Effects of Preceding Crops on Nitrogen Fixation and Mycorrhizal Colonization in Soybean and Soil Health
Research Funded by the United Soybean Board
Xinhua (Frank) Yin, Professor & Cropping System Scientist, University of Tennessee Nutifafa Adotey, Assistant Professor and Soil and Nutrient Specialist, University of Tennessee, and
Introduction
The amount of nitrogen fertilizer left over from the preceding crop and the dynamics of crop residue breakdown can both play into how much nitrogen is around when soybean is planted. This becomes more important as nitrogen fertilizer prices go higher. We would like to know how much nitrogen could be captured through cover crops. These are nutrients that can be used by the following cash crop.
On the other hand, cover crops must have economic value within rotation. Economic benefits from cover crops include increased yields in cash crops, reduced fertilizer requirements, and improved soil yield potential. The effects of cover crops on soil health are usually longer lasting.
In this project, we are going to looking at which crop sequences have the most stable soybean yields and which crops to grow before soybean. We focus on the effects of crop rotations on yield, mycorrhizal colonization, and biological nitrogen fixation in soybean as these are of special interest to optimize soybean profitability in the U.S.
Through this project, farmers will know (1) the best crops to plant before soybean; (2) the effects of the preceding crop on soil health, nitrogen fixation, and mycorrhizal colonization in soybean; (3) the economic profitability of cash crop rotation with and without a cover crop. All these are beneficial for more profitable and sustainable soybean production
Research Methodology
Field trials were conducted in a randomized complete block design with four replicates at two locations in Tennessee. The treatments of winter cover crop and summer cash crop for winter 2023 — fall 2024 include: 1. Hairy vetch with cotton; 2. Crimson clover with cotton; 3. Winter wheat with cotton; 4. Rye with cotton; 5. Radish with cotton; 6. No cover crop with cotton; 7. Hairy vetch with soybean; 8. Crimson clover with soybean; 9. Winter wheat with soybean; 10. Rye with soybean; 11. Radish with soybean; 12. No cover crop with soybean; 13. Hairy vetch with corn; 14. Crimson clover with corn; 15. Winter wheat with corn; 16. Rye with corn; 17. Radish with corn; and 18. No cover crop with corn. Nitrogen, phosphorus, and potassium fertilizers will apply to corn and cotton based on initial soil testing results. Neither cover crop treatment nor soybean will receive any fertilizer.
Soil samples collected at the depth intervals of 0-15, 15-30, 30-60, and 60-90 cm before cover crop planting and after cash crop harvest will be used for the following analyses of soil health. Weak organic acid (H3A-4) extractable phosphorus, potassium, calcium, magnesium, sulfur, manganese, zinc, boron, iron, sodium, and aluminum; available nitrogen; water extractable organic carbon; total nitrogen; and soil respiration. Based on the above analyses, a soil health score will be calculated. Available nitrogen, phosphorus, and potassium (lbs./acre) for crops will be calculated. In addition, soil pH; total organic carbon, cation exchange capacity, nitrogen fixation, mycorrhizal colonization, carbon to nitrogen ratio, and cash crop yields will be measured. The relationships of crop yields with major soil health parameters will be determined
Results and Discussion
Further reading
