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a Department of Crop, Soil and Environmental Sciences, 115 Plant Science Building, Fayetteville, AR 72701
b University of Arkansas Cooperative Extension Service, P.O. Box 351, Little Rock, AR 72204
* Corresponding author (nslaton{at}uark.edu).
Received for publication December 17, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Water quality issues related to hypoxia, eutrophication, or both in the Chesapeake Bay (Boesch et al., 2001), northern Gulf of Mexico (Diaz, 2001), and the Bosque River (Sanderson et al., 2001) have implicated animal and row-crop agricultural enterprises as significant sources of nonpoint nutrient pollution and heightened public awareness of agricultural nutrient use. Strategies implemented to reduce nutrient loading attributed to agricultural production have impacted the agricultural industry and have probably set a precedent for future nutrient management guidelines, making it necessary for the agricultural industry to review nutrient usage and implement best management practices to improve nutrient-use efficiency and ensure that nutrient applications are balanced with the rate of removal.
Summaries of soil-test nutrient data often indicate that some states and nations have nutrient accumulation or depletion (Fixen, 2001; Tunney, 1990). However, soil-test data and nutrient balances summarized on a state- or nationwide basis are often misleading in this regard, especially when animal and row-crop agriculture are segregated. For example, Sharpley et al. (1999) showed that excessive nutrient accumulation occurred primarily in the animal-producing areas of Delaware, but were balanced in the crop-producing regions of the state. Nutrient balances are usually determined on field (nutrient management planning), regional, or national scales since nutrient input and removal statistics are most available at these levels (Tunney et al., 2003). Nord and Lanyon (2003) showed that the components of modern agricultural production were not well integrated within a watershed making it difficult to achieve balanced nutrient management on a watershed scale. Tunney et al. (2003) reported that identifying nutrient surpluses in Ireland followed by the refinement of nutrient-use recommendations and public educational programs has reduced the use of inorganic P fertilizer and caused soil-test P levels to plateau and start to decline.
The literature contains only a few formal reviews of nutrient balance assessments for agricultural systems similar to that provided for Delaware (Sims and Wolf, 1994), Ireland (Tunney, 1990), and Virginia (Bosch and Napit, 1992). Nutrient management issues (i.e., accumulation and deficiencies) that threaten environmental quality, the productivity of agricultural lands, or both are generally reacted to rather than anticipated. It is appropriate, and, now more than ever, essential that periodic assessments of nutrient balances for the common agricultural enterprises within a state be conducted. These assessments would provide an accurate and unbiased estimate of nutrient management to identify geographical areas that require both immediate and future attention in terms of both excessive and deficient nutrient applications.
In 2003, Arkansas ranked high among all U.S. states in agricultural production: first in rice (Oryza sativa L.), second in broiler, third in turkey, fourth in grain sorghum [Sorghum bicolor (L.) Moench], fifth in cotton (Gossypium hirsutum L.), and ninth in soybean [Glycine max (L.) Merr.] (Arkansas Agricultural Statistics Service, 2003). However, to date, the overall nutrient balance for the various regions in Arkansas with distinctly different agricultural enterprises is poorly documented. Although the excess nutrient problem in northwestern Arkansas is well documented (Kellogg et al., 2000), it has not been adequately quantified or categorized into individual components. Therefore, the primary objective of this manuscript is to describe the balance between the predominant inorganic and organic agricultural nutrient (i.e., N, P, and K) sources and the amount of nutrients removed by harvested crops for nine geographically defined districts within Arkansas. Additionally, the nutrient balance for specific land use management practices will be evaluated by the major commodities produced within each district to assess whether soil-test P and K should increase, remain static, or decrease. This information will assist scientists, public officials, and land managers in developing solutions to nutrient management issues in Arkansas and surrounding states.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Nutrient Inputs to Agricultural Soils
The total inorganic fertilizer nutrient sales for N, P, and K were summarized by county and district from 1997 to 2001 (University of Arkansas Cooperative Extension Service and Arkansas State Plant Board, unpublished data). We assumed that all inorganic fertilizer nutrients were applied to soils used for agricultural purposes within Arkansas. Estimates of the nutrient content from broiler, turkey, dairy, and swine production were calculated and added to N, P, and K contents from inorganic fertilizers for total N, P, and K content. The nutrient content of inorganic fertilizers and animal manures will be considered and referred to as agricultural nutrient inputs that are collectable and transportable and applied to soils used for agricultural purposes. Nutrients contained in beef cattle manure were ignored in nutrient source estimates since a large proportion of these nutrients are obtained from the forage and deposited directly (i.e., recycled) to pastures during grazing rather than collected in lagoons or stockpiled from confined animal production facilities. Likewise, nutrients contained in biosolids (i.e., sewage sludge) were not considered in this analysis since estimates of the amount of biosolids applied to agricultural land within Arkansas are not available.
Total nutrient inputs attributed to Arkansas poultry production enterprises were determined using USDA historic estimates for annual broiler and turkey production (Arkansas Agricultural Statistics Service, 2003), referenced values for manure production per 1000 broilers, and standard referenced N, P, and K nutrient concentrations of broiler and turkey litter. Malone (1992) reviewed the literature on poultry-litter production and reported the average litter production by broilers was about 1000 dry kg (1000 broilers)–1. For simplicity and because turkey populations are relatively small compared with broilers, turkey litter production was calculated assuming the same litter production rate and nutrient concentrations as for broilers. Broiler litter nutrient concentrations cited by Edwards and Daniel (1992) were multiplied by the mean litter production rate to calculate nutrient content from broiler and turkey production. Nutrient concentrations cited by Edwards and Daniel (1992) are within the range of concentrations for N, P, and K observed for broiler litter in Arkansas.
