Inorganic fertiliser prices are at record or near record highs. Current prices for anhydrous ammonia (AA), potash (POT) and diammonium phosphate (DAP) are $1,155 per ton (5.8US cents per lb), $920:ton (46cents per lb) and $1131:ton (56.6cents per lb), respectively. Typical application costs for AA, POT and DAP are $7.00 per acre, $3.25 per acre and $3.25 per acre. As a result, many grain producers are looking for alternative soil amendments as sources for nitrogne (N), phosphorus (P) and potassium (K).

One of these alternative soil amendments is liquid swine manure or slurry. Slurry has some obvious advantages including lower cost, a good source of N, P and K, local availability, high organic matter content and the ability to enhance soil characteristics. However, just because swine slurry may be lower priced (cheaper) than inorganic fertiliser does not necessarily imply slurry is a more economical soil amendment for the environment. Historically manure has been land applied as a sole source soil amendment (fertiliser) for N, P and K. Applying manure to meet the crop's N requirement and thereby over applying P can result in soil P build-up. Whether or not this increase in localis-zed soil P has contributed to surface water hypoxia may be debated.

Certainly more research clarifying this issue is required. Regardless, management of liquid swine manure has become an important issue in American agriculture. The potentially negative environmental consequences for handling and land application of swine slurry must be addressed in relationship to the same consequences for using inorganic fertiliser sources. These consequences, including the runoff to surface water and leaching to ground water of nutrients (especially N, P and K) must be assessed from a scientific perspective based on actual data collection and not on theoretical projections.

The LUW Team is currently conducting or has planned several studies designed to evaluate these issues. In order to correctly design studies to evaluate the energetic, environmental and economic impacts of utilizing slurry, we must first recognise what we already know.

What Do We Know?

Assume the following scenario, a farmer has four grow-finish buildings, each with a one-time capacity of 1100 hogs and a turn-over rate of 2.25 times per year producing 9,900 finishers and generating 2,409,000 gallons of slurry per year.

The gallons of slurry generated per year is based on an average pig in the buildings weighing 200 pounds producing 1.5 gallons of slurry per day containing 0.06lbs. N, 0.02lbs P and 0.04lbs K per gallon. Total N, P and K production for the operation in one year is 144, 540lbs N, 48, 180lb P and 96,360lb K.

We know that one bushel of shelled corn production requires 1.33lb N, 0.22lb P and 1.10lb K. In this scenario, 180 bushels of shelled corn per acre requires 239lb N, 39.6lb (40 lbs) P and 198lb K.

For this scenario, the inorganic fertiliser choices are potash (60 per cent K₂O, 50 per cent K), AA (82 per cent N) and DAP (18 per cent N, 19 per cent P). Accordingly, 246lb AA, 208lb DAP and 396lb POT costing $142.68, $118.56 and $182.16, respectively are required. Total inorganic fertiliser cost per acre including application (at $13.50 per acre) is $456.90. (Prices relate to November 2008.)

If the 2,409,000 gallons of slurry are land applied to satisfy the 180 bushel per acre corn yield based on total N requirement (144,540lb N ÷ 239lb N per acre), each acre requires 3,982 gallons of slurry applied over 605 acres; based on total P requirement (48,180lb P ÷ 40lb P per acre), each acre requires 1980 gallons of slurry applied over 1217 acres; and based on total K requirement (96,360lb K ÷ 198lb K per acre), each acre requires 4,947 gallons of slurry applied over 487 acres.

Based on a survey of manure applicators in Illinois conducted during the fall of 2007, the average cost to land apply slurry via injection is 2.01cents per gallon for less than one million gallons, 1.67cents per gallon for between one million and four million gallons and 0.9cents per gallon for more than four million gallons. Therefore, for this scenario, the cost to land apply slurry is 1.67cents per gallon.

If the slurry is applied to meet the N requirement, N is met at exactly 239lb N per acre, P is over-applied by 40lb per acre or double the required amount and 159lb of K are applied which is 39lb short of the required amount. Therefore, 78lbs of POT is needed costing $39.13 (including application cost). Subtracting the cost per acre to land apply slurry (3,982 gallons × 1.67cents = $66.50 per acre) and the cost of POT ($39.13) from the total cost to apply inorganic fertiliser ($456.90), slurry has a fertiliser value of $351.27 per acre or 8.82cents per gallon returning $212,473 to the swine operator (2,409,000 gallons × 8.82cents per gallon). But, the operator over-applied P.

