Utilizing Compost or Manure? –
The Good, the Bad and the Ugly
By Neal Kinsey
Like all needed plant and animal products that contribute positively to soil fertility and plant growth, using compost and manures correctly can be of tremendous benefit. The effectiveness and value of applying them to accurately supply the required nutrients for the best in crop production is still greatly misunderstood by so many as to its full effects on the soil. All of the benefits compost and manures provide to encourage the biological life in the soil provides an excellent example. Release of “locked-up” nutrients that are present in soils in unavailable forms by stimulation of biological life in the soil is often completely overlooked or far underestimated in terms of both good and bad effects upon fertility.
What is true compost? How do you measure that? Is all “compost” good compost? Are some so-called composts not really compost? If so, how do you know? How about certified organic compost? Is it safe? Is the very best organic compost always good for the land? Can you apply too much compost?
How about manure? When should animal manure be considered as only manure and when should it be considered compost? Does it make any difference? When manure stops smelling like manure, is it then to be considered as compost? Can you even tell such things? Not only is it possible to do so, but as ranchers, farmers and stewards of the land, it is imperative knowledge that is often vital to real success!
To begin considering compost versus manure and their proper use, let’s go back to the first question asked above. What is true compost? How can you tell when manure has become compost? Well-made compost should have a carbon to nitrogen ratio of between 20:1 and 10:1, with 12:1 down to 10:1 generally considered as most preferable. We analyze hundreds of manures and composts for clients and find that many of those materials that are being sold as a compost are actually not a compost at all. How can you tell? Because the C/N ratio is 7:1 or 8:1. That is not a true compost – that is still manure! If the use of straight manure is of no concern, then fine. But sell manure for what it is, manure, not falsely calling it compost.
Why should this be a concern to those applying the material as to whether it is a compost or manure? Because the results of use on the soil are completely different. Once the proper C/N ratio has been achieved there is a different effect upon the soil that receives that material than the one that instead receives a manure application. Compost has been built to achieve a balance between the carbon and nitrogen content. This means that when it is applied, true compost will not tend to tie up nitrogen that will be needed by the crop that will be grown there.
Actually the use of a manure won’t either. It is the so-called composted material with a carbon to nitrogen ratio higher than 20:1 that tends to cause a nitrogen tie-up in the soil. That is the result when too much organic matter is used without sufficient nitrogen to break it down. In such cases, when these materials are applied to the soil there is too little nitrogen there to supply all that the microbes need and still supply enough to the plants. The microbes in the soil confiscate or metabolize the nitrogen for their own use at the expense of then having enough nitrogen for the crop to grow properly. This causes nitrogen deficiency for the crop. Here again, this is not a true compost, because the C/N ratio is above 20/1, which means it can cause nitrogen to be taken away at the time the plants need it for growth.
So then, why worry about the extra nitrogen contained in manures? What is the problem with the C/N ratio of manure which is below a 10:1? Microbes use nitrogen to build the energy they need to break down organic materials (carbon) as a food source. So long as the C/N ratio is between 10-20:1 there is a sufficient amount of both carbon and nitrogen so that both microbes and the crop can take up sufficient amounts. Within this range, the microbes break down organic matter to form humus in the soil. But when the C/N ratio is below 10:1, the nitrogen supply is so great that microbial populations proliferate and use up all the carbonaceous material present in that soil as the most available food source.
Now the microbes still have access to plenty of nitrogen as an energy source, but have run out of the most available food source – the under-composed organic matter which serves as their food and which in the process of being broken down forms and builds the humus content of the soil. Once all the organic matter is decomposed, suddenly these organisms now face the problem of finding another food source or dying. With all the extra nitrogen still remaining as an energy source, they are now forced to turn to another source of food that is much harder to obtain, but with the extra N they can do so. It is the humus that has been built up in that soil. This is where the term “burning up the humus” comes from in agriculture. When materials such as raw manure with a C/N ration of less than 10:1 is applied to a soil, this is what can happen if there is not enough residues to decompose and use up the nitrogen being added to that soil. That is an example of what over-applying raw manure to the soil can do to harm instead of help the carbon content of the land.
Click here to learn more about Kinsey Ag Services. Neal Kinsey has been consulting since 1973 after completing a program of study developed and taught by Dr. William A. Albrecht. Consultations include clients with small gardens up to large farms both conventional and no-till. This program is based on the system of providing soil nutrients to correctly treat the soil and the plants that grow there, using soil chemistry to correct the soil’s physical structure to build the “house” for the biology to flourish. Neal’s consulting includes soils received for analysis and recommendations from over 75 countries, principally from the U.S., Canada, Australia, New Zealand, South Africa, Great Britain, Germany, Austria, France and Mexico.
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Tim Bateman says
Did you get bored half way through writing the article and give up. I ask because you only seem to have answered half of the questions you pose at the beginning. You explain what makes a good compost but don’t explain how normal gardeners are supposed to tell what the c/n level of the compost is. How about answering the questions “When manure stops smelling like manure, is it then to be considered as compost?” and “Can you even tell such things?”. How can I tell when my organic compost or manure has broken down enough to dig into the soil without some gadget to test the c/n level.
Neal Kinsey says
Hi Tim, Never get bored writing about helping build soils to raise better crops. Sorry the article does not contain the answers or guidance that gardeners may wish were available. As a gardener myself, try to grow as much of what we eat as possible, and having worked with testing both soils and materials to be used on them since 1973, that experience just underscores the importance of using them to provide the best growth, nutrition and taste it is possible to achieve. But generally gardeners do not have access to a consistent supply of materials, and the content of whatever they apply varies greatly. So admittedly, the information was primarily written with those making large-scale use of compost and manure in mind.
Actually a careful reading of paragraph four does answer the manure question as well as the compost question. The difference between manure and compost is determined by the carbon to nitrogen ratio of each particular material. If looks, feel, or smell could give the right answer, then it would not be possible for so much manure to be falsely sold as compost.
Wish there was a “gadget” to supply such answers with certainty, but have not found one. We have hundreds of compost and manure samples tested and we pay in each case for a professional laboratory analysis. Another possibility that the majority of growers seem to resort to is to guess which is really not a very effective method to use if you spend as much time and money as those making large-scale use of such materials have at stake.
Determining the carbon to nitrogen ratio may not be the simple answer you wanted, but it is the only sure way to determine what is true compost.