Refining Needs for Phosphorus Fertilizers and Cover Crop Benefits in Vegetable Systems

Study author(s): Charles Rohwer, University of Minnesota
Growing season(s): 2026, 2027, 2028
Minnesota location(s): Waseca

Note: Complete reports with figures and tables will be made available for download for each crop year.

What is this project about?

Vegetable growers frequently apply high-P fertilizers at transplanting to aid crop establishment. However, many vegetable fields—especially those with a history of compost or manure applications—already test “very high” in soil phosphorus. Applying extra starter P in high-testing soils may add unnecessary cost without increasing yields or crop quality.

This project evaluates whether starter P fertilizers improve cauliflower yield or maturity in different soil P environments. It also studies whether planting a post-harvest fava bean cover crop can help draw down or cycle soil phosphorus for future crops.

Large cauliflower harvested on a sunny Minnesota day

Key Project Goals & Farmer Benefits

  • Clarify Starter P Needs: Determine if starter phosphorus (such as 10-34-0) actually boosts cauliflower yields or speeds up harvest when soil P is already high.
  • Reduce Fertilizer Costs: Help growers avoid over-applying unnecessary phosphorus inputs without sacrificing crop performance.
  • Soil P Mining via Cover Crops: Evaluate if a late-summer fava bean cover crop helps release or cycle active phosphorus in the soil ahead of the next growing season.
  • Practical Recommendations: Provide updated starter fertilizer and cover-cropping guidelines to commercial vegetable growers, extension specialists, and crop consultants.

How the Study Will Be Conducted

  • Location: Conducted across two cycles (2026–2027 and 2027–2028) at the Southern Research and Outreach Center in Waseca, MN.
  • Crop & Varieties: Cauliflower cultivars (‘Absolute’ and ‘Twister’) grown alongside Romanesco borders.
  • Soil P Levels: Compares normal soil P conditions against simulated high-P soil conditions created with broadcast fertilizer.
  • Starter Treatments: Evaluates three transplant watering solutions:
    • Plain water
    • Water + nitrogen-only starter (28-0-0)
    • Water + nitrogen and phosphorus starter (10-34-0)
  • Post-Harvest Cover Crop: After the cauliflower harvest in September, half the plots will be planted with a fava bean cover crop (80 lbs/acre), while the other half remain bare.
  • Measurements: The team will measure cauliflower head weight, size, and P concentration; calculate total P removal; test fava bean biomass; and analyze soil test P and organic carbon in the spring.
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