As you examine the BNF data presented in the table in section 3, it is evident that the amount of BNF that takes place in a given forage crop community can vary considerably. This implies that various environmental and management factors influence the magnitude of BNF in a given forage crop community. What are these factors? In general, virtually any factor that affects the growth and development of the forage crop will have some impact on BNF. This would include a wide range of factors including air/soil temperature, soil fertility, soil moisture, pest damage, weed competition, and soil pH. In the following sections specific effects of some of these factors are discussed.

Soil pH

Soil pH affects both the host plant and the bacteria involved in the symbiotic relationship. With regard to the forage plants, soil pH affects the uptake of nutrients from the soil. In general, a soil pH between 6.0 and 7.0 provides the best environment for optimum uptake by the forage plant. Therefore, maintaining the soil pH in this range tends to promote optimal plant growth and optimal BNF in many forage crops. However, some forage species are best adapted to soil pH values somewhat higher or lower than this range. The astute forage producer should become familiar with the preferred pH range of their crops. The bacteria most often involved in BNF in forages, those in the genus Rhizobium, are often negatively affected by soil pH values less than 6.0. If soil pH is less than 6.0, and where it is economical to do so, limestone may be added to increase soil pH and thus increase BNF. There are some exceptions to this general rule. For example the symbiosis between red clover and its microsymbiont Rhizobium leguminosarum biovar trifolii appears to function well at pH values less than 6.0.

Soil Fertility

Reduction of BNF due to soil fertility will often be related to either an excess of soil nitrates or a deficiency of some essential nutrient limiting plant growth and development. In general, excess soil nitrate levels will depress BNF. Thus the application of fertilizer N to forage crops that are able to utilize BNF is rarely recommended. If there is any suspicion of soil nutrient deficiencies, soil tests should be conducted. If soil tests indicate a significant deficiency of essential nutrients, appropriate fertilizer applications are advised.

Image: Chart: pH vs. Nutrient Uptake

Image: Forage Legumes Grown in Diff Fertility Levels



Virtually any aspect of climate that affects crop growth rate will also affect BNF. Specifically, factors such as low air/soil temperature, lack of sunshine, or drought will likely reduce BNF, at least temporarily. While climate control is currently beyond the control of the forage producer, some management steps can be taken to minimize the effect of climate. Planting date can be selected to alleviate problems with excessively cold or warm soils. Timely irrigation can be used to minimize the impact of droughty periods.