Readers Q&A

PHOSPHORUS AVAILABILITY

Q: The Q & A column in January (2002) said that the availability of phosphorus (P) in composted manure and manure was the same. However, I have read research that states composting reduces P availability. Why is there a discrepancy?

A: There is a discrepancy, and it is because researchers obtain different results. For example, the January Q&A referred to a research project that found little or no change in P availability of poultry manure before and after composting (P availability refers to the percentage of total of P that is water soluble and therefore available to plants). However, more recent research has shown that composting does affect P availability. (See page 48 of this issue, "Testing Composting Strategies To Control N and P.") This research found that P availability in a 1:1 volume mixture of poultry litter and ground yard trimmings decreased through the composting process. On average, the percentage of water soluble P decreased from 11.5 percent before composting to 3.1 percent after composting. This is good news if you are applying the compost to a situation where P is already abundant. It is bad news if you are relying on the compost for P fertility.

In any case, why did the two research projects find different results? It might be the feedstocks. While the first project looked at poultry manure and cattle manure, the second examined a mixture of poultry manure and yard trimmings. It could be the composting methods. In fact, the second project found that aeration and turning did affect the P availability. It might be the presence of P-binding minerals, such as aluminum. It could also be the timing, the location and the research methods. There are many variables that can influence composting research results -- especially because composting, and plant growth, are biological systems and biological systems are inherently inconsistent and variable.

ARSENIC AVAILABILITY IN SEAWEED

Q: The Readers Q&A column in the April (2002) issue of BioCycle discussed the qualities of seaweed as a composting material. I heard that seaweed has arsenic in it is that true?

A: The rich mineral content of seaweed is generally an advantage but, again, in large quantities, an excess of a particular mineral raises concerns (in addition to the salinity effects). Rufus Chaney, with the USDA Agricultural Research Service (ARS) observes, "Some algae accumulate high levels of arsenic (As), in excess of 100 ppm dry weight. Many of these algae are used as food, and they are safe because the As is in an organic form. But when this is composted and/or used directly as an organic fertilizer, it becomes inorganic As, which is the toxic form. If you add enough seaweed to soil, you can end up with an arsenic-contaminated soil." Fish and some other marine residues can also contain high levels of organic As. (One specific paper on the topic, noted by Dr. Chaney is: Whyte, J.N.C., and J.R. Englar. 1983. Analysis of inorganic and organic-bound arsenic in marine brown algae. Bot. Mar. 26:159-164.)

CEDAR VS. PRESSURE TREATED WOOD

Q: The Q&A column in the March (2002) issue suggested that pressure treated wood should not be used for composting bins. What about wood from cedar trees? Is cedar a good substitute for pressure treated wood, or are there compounds in the cedar that also may also cause problems in the long run?

A: "Cedar" can refer to one of several different tree species, depending on where you live. More than likely, you are referring to a common lumber yielding species of cedar like Western Red Cedar. Most species of cedar are resistant to decay. Therefore, lumber from some species of cedar is used for wood applications that require durability, like roof shingles or compost bins. Cedars owe their decay resistance to resins and oils that act as preservatives, resisting attacks by insects and microorganisms (e.g. fungi). Conceivably, those same qualities could render the wood, or compounds from the wood, toxic to plants or composting microorganisms.

However, cedar does not appear to be a problem for composting or compost. Although it may compost more slowly than other species of wood, there have been no reported problems in composting bark, chips or sawdust from cedar species (e.g. Western Red or White Cedar). With sufficient time, any biohindering compounds will likely decompose during composting. In fact, bins sold specifically for backyard composting are often made from cedar (most likely Western Red Cedar). In addition, the use of mulch made from cedar woods does not appear to affect plants so compounds lost from cedar should not be a problem for composts either (see the University of Missouri newsletter: News for Missouri's Gardens, Yards and Resources, May 4, 2001; Volume 7, No.3).

It seems that the cedar trees that are currently harvested for lumber are less decay resistant than the cedars of old. This is because the timber from the former trees contained more heartwood, which in turn accumulated more of the compounds that resist biological attack.

Readers are invited to submit questions about any area of our editorial coverage. Due to time and space constraints, not all questions can be answered. Send questions to:

Robert Rynk/ Q&A

BioCycle

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