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With spring fast approaching and the early onset of winter in much of the Northern Plains, there will likely be some questions circulating about what is going to happen in fields that were not tilled
in the fall. We know that high residue crops can cause significant problems in the spring if left on the surface. Increased surface cover will insulate the soil and make warming and drying the soil take a little
longer which is problematic when time is tight during planting season. Along with some of these fears is also the notion that farmers may have to modify their management system, particularly fertilizer application,
to account for residues still in the field. Most of this has circled around the carbon-to-nitrogen (C:N) ratio of crop residues. While there is a scientific basis to this concern there are a few things that must be
taken into account following what is happening when soil organisms are decomposing residues. Crop residues provide a significant source of carbon for organisms. Carbon is essential to supply the building blocks for
organic compounds and energy for growth and development. Along with carbon, nitrogen is essential in the formation of amino acids, enzymes, and DNA (Deoxyribonucleic acid), which are all important elements in the
function and growth of organisms. Without carbon and nitrogen life would not function on this planet. On average a soil micro-organism must assimilate eight parts of carbon for every part of nitrogen used. Of the
total carbon utilized by organisms, only about a third of the total is incorporated into cells. The rest of the carbon taken in is utilized for respiration (process by which an organism obtains energy) and lost as
carbon dioxide. Thus for every part of nitrogen used by organisms in the soil around 24 parts of carbon will be used. Of course situations may exist where this number will not hold true, but in most cases a C:N
ratio between 20 to 30:1 will usually result in no net gain or loss in soil N. In situations where residues have a C:N ratio below 20 nitrogen will be added to the soil as the decomposition of residue progresses.
When values are higher than 30 then soil N must be used in order to decompose plant residues. Does this mean that over time all N used to decompose these residues will be lost forever? Not exactly. You have to view
the soil processes in terms of the whole system. All soil organism, both large and small, have similar C:N ratios. Larger organisms that consume the smaller ones as their food will concentrate nutrient in their body
and any wastes excreted will generally have a low C:N ratio, which allows for the cycling of nutrients back into the system. As time progresses some of the N used will become available again. The question is how
much and if it will be available when a crop needs it.
Residues will differ in their relative amounts of carbon to nitrogen. Specific values can vary between and within plant species. On
average, carbon-to-nitrogen ratios range from 60 to 100:1 for corn stover, 40 to 80:1 for wheat and other small grain straws, and 20 to 40:1 for soybean residue. Wood products such as paper or sawdust typically have
high C:N ratio ranging from 100:1 or significantly higher depending on the product (in some cases the ratio can be 1000:1). Green materials tend to have a lower ratio compared to straws and stovers, which can be
effective at increasing nitrogen content in soils if they are incorporated into the soil. In the case of stover or straw, should producers be worried about high carbon residues in the soil and apply extra fertilizer
to prevent tie-up? The answer to this in most cases is no. Recommendations for high residue crops already incorporate additional N needed. For instance, in corn following corn or wheat nitrogen recommendations are
higher than when either of these crops would follow soybeans or alfalfa. When the research was conducted, it was done so with conditions similar to any of those seen in a production field. Therefore the addition of
nitrogen to decompose residues should be no more worrisome than the things to go bump in the night. There has been no definitive evidence that extra N should be applied to allow for microbial decomposition from
research conducted in this area. Even in high residue systems such as corn, tillage and time works the best to help break down residue before the next planting season. Tillage effects are due to increased contact
between residue and the soil when it is buried and increased aeration and any warming of the soil which will speed up microbial activity. The simple fact is that we can control decomposition more with tillage than
with any other practice.
The rate that organism decompose materials depends on the composition of the material, the C:N ratio, and a favorable environment
(adequate soil moisture, temperature, and pH). Sugars, starches, and simple proteins are easily used by organisms and will rapidly disappear if in the soil. Crude proteins, cellulose, lignin, fat, and wax all
decompose slowly. Other carbon sources such as charcoal are relatively stable in the soil and do not decompose easily. When a material with a high C:N ratio is mixed with the soil the organisms respond to the
additional food source and begin to grow. In response to the low relative nitrogen supply in the residue, the level of N in the soil will decline as organisms use this source to satisfy their needs. Over time the
C:N ratio in the residue will decline as soil N is used and carbon in the residue is lost as carbon dioxide through respiration. Eventually the ratio will be low enough that N will be mineralized out of the residue
and the content in the soil will be increased. However, the depression period between when soil N decreases then increases can significantly affect growing crops. The overall time of the depression period is not
easy to predict. However, if there is concern over tie-up of N, the best alternative may be to place fertilizer below the surface away from the residue and within the root zone. Probably the biggest risk for tie-up
would involve direct application to the crop residues such as a 28% urea ammonium nitrate solution sprayed on the top. While organisms may response to an influx of N in the soil if it has been limiting, with high
priced fertilizer inputs should you be more concerned about feeding the soil micro-organisms or feeding the crop?
The most limiting factor in Northern climates when dealing with residue decomposition is climate. While it is a problem some years it
is one factor that has made our soils as productive as they are. While we cannot control the weather through cultural practices (such as tillage) we can effectively manage high residue situations. While it can be
worrisome to think about dealing with large amounts of crop residue in the spring, most of these issues are not new and were thought of in many of the crop recommendations. While there may be some products out there
that are sold to help manage crop residue, remember that climatic factors are still likely to dictate how fast any residue will decompose. It is not for a lack of the organisms required to complete the process that
limits us the most. Unless you are in a management system that wants residue left on the surface, the best alternative is to get in the field as early as possible to get the decomposition process started.
High residue crops can cause significant problems in the spring if left on the surface. Unless
you are in a management system that wants residue left on the surface, the best alternative is to get in the field as early as possible to get the decomposition process started early.
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