[USCC] P accumulation in manured fields

[Rufus Chaney]

Dear USCC Folks:

Jarry d'Aquin wrote me asking about the long term accumulation of Al with P in manured soils. It is a valid issue, and we need to consider a longer time scale -- centuries at least. This question is a proxy for "what should we do with livestock production wastes/byproducts in the US?". Recall the older definition of "waste" as "a resource out of place".

For issues of pathogens, malodor, stabilization, rate of N release and some other manure issues, composting is a good answer. But then the slow N release of composted-manure allows one to apply higher rates of P, and P-limited applications will stop use of composts much like it does the manure. By itself, composting with local application is not the solution needed by livestock producers.

The central issue is concentration of production and manure disposal; when large number of livestock are housed in a small area for more economic production, feeds are imported from more distant farms (states), but P, Zn, Cu, Cd, N, etc., accumulate where it is applied. Wet manure is expensive to haul. Nutrient Management Plans are being required, so application of N and P will be or are already limited the amount which can be removed by the current crop. 

Technologies being considered to deal with the excessive local N and P from manure include (at least):
     Reduce P in rations; use High-Available-P grains (low phytate); add phytase to diets; makes local use more nearly sustainable.
     Compost the manure and sell the compost to urban markets.
     Compost the manure and use the compost in environmental remediation off farms.
     Haul the manure to more distant fields that need the P (cost eventually paid from sale of meat).
     Burn manure and put ash in landfill.
     Burn manure and sell ash as P and K fertilizer at distance from site of livestock production.

What should be done with manure is complex and very political. Research has been being conducted to assess risks from P runoff and leaching, and to seek methods to prevent identified risks. The industry had shifted fairly well to application at rates which satisfied the N requirement of the current crops rather than apply excessive N, and high nitrate in groundwaters has declined since Nutrient Management Plans were required for livestock production units.

High P accumulation in soil is already regulated in some states. Imperfectly regulated. Composts are not yet strongly regulated in relation to plant available or water soluble P in the amended soil, but regs are moving toward limiting the Water Extractable Phosphate (WEP) of the soil amendment or amended soil. Slopes and other factors which affect runoff and erosion are increasing included in planned regulations for manure P applications.

In a perfect world, it would seem wise to haul the manure to fields which need the phosphate as fertilizer for crop production. But the cost of hauling soon overcomes the value of the nutrients in the manure. If government subsidized hauling manure, it would likely violate international agreements about subsidies to farmers. The US already provides low cost loans and planning assistance to livestock farmers to construct manure handling and storage facilities.

I sometimes joke that the only solution is to require that when you purchase meat at the grocery store that you have to take along a bag of the manure that was produced by the portion of the animal you purchase. This would be at last a 10 lb. bag for a serving of meat. Obviously this is unlikely to occur, but it shows the magnitude of the problem and why sale of meat needs to cost more so the manure can be safely utilized.

One conceren that I am sensitive to is the number of smaller livestock producers who have applied the manure produced in their operation on their own land. Because they import feeds, they have more N than they really need. And the amount of P they applied in the past has pushed the soil WEP to levels above those acceptable in relation to runoff or leaching of P to surface waters where eutrophication may occur. For those producers, enforcement of P-based application rates for manure means they will not be allowed to apply any manure on their field (no P allowed until WEP drops, and then only replacment of P removed by crops until soil P drops a lot). If no nearby farm needs the manure, the producer can no longer produce the manure. He could pay to haul it further to a farmer who needs the P, and that farmer may require some payment to accept application of the manure. When manure handling become severely limiting, you can imagine someone charging to accept manure rather than paying for the value of manure. Some companies who have worked with biosolids are looking at this market because they are equipped to both haul organic residuals, apply them appropriately, and obtain permits for manure applications and follow regulations. Proper equipment can reduce the cost per ton of manure handled.

In my opinion, the incineration approaches are based on government purchase of the incinerators and landfills. Incinerators cannot produce actual positive energy from wet manure. It can be done, but it is not cost effective. If the poultry purchasing company built the incinerator, and paid for the ash disposal, the cost could pass thru to the meat sale price, and P contamination of surface waters could be prevented. Some companies may try this, but so far they have generally lobbied for government to build the incinerator.

So what should society do about livestock manure phosphate accumulation in soils? First, we need to recognize that in longer term production regions soil P has accumulated to levels which are not acceptable regarding P runoff and leaching. In some areas of poorly drained soils, ditches are required and if the grower farms right up to the edge of a grasses ditch, he may spread manure directly into the ditch by accident. 

Not making the problem worse seems important to most of us. I have argued that adding Fe and Al rich byproducts or treatment chemicals to manure or excessively-manured fields could alleviate the risk from this P in existing over-manured soils. Several groups have published research papers showing that the P-risk could be reduced by these treatments.

Most evidence shows that the P is adsorbed on Fe and Al hydroxides, although some is slowly converted to inorganic compounds of P in the soil or diffuses inside the micropores in the oxides. Plants can utilize the adsorbed and organic P, and even the P compounds, by the mechanisms they use within the rhizosphere to dissolve and absorb soil P for normal soils. We did one test of the P available to wheat from soils on my long term biosolids plots at Beltsvsille and found that with no P fertilizer the wheat obtained adequate P for full growth. So the P adsorbed on Fe and Al has remained plant available. Other's research also supports this conclusion.

As I have tried to say over the months of discussion on P from manure and biosolids, I don't know what the long term solution for US farmers and livestock producers will be. Shipping grains across the country but leaving the manure where the livestock are produced is not going to work. Economics rules in the end. Hauling manure longer distances from livestock producers farms will only become more costly with time because of fuel costs and increasing saturation of the P adsorption capacity of the soils nearer areas of livestock production. Excessive P accumulation will remain a significant issue until society works out a new way to pay for the joint benefits of meat consumption and environmental protection. The pattern of 60 years ago in which crop producers fed a few livestock with the crops they produced and shipped the livestock to market (and recycled the manure to the crop production fields) has shifted toward larger livestock production units. Farmers prefer to do one or the other, but not both parts of this production system, so it is not cost alone which has driven us toward larger livestock production units. I can only watch to see how this comes out.

Regards,

Rufus Chaney
Beltsville, MD


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Jerry d'Aquin, Con-Sul, Inc.   
     One area which is of concern to me on a multi-decade time horizon is the practice of applying alum to fields receiving high doses of poultry litter. This approach has been promoted by the U of Arkansas as a means to convert soluble P in litter to an insoluble form. This permits high-volume litter applications while limiting P runoff and percolation into surface  and ground waters. In my mind, we are (at best) over-accumulating ground P until the point when it again becomes an issue (if it ever ceased being so) AND artificially injecting Al as aluminum phosphate into a long  term equation -- rather than simply "biting the bullet" and  dispersing litter over a larger area and at doses appropriate for requisite  nutrient uptake of those crops. Am I wrong regarding the use of alum?