Since much of no-till ground is in areas of low annual precipitation, and since such areas are considered stress areas for biological N-fixing Inoculants, potential problems exist.
1. The general no-till suggestion is to plant soybeans early and shallow. This can place a seed-applied Inoculant close to the soil surface exposed to the sun and drying winds. Also, early-planted soils often are more moist soils that can easily be compacted. Compaction not only makes it difficult for the soybean to emerge, but also it makes it difficult for air to get to the nodule and thus provide the gaseous nitrogen needed for the Nitrogen Fixation process. Further, it has been observed in some cases that compaction can result in decreased nodule number.
2. Often seed treatment fungicides are recommended for early plantings and thus fungicide/Inoculant compatibilities can be a problem – i.e. just as fungicides kill biological disease organisms in the soil, they can kill the Rhizobia bacteria in the Inoculant or in the soil. The level of toxicity depends on the active ingredient in the fungicide, and on the formulation of the active ingredient.
3. In some areas, pH can be a problem. Especially in areas where wheat has been continuously grown for a long time. The pH optimum for growing wheat may be below or close to 6.0 while the optimum for soybeans is closer to 6.5. A low pH kills the Rhizobia bacteria in the Inoculant and/or prevents the bacteria from surviving and growing in the soil.
4. Many no-till farmers use starter fertilizer with soybeans because of low initial nitrogen levels. If too much nitrogen is used, the soybean roots may not pick up the Rhizobia early when the bacteria are readily available; and then when the soil N is totally depleted by normal plant growth, the Inoculant Rhizobia bacteria are dead or not available. Also, direct contact between starter fertilizer and the Inoculant bacteria can kill the bacteria in some cases (usually because of low pH fertilizer.)
5. Drought! Drought shuts down plant growth as everyone knows. One of the initial things a soybean plant does in a drought stress is to redirect photosynthate from the nodule to the root tips -- most likely to help the roots grow and explore for more water. When the Rhizobia bacteria in the nodule don’t receive the photosynthate energy source from the plant, they stop fixing nitrogen.
There is not a lot a farmer can do about drought and some of these other factors, but it is important to understand their effect on the soybean plant and on the process of Nitrogen Fixation. Inoculants are not super fertilizers or magic bullets but rather just provide the living bacteria (Rhizobia) to soybean roots so that the plant can use the free nitrogen in the air. Both the bacteria and the soybean plant must be in good health for this symbiotic process to develop and work efficiently.