Main Limiting Factors of BNF in Tropics

Although inoculation with efficient and competitive strains can increase grain yield of soybean, common bean and other legumes in Brazil some limiting factors to the biological process are often reported under field conditions.

High temperature and low soil moisture are major causes of nodulation failure, affecting all stages of the symbiosis and limiting rhizobial growth and survival in soil. They may also contribute to undesirable changes in rhizobia, including plasmid deletions, genomic rearrangements and reduced diversity. Practices such as use of cover crops and no-tillage management decrease soil temperature and increase soil moisture, benefiting BNF, as discussed earlier.

Seed treatment with pesticides is used by most farmers and can drastically reduce nodulation and BNF. In Brazil, when soybean seeds were treated with ten fungicides recommended for the crop, decreases in nodulation varied from 8 to 88%, affecting BNF and yield (Campo, HungrĂ­a 1999). Soil acidity affects several steps in the development of the symbiosis, mainly nodulation.

Nutrients such as phosphorus and molybdenum often limit BNF, mainly under acidic conditions. For example, in an experiment performed in the State of ParanĂ¡, Brazil, supplying additional molybdenum increased soybean yield from about 3100 to 3400 kg ha"1 (Campo et al. 1999).

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