Types of Organic Matter Additions
In general, organic matter additions to a soil will increase its ability to hold nutrients in an
available state.Organic matter additions will also increase soil biological activity which will
affect the availability of nutrients in the soil. Soil which has received organic matter has
increased microbial populations and more varied fungal species than soils receiving synthetic
fertilizer applications. The long-term objective of organic matter addition is to build up soil
humus. Humus is the more or less stable fraction of the soil organic matter remaining after
decomposition of plant and animal residues.
Cover Crops
Cover crops are those planted to improve the soil or for weed control rather than for harvest.
They are discussed in more detail in Cover Crops. Cover crops can gradually add organic matter
to the soil and help retain soil nutrients from one season to the next. Cover crops vary greatly,
however, in their relative contribution to soil organic matter and soil fertility. For example,
legumes decay quickly because residues are high in nitrogen. Thus they are more valuable as N
sources than as organic matter sources. Grass crops, such as rye, will have a much greater effect
on soil organic matter content than legumes because they have a higher carbon to nitrogen (C:N)
ratio and decay more slowly.
While most cover crops will add organic matter to the soil, prevent erosion, and hold nutrients
from one season to the next, for a cover crop to be an effective fertilizer, it must also accumulate
nutrients that would not otherwise be available to the following crop. Legume cover crops supply
some or most of the following crop's nitrogen needs, but some other cover crops also increase
plant nutrient availability. For example, buckwheat and sweet clover are able to accumulate
phosphorus even in soils with low available phosphorus. Others may have root systems that
scavenge deeply for nutrients. The chapter on Cover Crops describes ways to calculate the
approximate amount of nitrogen and other nutrients that will be available when using a particular
cover crop. In general, cover crops need to be incorporated to speed up nutrient availability to the
following crop.
Manure
Many soils in the southern United States, especially in the Piedmont, have poor tilth. Manures
and other materials with a high organic component are especially useful where topsoil has been
lost. They are also beneficial in soils with micronutrient deficiencies or where specific nutrient
needs cannot be determined by soil testing. Manures and composted materials also are used to
improve the water-holding capacity of sandy soils and to improve the structural stability and tilth
of heavy clay soils.
Manure, along with crop residues, is a primary means of replenishing soil organic matter.
Although crops respond readily to the nutrients in manure, the total yield benefits from manure
sometimes take three or more years to become apparent. Although a portion of the nutrients and
organic matter in manure is broken down and released during the first year or two, a portion is
also held in humus-like compounds which decompose more slowly.
Uncomposted manures are difficult to apply, not only because of their bulk, but because it is easy
to apply more nitrogen than the plants can absorb. For this reason direct application of manure or
other raw animal wastes is not recommended nor is it usually allowed in certified organic
production. Direct application can lead to problems of excess nitrates in the plant and runoff of
nitrates into surrounding water supplies. Excessive raw manure can burn plants and lead to toxic
levels of nitrates in leafy greens.
Nitrogen, the most unstable nutrient in manure, can easily be lost by volatilization in the form of
ammonia gas. For example, when manure is spread on frozen ground, 25 percent of the nitrogen
can be lost immediately and 75 to 80 percent can be lost within two weeks. Delaying application
until manure can be immediately incor-porated into the soil will lessen the amount of nitrogen
lost.
Determining the correct amount of manure to apply is difficult. Manure samples should be
analyzed for nutrient content and levels of metals such as copper which are often present in
poultry litter. Nitrogen available to the plant is lower than the content in the samples since some
loss occurs through volatilization with spreading and since only a portion of the organic N
becomes available to the plants through mineralization during the growing season. Also, the rate
of manure application needed to supply the nitrogen needs of the crop will usually supply
phosphorus and potassium in amounts in excess of those the plant can use. This excess
application generally does not affect crop growth but can, in the case of phosphorus, pollute
water if runoff or erosion occurs. Phosphorus runoff can be minimized by controlling erosion
with cover crops or mulches.
