[USCC] P accumulation in manured fields

[Rufus Chaney]

Tim et al.:

One of the logical reasons we need to recycle the P in biosolids, manures and composts is this evidence of a limited supply of mine grade phosphate on earth. 

In general, the highest quality (in terms of Cd) phosphate ores have been used for decades and the largest reserves of phosphate are much higher in Cd. Even today some companies have to mix ores to be able to meet limits for US states and the EU. The US has large reserves in Idaho and that region, but these ores are highly contaminated with Cd, V, and some other elements. The other major sellers are the large African sources, Morocco and Senegal. The fertilizer from Morocco contains about 300 ppm Cd, higher than allowed in WA and EU. Inexpensive technologies to remove these contaminants have not been commercialized. The one method known to work was electric furnace distillation of the P which gave very pure phosphate products, low in Cd, U, and fluoride. This "electric furnace" phosphate production method is still often used for feed-grade to remove fluoride which would harm livestock if used in feeds. 

The Cd in phosphate fertilizers is more plant available and food-chain mobile than the Cd in manure, biosolids and composts. The Cd:Zn ratio of P-fertilizers is about 16:100 while for our other materials, it is about 0.5:100, and the Zn in our materials inhibits the flow of Cd from amended soils into crops and reduces the bioavailability of Cd in crops for animal feed/food. A long term study in the UK showed that even though application of manure raised soil Cd faster than fertilizer did, it caused a reduction in Cd uptake to wheat grain while the fertilizer treated crop had significantly higher Cd over time. Again showing that plant availability is affected by the rest of the manure.

The EU has dealt with Cd in phosphates by setting a relatively low limit and making long-term contracts for lower Cd ores. Leaving the higher Cd ores for lesser developed countries. The US has Florida phosphates which are very low in Cd, and they stopped selling ore for export many years ago; they sell only finished fertilizer products to get the maximum value from their investments.

So I agree with Tim that placing manure, or ashed manure, in landfills is not a good idea, even if it would reduce the potential for P runoff and leaching from over-manured soils. 

Recycle. Reuse. Seek new sources and technologies. Use manures, biosolids and composts at maximum resource value rates where possible. That is the P-fertilizer rate for annual applicaitons. But for soil improvement or remediation (disturbed soils, mined soils, etc.), high rates are valuable to improve soil OM and structure, water holding capacity, etc. And there we will need to deal with P solubility in order to be allowed to make the higher rates of application. I don't want to try to imagine "Big Brother" enforcing phosphate applicaitons from all sources on all land, rural and urban, livestock farmer and crop producer. But the compost industry benefits from the high phosphate fertility of our products, so the industry needs to be prepared to deal with phosphate when it comes up.

Regards,

Rufus Chaney
Beltsville, MD


>>> "Tim Evans" <tim at timevansenvironment.com> 08/30/06 11:13AM >>>

Is it a relevant question to ask what we mean by sustainable development?

The phosphate industry says that at the current rate of exploitation we shall run out of useful rock-P in 100 or at best 250 years.  We cannot exist without P so I believe that when we have P in the anthropogenic cycle we should not squander it by seeking to immobilise it when we have it in organic resources just so that more can be applied to a particular piece of land.  Wherever farmers are using mineral-fertiliser-P there is an opportunity to substitute P recycled in organic resources.  "New" P is always associated with some cadmium so whenever "new" P is applied to land we are applying "new" Cd - this might be a miniscule risk, but it is part of the story.  When we recycle P we recycle Cd that is already in the anthropogenic cycle.
 
For future generations we need to recognise that P is a finite resource and to recycle it when we have it in the anthropogenic cycle.  We should not squander it.  

Of course we also need to consider the rest of the environmental footprint of our recycling activities, so AD with maximum solids destruction and biogas utilisation and improved ease of dewatering are useful components for many situations.  It's all a balance, or as Barry Commoner said  Everything is connected to everything else".

regards

Tim

Dr Tim Evans
TIM EVANS ENVIRONMENT
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