Rhizobium leguminosarum populations in the soil. These bacteria are responsible for nitrogen fixation by nodules on the legume roots. This would reduce the nitrogen supply for the legume crop as well as carryover nitrogen for the following cereal crop.

Winter Wheat

Winter wheat showed a higher tolerance to soil acidity than did lentils and peas. Minimum acceptable pH values ranged from 5,19 to 5.32 depending on the variety. Although Daws had the lowest minimum acceptable pH value for maximum yield (pH 5.19), Hill 81 maintained the highest percent maximum yield as soil pH declined (Table 2). An estimated 16 percent of northern Idaho cropland has soil pH values less than the minimum acceptable for maximum production for Daws, 27 percent for Hill 81 and 30 percent for Stephens.

Spring Barley

Spring barley responded to soil pH much like winter wheat with a minimum acceptable pH value for maximum production of 5.23 (Table 1). Advance and Steptoe varieties responded similarly. At pH values 5.2, 5.0 and 4.8, Mahler and McDole estimate that spring barley yields could decline to 93, 74 and 54 percent of maximum yield potential, respectively. Approximately 19 percent of northern Idaho cropland has soils below the minimum acceptable pH for maximum production of spring barley.

Table 2. The effect of soil pH on percentage maximum yield of three winter wheat varieties in northern Idaho (Mahler and McDole, Ul).

Management Options

Mahler and McDole point out that different crop and variety tolerance to low soil pH provides a useful, though temporary, management tool. Looking at the long term, it is apparent that soil pH values will probably continue to decline. Consequently, they feel the ultimate solution to soil acidity is the development of a liming program. The development of lime application guidelines is currently underway.