Animal waste must be applied to the soil so that the nutrients in the waste are not
greater than the soils capacity to adsorb and store them. The rate and timing of animal
waste applications are important to the protection of our natural resources (soil, water,
air, plants, and animals).
Applied waste should be incorporated into the soil as soon as possible to preserve
nutrient value and reduce the opportunity for runoff or odor complaints.
Incorporating manure into the soil conserves more of the ammonia (NH3)during
periods of warm, dry weather and prevents NH3 toxicity to the growth of plants.
Animal waste should be applied based on a limiting nutrient, either nitrogen (N) or
phosphorus (P). The rate is usually governed by the amount of N that is mineralization and
the crop N requirement. However, it is important to know that using manure as a N source
can result in an over-application of P).
The goal is to match the timing of the crop’s nutrient requirement with the
release of nutrients from the manure. Less than ½ the N and P are available the first
year after application.
N that is mineralized from manure applied in previous years should be considered, i.e.,
subsequent manure rates should be reduced.
A good animal waste management program should include: achieving high irrigation
efficiencies and tillage practices that maintain or improve soil tilth and reduce soil
compaction and/or crusting (i.e., to maintain infiltration, permeability, aeration, and
enhance the biological decomposition process).
Building a soil P residual can be beneficial in soils that readily fix P into an
insoluble, unavailable form for plant uptake (e.g., clayey soils and those high in lime).
The concentration of soluble and labile P can increase significantly at high
application rates of manure. Therefore, knowing the amount of clay (i.e., mineralogy),
oxides, organic matter, and lime in a particular soil gives the upper limit at which P
applications can occur in the soil before soluble P concentration increases, thus causing
potential problems.
The soluble salt content of manure can be high and the amount must be considered when
applying to farmland, where crops to be grown have a low salt tolerance. The % salt in
waste may be estimated by multiplying the combined % of K+, Ca2+, Na+,
& Mg2+ as determined by lab analysis by a factor of two.
The salt concentration in manure are directly related to salt levels in the cattle
rations, as most of the dietary sodium and chloride is excreted. Consequently, since
manure contain variable salt levels, precaution must be taken when they are used to avoid
excessive salt accumulation, especially on poorly drained soils (e.g., clayey soils or
soils with shallow water tables).
Manure applications can actually increase P movement into the soil because organic
phosphorus is more mobile through the soil profile than inorganic phosphorus.
Microbial Decomposition
Manure is the food and energy source of micro-organisms. Consequently, the microbial
population size depends on its food supply (e.g., plant residues and manure) and other
environmental factors such as soil moisture, aeration, temperature, pH.
The waste from warm-blooded animals have countless microbes (bacteria, viruses,
parasite, and fungi; some of the organisms are pathogenic).
Aerobic microbes require oxygen to survive, while facultative organisms function in
both aerobic or anaerobic environments.
Soils that are warm, moist, & well aerated have the highest potential microbial
activity and the highest rate of organic waste mineralization.
By-products of aerobic digestion are carbon dioxide, water, NH3, and the
release of other essential nutrients (note: organic matter begins to decay in the
effluent, i.e., in a aerobic or anaerobic environment).
Factors that influence the rate of decomposition are the carbon/nitrogen ratio, lignin
content, temperature, pH, aeration, and water content.
Composting of organic matter to reduce its reactivity or to stabilize the material is a
viable waste management component. Composting converts organic matter into a stable
organic product by converting N from the unstable NH3 form to a more stable
organic form.
Nitrogen Mineralization
Organic nitrogen in the solid fraction is mineralized, i.e., converted from the
organic nitrogen to an inorganic form, much more slowly than the liquid fraction. About
40-50% of the organic nitrogen in the solid fraction is mineralized to NH3
within 4-5 months after application to the land. However, under the right temperature and
moisture conditions, mineralization can be essentially complete in about 60 days. The
remaining N is mineralized in the 2nd and 3rd year after incorporation.
Organic nitrogen in the liquid fraction converts very rapidly to NH3.
Mineralization to NH3 can occur either under aerobic or anaerobic environments.
The mineralization of NH3 to nitrate can only take place in an aerobic
environment (e.g., upon application and incorporation to a moist well-drained soils).
The positively charged ammonium is held on the negatively charged clay and humus
colloids (very small soil particles) and generally remains in place until converted to
other forms (e.g., nitrate; this form does not occur in manure).
Nutrient Availability
Animals excrete a large portion of the nutrient elements consumed by them in their
feeds. About 3/4 of the N, 4/5 of the P, and 9/10 of the K ingested is voided by the
animals and appears in the manure. Thus, animal manure are a valuable source of both macro
and micronutrients.
On the average, a little more than ½ of the Nitrogen (N), almost all the
Phosphorus (P), and about 2/5 of the Potassium (K) are found in the Solid Fraction.
