Supplying a ready source of organic matter to the soil is an important management practice for all persons using the soil for plant production. Organic matter provides nutrients, tilth, and makes the soil "healthy". The USDA-NRCS has developed a program of increasing soil organic matter as being the key to better air and water quality. Soil Organic Matter
Soil Health - Soil health is an indicator of environmental health and, like human health, provides an overall picture of the condition of the soil. The terms soil health and soil quality can be used interchangeably. Soil health or quality is the soil's fitness to support crop growth without resulting in soil degradation or otherwise harming the environment.
Soil quality changes slowly because of natural processes, such as weathering, and more rapidly under human activity. Land use and farming practices may change soil quality for the better or for the worse. Soil health deteriorates mainly through wind and water erosion , loss of organic matter, breakdown of soil structure, salinization, and chemical contamination. Soil Health WEB Links & Introduction to Soil Health & Soil Health.com
In order for soil organisms to decompose all the organic debris that is
added to the earth's surface, their growth conditions must be met.
Factors for OM decomposition
that would be ideal include:
a) soil temperatures near 68 degrees F.;
b) moisture of 50 to 70% of the soil's water holding capacity;
c) aeration--oxygen must be in adequate supply for aerobic decomposition;
d) and, lastly, a food supply or fresh organic matter additions.
Humus is a by-product of organic matter decomposition. It is resistant to further decomposition and is the source of nutrient storage capacity. In the formation of soil humus, there is a rapid decomposition of the water soluble constituents: sugars, organic acids, amino acids, lipids, and nucleotides.
Polysaccharides form the bulk of organic matter naturally added to the soil; the most abundant polysaccharide is cellulose, a linear polymer of the sugar glucose. Cellulose is relatively resistant to decay.
Soil humus becomes an important reservoir of nitrogen, phosphorus and sulfur. It also is a source of negatively charged sites which can attract basic cations called cation exchange capacity or CEC. More on this in a later unit.
The maintenance of organic matter in soils used for agricultural production is an important practice. This is usually accomplished by incorporating plant residues into the soil and the more mixing of soil and residue, the faster the decomposition. It is very difficult to increase the percentage of organic matter in cultivated soils over that which was in the virgin soil. This is especially the case for prairie soils because of the increase in temperature and aeration after cultivation. For more information about organic matter maintenance go to Why Manage Soil Organic Matter?
Note the graphs here which show organic matter levels in soil and what can happen over time. After farming for a number of years, most organic matter added to the soil only maintains the present level of organic matter. When more organic matter is added, the populations of organisms also increase.
The bar graph shows some typical cropping practices and how they affect the increase or decrease of organic matter.
The amount of soil organic matter at any one time is dependent on the gain or loss of the organic matter pool.
Change in Soil Organic Matter=Amount OM added - Amount OM decomposed
Equilibrium concept of soil organic matter: for a plow layer soil with 2 % SOM can be demonstrated by the following:
Assume an AFS is 2,000,000 lbs and is 2% SOM or it contains
40,000 lbs of SOM,
Assume a 2% annual decomposition rate or .02 x 40,000=800 lbs of
SOM lost or 39,200 lbs left.
Assume during the year 2400 lbs. of crop residues were added to
the soil.
Assume 2/3 of residues were decomposed and 1/3 was humified; or
1/3 of 2400=800 lbs of new SOM.
Therefore- Total humus at end of year=39,200 + 800=40,000.
It is interesting to note that the decomposition of 800 lbs of SOM if the SOM was 5% nitrogen would mineralize (or release for plant use) ( 800 x .05) 40 lbs of nitrogen.
For more information about how tillage and cropping systems affect SOM see Residue Management Systems.
Green Manure
Green manure crops or cover crops are plants sown to enrich the soil when plowed or tilled under at a later date. Organic matter is added to the soil as well as the potential for nutrients such as nitrogen. An added benefit is reducing soil erosion during winter months due to blowing winds.
Legumes used as green manure crops can provide a significant amount of nitrogen to soil the following year. Typically, green manure crops are sown in the fall and rototilled under the following spring. However, fallow ground can be covered with plant material for a year or more before plowing under. Crops should be chosen carefully to avoid a potential weed problem down the road. While the crop can be plowed under, it may set seed or reproduce through root cuttings.
Common winter green manure crops include: rye, wheat and ryegrass. Most are sown between August and September to allow for fall growth. Summer cover crops include oats, soybeans and buckwheat. Other plants used for green manure crop include alfalfa, clover (red, alsike, crimson, sweet), barley, bromegrass, lespedeza. Most are sown in the early spring and plowed under in the fall.
