Soil pH can have an influence on the outcome of root diseases caused by soilborne pathogens, but it depends on the pathogen. Cereal growers in the Pacific Northwest (PNW) have been experiencing an increase in soil acidity (lower pH) primarily due to a long history of ammonium fertilizer use.
In eastern Washington and northern Idaho, soil acidification tends to be worse in areas that are annually cropped, do not include nitrogen-fixing legumes in the crop rotation, and in areas that were historically forested. Forested soils tend to have a lower pH buffering capacity, making them more prone to shifts in soil pH. These same areas also typically include more forage and seed grass production and seldom include legumes in rotation, meaning that there is more intensive nitrogen application to the soil.
In addition, direct seeding can result in a stratification of soil pH in which the top few inches of soil are more acidic. This is because acidification caused by fertilizer application in the top soil layers is not diluted by mixing with the more alkaline soil below the fertilizer zone. However, the contribution of this stratification on management of soil acidity in direct-seed systems has not been evaluated.
Soil pH and Root Diseases
How does soil pH affect diseases? Plant disease is the outcome of three interacting components: the pathogen, the plant, and the environment. Soil pH, a component of the environment, influences both the pathogen and the host. Root diseases are caused by microscopic soilborne fungi. These organisms form a network of tiny threads, which can grow through the soil and infect plant roots.
Fungi absorb food as simple molecules from organic matter or living plants. These molecules must be transported across the membrane from the outside to the inside of the cell. The fungus expends energy and uses a proton pump to transport many of these molecules across the membrane by maintaining a proton (H+) gradient.
The external pH (proton concentration) can affect its ability to take up food, but in general, only extremes of pH (greater than 7 or less than 5) impact the growth of most fungi. Put another way, at the pH of most agriculture soils, the growth of most fungi is not pH limited. However, pH may influence the availability of trace nutrients, such as iron, zinc, or manganese, in the same way as its availability to plants is affected. Thus, fungi must work harder to get these less available nutrients.
The following is a summary of some diseases in the PNW that may be influenced by acid soils. Much of this evidence is based on research done in the PNW over 20 years ago, but some was conducted in other areas of the world.
The research also addressed the type of nitrogen (N) fertilizer—ammonium vs. nitrate. When plant roots take up the positively charged ammonium ion (NH4+), they balance the charge by excreting hydrogen ions (H+), making the pH lower in the root zone. When plants take up a negatively charged nitrate ion (NO3–), they balance the charge by excreting hydroxide (OH–), making the root zone more alkaline.
Take-all (Gaeumannomyces graminis var. tritici). Although widespread, take-all generally is not a serious problem in dryland wheat production, except in irrigated, continuous wheat or in the high rainfall areas of western Washington. It can be easily controlled with one year of a broadleaf, non-host crop such as pea, lentil, chickpea, or canola.
With take-all, the evidence is strong that it is more severe in alkaline than in acid soils, and that disease is reduced when ammonium forms of N are applied, as opposed to nitrate forms, an effect related to pH of the rhizosphere. The disease is greatly reduced when the rhizosphere pH is below 6.6, but the correlation with bulk soil pH was poor (Smiley, 1974).
The rhizosphere pH for wheat supplied with ammonium nitrogen was 5.5, compared to 7.5 for plants supplied with nitrate. The best control occurred with ammonium sulfate, and the addition of lime negated the control.
Rhizoctonia Bare Patch and Root Rot (Rhizoctonia solani AG-8, R. oryzae, and Waitea circinata). There is not much information on the effect of N or pH on Rhizoctonia root rots of cereals, and the results are mixed. There is no literature on the effect of pH on R. oryzae, which is widespread in eastern Washington.