In the last decade, soybean acres have increased by 707,000 acres in NW MN. Spring wheat yields have risen from 47.8 in 2011 to 68.5 bu/acre in 2017, while reported soybean yields are relatively flat ranging from 35.7 in 2007 and 34.1 bu/acre in 2017. A 40 bu/acre soybean crop will require 33.6 # P205 and 52 # K20. With increased wheat yields and the potential increase in soybeans yields, P and K may be a limiting factor for plant growth development and yield in a wheat -soybean crop rotation.
A 2017 AGVISE soil survey in northwest MN indicated 67% had Phosphorus (P) levels of < 10 ppm and 39% had Potassium (K) levels < 150 ppm. This soil survey suggests over 2/3 of the soil samples for P and over 1/3 of samples for K may have fertility levels that limit the production of high yield soybean and wheat. The objective of this project is to utilize a combination of small plot replicated research and large on-farm trials to determine if P and K levels are limiting soybean and wheat yields. Field trials will be conducted over four consecutive years in a wheat-soybean cropping sequence in northwest MN. The 2021 growing season will be year 3 of 4 for this project.

Project Goal:
Determine optimum levels for Phosphorus (P) and Potassium (K) in a high yield, 4-year wheat-soybean rotation in northwest MN.


Project objectives:
1) Establish long term crop rotation trials (4 year minimum) in wheat and soybeans using a combination of small plot replicated research trials and on-farm research sites
2) Conduct small plot replicated research to determine the influence of elevated levels of P and K on wheat and soybean growth, development, and yield
3) Partner with the MN Wheat On-Farm Research Network (OFRN) to evaluate enhanced P and K fertility utilizing large on-farm research trial methodologies to determine if current P and K recommendations provide adequate fertility in a high yield wheat-soybean crop rotation.

Background
In the last decade, soybean has become a popular crop choice for farmers in northwest MN. The USDA National Agricultural Statistics Service (NASS) details an 11-county area of northwest MN as District 10. District 10 includes the following MN counties: Becker, Clearwater, Clay, Kittson, Mahnomen, Marshall, Norman, Polk, Pennington, Red Lake and Roseau. According to the USDA, soybean acres in District 10 were 1,106,000 in 2007 and had risen to 1,813,000 in 2017. This is an increase of 707,000 acres of soybean during this ten-year period. In this same time frame, spring wheat acres have ranged from a high of 1,284,000 in 2008 to a low of 944, 000 in 2017. This data would indicate a spring wheat-soybean crop rotation is now a common cropping sequence for farmers in District 10 of northwest MN.

In the last ten years (2007-2017), USDA NASS data indicates spring wheat yields have ranged from a low of 47.8 in 2011 to a high of 68.5 bu/acre in 2017. That’s a wheat yield increase of 20.7 bu/acre from low-to-high during the last decade. However, reported soybean yields have been relatively flat during this same ten-year time frame. As an example, soybean yields were 35.7 in 2007 and 34.1 bu/acre in 2017.

Agvise Laboratories conducts a soil survey each year in their trade area. In 2017, the area that roughly corresponds to USDA District 10 had 67% of soil samples with a Phosphorus (Olsen Test) level of less than 10 ppm. Further, 39% of submitted the soil samples had Potassium levels less than 150 ppm. This soil survey data suggests that over 2/3 of the fields sampled for P and over 1/3 sampled for K, may not have adequate nutrients for the production of high yield soybean and wheat. With the recent increase in wheat yield and the potential increase in soybean yield, the soil levels of P and K may be a limiting factor for plant growth, development, and yield.

Methods and Timetables:
Objective 1
The objective of this research is to establish two small plot replicated research sites and five on-farm locations in 2019 and continue for four years. These sites will serve as in-field laboratories to determine if P and K levels are limiting crop production in a spring wheat-soybean rotation. A complete analysis will be conducted on collected soil and tissue samples to determine potential nutrient interaction with elevated applied levels of P & K. Previous research suggests elevated levels of P can interact with zinc. Further, high levels of K can have a negative impact on the uptake of calcium, nitrogen and magnesium. All sites will include an alternating spring wheat-soybean rotation for at least four years. The on-farm cooperators will be selected with input from the MN Wheat OFRN. Farmer cooperators will agree to have this site in a wheat-soybean rotation for at least four years and will follow best management practices. Wheat and soybean varieties will be selected from varieties in the upper 25% of yield trials for the geography near the small plot or on-farm research location.


Objective 2
Conduct small plot replicated research to determine the influence of elevated levels of P and K on wheat and soybean growth, development, and yield. The nutrient sources will be 0-46-0 and 0-0-60 and will be applied broadcast and incorporated prior to seeding. Experimental design will be a RCB with a factorial arrangement of treatments. Broadcast fertility treatments will be blended and applied by hand. Individual plot size will be 6 x 15 feet This small plot fertility trial will have 5 fertility rates for P, 5 for K and the combination of each plus a no treatment (no added P &K). Total number of treatments will be 15 plus a control = 16 treatments replicated 4 times for a total of 64 individual plots. Proposed treatments:
• 0-46-0 @ 20 units; 0-0-60 @ 20 units; 0-46-0 + 0-0-60 @ 20 units of each
• 0-46-0 @ 40 units; 0-0-60 @ 40 units; 0-46-0 + 0-0-60 @ 40 units of each
• 0-46-0 @ 60 units; 0-0-60 @ 60 units; 0-46-0 + 0-0-60 @ 60 units of each
• 0-46-0 @ 80 units; 0-0-60 @ 80 units; 0-46-0 + 0-0-60 @ 80 units of each
• 0-46-0 @ 100 units; 0-0-60 @ 100 units; 0-46-0 + 0-0-60 @ 100 units of each
• Untreated control

