Beneath the Surface: Phosphorus

Expert Opinions on Phosphorus Management Considerations

A hot topic in soil health and nutrient loss over the last few years has been Dissolved reactive Phosphorus (DRP). DRP is a form of Phosphorus that is dissolved and readily available to plants and algae. This differs from the phosphorus loss we usually hear about, which is tied to soil particles. We sat down with two ISAP network experts, Dr. Andrew Margenot, associate professor of crop sciences at the  University of Illinois, and Dr. Shalamar Armstrong, associate professor of agronomy at Purdue University, to get their take on DRP and what it means for IL farmers. Following are responses in interview format.  For a summary of key points, read ISAP’s “Quick and Dirt-y” on DRP.

Why is DRP important and getting more attention recently?

Dr. Margenot: “Dissolved reactive P (DRP) is important because it can be transported via surface runoff and to drainage tiles and is a form of immediately bioavailable P, not just to plants but also to algae. DRP is therefore a contributor to algal blooms and eutrophication of surface waters.”

 

If P can be lost in several different forms, what are the most significant routes of P loss from cropland in Illinois?

Dr. Margenot: “The major pathways of P loss from croplands are surface losses as runoff and subsurface losses as leaching. Surface runoff can entail P loss both as dissolved P, often dissolved reactive P (DRP) also known as orthophosphate, and via sediment P, in which soil transported by water contains P. In general, surface runoff and via sediment P (also termed particulate P (PP) is the dominant pathway. DRP can be lost vertically via leaching in very coarse soils (e.g., sand textural classes) or via tile drainage, but these magnitudes are often much lower than surface losses even in relatively flat fields. Because the majority of P is lost via soil erosion, soil conservation practices can double as P loss mitigation practices.”

How do tillage practices affect total P and DRP loss? What about fertilizer placement?

Dr. Margenot: “It is challenging to summarize general effects of tillage on P. Total P is unaffected by tillage, but its distribution vertically in the soil (i.e., stratification) is often enhanced by reduced or no-tillage, especially for broadcast applications of P. This can lead to very high total and available P concentrations in the surface 1-2”, with much lower concentrations in the remaining 4-5” of the surface 0-6” depth. This can pose a risk of DRP loss, which has been documented to occur under no-tillage. On the other hand, [full width tillage] poses a risk of greater sediment loss by soil erosion and thus particulate P loss. Banding P in the subsurface and ‘occasional tillage’ that is strip tillage, is a means to navigate these trade-offs of tillage vs no-tillage on particulate P vs DRP loss risk, respectively.”

Dr. Armstrong further clarified, “Tillage has the potential to greatly affect the loss of soil TP from an agricultural field due to the fact that it increases soil loss via surface runoff. Tillage alters the structure of soil in the plow depth (0-6 inches), which results in less infiltration and a greater susceptibility of surface runoff. Additionally, disturbance of the soil via tillage reduces surface coverage from crop residues that protect the soil surface against the soil detachment energy of raindrops and drop out sediments due to surface roughness. Strip-till provides residue cover and surface roughness similar to no-till, but should be orientated perpendicular to the slope [so that] less erosion occurs in the tilled strips.”

 

What impact do cover crops have on DRP loss?

How do different types of cover crops affect DRP? 

Dr. Armstrong: “In my study of cover crops and DRP, I found that all cover crop species are not created equal as it relates to their influence of the soil’s sorption capacity of DRP after long-term adoption. We published a recent paper demonstrating that brassica cover crops tend to increase or have equal DRP desorption as soil from a no cover crop control. While, cereal rye decreased DRP loss potential significantly relative to the brassica  and control treatments and annual rye significantly reduced DRP loss potential relative to all treatments. Thus, cover crop species does matter and grasses that over winter and that establish deep and robust rooting systems have the greatest potential to reduce DRP loss relative to no overwintering cover crops.”

What is the effect of cover crop residue on DRP?

Dr. Margenot: “Depending on the cover crop species and timing of termination and then the ensuing rainfall, residues may lead to immobilization or mineralization of P (i.e., orthophosphate), which may be susceptible to runoff as DRP. 

Dr. Armstrong: “Another factor is cover crop species and resultant Carbon to Phosphorus ratio. Again overwintering grasses put on lots of carbon in the reproductive stages (termination timing is important), which widens the Carbon to Phosphorus ratio and decreases the decomposition rate of the residue and mineralization of P.”

 

What 4R management strategies could reduce DRP loss?

Dr. Margenot: “Cover cropping may reduce it by taking up DRP into biomass, though freeze-thaw of cover crop biomass has been found to increase DRP losses in some cases. For reducing DRP loss from freshly applied fertilizer (DAP, MAP, TSP, manure), subsurface incorporation by banding, knifing (for manure slurry), or tillage incorporation is best.” 

What’s a farmer to do?  

Edge of Field Practices 

Dr. Margenot: “Buffers are great for halting sediment and thus TP transfer to surface waters. Wetlands are not permanent solutions; they will transiently be a sink to catch sediment and thus TP, but once they are saturated with sediment, they will start to release large amounts of P. Even before then, waterlogging of wetlands actually increases production of DRP from soil P via reductive dissolution, making wetlands potentially a slow-release DRP ‘drip’.”

In field practices 

Dr. Margenot: “I think erosion control is #1 (for TP), followed by managing fertilizer carefully (for DRP) and reducing run-off (for DRP and TP) via surface cover and terraces. Surface “roughness’ via no-till corn stalks and residues of cover crops can also help decrease runoff transfer, as can improved soil structure that promotes infiltration.” 

Dr. Armstrong agreed, adding, “Also, I recommend to farmers to understand their geographic location and their cropping system goals. Your system conservation practices in flat versus rolling ground should be different. If your drainage contributes to a P sensitive watershed, P source and application method and cover crop species will have to be taken into consideration. If your system contains manure applications, different adaptive management practices to conserve soil and nutrients have to be considered.”

 

What’s the take home message for phosphorus loss? Is it better to reduce erosion and therefore TP or avoid potential increase in DRP? 

Dr. Margenot: “Impossible to answer. The trade-off that makes the ‘either/or’ make sense in one context will differ in another context. Overall, more total P can be delivered via TP and thus erosion than by DRP, but it is less available. In the long-term, TP likely matters more due to its legacy effects once eroded into stream channels and lake beds.

Dr. Armstrong agreed, saying “Also do realize that total P can mineralize in the streams as it is in transport.”

 

Summary

While more research certainly needs to be done on DRP and its management, continue to focus on the strategies that have been proven successful. Whether they be in-field or edge of field practices, farmers can and do have a real impact on the amount of nutrients leaving their fields. ISAP and its network of specialists and researchers can be a great resource on the current best practices for reducing nutrient loss. 

Special thanks to ISAP network experts Dr. Andrew Margenot (UIUC), Dr. Shalamar Armstrong (Purdue), Pete Fandel (ICC), Dr. Shani Golovay (NREC)