The use of cover crops in an integrated production system can provide many benefits, including biomass production to increase soil carbon and improve soil health. In addition, some researchers also hypothesize that cover crops may take up P from less available fractions in the soil and recycle it back to the soil through plant residues, resulting in better P use efficiency. However, there is currently limited field data to support this hypothesis and quantify the effects on soybean yields.

Optimum fertility management for new soybean genotypes and yield potential also required continuous evaluation and adjustment of diagnostic tools, including traditional soil tests and tissue analysis as well as novel methods. In addition to improved genotypes, changes in farming practices aiming to increase soil carbon and health can affect nutrient cycling and availability to crops.

Proper phosphorus fertility management can have significant economic and environmental benefits. The expected results from this project can inform soybean producers on the best management options integrating cover crops and fertilizer management to minimize cost, increase yields and reduce environmental risk.

The co-PIs are extension specialists with active programs on soybean production, soil fertility, and soil and water quality. The outcomes from this research will be shared as early and frequently as possible as reliable findings become available. Results will be communicated to Kansas producers and crop advisors in various ways. Information will be disseminated through field days, the regular KSU Soybean Production Schools, extension publications, and the KSU nutrient management webpage. Results of field experiments and any revised recommendations that arise will be summarized and distributed to the public via news releases. Brief articles will be prepared for publication periodically in the Kansas State University Agronomy eUpdates. Results will be shared with county/district extension agents in the state who provide information to soybean growers on a regular basis.

Finally, this work will be completed in close collaboration with the KSU soil testing laboratory. The results will be shared with the KSU soil testing lab and private laboratories that advise Kansas growers each year.

The overall objective of this project is to improve phosphorus (P) management for soybean production in Kansas, increasing yields using improved diagnostic tools and fertilization strategies, and leveraging opportunities for application timing and placement with cover crops in the rotation.

Specific objectives include:
• Determine agronomically efficient and cost-effective P fertilizer application time and placement to maximize soybeans yields.
• Assess the interaction of cover crops ahead of soybean and P availability from the soil fraction and fertilizer application to maximize P use efficiency and reduce environmental losses.
• Evaluate current soil test interpretations for accurate P fertilization rates with increasing soybean yields, including the evaluation of new chemical and biological soil test methods.

• Establish multi-location research on the integration of fertilizer placement, timing, and cover crops ahead of soybean to improve phosphorus (P) use efficiency and soybean yields.
• Development of P management strategies that minimize environmental risks for surface water in Kansas.
• Increase profitability for soybean growers through improved P management practices.
• Measure and enhance the overall soil health and soil carbon levels in soybean production systems.
• Generation of applied knowledge for effective phosphorus management in the field.
• Transfer research findings and information directly to soybean growers and agronomists through multiple extension programs.
• Contribute with applied and current local information for the soybean production schools and disseminate the latest P management techniques and practices to stakeholders in the soybean industry.

Updated December 1, 2023:
The biomass of CC showed a significant difference comparing oat and triticale, with
higher values when P fertilizer was applied (Figure 1). The difference between the CC
was mainly due to the longer time given for triticale to grow, as it was planted in the fall
of the year before soybean planting, while oat was planted in the spring.
Early-season phosphorus uptake (V3-V4) showed no significant difference between
CC treatments with or without fertilizer P application I non-responsive locations (Linked document Figure 2
– Non-Responsive). In locations responsive to the application of P fertilizer (Linked document Figure 2 -
Responsive), there was a penalty in P uptake when a CC was added, showing a tendency
to reduce even further when the CC was triticale.
The CC undergoes a decomposition process that lasts several days, during which time
the nutrients they contain are gradually released into the soil. In scenarios where soil P
availability is limited (Linked document Figure 2 – Responsive), delayed decomposition of cover crops can
result in slower release of P. Consequently, this delay can negatively affect soybean
crops, particularly during the early season, as the slow release of phosphorus from cover
crop residues may not readily satisfy soybean nutrient demand. This delay can potentially
interfere with the development of soybean plants and their P uptake (Varela et al. 2017).
In locations where the crop was non-responsive to P fertilization, the treatments with
or without cover crops did not exhibit a significant difference in grain yield (Linked document Figure 3 –
Non-Responsive). However, the scenario changes in areas with low P levels (Linked document Figure 3 –
Responsive). The decomposition of cover crops may not occur timely or completely by
the time the main crops need to uptake this nutrient for optimal growth, resulting in a
penalty by using CC (Poudel et al. 2023). The disparity in grain yields in these cases can
also be attributed to the disadvantage faced during the soybean early season, where
nutrient demand is high but supply from cover crop decomposition was slow.
In summary, there was no significant response to CC treatments in non-responsive
locations. In locations responsive to P fertilization, there was a penalty in soybean growth
and yields when adding CC to the system, rejecting our hypothesis that CC treatments
would act as a slow-release source of P into the soil for the next cash crop.
The situation where cover crops were at a disadvantage could also result from the
dryer Kansas environment, which might have impacted the rate of decomposition and/or
the availability of water to the main crop. However, in scenarios where no significant
differences in grain yield were observed, employing CC may still present benefits as they
can enhance soil health and protection, contributing to a better soil structure or playing
as a weed suppressor

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Phosphorus is the most limiting nutrient for soybean yields, and also poses a high environmental risk for surface water in Kansas. Integrating fertilizer placement, timing, and cover crops ahead of soybean may increase P use efficiency, soybean yields, and profitability while minimizing losses. Proper management can also improve overall soil health and soil carbon in production systems in the medium and long term.

This project will generate needed, applied knowledge for P management in the field. And the information will be transferred directly to soybean growers and agronomists through extension programs and regular soybean production schools.