C to N Ratio of Organic Materials & Composting
If organic material is added to soil that has a wide carbon to nitrogen ratio, the nitrogen in the soil will be used by the organisms to decompose the organic matter. This can lead to nitrogen deficiencies for plants growing in the soil. In order to avoid N immobilization it is best to compost organic materials that have a C:N ration wider than 30:1 before they are added to the soil.
Everything organic has a ratio of carbon to nitrogen (C:N) in its tissues.
Manure (Fresh) C: N=15:1
Legumes (peas etc.) 15:1
Grass Clippings 20:1
Manure w/Weeds 23:1
Weeds (Fresh) 25:1
Hay (Dry) 40:1
Leaves (Fresh) 40:1
Leaves (Dry) 60:1
Weeds (Dry) 90:1
Straw, cornstalks 100:1
Pine Needles 110:1
Sawdust 500:1
Wood Chips 700:1
It is the combination of materials that creates the ideal climate for compost microbes which is around a C:N ratio of 30:1. This combination, along with moisture, oxygen and surface area, is what makes a fast, hot pile.
Some composters like to keep things simple and use the terms brown (carbon) and green (nitrogen), and follow the general rule of 1 part brown for every 2 parts green. Other composters like to build their piles using a variety of materials and using the following formula to determine the C:N ratio of their compost pile. This balance is difficult to achieve exactly, but you can come close by combining materials and calculating the resulting (C:N) ratio.
For example, mixing two parts green grass clippings (which have a C:N ratio of 20:1) with one part dry leaves will give you a C:N ratio of 33:1, very close to the ideal.
The process is this: add 20/1 + 20/1 (the 2 parts green) with 60/1 (the
leaves).
The total is 100/3 or 33/1. Another sample: Making a pile of 2 parts green
grass, 1 part pine needles and 1 part fresh manure yields a C:N ratio of _____?
20/1 + 20/1 + 110/1 + 15/1=165/4=41/1 or this would need some additional
"green" material.
Using the C:N ratios above work out the following problems.
Keep in mind that all volumes are equal - each pile of material in the following problems is approximately the same size ( 1 gallon, or 1 pail , or 1 barrel) .
A. Your pile is a mixture of 1 part straw, 1 part dry leaves, and 3 parts green grass. What is the C:N ratio? _____. 3.
B. You have all the waste from your yard and garden and you have some sawdust to add to the pile. The ingredients are: 1 part sawdust, 2 parts green grass and 1 part fresh weeds. The C:N ratio of this pile is _____.
C. If you make your pile with 1 part manure, 1 part straw and 2 parts green grass. what is the C:N ratio? _____.
Answers- A=44:1 & B=141:1 & C=39:1
For more information on the composting process go to Composting Information Web Page
Native SOM Levels
Under a forest ecosystem,
organic matter will be maintained at high levels. However, when compared to
grassland soils, the total quantity of organic matter in the soil is much
lower. In a forest, organic matter is only added to the surface of the soil
once each year when the leaves fall. It will take 1 to 3 years for the leaves
to decompose and be added to the soil organic matter pool. It will take time to
redistribute this organic matter into the soil. This will be aided by
earthworms and other organisms.
Grasses have a fibrous root system that
extensively explores the upper 18" of soil. During the year roots
continually contribute fresh organic matter to the soil root zone. Thus the
soil organic matter pool is throughout the fibrous root zone and not just as
the surface as in the forest. See this graph.
Climate has an influence on the quantity of organic residue. With increasing precipitation from areas of low rainfall to areas of high rainfall there is an accompanying increase in the amount of vegetation produced annually. Thus, the organic matter content of the soil increases from west to east across the central U.S.A... However, the difference between prairie and forest vegetation complicates this interpretation. Also, an increase in soil temperature speeds up the decomposition of organic debris and results in less OM in the soil.
This map shows the amount of organic matter in the upper 40 inches
of soil throughout the US. This change in organic matter depends on the
temperature and precipitation for each area.
In general, Minnesota soils with native grass vegetation (prairie) will have higher organic matter levels in the upper 2 feet of soil than those formed under trees. However, as we go from west to east across the prairie of southern Minnesota, the organic matter content of the soils increases due to higher rainfall, and thus more vegetation.
See Map for a look at Minnesota's native vegetation.
Chapter 2 Peatlands
Unit 10 Chapters
© Terence H. Cooper & Regents of the University of Minnesota, 2007 The University of Minnesota is an equal opportunity educator and employer.