Depending on ration and manure handling practices, cattle manure can be expected to
contain about 2% N, 1% P2O5, 2% K2O, & 25% Carbon on
a dry weight basis (note: lab analysis of manure constituents show that values can have
significant ranges).
Organic phosphorus in animal waste is made available for plant growth through the
mineralization process.
Nitrogen Form
Organic nitrogen in fresh manure is the dominant form of N (organic nitrogen is
60-80% of the total nitrogen).
Total Nitrogen consists of organic nitrogen, Ammoniacal N (ammonia +Ammonium), Nitrate
+ Nitrite; while Total Kjeldahl Nitrogen (TKN) includes only organic nitrogen + ammoniacal
nitrogen.
Mammals excrete about 50% of their N in urine (i.e., Liquid Fraction, which is in the
form of urea, and the rest in the feces (i.e., solid fraction) as complex molecules
(digested food and synthesized microbial cells).
If the manure has been stored under anaerobic conditions, greater than 50% of the total
nitrogen is in the ammonia form.
Ammonia is a molecule and ammonium is a cation (ammonia and ammonium are in equilibrium
with each other, with the concentration of each depending on pH and temperature, e.g., as
both pH and water temperature increase, more of the N is in the ammonia (un-ionized)
form).
Nitrogen Losses
Upon irrigation with a sprinkler head, as much as 25% of the ammonia from the
animal waste lagoon can be lost (volatilized) between the sprinkler head and ground. The
temperature, wind, and humidity will affect losses.
On warm and windy summer days, all of the initial ammonia in manure can be lost within
24-48 hrs., i.e., for manure that is surface applied and not incorporated.
Nitrates are anions which are very soluble in water and can leach readily through the
soil profile. However, if nitrates are present under anaerobic conditions, microbial
activity can convert (denitrify) nitrates to a gaseous form of N.
Manure
The major component of Waste is Manure; fresh manure has a high water
content, commonly varying from 70-80% for dairy cattle.
Manure is a combination of feces and urine. The water content and Total Solids (TS) are
given as a % of the total wet weight of the manure.
The common carbon/nitrogen ratio of "excreted" manure is below 20:1.
Solid manure from either feedlot or dairy source, when collected from open lots at
regular intervals, usually contains 15-45% water.
Effluent
The effluents accounts for about 15% of the total solids, with the milking center
producing about 50% of the waste volume (mostly water).
In an anaerobic lagoon (e.g., lagoons that receive a significant loading of manure,
such as from the holding area or the cow yard, generally operate in an anaerobic mode) the
organic nitrogen fraction is typically 20- 30% Total Nitrogen (note: organic nitrogen
& ammonia generally are the only forms of N in anaerobic lagoons and waste storage
ponds).
If a dairy waste lagoon receives wastewater only from the milk house or the milking
parlor, the lagoon generally exhibits a very dilute supernatant and operates in an aerobic
mode.
Anaerobic dairy lagoon sludge accumulates at a rate of about 0.073 cf/lb. of total
solids added to the lagoon.
Waste Characterization
The quantity, quality, and consistency of manure varies with animal type, age,
health, feed ration, waste storage, time, climate, and other environmental factors.
The effluent waste can be characterized as the following components: (1) Total Solids =
Dry Matter (i.e., Dissolved Solids + Suspended Solids); (2) Volatile Solids = Organic
Matter (about 6 times more volatile solids are produced than fixed solids); (3) fixed
solids = mineral Ash (note: both dissolved and suspended solids contain volatile solids
and fixed solids); and (4) Settleable Solids, e.g., sludge, which is that matter in waste
water that will not stay in suspension during a preselected settling period, such as 1 hr.
(a sedimentation tank is a unit in which wastewater containing settleable solids is
retained to remove by gravity a part of the suspended solids).
Waste contains manure, bedding (e.g., straw), soil, and wasted feed (about 3% is
common; feed consumed by animals is 50-90% digested). Soil or other inorganic materials
commonly added to manure can result in a waste that has double the Fixed Solids content of
"as excreted" dairy manure.
Much of the P settles out and is lost from the liquid applied on the land (about 73% of
P is in the organic form and typically 70-90% of the P in waste will settle and be
retained in the sludge on the bottom of the lagoon).
Organic Phosphorus is the principal form of P in the metabolic by-products (waste) of
most animals.
Questions about HIT may possibly be quickly answered by
contacting:
Rudy Garcia
Natural Resources Conservation Service
Soil Conservationist & Water Quality Specialist
e-mail: rgarcia@nm.nrcs.usda.gov
or call: 1-505-522-8775, extension 116
Note: Technical Questions
may possibly be quickly answered by contacting USDA, Natural Resources Conservation
Service Soil Conservationists:
