Soils across the northern Great Plains and Canadian Prairies generally have high soil pH (greater than 7.3); however, soil samples with moderately to strongly acidic pH (5.0-6.0) are becoming more common, particularly from long-term no-till fields in north-central Montana and southwest North Dakota. Soils with low pH can reduce the availability of certain nutrients, such as phosphorus. When soil pH is below 5.0, aluminum toxicity becomes a serious concern; aluminum is toxic to plants and reduces root growth, nutrient uptake, and yield.
We know that these soils have become more acidic over the past few decades; continuous application of nitrogen fertilizers is well documented to lower soil pH. In long-term no-till fields, this effect is more pronounced because surface fertilizer application and accumulation of organic matter concentrates acidity at the soil surface (0-2 inch depth). These no-till fields may have surface soil pH in the 4.0-5.0 range, which can cause aluminum toxicity for young seedlings.
In areas with limited access to lime, farmers may apply high rates of seed-placed phosphorus (about 40 lb/acre P2O5) to tie-up the soluble aluminum, improving wheat seedling establishment; this is a strategy used on acidic soils in western Kansas and Oklahoma. In the Southern Plains, most wheat varieties are rated for aluminum tolerance; however, we currently lack aluminum-tolerance ratings in northern wheat varieties because this problem is relatively new to our region.
There is interest in liming these no-till fields to correct aluminum toxicity. A buffer pH test, which can be requested, is used to determine the required lime rate. Surface application of lime without incorporation can effectively raise pH in no-till fields because the acidity is concentrated at the soil surface. This year AGVISE will be starting a long-term project to evaluate surface applied lime in no-till fields. We will keep you updated as we learn more about managing acidic soils in the Northern Plains.