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.

Update:
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

View uploaded report

View uploaded report 2

Update:
Improving Phosphorus Management for Soybean: Integrating Cover Crops and Fertilizer Placement and Time

Progress Report, April 2024

D. A. Ruiz Diaz and J.O. Demarco, Kansas State University, Manhattan, KS


SUMMARY

This study aimed to maximize the soybean crop's phosphorus (P) use efficiency by using cover crops (CC) planting as a window of opportunity for better P fertilizer placement and timing. Specifically, P fertilizer can be combined with cereal CC seeds to place the fertilizer below the soil surface, and two operations (CC planting and fertilizer application) can be combined in one pass. Other benefits include eliminating the environmental risk of P fertilizer runoff and potentially creating a synergistic benefit of the CC and fertilizer combination on P availability to the soybean crop. The specific objectives of this study were to improve phosphorus management for soybean production in Kansas, increase yields using improved diagnostic tools and fertilization strategies and leverage opportunities for application placement with a CC in the rotation. Nine sites were established during the project, with five locations under supplemental irrigation and four rainfed locations. Phosphorus treatments included a control with no P application and three P rates of 40, 80, and 120 lbs P2O5/acre, using mono-ammonium phosphate (MAP). Cover crop treatments included oat and triticale with no P application and with P application of 45 Kg P2O5/ha. Cover crop samples were collected before soybean planting to measure biomass and P uptake. Soybean whole plant samples were collected at the V3-V4 stage for P Uptake analysis. At harvest, grain yield was recorded for each plot. The results obtained with this research showed that there was no significant response to CC treatments in locations that are non-responsive to P fertilization. In responsive locations to P fertilization, there was a penalty in soybean growth and yields when adding CC. However, results showed that excessive CC growth ahead of soybean could be detrimental to soybean yields, and CC termination should be timed to minimize potential yield penalty. Large water use by the CC could have affected the soybean growth and yield when biomass in the CC was excessive.

BACKGROUND

Phosphorus is an essential nutrient for plant development and can be scarce in some ecosystems, in addition to being an important cost for agricultural production and being a non-renewable resource. Phosphorus management can alter plant use efficiency, just as tillage and fertilizer placement can alter nutrient availability and stratification in the soil (Mallarino and Borges 2006).
The creation of many agricultural best management practices have been proposed to reduce fertilizer P losses, and their implementation is important since most fertilizer recommendation systems for agricultural crops were developed based on maximizing yields and not on avoiding possible environmental impacts (Withers et al. 2014).
Keeping the soil exposed, in the period without crops growing, can cause soil disaggregation by the impact of rain, and consequently runoff of soil and nutrients by water or even losses by wind (Havlin et al. 2005). Cover crops have been encouraged to be used before crops such as corn and soybeans, seeking the principles of a more conservationist agriculture. Cover crops can decrease sediment losses as they cover the soil surface during the time when there are no crops growing in the field, reducing the energy of raindrops and the speed of water runoff, increasing water infiltration into the soil and avoiding nutrient losses (Blanco-Canqui et al. 2011).
The soybean crop provides one of the best opportunities to include a cool season cover crop before planting. Combining P fertilizer with cereal cover crop seeds will place the fertilizer below the soil surface and combine two operations (cover crop planting and fertilizer application). This study aims to maximize phosphorus use efficiency by the soybean crop by using cover crop planting as a window of opportunity for better P fertilizer placement and timing. The hypothesis of this study was that, in locations responsive to P application (low P levels in the soil), CC would be beneficial for soybeans as it would act as a slow-release source of P into the soil.

METHODS

This study was conducted in 2022 and 2023 at nine locations across Kansas. Among the nine locations, five were established under supplemental irrigation and four rainfed locations. Before fertilizer application, soil samples were collected at a depth of 0 to 15 centimeters using a hand probe. The average soil test P (Mehlich 3 and Bray 1), pH, and organic matter (OM) are presented in Table 1.
Phosphorus treatments included a control with no P application and three P rates of 40, 80, and 120 lbs P2O5/acre, using mono-ammonium phosphate (MAP). CC treatments included triticale (planted in fall) and oat (planted in spring) with no P application and with P application of 40 lbs P2O5/acre. P rates and CC were arranged in a factorial combination of treatments.

