Article courtesy of Monica Day | June 1, 2015 | AG Professional | Shared as educational material
The March/April 2015 Journal of Soil and Water Conservation features “What is causing the harmful algal blooms in Lake Erie?” In it, the authors document the recently suggested scientific theories about natural and agricultural sources of increasing soluble phosphorus (SP) to Lake Erie. Their list excludes urban and suburban contributions.
New technology has improved the efficiency of food production, but may also change conditions resulting in more soluble phosphorus.
- Newer crop varieties are more efficient users of nutrients, while fertilizer recommendations have not kept pace resulting in P application in excess of crop demand.
- Switch from triple superphosphate (TSP) to diammonium phosphate (DAP) and monoammonium phosphate (MAP) lowered acidity and it is difficult to calibrate equipment to apply the newer fertilizers below 100 pounds per acre.
- Glyphosate applications may alter soil microbial communities, microbial processing of P and increase the amount of P available for transport.
- Increase in no till creates soil structure and worm channels that may increase movement of SP to tile lines.
- Products sold to increase soil phosphorus solubility make more P available than the crop can use.
- When soil microbes digest GMO residues P become more soluble.
- An increase in tile drainage may lead to an increase in SP.
- Severe spring storms are delivering more P to Lake Erie during the spring.
- Decreasing air pollution resulted in increased soil pH. Higher soil pH results in greater P availability and greater P loss.
- Declining sediment loads from uplands result in increased P-saturated bank erosion downstream.
- Excess nitrogen favors algae that produce microcistin toxin, intensifying the harm from phosphorus alone.
- Higher P concentrations in surface soils pose a risk of higher P concentrations in surface runoff and drainage water.
- Zebra Mussels filter sediment and algae, but excrete P.
Economic drivers and farming practices
- High commodity prices incentivize producers to increase P.
- Less diverse cropping systems require greater nutrient inputs.
- Demand for ethanol has resulted in more acres planted in continuous corn, a high nutrient demand crop.
- Surface application of fertilizer.
- Larger farm size shortens time available for field operations, resulting in more concentrated applications during non-growing season.
- Manure represents a continuous source of P “especially where applied at disposal rates for many years and have resulted in high soil test P concentrations.”
- Rental agreements do not allow the producer to modify inputs according to field needs and the short term agreements prevent the adoption of structural conservation practices.
- Timing of application. Application of fertilizer during the non-growing season (November to April) increases risk of P loss in storm runoff. One fertilizer application for two years crop removal may increase loss of P.
- Fertilizer is applied at rates greater than expected benefits to the crop.
- Tri-State recommendations are generalized to ensure productivity on less productive soils, resulting in over application on the productive soils in the WLEB.
- Misconceptions about Phosphorus Loss. Agronomically insignificant phosphorus loads of one pound per acre are equivalent to the loads causing the Lake Erie HNABs.
- Soil tests and their analysis are outdated and often misinterpreted.
With 25 causes for excess phosphorus loss from fields given by Michigan State University Extension, there are many alternatives for helping to reduce impacts to Lake Erie. One important sobering lesson is that “managing for total phosphorus does not necessarily mean we will improve losses of soluble phosphorus”. This is because managing the two forms is frequently contradictory. As we learn more about the unintended consequences and engage in an open dialog what is considered best practices will also change. With growing demands for food and water, we owe it to ourselves and those to come after us to accelerate our progress on this problem but do so thoughtfully.