MANAGING AGRICULTURAL WASTES

J.P. Murphy, Biological and Agricultural Engineering

Land Application

Manure nutrients help build and maintain soil fertility. Manure also improves tilth, increases waterholding capacity, lessens wind and water erosion, improves aeration, and promotes beneficial organisms. When wastes include runoff or dilution water, they can supply water as well as nutrients to crops. The economic value of manure fertilizer is calculated from its available N,P, and K at commercial fertilizer prices. These values change with the costs of fertilizer and handling practices.

Applying excess manure can harm crop growth, contaminate soil, cause surface and groundwater pollution, and waste nutrients. While most soils have a tremendous capacity to absorb phosphorus, very high soil phosphorus levels can interfere with plant nutrition by inhibiting uptake of metallic trace elements such as iron, zinc, and copper. When plant residue or manure is added to soil, there is an immediate and marked drop in O₂ and an increase in CO₂ in the soil air, which can inhibit plant growth.

The carbon-nitrogen ration (C/N) of applied wastes affects both microbial and plant growth. If a waste having a high C/N ratio, such as manure with a lot of bedding, is added to a soil, organisms decomposing the organic matter grow until available mineral and nitrogen become limiting. All the immediately available nitrogen is bound by the microorganisms. In the short run, nitrogen is unavailable for plant use and more chemical fertilizer may have to be added than before the waste application.

Heavy manure applications can increase soil salinity, especially in arid regions where little or no leaching occurs. Salts can inhibit plant growth and depress yields. If salinity becomes a problem, consult a soil specialist. Sodium and potassium can alter soil structure and reduce water movement rates. Field equipment, such as heavy manure wagons, compacts wet soils, alters soil structure, and reduces water movement.

Nutrient Losses During Collection and Storage

Table 1 gives the nutrient content of swine manure as produced. Housing and waste handling systems affect the nutrient composition of wastes. Bedding and water dilute manure, resulting in less nutrient value per pound. Much nitrogen can be lost to the air as ammonia. Runoff and leaching in open lots can remove nitrogen. There is much less nitrogen loss from compost pits, liquid storage systems or roofed feeding areas as shown in Table 2.

Phosphorus and potassium losses are negligible except for open lots or lagoons. About 20 - 40% of the phosphorus and 30 - 50% of the potassium can be lost by runoff and leaching in open lots. However, much of the P and K can be recovered by runoff control systems such as settling basins and holding ponds. Up to 80% of the phosphorus in lagoons can accumulate in bottom sludge and is not applied to land unless the sludge is removed.

Application

Manure is usually:
l Broadcast (top dressed) with plowdown or disking.
l Broadcast without plowdown or disking.
l Knifed (injected under the soil surface).
l Irrigated

Table 3 shows average nitrogen losses by method of application. The greatest nutrient response follows land application and immediate incorporation into the soil. Plow down solid manure as soon as possible to minimize nitrogen loss and to begin release of nutrients for plant use. Most losses occur in the first 24 hours after application, so incorporate manure into the soil as soon as possible. Injecting, chiseling, or knifing liquids into the soil minimizes odors and nutrient losses to the air and/or to runoff.

Nitrogen loss as ammonia from land is greater during dry, warm, windy days than during humid or cold days. Ammonia loss is generally greater during the spring and summer months. Uniform application of manure prevents local concentrations of ammonium or inorganic salts that can reduce seed germination and yields.

Crop Nutrient Removal

Apply manure so additional nutrients do not greatly exceed crop needs (Table 4). Before heavy manure applications, have your soil tested for fertilizer needs and nutrient imbalance. Adjust waste application rates for your soil conditions against soil tests for nitrogen, phosphorus, and potassium.

Available nitrogen (N) is nitrogen the plant can use. Total N is mostly organic and ammonium nitrogen. Organic N is slow releasing N. Ammonium N is equivalent to commercial fertilizer and, except for the N loss to the air, can be used by plants in the application year. Organic nitrogen must be released before plants can use it.

Variable amounts of organic nitrogen are released in a plant-available form during the first cropping year after application. Organic N released during the second, third, and fourth cropping years after initial application is usually about 50%, 25% and 12.5%, respectively, of that mineralized during the first cropping season.

Nearly all of the phosphorus and potassium in animal wastes are available for plant use the year of application. After a few years of regular waste applications, the amounts available are about the same as one year’s application.

Manure Management Plan

Every swine producer is encouraged to develop a manure management plan for their operation. The nitrogen loss tables point out the variability of nitrogen content of swine manure. Couple that variability with the inherent variabilities of soil type, tillage practices, weather patterns, and crop response and the swine producer can only estimate nitrogen application from manure within broad ranges. For three different swine production systems, Table 5 gives an estimate of acres needed for 100 lbs. of available nitrogen per year using various methods of manure collection, storage, and field application.

Each producer must fine-tune their manure management. Swine producers should develop a fertilizer application plan that first maximizes the use of manure nutrients and then supplement these nutrients with commercial fertilizer only if additional nutrients are needed for the corp. The major elements of such a plan include: 1.) periodic analysis of the manure produced, 2.) a routine soil testing program, 3.) keeping records of manure applications and soil tests over time to minimize nutrient and salt build up.

Diligent and conscientious management of swine manure is necessary to minimize water contamination and odor production while maximizing the nutrient value of the manure.

Table 1. Nutrients in Swine Manure as Produced

Nutrients produced per animal per year.

Table 2. Nitrogen Losses During Handling and Storage

Typical losses between excretion and land application adjusted for dilution in the various systems.

These values are in addition to land application losses, based on Purdue University data.

Table 3. Nitrogen Losses During Land Application

Percent of nitrogen applied that is lost within 4 days of application.

_{Table 4. Crop Nutrient Utilization}