Updates on Phosphorus Recycling

Phosphorus (P) runoff has long been blamed for excess harmful algae blooms in Lake Erie. While most experts blame agriculture, humans are also a significant source of P runoff in the form of human sewage. The City of Maumee by Toledo is an example. US EPA recently announced that the City of Maumee has been dumping 150 million gallons of raw sewage into the Maumee River for the last 20 years.

The problem occurs because during heavy rains, relatively clean water from downspouts, sump pumps, and other sources of stormwater is mixed with raw sewage, and the sewer system can not handle that much water so it is dumped into the river. Cities are now required to have two piping systems, one for raw sewage and one for relatively clean stormwater. However, the cost is very expensive to replace and update all those sewage systems. It is not only problems in major Cities, it is also rural home systems that need updating. So, everyone needs to do their part to keep water clean.
Another new development is how P is recycled on land. Researchers from Northwestern University have recently “discovered” that not only microbes release P enzymes that free up P but also iron oxide can release P from soil organic matter (SOM). Actually, Dr. Rafiq Islam and myself have been reporting this for the past 10-15 years. The big difference discovered by Northwestern University is that when they tried to account for how much P was being recycled in the soil, they had a big void. Something else besides microbes was releasing P into the soil. Their research showed that as much as 50% of all plant available or inorganic P in the soil is released from SOM through interactions with iron oxide. That’s a lot of recycled P that was not previously accounted for in the P cycle. A lot of P is tied up by SOM.

In the SOM, P is tied up by carbon bonds and is called organic P. Plants, microbes, and humans absorb and utilize inorganic bioavailable P which is lacking carbon. The relatively new Northwestern University research showed that iron oxide which occurs naturally in soils can
 
combine with the SOM to release bioavailable inorganic P. Northwestern Ohio soils contain, on average, about .43% iron. Iron has many forms and the chemistry is difficult, to say the least. Many rocks and minerals contain iron in various forms. Here is a quick review.

Iron is a major element in our soils and is a major problem in Northwest Ohio soils because it releases the P to surface water. Under saturated soil conditions, ferric iron or Fe3+ releases inorganic P or dissolved reactive phosphorus (DRP) quickly and converts to ferrous iron or Fe2+ . When water is standing on your field, the iron mineral is releasing DRP to our creeks, ditches, and streams. Saturated soils are a common problem due to poor soil structure and soil compaction.

Most P is mined to make P fertilizer. Building SOM and keeping the P in the soil is much cheaper and much more efficient. Current estimates are that within 50 years, many sources of mined P will be mined out. For food security, keeping P recycling in the soil is a cheaper and better plan. Keeping soils covered with live plants or cover crops after the crop is harvested helps keep the P recycling. With wheat harvested, if double crop soybeans have not been planted, planting a cover crop is a good way to conserve and improve your soil.

For years, soil health experts and farmers have said that they need to apply much less P fertilizer to get the same yield. Healthy soils have healthy populations of microbes that secrete enzymes to make P and other nutrients plant available. Another source of P, may now be coming from iron oxide which releases P from the SOM and makes it available. Some scientific experts think that healthy soils are leaky. Not really, only if you break the cycle and forget to keep live plants growing so the roots can take up the recycling P in the soil.

Perhaps, healthy soils need less P fertilizer and perhaps the soil tests rules should be different. I have argued for years that farmers who use cover crops and no-till and have healthy soils should have different rules. Unfortunately, regulators want to treat every farm the same and not have different rules for different farming systems. Maybe one day they will look more closely at what the science is telling us.