Is it possible to build soil organic matter while simultaneously
decomposing soil organic matter to supply nitrogen?

By Dr. Sieg Snapp, Soils and Cropping System Ecologist, Dept. of Crop and Soil Sciences, Kellogg Biological Station, Michigan State University - for more information: snapp@msu.edu

This article was first printed in the Jan/Feb 2009 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.

The Red Queen in Alice in Wonderland said that ‘Sometimes I've believed as many as six impossible things before breakfast.’ Sometimes farming is like that, impossible tasks need to be preformed before breakfast. One of impossible tasks farmers face is to simultaneously build soil organic matter – for all of the benefits that accrue from improved soil quality – while at the same insuring that sufficient soil organic matter decomposes each year to supply crop demand for nitrogen. The good news is that long-term research on organic and sustainable field crop production practices conducted in SW Michigan is showing that it IS possible to build soil organic matter, even on sandy soils. We have been able to build organic matter by 20% over 10 years in a corn-soybean-wheat organic rotation, just from maintaining winter cover through cover crops (red clover and cereal rye). The addition of dairy compost (5000 lb/acre per year) built organic matter even faster.

To build soil organic matter  (also referred to as soil carbon) depends on the following factors: 1) the amount of carbon added, 2) the quality of that carbon, and 3) the rate of carbon loss. Field crop farmers in the upper Midwest are experimenting with conservation tillage, particularly for soybean production as this crop is adapted to no-till. This reduces soil disturbance, which protects soil carbon from being lost. Adding carbon is achievable through optimizing organic inputs from practices such as applying manure, maximizing crop residues or growing cover crops.
Most organic farmers already strive to augment the use of organic inputs on all their fields. Here we present information on how the quality of organic inputs influences soil organic matter, and the value of maintaining continuous living cover on all fields through integrating cover crops whenever the soil would otherwise be bare. Even if the cover crop is quite small, in cases where there is insufficient time or space to all the cover crop to grow higher than 6inches (ideally covers should be allowed to grow to at least 10 inches, which provides about 2500 lb of organic materials in residues), still the ‘out of sight’ but critical inputs from roots are providing habit – homes - and food for a healthy soil community of diverse organisms.

Questions arise regarding which quality and quantify of organic amendments are worth the most, and for which purpose: building the soil, feeding the crop, or both? The costs can be high for compost or manure application, and not all manure quality is the same. This highlights the need for more information about organic amendment quality and predicted impact on soil quality. The concept of quality depends on the goals of the farmer. If the main goal in applying compost is to provide nutrients, then the nutrient content needs to be analyzed. If the main goal is to build soil organic matter, then the ideal would be to know how much carbon is being applied and if that carbon is ‘recalcitrant’ or not rapidly available to microbes to decompose. If no information is available on the amount of carbon in the manure, then the type of animal that produced it and the type of bedding use provides a guide to expected quality (amount of carbon and nitrogen applied in the compost or manure).

Compost. Compost varies in quality, as shown in table 1 below. From poultry compost there often short-term benefits as illustrated in Figure 1, which shows snap bean yield response in a long term trial from Central Michigan.

It is also important to consider the long-term effects of compost or manure, which often are highest if a mixed quality compost is applied (e.g, the compost is made with different sources of carbon, such as straw and leaves, or the compost is applied in combination with a cover crop, see below for more on this practice). There are tradeoffs, and compost that provides immediate benefits from high levels of nitrogen and phosphorus may have lower long-term benefits. Table 1 below provides a summary of the different types of manure commonly available in Michigan, and the quality of the manure for different purposes is presented for each manure source. Note that the feed provided the livestock, and the bedding used which ends up in the manure, are important in determining manure quality. For compost it is also important to consider what type of carbon was applied in the compost preparation process.

Table 1. Manure type from different livestock systems and the common nutrients, organic carbon and pH associated with different qualities of manure.

