You may be wondering why there are such problems in Lake Erie. Algal blooms have returned to the lake and are causing some major environmental and human health impacts. Lake Erie is the shallowest, warmest, smallest, and southern-most of the Great Lakes. This makes it particularly vulnerable to eutrophication, which is the enrichment of water by growth-limiting nutrients, such as phosphorus (P). What role does phosphorus play and how can the risk of P loss be reduced while maintaining productivity?
When it comes to phosphorus fertilizer, common knowledge used to be that applied P was tightly bound to soil and mostly did not move within the soil profile. Today, we know that in addition to the loss of particulate phosphorus through soil erosion, phosphorus in the form of dissolved P can also leave the field through tile drains. In fact, this has been identified as a major contributor to P loss from farm fields in Ohio. We also know that the majority of phosphorus (anywhere from 60-80%) is lost from farm fields is lost during the non-growing season.
Does Soil Test P Matter?
As for soil test level, it does matter. If your Olsen P test level is very high, dissolved reactive phosphorus is at greater risk of leaching. Research shows, however, that there appears to be a soil test level threshold, or change point (anywhere near 40 ppm Olsen P and above, depending on the study), below which risk of dissolved phosphorus loss through tiles is much lower. Fortunately, this threshold is well above the critical soil test P levels required for optimal crop production in Ontario (see Pub. 811, Agronomy Guide for Field Crops).
Phosphorus in surface waters
Particulate P: Also known as sediment P, it is the form of P that can be attached to soil particles and in suspension.
Dissolved P: The portion of phosphorus that passes through a 0.45-micron filter. The majority of it is comprised of dissolved reactive phosphorus (DRP).
Total P: The sum of particulate and dissolved P.
4 Key Practices That Ensure Proper Fertility and Reduced Risk of P Loss:
- Maintain soil P test levels within a moderate, agronomic range. Excessively high phosphorus test levels can increase risk for loss of both particulate and dissolved phosphorus and also interfere with zinc uptake.
- Consider practices that improve water infiltration. Although losses of phosphorus through tile drains have been recognized to play a significant role in parts of Ontario, surface run-off and loss of particulate P through water erosion is a major factor. Reduced tillage, lengthened crop rotation, and cover crops are practices that, especially when combined, can increase infiltration.
- Minimize phosphorus applications during the non-growing season. The best available data show that P from fall and winter applied manure and fertilizer is at higher risk of loss than spring applied P.
- Avoid broadcast and surface applications of phosphorus if possible.Banded fertilizer P has been shown to be more effective than broadcasted P in Ontario. In no-till scenarios, consider sub-surface banding of P to minimize risk of loss.
It's Not Only About Phosphorus
Although there will undoubtedly be a focus on phosphorus in the coming months and years, it is very important to bear in mind that good soil management and soil health is key. Often, practices that reduce phosphorus loss from fields are the same practices that increase soil health and boost your bottom line. Healthy soil infiltrates water and reduces soil erosion and the associated phosphorus loss. Healthy soil has higher natural fertility and allows you to get more out of your fertilizer inputs. And healthy soil handles stresses, such as heavy rains and extended dry spells, much better. It will be critical to keep the big picture in mind as we address this challenge as an industry.
The issue of phosphorus in Lake Erie has been gaining widespread attention recently. This past June, the Premier of Ontario and Governors of Michigan and Ohio signed a Collaborative Agreement that set a target of a 40% phosphorus load reduction (from 2008 levels) to the western basin of Lake Erie by 2025. The interim aspirational target is a 20% reduction by 2020. Although other targets will also be set shortly, phosphorus loading into Lake Erie has become a major issue in southwestern Ontario. Algal blooms have returned to Lake Erie, after many years since the blooms of the 1960s and 70s. The summer of 2014's algae bloom that left 500,000 Toledo, Ohio residents without drinking water sparked public awareness of the issue and its impacts.
Figure 1 - Surface runoff from a field in the non-growing season - a major pathway of phosphorus loss. Photo credit: Christine Brown.
Source: OMAFRA