Switching Maximum Manure Application Rates from a Nitrogen Basis to a Phosphorous Basis: The Issues

Introduction

The modern trend in livestock agriculture has been the development of large confined feeding operations (or intensive livestock operations), the so called "factory farms". There is also the tendency for confined feeding operations to congregate in certain regions. A large concentration of livestock in a given region means a large production of manure. The most efficient method of utilizing manure is to apply it on agricultural land. Manure can improve soil fertility and soil physical properties, particularly for degraded soils. However, mismanagement of manure can have negative effects on the environment, such as degradation of soil, water, and air quality. Cumulated nutrients in soil are a potential source for groundwater and surface water contamination.

The best approach to preventing or reducing the negative impact of manure on water quality is to apply manure on the basis of nutrient requirements of crops. In Alberta, through previous codes of practices and now through the regulations in the Agricultural Operation Practices Act (AOPA), manure should be land-applied based on the nitrogen requirements of crops. However, when manure is applied based on nitrogen requirements of crops, other nutrients, such as phosphorus, are applied in excess of crop needs. Because of this, many jurisdictions have adopted or are considering adopting phosphorus-based application of livestock manure.

The Problem with Phosphorus

Excess phosphorus in soil is a significant risk to surface-water quality. The movement of phosphorus from agricultural land to surface water can lead to accelerated eutrophication (Correll 1998), which is ranked as the most widespread water quality impairment in the United States, and agriculture involving intensive livestock production has been identified as a primary source of phosphorus in surface waters (Sharpley et al. 2003).

As part of the Canada-Alberta Environmentally Sustainable Agricultural (CAESA) Agreement Program, a five-year water-quality assessment study revealed that agriculture practices contribute to the degradation of water quality in Alberta (CAESA 1998). Generally, extractable phosphorus levels in Alberta soils are deficient or marginal for crop production (Manunta et al. 2000), and much of the agricultural land in Alberta can benefit from added phosphorus to obtain optimum crop yield. However, over- application of nutrient sources can greatly increase soil phosphorus levels. Olson et al. (2003) showed that eight years of annual application of manure in southern Alberta increased soil-extractable phosphorus well above the levels required for optimum crop growth (Figure 1). Phosphorus has a tendency to remain in the top soil layer, and thus is more vulnerable to losses by surface water runoff. Research has clearly shown that as extractable phosphorus (bioavailable phosphorus) increases in soil, the concentration of phosphorus in runoff water will also increase (Pote et al. 1996).

Figure 1. Net cumulation of extractable soil phosphorus in a loam-textured soil near Lethbridge after eight years of cattle manure application (Olson et al. 2003).


The Phosphorus Challenge for Manure Management

It is clear that allowing phosphorus to cumulate in agricultural land poses a significant risk to surface water quality. When soil phosphorus approaches high levels, changes in manure management must be considered. The most obvious better management practice is to apply manure based on crop phosphorus requirements. However, there are several issues and challenges associated with a phosphorus-based approach.

Land-base requirements
The main implications of switching from nitrogen-based application rates to phosphorus-based application rates is the requirement of more land to accommodate manure. This can be illustrated by the following example. Based on the Alberta Fertilizer Guide (Alberta Agriculture 1995), fertilizer recommendations for the major, annual dryland crops ranges from 0 to 110 kg ha-1 for nitrogen (N) and 0 to 22 kg ha-1 for phosphorus (P). Let’s use the mid-way values of 55 kg ha-1 nitrogen and 11 kg ha-1 phosphorus as average fertilizer recommendations. Let’s use the nutrient content values for manure as given in the AOPA (Province of Alberta 2001; Schedule 3, Table 5). Using this information and making certain assumptions on manure-nutrient availability to crops, Table 1 shows calculated nitrogen-based and phosphorus-based application rates. The calculations in this example show that by switching from nitrogen-based to phosphorus-based application rates, the land-base requirement will have to at least double to accommodate the manure. If the phosphorus recommendations are lower, and/or the phosphorus content in the manure is higher, then even more land will be required.

Table 1. Comparing Nitrogen-based and Phosphorus based application rates.


Nutrient balancing and crop rotation considerations
Phosphorus-based manure application will generally result in the under-application of nitrogen in terms of meeting crop requirements. This can be more of a problem if manure contains a significant amount of bedding, such as added straw. Carbon in the straw can cause a large proportion of the crop available nitrogen to be tied up, or immobilized, by microorganisms. Minimizing the use of bedding material may help. Plus, using crop rotations with legumes in the sequence may also help to balance nutrient levels. However, there are many other factors, besides manure application, to be considered when deciding on crop rotations.

Manure-application technologies
The adoption of phosphorus-based management will result in lower application rates. When manure was applied in excess of nitrogen requirements, there was generally no concern about supplying nutrients and application uniformity in terms of crop performance. However, when managing manure as a fertilizer for crop production, applying, lower, specific, and uniform rates becomes more critical. Therefore, improved application technologies are needed to handle lower rates of manure. These technologies should emphasize incorporation or injection of manure to conserve nutrients and minimize environmental impact.

Livestock feed diets
The nutrient-use efficiency of livestock is relatively low. For example, 40 to 82 percent nitrogen, 36 to 73 percent phosphorus, and 75 to 93 percent potassium fed to livestock are excreted by the animals (Barrington 1991). However, careful planning of feed rations can influence the amount of manure produced and the nutrient content in the manure. Research has been carried out to improve nutrient-use efficiencies by livestock, and more recently, the environmental implications of nutrient-use efficiency has been pursued. Poulsen (2000) listed four methods to improve phosphorus utilization in hog production: (1) define precise recommendations for rations, (2) improve phosphorus digestibility in feed, (3) better choice of inorganic phosphorus supplements, and (4) plant breeding. For example, the addition of phytase has been shown to improve phosphorus digestibility in feed for hogs (Poulsen 2000). Many of these methods can apply to other types of livestock. However, it should be recognized that nutrient-use efficiencies probably can only be improved to a certain degree, with the genetics and biology of animals being limiting factors.

Manure processing
As already indicated, switching to phosphorus-based application rates will result in the need for a larger land base for manure application. This will result in greater transportation costs. Composting raw manure has been suggested as a method to reduce the amount of transportable material (Larney et al. 2000). Composing can greatly reduce the mass and volume of manure. Freeze et al. (1999) reported that the breakeven hauling distance is about twice as far as that of raw manure. During the composting process, a significant amount of nitrogen is lost, whereas phosphorus is conserved (Larney 2001). In fact, because of the loss in mass, phosphorus is more concentrated in compost than in raw manure. Therefore, the ratio between nitrogen and phosphorus in compost widens and the nutrient balance, in terms of crop requirements, is worse in compost.

Soil extractable Phosphorus analytical methods
Commercial laboratories and researchers have used a variety of methods to determine soil extractable phosphorus in Alberta, including the Miller-Axley, Olsen, Bray, Kelowna, and Modified-Kelowna (two versions) methods. If phosphorus limits are adopted in the future, and particularly from a regulatory perspective, a standard methodology will need to be developed. This does not necessarily mean the adoption of a single method. Standardization may be accomplished by developing conversion factors among the various methods.

Economic and policy frameworks
Applying manure on a larger land base will result in additional time required to handle the manure and more transportation costs. Freeze and Sommerfeldt (1985) concluded it was economical to custom haul manure up to 18 km in comparison with commercial nitrogen and phosphorus fertilizer in southern Alberta. Freeze et al. (1993) also showed that hauling distances could be extended if manure was used to restore eroded soils.

It is often argued that suggested manure-management changes, which require a reduction in application rates for the purpose of improved environmental sustainability, are not economical due to increased handling and transportation costs. Perhaps, the economic analysis is too narrow by simply focusing on the replacement of commercial fertilizers. Manure production is an inevitable consequence of confined livestock production, and this manure must be removed and utilized. There also seems to be a paradox with animal production systems. Why does the system allow for the transportation of feed, fertilizers, live animals, animal carcasses, and processed food across vast distances, whereas we can hardly transport manure beyond the ‘farm gate’? The economics and associated policies required to make manure management environmentally sustainable are major issues and need to be addressed.

Soil phosphorus limits
Several better management practices have been suggested to control agricultural phosphorus transfer from soil to water (Sharpley et al. 2000). In addition to better management practices, soil phosphorus limits or phosphorus indices have been adopted in many jurisdictions (Sharpley et al. 2003). The United States Department of Agriculture Natural Resources Conservation Services initiated national guidelines on nutrient management (Mallarino et al. 2002). The national guidelines suggested the use of one of three phosphorus-risk assessment tools: (1) agronomic soil test phosphorus interpretation classes, (2) environmental soil phosphorus limits, or (3) phosphorus index. In Alberta, a project was initiated in 1999 to develop phosphorus limits for agricultural soils in the province (Paterson et al. 2004).

Conclusions

Environmentally sustainable manure management is essential for the livestock industry in Alberta. Industry and government developed regulations for intensive feeding operations, with land application of manure based on nitrogen requirements of crops. However, there is strong evidence that with land application of manure based on nitrogen management, phosphorus levels can increase in the soil and pose a significant risk to water quality. Switching the management of manure application from a nitrogen basis to a phosphorus basis poses significant challenges. The primary challenge is that a larger land base is needed to accommodate the manure. This may be particularly challenging in areas that have high densities of confined feeding operations and significant portions of the land base with high levels of soil phosphorus. Other issues include, meeting proper nutrient balances for crops, manure application technology for low rates, manure processing to allow for greater hauling distances, and economic considerations. Science-based soil phosphorus limits are currently being developed for agricultural soils in Alberta.

References

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