As shown in table 1, if the primary goal of applying manure or compost is to provide a nitrogen source then poultry manure is a good choice. We also have evidence from our long-term research trials on potato and snap bean rotations that applying poultry manure in some cases can improve the health of the crop by suppressing some soil-borne diseases, leading to improved yields. It is important to learn more about what nutrients are being added with the manure and to reduce the amount of fertilizer applied accordingly. There is potential for long-term benefits in terms of nutrient build ups, so monitoring of soil nutrient status should be frequent if manure is applied.

If the primary goal is to improve soil organic matter status then this will be optimized if manure or compost is applied that is high in straw content, or from cattle fed on pastures and hay (Table 1). A combination of manure or compost is applied PLUS a high biomass producing cover crop, this is ideal for the mix of quality organic inputs and the inputs from living roots. For example, compost or manure that is applied in the fall with a rye cover crop is an easy way to successfully increase organic inputs. If summer cover crops are feasible then sorghum-sudan grass can be grown and incorporated with swine manure or some other medium quality manure to provide a mixture of qualities in the manure and cover crop residues. Application of manure will – from our experience with Michigan field trials - enhance growth of cover crops by 30 to 60%, and lead to reinforcement of benefits from the two organic sources. Once source of further information is the MSU Extension Bulletin E2893 ‘Managing manure in potato and vegetable systems’ by Snapp et al., 2003.

Cover crops. Cover crops grown over the winter provide a unique – but hidden – benefit in terms of living roots. These provide constant organic inputs from root exudates and root turnover. Roots grow, die back during cold spells, and then regrow. This is a key source of food and habitat for soil microorganisms, which in turn help build soil aggregates that protect soil carbon, and this leads to rapid soil organic matter formation. Choosing the right cover crop for your cropping systems will depend on resources and goals, but there are many sources of information to provide guidance. See for example the MSU Extension Bulletin E2884 ‘Cover crop choices for Michigan’ by Mutch and Snapp, 2003, or the website: www.covercrops.msu.edu/general/general.html

Cover crops are an efficient means to achieve multiple objectives: protect against soil erosion, build long-term soil organic matter, and augment the ‘active’ organic matter pool of ready to decompose materials that release nutrients and supply crops. So it is possible to achieve the impossible: building soil organic matter (long-term pool) while simultaneously building decomposable soil organic matter (short-term pool) to supply nitrogen and other nutrients. Both pools need to be fed, so a mixed quality of residues is your best bet to achieve these dual objectives. Mixed quality residues can be achieved by combining manure types shown in table 1, or applying manure in combination with cover crops and other high quality crop residues (for example, rotating with a pasture or alfalfa stubble). If a ‘low quality’ residue that is high in carbon and low in nitrogen - with a carbon nitrogen ratio of greater than 25 - is incorporated into the soil then soil microorganisms will compete for available nitrogen to decompose these residues. This will result in nitrogen tie up, and insufficient nitrogen for nitrogen-demanding crops such as corn. It is essential in this case to provide a high quality, N-enriched residues or organic N-enriched fertilizers mixed with the low quality residues. Ensuring sufficient nitrogen is particularly a challenge if the spring is cold or wet.

There is emerging scientific evidence that this it is a mixed quality of residues that is most reliable at enhancing soil and crop health, apparently by supporting a diverse soil food web. However, this is an area of research where responses are not consistent and economic gains have rarely been achieved in a reproducible manner, particularly in a field environment. It is not yet known which types of residues are the best to use - nor which are the most beneficial soil microorganisms. It is important to check for your self – and to test on an economically modest scale - any soil health promotion claims for residues or manure, to see what works for your soil type and for your farming system.

To learn more, see New Ag Network articles on nitrogen management for organic production and soil organic matter building research findings from long-term field crop trials at the Kellogg Biological Station in SW Michigan, or contact Sieg Snapp at snapp@msu.edu (http://www.ipm.msu.edu/cat07field/fc09-20-07.htm)