Soil Basics Part III

Part III: Organic Matter: Key To Soil Management

In previous fact sheets we have discussed physical and chemical aspects of soils. We have seen how soil organic matter (SOM) improves moisture holding capacity of sandy soils, aeration of clay soils and how it helps over-all tilth of any soil. In New England soils, SOM is the chief contributor to cation exchange capacity, a measure of a soil’s ability to retain nutrients. The break down or decomposition of SOM releases nutrients which can be used by plants.

What is organic matter?

By definition, organic matter contains carbon. Carbon is a source of energy for microorganisms (microbes) in the soil. These are microscopic plants and animals such as bacteria, fungi, and actinomycetes. Some of these are pathogens which cause plant disease, but in a healthy soil the vast majority are beneficial. Organic matter provides food for a diverse population of microbes in the soil and this helps prevent any one type of organism such as a plant pathogen from dominating.

SOM is continuously being produced and broken down by living plants and animals. Dr. Fred Magdoff of the University of Vermont coined an appropriate statement: There are three kinds of SOM; “the living, the dead and the very dead”. The living fraction of the SOM is made up of living plants and animals including microbes that are found in the soil. When they die, stalks, leaves and other plant parts retain recognizable characteristics for a while. This is the dead fraction of the SOM. Sooner or later the dead organic matter decays and can not be recognized for what it was and eventually becomes humus. This is very dead organic matter. In addition, animals eat plants or other animals and pass some of their food through their bodies as manure which is rich in nutrients and organic matter.

SOM is broken down by microbes as they consume it for food. Any factor that affects soil microbial activity also affects SOM break down. In the microbe, respiration combines most of the carbon from SOM with oxygen to form carbon dioxide gas. For this process to continue, there must be an exchange of oxygen and carbon dioxide between the atmosphere and the soil pore spaces. Gas exchange can be restricted if the soil is compacted or saturated with excess water. This slows the rate of SOM decomposition. While excess water inhibits decomposition, a certain amount is necessary to support microbes. Therefore, conditions of moisture stress can be expected to slow the decomposition of SOM.

Soil microbes are also influenced by soil pH. This is especially true of bacteria. Under acid conditions, bacterial activity in breaking down organic matter is greatly reduced. Soil fungi responsible for break down of SOM are generally less affected by low pH. In most cases, however, bacteria are responsible for most of the decomposition of SOM, and as a rule this process is process is markedly slowed if pH levels drop below 6.0.

Soil temperature has a marked influence on microbial activity. The optimum soil temperatures for bacterial activity are in the 70o F to 100o F range, but activity occurs as low as 40o F, although at greatly reduced rates. In the South where soils are warm for most of the year, there is more annual decomposition of SOM than in the North and generally soils are lower in organic matter.

Organic matter consists of numerous compounds which vary greatly in their ease of decomposition. Sugars, starches and proteins are rapidly decomposed by microbes while lignin, fats and waxes are resistant to this process. Fresh organic residues consist mostly of easily decomposed compounds which break down rapidly under favorable conditions. The result is a rapid reduction of in the volume of SOM.

The resistant materials remain and form the dark colored material called humus. Humus continues to decompose, but at a very slow rate. Carbon dating has shown some humus to be thousands of years old. Humus forms the colloids which contribute to increased cation exchange capacity and good soil structure.

In summary, a moist, warm, well aerated soil with a pH between six and seven provides ideal conditions for decomposition of SOM. These are the conditions that promote optimum growth of most vegetable crops. It seems clear that productive farming practices are usually quite destructive to SOM! But this isn’t as bad as it sounds. Imagine what would happen if SOM didn’t break down. We would be buried under tons of undecomposed dead plant material. The decomposition of SOM is a beneficial process. It provides energy for a diverse group of soil microbes, releases nutrients for plant growth and leaves us with humus. The challenge is to continuously replace what is lost and, if practical, increase SOM.

Adding To Soil Organic Matter

Compost Compost quickly comes to mind when thinking of ways to add organic matter to the soil. Everyone from homeowners to farmers can make compost. Most farmers don’t have enough raw materials to satisfy their needs. Some are bringing in additional materials such as municipal yard wastes to compost on site. Others are purchasing compost from the increasing number of commercial composters. Regardless of the source, compost should be finished before use. Finished compost has no recognizable bits of matter and will not heat up after turning. Compost should also be tested for nutrient content. Finished compost should have a low ammonium content, high nitrate level and a pH near neutral. Repeated use of a compost high in a particular element could cause a nutrient imbalance.

Manure Animal manure is an excellent source of nutrients and organic matter. About half of the nitrogen in fresh dairy manure and 75% of the nitrogen in poultry manure is in the form of ammonia. Ammonia is subject to loss through volatilization if not incorporated immediately after spreading. In the soil, ammonia is converted to nitrate and is available for plant use. However, nitrate is subject to leaching and large applications should generally be avoided. There are times when readily available nitrogen is needed, but fresh manure should be applied with caution. Many people prefer to compost manure before field application. This stabilizes the nitrogen. Manure can be mixed with other materials for composting.