If the slurry is applied to meet the P requirement, P is met at exactly 40lb P per acre, N is deficient 120lb and K is deficient 119lb. Therefore, 146lb of AA costing $94.68 (including application cost) and 238lb of POT costing $112.73 (including application cost) is needed. Subtracting the cost per acree to land apply slurry (1980 x 1.67cents = $33.07 per acre), slurry has a fertiliser value of $219.42 per acre or 11.08cnets per gallon ($456.90 - $94.68 - $112.73 - $33.07 = $219.42) returning $266,917 to the swine operator (2,409,000 gallons × 11.08cents per gallon). Applying slurry to meet P requirements and supplementing slurry to meet N and K requirements provides a nutrient-balanced fertiliser programme and returns the most potential income to the swine operator.

If the slurry is applied to meet the K requirement, K is met at exactly 198lb K per acre, and N and P are applied in excess (+58lb N per acre and 59lb P per acre). In other words, slurry is applied as a sole source of fertiliser to guarantee the minimum amount of N, P and K required to grow 180 bushels per acre of shelled corn. Applying 4,947 gallons of slurry to meet the K requirement costs $82.62 per acre (4,947 × 1.67cents) and the fertiliser value for slurry (not counting the excess N and P applied) is $374.28 per acre or 7.57cents per gallon of slurry ($456.90 minus $82.62). This rate of application returns $182,361 to the swine operator (2,409,000 gallons × 7.57cents).

However, regulations for nutrient management plans (NMP) in most states limit manure application per acre to the amount of N required to grow the specific crop and many NMPfs are beginning to prohibit applying excess P per acre above what the crop requires. In several states, the soil P rating (low, medium or high) provides a recommendation(s) to not limit P application (lb per acre) if the P soil test is low (in Illinois, <50lb per acre), to limit P application to the amount required for crop production if the P soil test is medium (in Illinois between 50 and 70), and to not apply any P if the P soil test is high (in Illinois above 70lb per acre).

From a fertiliser perspective, slurry has greatest economic value per gallon if it is applied to satisfy the crop's P fertiliser requirement. Applying less slurry per acre provides more value per gallon but requires more acres to apply a given amount of slurry. If costs for transporting slurry to the field are included, more acres translates to a lower total return to the swine operator. Applying slurry to meet N or K requirements may not be allowed under NMP guidelines and may not be environmentally sustainable.

What Do We Not Know?

We do not know the effect of:

1. utilising slurry as the sole source of fertiliser, or
2. utilising slurry in combination with inorganic fertiliser compared to using inorganic fertiliser as the sole source of soil amendment on the following: crop growth and yield, soil structure, soil cation exchange capacity, soil organic matter, nutrient overload in ground water, net dollar return per acre and net dollar return for slurry generation. Hopefully, the LUW Team studies will help provide answers to what we do not know.

Suggestions for Land Applying Slurry

1. Obtain pit specific slurry samples for chemical analyses prior to land application. Slurry composition varies by storage method. Slurry stored in lagoons is more dilute than slurry stored in deep pits. Fresh slurry (less than two weeks old) is more dilute than slurry stored for prolonged periods. Slurry from shallow pits where pit flushing and/or pit recharging with water is used is more dilute than slurry stored in deep pits for prolonged periods. The liquid manure generated by sows, nursery pigs and grow-finish hogs varies in nutrient composition and N, P and K concentration. The concentration of solids and N, P and K will vary substantially from pit to pit. Separate samples from each pit are required.
   
   
2. How samples are obtained affects N, P and K concentration analyses. The solids are concentrated on the bottom of a pit or lagoon. The effluent containing little biosolids is on top. Therefore, constant and complete agitation of the slurry is important to obtain a representative sample. Even with agitation, a slurry probe should be used to collect truly representative slurry samples. Be sure the probe can reach to the bottom of the pit.
   
   
3. Obtain multiple samples from each pit or lagoon. Collect probe samples from multiple locations within the pit/lagoon. Obtain a minimum of one sample for every 50,000 gallons if multiple pits are sampled. If one large slurry storage facility (one million gallons or more) is sampled, obtain one sample for every 200,000 gallons.
   
   
4. Be sure to use the slurry analyses when calculating how much slurry to apply per acre. Do not average all samples to determine how much slurry to add per acre. Only average the sample values for each storage facility. When the source of the slurry is changed, use the analysed values from that specific storage facility and re-calculated gallons of slurry required per acre.

For more information like this from SWEETA, click here.

February 2013