An innovative farmer brings his crop residues into the chicken coop. Hens scratch
through the residues, eat weed seeds and insects.
Their droppings add nitrogen which speeds decomposition.
Composts In comparison to chemical fertilizers, composts have relatively low amounts of
nitrogen, phosphorus, and potassium. However, the total amount available over time is greater
than that immediately available. Some nutrients are held in humus-like compounds which
decompose very slowly so that their effects on soil continue for years after application.
Composts can be made on-farm, but they are also available from municipal and commercial
sources. Municipal sources usually consist of yard wastes, minimally processed, and distributed
at low or no cost. These composts vary in quality, but tend to have low fertility and such high
C:N ratios that little N is available. Since plastic, trash, and industrial waste may also turn up in
municipal wastes, some state organic certification programs do not allow their use.
Commercially composted chicken and turkey litter is also increasingly available in the south.
Poultry litter usually has a higher nutrient level than other composts and few weed seeds, but
may contain feed additives, including heavy metals such as copper.
Composting raw materials before applying them to the soil has several advantages. It:
1. prevents the loss of N through ammonia gas (NH3 by fixing N into organic forms.
2. allows the safe use of materials such as cannery wastes and sawdust that otherwise tie up
soil nitrogen.
3. distributes mineral soil amendments more evenly than direct application.
4. reduces pathogen populations when temperatures reach 150 degrees F and destroys weed
seeds when temperatures reach 175 degrees F.
5. reduces the volume of material that has to be applied.
6. stores and handles more easily.
7. reduces N availability below levels that will burn plants. For example, in a greenhouse
study with barley and sudangrass, N availability from fresh dairy manure was reduced
from 22 percent to 14 percent in composted manure, while that of poultry manure was
reduced from 60 percent to 43 percent.
Composting also has several disadvantages. It:
1. is not as effective as raw organic matter in improving soil structure. As microorganisms
work to decompose raw organic matter, they excrete gels and slimes that bind soil
particles together and enhance soil structure.
2. results in loss of nitrogen as NH3 when the compost is turned.
3. reduces nitrogen availability in comparison to the raw material from which it was made.
Recommendations for on-farm composting. Successful composting requires a moisture content of
50 to 60 percent, oxygen content of 5 percent or more, and a carbon to nitrogen ratio of 30:1. If
the oxygen content falls below 5 percent, decomposition becomes anaerobic, slows down, and
creates unpleasant smells. The optimum temperature is from 122 to 140 degrees F. Under
optimal conditions, the temperature of the compost increases rapidly as heat is given off by the
composting process. Within a few days it will reach its maximum temperature. At this point,
organic matter burns up rapidly and the pile is said to 'cook.' Within a week or so the pile starts
to cool. The temperature during the heating part of the cycle is critical.
If temperatures are hot enough, pathogens (150 degrees F required) and weed seeds (175 degrees
F required) are killed. However, if temperatures are too high, the process self-destructs, killing
the various bacteria, fungi, and actinomycetes whose activities drive the composting process.
Thus the pile is turned or mixed to maintain temperatures from 122 to 140 degrees F and to keep
oxygen above 5 percent. As the pile cools, decomposition continues at a slower rate. This
maturing stage may take weeks or months. At some point, however, the compost temperature
returns to that of the surrounding air and soil and further decomposition is minimal. This stable
compost will not heat further when the pile is turned. Usually the stable product has a high C: N
ratio (greater than 30:1), but there is no standard value for this ratio. See the list of resources for
additional information on composting.
Residues of Organic Mulches
A mulch is any material placed on the soil surface to protect the soil from the adverse effects of
raindrops, freezing, and water loss. Mulches are also used to control weeds and reduce erosion.
Organic mulching materials will break down over time, contributing organic matter to the soil.
The use of mulches for weed control is discussed further in Weed Management.