Many organic farms have high phosphorous levels, often exceeding recommended thresholds
Livestock manure is a readily available and cost-effective fertilizer, making it a key treatment in both conventional and organic farming. However, manure is also rich in phosphorous, which can leach into the water system and accelerate eutrophication. Data show organic farms have much higher phosphorous levels in their soil, many above the recommended threshold.
Livestock manure is a readily available and cost-effective fertilizer, making it a go-to in both conventional and organic farming. Manure provides important plant nutrients, like nitrogen, and can be an excellent soil conditioner. Properly managed manure applications recycle nutrients to crops, improve soil quality, and protect water quality.
Because organic farmers cannot use chemical fertilizers, they often apply large amounts of manure to boost their nitrogen levels. However, manure is also rich in phosphorous, which can build up in soils over time. Phosphorus is an essential element for plant and animal growth, but it also increases the biological productivity of surface waters by accelerating eutrophication. Because it causes increased growth of algae and aquatic weeds, as well as oxygen shortages resulting from their die-off and decomposition, eutrophication restricts water use for fisheries, recreation, and drinking.
Conventional farmers also apply manure but often in much smaller amounts, because they rely more on chemical fertilizers to supply crops with nitrogen. Additionally, many conventional farms have crop consultants that test their soil regularly and adjust the amendments to maintain soil health and nutrient levels.
Data from the Citizen Science Soil Health Project show that many organic farms in Boulder County have much higher phosphorous levels in their soil than conventional farms. In addition, phosphorous levels often exceed the threshold recommended for water quality protection.
Here we explore the data and make recommendations for monitoring soil health in Boulder County. We suggest that Boulder County continue to help farmers regularly test and restore their soils, as well as incorporating legume cover crops and establishing buffer strips along fields, to absorb nutrient runoff.
explore the data
Phosphorous levels in Boulder’s soil: Conventional versus Organic farms
Figure 1. Phosphorus levels (mg P kg-1) in the soil of conventional versus organic farms. Blue bars represent conventional farms, while gray bars represent organic farms. Note that organic farms more often surpass the 100 mg P kg-1 recommended phosphorus threshold for water quality protection. Source: Data collected from the Citizen Science Soil Health Project. Soil health was calculated from soil health tests and annual soil health scores for each participating grower, using the Haney/Phospho-Lipid Fatty Acid (PLFA) soil tests from Ward Labs.
Percentage of farms at or above the recommended phosphorous threshold
Figure 2. Percentage of farms at or above the phosphorus threshold, split by conventional and organic farm types. Only 10.5% of conventional farms, compared to 47.8% of organic farms, are at or above the recommended phosphorous threshold. Source: Data collected from the Citizen Science Soil Health Project. Soil health was calculated from soil health tests and annual soil health scores for each participating grower, using the Haney/Phospho-Lipid Fatty Acid (PLFA) soil tests from Ward Labs.
recommendations
Partner with organic farms to improve soil monitoring and remedy unhealthy soils.
Healthy soil safeguards Boulder’s potential for local, healthy foods / Unsplash
Large-scale and conventional growers often hire consultants to assess soil health and provide recommendations for fertilizer applications. These services include calculations for how much manure will be needed per acre of land and suggestions for application timelines.
Because they are often able to monitor nutrients in their soil through these consulting services, farms can avoid harmful buildups of phosphorus in the soil. However, many organic farmers cannot afford such services, and the soil testing kits readily available on the market have varying quality.
But without reliable and consistent soil testing, it can be difficult for farmers to make informed decisions about their fertilizer applications and avoid applying too much.
We suggest that Boulder County invest in monitoring systems for small and organic farms, to ensure all farms have access to quality testing for their soil.
Additionally, Boulder can help small and organic farms afford more costly alternative fertilizers (such as blood meal or fish fertilizer), which can help reduce the amount of phosphorus left over in the soil.
Boulder can also help growers incorporate legume cover crops into rotations to boost nitrogen but not phosphorus, so they apply less manure. Boulder can also help growers establish adequate buffer strips along fields, such as those used very successfully on Iowa farms, to slow and absorb nutrient run-off and protect waterways.
