The Effect of Nitrogen Fertilization and the Liming of Acid Soils on Kidney Bean Root Rot in Minnesota
April 13, 2004
Overview: Root Rot Problem
Decreasing red kidney bean yields have been the result of a combination of factors such as growing root rot susceptible varieties, increasing populations of the primary soilborne fungal root pathogens (Fusarium solani, F. oxysporum, and R. solani), the planting of susceptible rotational crops, reduced tillage which results in the survival of the root rotting fungal pathogens in previous crop residues, and soil acidification, probably caused in part by the use of nitrogen fertilizers. In order to maintain high Kidney bean yields it was thought that growers must use high rates of nitrogen fertilizer. Acid soils with a pH at or below 5.5 are fairly common in north central Minnesota, where Kidney bean root rot problems continue to be a major concern for decreasing yields. Dry bean growth can be affected in acid soils by aluminum, iron and/or manganese toxicity as well as by deficiencies of calcium, magnesium, boron or phosphorus. Also, the adaptation of reduced tillage practices has been associated with the acceleration of soil acidification. Current information concerning the effect of soil pH and the impact of high nitrogen fertilizer on dry bean root rot is limiting and at best confusing.
Research Summary 2003 (Year-One):
1. Various bacterial biocontrol seed treatments (Bacillus subtilis and Rhizobium tropici) combinations were effective in reducing root rot and significantly increasing Kidney bean yield when compared to a chemical seed treatment plus 90 lb./A of nitrogen (Table 1). The site was highly infested with the root rot pathogens and a deep tillage was used.
2. On-farm Kidney bean yields increased 22% without the use of nitrogen fertilizer.
Nitrogen fertilizer above 30 pounds per acre at sowing reduced Rhizobium infection, nodule formation, and the fixing atmospheric nitrogen.
3. Nitrogen fertilizer applied at 30 and 90 pounds per acre did not significantly increase Kidney bean yields when compared to seed inoculated with Bacillus subtilis and Rhizobium without nitrogen (Table 2). This on-farm study is trying to determine if nitrogen fertilization has any significant yielding enhancing benefits when biocontrol seed treatments are used in conjunction with deep tillage. (year-one of a two-year study).
4. Kidney bean yields were significantly increased 36 to 68%, depending on biocontrol seed treatment, in root rot pathogen infested deep tilled (11-14 inches) soil when used with Bacillus subtilis/Rhizobium treated seed.
5. On-farm studies in a soil infested with the root rot pathogens, having a low pH (5.1), and compacted resulted in Kidney bean yield increases of 36-44% with the use of deep tillage (11-14 inches), regardless if the soil was limed or the seed treated with Bacillus subtilis plus Rhizobium when compared to reduced tillage (year-one of a two year study).
6. The effect of Kidney bean (Montcalm) seed treated with different Bacillus subtilis strains on yield in an acid soil (pH 5.1) at Staples, MN in 2003 was quite variable depending upon the stain (Table 3). Some strains were able to infect, form nodules, and fix nitrogen better than others. However, all the strains resulted in higher yields than the untreated seed with 90 pounds of nitrogen per acre. These B. subtilis strains (Dr. P. Graham, Univ. of Minn.) are very promising in that they can successfully infect and fix nitrogen in an acidic soil.