Cover Crops Cover crops are gaining favor as a way of increasing organic matter. Winter cover crops have been used for years, primarily to protect soil from erosion. Winter cover crops can also take up much of the nitrogen left over at the end of the growing season. Winter rye has been an old stand by. It can germinate and make quite a bit of growth, even if planted as late as October. Winter rye is efficient at taking up left over nitrogen. It remains green over the winter and resumes growth early in the spring. It adds little organic matter if plowed under in early Spring while still small. If allowed to grow until late may, it can reach three to four feet and contribute a fair amount of organic matter. Unless plowed under while quite small, it can be difficult to break up the clumps of winter rye, making it difficult to seed crops.

Some growers prefer oats as a winter cover. Oats should be planted in late summer to make good growth in the fall. Oats winter kill and don’t re-grow in the spring, making it easy to incorporate them into the soil. If there is time to make good growth in the fall, oats can contribute several tons of organic matter per acre.

Hairy vetch is a legume which some farmers are using as a winter cover. If planted in mid August and plowed under in spring, it can grow enough to contribute 100 lb or more of nitrogen per acre. Hairy vetch can be planted as late as mid-September, but it won’t grow much in the fall, and must be allowed to grow until late May to reach its potential for nitrogen contribution. Most farmers plant vetch in combination with rye or oats.

A number of vegetable growers are growing a summer cover crop to increase SOM. This may involve taking a field out of vegetable production for a season, but an early vegetable crop can be grown and followed by a summer cover crop. A good choice is sorghum-sudan grass. This is a fast growing plant that looks like skinny corn. It can produce a high amount of organic matter if planted by early July. It grows up to ten or twelve feet high and is difficult to turn under. However, this can be made more manageable by mowing two to three times during the season, whenever it reaches three to four feet.

Carbon-To-Nitrogen Ratio As we discussed, organic matter is broken down by microbes which use carbon for energy. They also have a high requirement for nitrogen. Microbes have a requirement of about one nitrogen atom for each 25 carbon atoms. This is a carbon-to-nitrogen ratio (C:N) of 25:1 or 25. If the organic matter has a higher C:N, microbes will need more nitrogen than is in the organic matter and will take it from the soil. Microbes are more efficient than crops in obtaining nitrogen from the soil. If there is not enough nitrogen for both the microbes and the crop, the crop will not obtain what it needs. Eventually there will be a net gain in nitrogen, but crops can suffer in the short term. If organic matter with a high C:N is applied to soil shortly before planting a crop, additional nitrogen may be needed to assure that the needs of both the microbe and the crop are met. Organic matter with a C:N of less than 25:1 (25) should not be a problem and in some cases can contribute nitrogen for crop use. See Table 1 for examples of C:N’s of some sources of organic matter.

Table 1. Typical carbon-to-nitrogen ratios.

Material C:N Ratio
Legume hay 15-19:1
Non-legume hay 24-41:1
Corn stalks 42:1
Oat straw 70:1
Rye straw 82:1
Cow manure 8:1
Finished compost 17-20:1
Agricultural soils 8-14:1
Hardwood sawdust 500:1

We should consider the C:N when incorporating a cover crop into the soil. In recent experiments peppers were planted following:

1) no winter cover crop; 2) winter rye, or 3) winter rye plus hairy vetch. The cover crops were plowed down at the end of May. On plots with no nitrogen added the plants following rye were stunted and yellow compared to those following no cover crop. At the time of incorporation rye would have a C:N ratio well over 25 causing a reduced availability of nitrogen to the peppers. The peppers following vetch/rye grew better than either of the other treatments. Apparently, the vetch contained enough nitrogen to supply the needs of both the microbes and peppers.

Crop residues contribute nutrients and organic matter if they are returned to the soil. Residues are usually returned by incorporating them directly into the soil, but on a small scale, they can be removed from the field, composted and then returned to the soil. Crops vary considerably in the amount of organic matter and nutrients their residues contribute. An acre of corn stalks can contain several tons of organic matter. About two-thirds of the nutrients taken up by a corn crop go into the stalks and leaves. This is returned to the soil if only the ears are harvested and the stalks are plowed under. Some sweet corn growers sell their corn stalks to dairy farmers who chop them for silage. The going price is about $40 per acre based on the feed value of stalks without ears. This doesn’t pay the cost of fertilizer to replace the nutrients lost in the stalks, not to mention value of the organic matter. The amount of organic matter and nutrients in the unharvested parts of plants varies considerably depending on the crop. For instance, onions or cabbage leave little in the field after harvest. The New England Vegetable Management Guide contains a table and other information regarding nutrient uptake of crops. This information is presented for the part of the crop harvested and the residue left in the field. From this you can estimate the nutrient value in residues of various crops.

In the next fact sheet, we will discuss how to interpret a soil test and put this information to practical use.