To view the article abstracts from this months research update follow this link:

Nitrogen in Compost

This month’s library is all about nitrogen availability from composts -- all composts, not just biosolids composts.  It comes at the request of Calla Rose Ostrander.  Calla has worked with the Marin Carbon project and others to help build soil carbon.  She asked for papers to help understand the differences in availability of nitrogen (N) from different composts.  It turns out that this is a question that people have been researching for a long time, and it also turns out that the Pacific Northwest is a hotbed for answers to all questions about compost.  To show how long we’ve been working on this, the first paper dates back to 1991.  While for some of us reading this library, 1991 seems like just yesterday, for others reading the library they may still have been in diapers. 

The authors of the first paper tested N availability in 19 different materials. including biosolids (had to get them in at least 1 paper), animal manures, and different composts.  Nitrogen availability was determined by incubating the organics in sand in test tubes in a lab for 67 days.  A range of tests was also conducted, including total N, and Walkely-Black N (WBN).  They found the best relationship between the WBN and N released.  Most importantly, they found that N release varied greatly across the different materials with some (composts) actually tying up N rather than releasing it. 

Fast-forward 15 years to paper #2 -- the same topic as paper #1, but more sophisticated methods.  Here the authors have tested two green waste composts, pelletized poultry manure, a straw compost and worm castings.  Materials were incubated for 142 days.  Clearly those 15 years upped the ante on incubation times required for publication.  Anyway, these scientists also included characterization of the carbon fractions of the amendments.  All the tested amendments, except for one of the composts, released some small amount of N; the poultry pellets were the only amendment to provide a significant amount of nitrogen.  This shows again that not all composts are created equal, failing to provide an easy tool to estimate N release. 

The third article brings us to the Northwest.  Dan Sullivan, currently at Oregon State, along with Andy Bary and Craig Cogger from WSU, tested grass response to food waste compost alongside conventional nitrogen fertilizer.  Food waste based composts (here made with higher carbon feedstocks as well) are typically richer in N than yard waste composts.  While I really respect these authors, I have to confess that this isn’t a thrilling read -- lots of stuff on regression responses.  But the take home is important.  What they saw was that the compost really only started to make a difference in plant available N in the 2nd and 3rd years of the study.  The reduction in quantity of fertilizer required as a result of the compost was relatively small.  Compost reduced N requirements by less than 0.3 kg of N per hectare per day.  The actual requirements were 1.4 - 1.9 kg of N per hectare per day.  A follow up study (email me if you’d like it) saw that the compost kept providing very small amounts of N for subsequent years. 

The 4th paper, from the same research group, goes into greater detail and has a broader scope than the previous 3 papers.  Here the authors are attempting to define guidance for estimating plant available N from a range of organic amendments.  There is a lot of pretty dense stuff here as well, comparing decomposition from lab incubations to field incubations and discussing the utility of a DECOMPOSITION model for this type of estimation (conclusion: it worked well).  For me the big points are shown in Figures 3 and 4.  Figure 3 shows that the carbon to nitrogen (C:N) ratio is a good predictor of plant available N: The higher the ratio the lower the plant available N.  The next figure shows that composting process itself also reduces the amount of available N for most of the amendments tested.

From here we go to the last paper in the library and likely the most important guidance.  Andy Bary, co-author of the last three papers in the library, and all around good guy. has this to say:

‘When using composts or manures for crop production it’s important to have a general idea of what the nitrogen availability is from the material you are applying.  As a general rule composts and manures with a total nitrogen value greater than 2% can be used as a fertilizer.  Manures and composts with a nitrogen concentration less than 2% are low in available nitrogen and should be considered a soil builder. It is particularly important to know when the total N concentration of your compost or manure is low, as the addition of the compost or manure can lead to short term nitrogen immobilization (tie up).  Examples of materials that can provide nitrogen for your crop are broiler litter compost and biosolids compost.  Examples of those that have a low nitrogen availability are rabbit manure compost, separated dairy solids compost and horse manure with or without bedding.  With compost you can expect 0 - 10% of the total nitrogen to be available the first year depending on the nitrogen concentration.  A handy reference that provides general information on nutrient availability for different manures and composts is “Fertilizing Manure and Other Organic Amendments PNW 533 ‘

That pdf link is also the last paper in this month’s library.  Hope this was helpful.

Sally Brown, University of Washington