These small plots will be managed for a yield goal of 80 bu/acre for spring wheat and 50 bu/acre for soybeans. Individual plots will be harvested with a small plot combine for yield with a sub-sample collected from each plot for seed quality assessments. Soil samples will be taken prior to seeding in the spring of 2019. Annual soil samples will be taken after each crop harvest to determine soil residual levels of P & K. In-season tissue samples will be collected at early tillering in wheat and at the second to third trifoliate stage in soybeans. Crop vigor 30 days after planting, a relative chlorophyll index (RCI), and plant height will be collected for each plot.

Objective 3
The large on-farm trials will be conducted at various locations in USDA District 10. The goal to establish five field locations for this long-term wheat-soybean rotation project was reached in 2020. The trial design will be a randomized complete block with four replications. The farmer cooperators selected to participate in these research trials will strive for high yield wheat 80 and soybeans 50 bu/acre. Farmer cooperators will agree to utilize best management practices in each year of this project. Each site will have two treatments replicated four times (eight strips). All sites will have a standard farmer practice (FP) P & K rate (treatment 1) and an enhanced P & K strip (treatment 2).
• Treatment 1: FP - The current P & K fertility program
• Treatment 2: Enhanced P & K fertility program = FP P & K rate + an additional 50 units of P and K
Plot size will be one or two passes of the application equipment (70 or 140 feet) wide by the length of the field to accommodate the cooperator’s production practices and equipment. Commercial equipment will be used in all facets of the on-farm trials with a weigh wagon and/or combine yield map to determine wheat and soybean yield. Other data to be collected include: Crop vigor and color 30 days after planting, plant tissue samples at the same timing as small plot trials, plant height at physiological maturity, and soil samples from each strip after harvest. If aerial imagery or combine yield data suggests pockets of low or high yields, these areas will be marked for intensive soil sampling and analysis to determine the cause for this variable yield response. Soil cores will be taken from the same location within each strip for each year of this project.


Project Outreach and Deliverables
The Minnesota Wheat OFRN has an established track record of conducting on-farm research. This project will partner with the MN Wheat OFRN to select farmer cooperators, assistance in site selection, coordination and collection of research results and will assist with data analysis and summaries.

Minnesota Wheat has a tradition of disseminating research results in various methods of communication. Annual project results will be published in the Minnesota Wheat Research Review book (1,200 copies) and in the bi-monthly magazine publication titled: Prairie Grains (18,000 copies printed and mailed). This magazine reaches the spring wheat growing region of MN, MT, ND, and SD. Project presentations will be given at the annual Prairie Grains Conference, organized in part by Minnesota Wheat, Minnesota Soybean, and Minnesota Barley. The Prairie Grains conference is a two-day conference held annually in December at the Alerus Center in Grand Forks, ND. The project results will be presented during the annual half-day OFRN Research Summit (6th annual in 2018 had over 100 attendees). Farmers and interested stakeholders attend this research summit to learn and offer opinions on research priorities to help improve crop production and the “bottom lines” for their operations.
Research results will be printed in the annual On-Farm Research Report and distributed to attendees at the Prairie Grains Conference, and at the Small Grains, Wheat, Soybean and Corn Update Meetings held at 8 locations each January. On average, 50 people/location attend these meetings held in Fergus Falls, Moorhead, Crookston, Ada, Hallock, Argyle, Roseau, and St. Hilaire. Research results be will be presented at the Minnesota Soybean Growers Association regional and county meetings. Project results will be summarized for on-line publication in Minneline (MN Soybean), and U of MN Crops News Blog, and in the smallgrains.org research database (MN Wheat).

Update:

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Update:

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Spring wheat and soybeans is a common crop rotation for producers in NW MN. In a ten year period from 2007 to 2017, soybean acres increased from 1.1 million to 1.8 million acres in USDA District 10 in NW MN. Reported wheat yields in 2007 were 50.4 and in 2017 was 65.8 bu/ac, but soybean yields were relatively flat at 35 bu/ac. One of the theories for the flat response in soybean yields was a lack of phosphorus (P) and potassium (K).

The project objective is to determine if elevated levels of P&K will improve wheat and soybean yields compared to standard P&K rates. These wheat and soybean research trials were conducted at two sites with small plot replicated trial methods and five locations with on-farm strip trial methodologies.

Small plot wheat yields ranged from 60 to 84.8 bu/acre and soybean yields ranged from 46 to 54.1 bu/acre. Wheat yields from the low P rate (20) were increased by 12.6 bu/ac and all P rates produced 19.6 bu/ac or more than untreated. Soybean yield were flat, probably due to drought conditions in August. On-farm soybean trials gave a positive response to additional 50 units P&K in 25% of the sites.

These data will help farmers in MN determine when and where increased P and K fertility can increase yield and profitability, and determine where in the rotation P and K is most used.