Soil Acidity and Aluminum Toxicity in the Palouse Region of the Pacific Northwest

Soil Acidity and Aluminum Toxicity in the Palouse Region of the Pacific Northwest

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Rich Koenig, Washington State University, Pullman, Kurt Schroeder, Washington State University, Pullman, Arron Carter, Washington State University, Pullman, Michael Pumphrey, Washington State University, Pullman, Tim Paulitz , USDA-ARS, Kim Campbell , USDA-ARS, Dave Huggins , USDA-ARS
Soil pH/soil acidity continues to be a major concern among wheat growers in the higher rainfall areas of the Palouse region. This publication is an update on earlier work conducted by Robert Mahler at the University of Idaho and provdies more recent guidance on where farmers should be concerned with soil acidity problems, how they can further diagnose the problem, and genetic (resistance) solutions for dealing with soil acidity in the short term.
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In the early 1980s, Dr. Robert Mahler of the University of Idaho published several research articles and Extension bulletins on soil acidification on the Palouse (Mahler et al. 1985; Mahler and McDole 1985; 1987a,b; 1994). At that time, Mahler and other researchers cited recent and archival data that indicated pH in the surface foot of soil had declined from near neutral (7.0), before farming began, to values below 6.0 in up to 65% of the fields surveyed (Mahler et al. 1985). A large number of fields had a soil pH below 5.5 and a few fields were below 5.0. Soil pH has continued to decline throughout the Palouse, and practices such as conservation tillage have led to the development of an intensified layer of acid soil at the depth of fertilizer placement (Figure 1).
Figure 1. Soil pH (1:1 soil:water ratio) profile from a no-till field near Rockford, WA, Fall 2010 (Koenig et al. unpublished).
Soil acidity has a dramatic impact on most chemical and biological processes. In plants, soil acidity can cause aluminum toxicity that leads to severe yield reductions. In general, legumes are less tolerant of soil acidity than cereals such as wheat and barley. This is because acid soil affects the legume’s ability to fix atmospheric nitrogen by reducing populations of Rhizobium bacteria (Mahler and McDole 1985). Mahler and McDole (1987) defined critical soil pH values (1:1 soil:water ratio) in the first surface foot of soil for peas (5.5), lentils (5.6), wheat (5.2 to 5.4), and barley (5.2) in northern Idaho. Below these soil pH values, crop yields declined dramatically. They also showed substantial yield responses to lime applications at sites with soil pH below these critical values (Mahler and McDole 1985).

Recent field trials involving lime applications across a range of Washington Palouse locations showed no response to lime in spring peas, wheat, or barley in fields with a pH as low as 5.0 in the first surface foot of soil (Brown et al. 2008; Koenig unpublished data). These studies were conducted at sites

historically covered by grass vegetation. An important feature of these sites is that the soil’s base saturation is still relatively high and its exchangeable aluminum is low, even though pH has dropped below critical levels (as established for these crops in the 1980s by Mahler and McDole). Due to higher base saturation and lower aluminum concentration, historical grassland soils are at lower risk of problems with aluminum toxicity. Areas historically covered in forest vegetation (Figure 2) are currently at greater risk of aluminum toxicity, since these areas commonly have lower pH throughout the soil profile depth, lower base saturation, and higher concentrations of exchangeable aluminum. Recent field trials (Paulitz and Schroeder unpublished) conducted at a northeast Washington site historically covered by forest vegetation showed dramatic spring wheat responses to lime applications. Also, Dr. Mahler’s original liming work was conducted at locations historically covered by forest vegetation.

As farmers and consultants on the Palouse continue to observe declines in soil pH, many are asking what they should do. It would be useful to review the map in Figure 2 to determine if a field is in a historic forested area. If it is not, the current risk of having soil pH affect crop production is low, although soil pH and crop health should continue to be monitored. If the field is in a historic forest area, and if soil pH is below 5.5, further investigation should be carried out by submitting a sample of the top foot of soil to a qualified testing lab (Daniels 2011). Lab tests should provide an analysis of exchangeable acidity as a percentage of the cation exchange capacity. Values above 60% exchangeable acidity (or less than 40% base saturation)

Figure 2. The historic distribution of forests in the inland Pacific Northwest. Shaded areas indicate current or historic forested areas at higher risk for aluminum toxicity when soil pH is below 5.5 (Hessburg et al. 1999).


Copyright 2011 Washington State University

Published September, 2011

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