CC samples were collected before soybean planting to measure biomass and P uptake. Soybean whole plant samples were collected at the V3-4 growth stage to be analyzed for P uptake. The plant tissue samples were digested using nitric-perchloric acid digestion and analyzed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). At harvest, grain yield was recorded for each plot.
Data was analyzed by location and combined using lmer4 package in R 4.3.1, using RStudio (Version 2023.06.1+524), assuming block as a random factor in the model. When locations were combined, it was also considered as a random effect.

RESULTS

This study looked at how adding different cover crops (CC) before planting soybeans affected their growth and yield, especially when combined with phosphorus fertilizer use. When comparing oats and triticale (the two cover crops), we found that triticale produced more biomass (plant material) in part because it had more time to grow before soybeans were planted. When phosphorus fertilizer was used, the biomass increased even more.

The study also looked at how early in the season the plants took up phosphorus. In some locations where the soybean didn't respond much to phosphorus fertilizer (Non-Responsive), adding cover crops didn't show a significan effect on soybean. However, in locations where phosphorus fertilizer was needed (Responsive), adding cover crops seemed to slow down phosphorus uptake in soybean, especially with triticale cover crops.

Cover crops break down over time, releasing nutrients into the soil. In places where phosphorus is limited, this breakdown may be too slow, which could penalize soybean growth early in the season when the phosphorus demand is high. In locations with low soil phosphorus levels, adding cover crops seemed to penalize soybean yields. This might be because the cover crops didn't break down fast enough to provide phosphorus when the soybeans needed it early in the season.

Overall, adding cover crops didn't show a significant effect in locations where soil phosphorus wasn't a limiting factor. However, in locations where soil phosphorus was low, adding cover crops seemed to contribute to lowering soybean growth and yield. Dry conditions in Kansas might have also played a part in this, affecting how quickly the cover crops broke down and how much water the soybeans could access after CC termination.

Even though adding cover crops might not always help soybean yields, they can contribute to overall soil health and preventing weeds, especially in locations where soil phosphorus isn't a limiting factor. Additional studies should explore in more detail the effects on soil health and soil carbon as key components of sustainability in soybean production.

View uploaded report

View uploaded report 2

This study looked at how adding different cover crops (CC) before planting soybeans affected their growth and yield, especially when combined with phosphorus fertilizer use. When comparing oats and triticale (the two cover crops), we found that triticale produced more biomass (plant material) in part because it had more time to grow before soybeans were planted. When phosphorus fertilizer was used, the biomass increased even more.

The study also looked at how early in the season the plants took up phosphorus. In some locations where the soybean didn't respond much to phosphorus fertilizer (Non-Responsive), adding cover crops didn't show a significan effect on soybean. However, in locations where phosphorus fertilizer was needed (Responsive), adding cover crops seemed to slow down phosphorus uptake in soybean, especially with triticale cover crops.

Cover crops break down over time, releasing nutrients into the soil. In places where phosphorus is limited, this breakdown may be too slow, which could penalize soybean growth early in the season when the phosphorus demand is high. In locations with low soil phosphorus levels, adding cover crops seemed to penalize soybean yields. This might be because the cover crops didn't break down fast enough to provide phosphorus when the soybeans needed it early in the season.

Overall, adding cover crops didn't show a significant effect in locations where soil phosphorus wasn't a limiting factor. However, in locations where soil phosphorus was low, adding cover crops seemed to contribute to lowering soybean growth and yield. Dry conditions in Kansas might have also played a part in this, affecting how quickly the cover crops broke down and how much water the soybeans could access after CC termination.

Even though adding cover crops might not always help soybean yields, they can contribute to overall soil health and preventing weeds, especially in locations where soil phosphorus isn't a limiting factor. Additional studies should explore in more detail the effects on soil health and soil carbon as key components of sustainability in soybean production.

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.