Annual nutrient production from dairy and swine animals were determined using the number of milk cows and hogs (Arkansas Agricultural Statistics Service, 2003) and Natural Resources Conservation Service (Barth, 1999) values of 0.204 kg N, 0.032 kg P, and 0.118 kg K d–1 454 kg–1 animal weight for excreted dairy manure and 0.191 kg N, 0.073 kg P, and 0.10 kg K d–1 454 kg–1 animal weight for excreted hog manure. Nutrient content in dairy and hog manures were calculated on an "as excreted" basis as defined by the Natural Resources Conservation Service (Barth, 1999). The assumptions used to estimate excreted nutrient content for dairy manure were that the average milk cow weighed 454 kg and was lactating. The assumptions used to estimate excreted nutrient content for hogs were that the average hog weighed 59 kg and was classified as a grower hog (18–100 kg). Total nutrient input accounts for total inorganic fertilizer sales and production estimates of organic source nutrient content from broiler, turkey, hog, and dairy animals. Calculated nutrient contents from manure sources represent reasonable estimates from animal-production enterprises in Arkansas that are considered both collectable and transportable.
Nutrient Balance and Distribution
The net nutrient balance for each district was calculated by subtracting total soil nutrients removed from the total amount of agricultural nutrient inputs, with the difference representing either a net deficit or excess. The net nutrient balance was then expressed on an area basis for the categories of harvested row-crop, total-harvested cropland, and total-land hectarage in agricultural use. Urbanized land that may receive nutrient applications was not considered in net nutrient balance calculations. Harvested row-crop hectarages were the sum of the seven previously listed row-crop commodities. Total-harvested cropland included the seven row-crop commodities plus hay hectarage. The total-land hectarage in agricultural use was the total-harvested cropland plus pasture hectarage. We assumed that all excess or deficit nutrients were uniformly distributed across these land-use categories. Expression of the nutrient balance per unit of each defined land-use category provides an indication of whether the land-use categories are sufficient or inadequate to handle the net nutrient surplus or deficit for each district and should coincide with trends shown in soil-test summaries during the past decade. While this type of information is generally used to identify areas of nutrient surplus, it also has value for the identification of areas where potential nutrient deficits could occur.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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During the 5-yr period from 1997 to 2001, Districts 3, 6, and 9 (Fig. 1) accounted for 95% of the row-crop hectarage and only 16% of the hay and pasture hectarages in Arkansas (Table 2), and for only 6% of poultry, 0% of turkey, 2% of hog, and 11% of dairy animal populations. The three districts (1, 4, and 7) located in the western one-third of Arkansas accounted for 55% of the hay hectarage, 50% of the pasture hectarage, and only 3% of the row-crop hectarage. Animal production was also concentrated in the western one-third of Arkansas with 76% of poultry, 88% of turkey, 85% of hog, and 49% of the dairy populations in Districts 1, 4, and 7 (Table 2). Although beef cattle were not considered in this analysis, 56% of the beef populations were located in Districts 1, 4, and 7, while only 15% of the beef populations were in Districts 3, 6, and 9 (data not shown).
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Although the nutrient input statistics alone (Table 3) do not directly indicate nutrient management problems within the state or within certain districts, the N to P ratio of total nutrients within each district does describe an unbalanced nutrient distribution assuming that the nutrients are applied to agricultural land within each district. The three largest row-crop production areas, Districts 3, 6, and 9, have a wide total N to P ratio (6 to 11:1). The remaining six districts, which also contain the highest animal populations, have total N to P ratios of 
5:1. The narrow total N to P nutrient ratios combined with the lack of harvested cropland in the central and western districts suggest the potential for P to accumulate in the soil assuming animal manures are applied within the district boundaries. The N to K ratios for the nine districts ranged from 1.8 to 3.2 with a state N to K ratio of 2.4. The total N to K ratios for each district do not indicate a significant imbalance of N or K. The ratio of total nutrient inputs in Districts 3, 6, and 9 approximates the nutrient ratios in inorganic fertilizer blends recommended for crops grown on soils that have low to medium soil-test P and K levels.
Nutrient Removals from Agricultural Soils
Districts with predominant row-crop agricultural enterprises (e.g., Districts 3, 6, and 9) removed the largest amounts of soil N and P (Table 4). For K, however, districts with a large hay hectarage (e.g., District 1 and 4, Table 2) had slightly higher K removal than District 9. Row-crop agriculture accounted for 94 to 99% of total N, 89 to 97% of total P, and 77 to 94% of total K removals in Districts 3, 6, and 9, but only a minor portion, 2 to 43% for N, <1 to 28% for P, and <1 to 17% for K, of nutrient removals in the other six districts (Table 4). Kellogg et al. (2000) also found that the soils and their associated uses in eastern Arkansas had a greater capacity to assimilate (i.e., remove) N and P than soils in western Arkansas.
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Net Nutrient Balance
The net nutrient balance is the difference between total nutrient inputs and removals with a positive value indicating an excess of nutrients within the state or district (Table 4). The state summary shows a net excess of N, P, and K. The calculated nutrient balance is affected by inorganic fertilizer sales, animal populations, harvested crop area, and crop yields. Districts 3, 6, and 9 had net balances that were negative or near zero for N and P and positive for K. For districts in central and western Arkansas, the net balances for N and P were positive and negative for K. The amount of inorganic P fertilizer sold (Table 3) accounted for 35 to 88% of the total P removal (Table 4) for districts in central and western Arkansas. Therefore, a major portion of the poultry litter would have to be transported outside of these western districts to establish a balanced situation for P.
Net Nutrient Distribution
Although the total net nutrient balance values (Table 4) show the relative magnitude of nutrient accumulation or depletion among districts, they do not indicate the distribution of nutrients within each district. The net nutrient distributions (Table 5) suggest the extent of nutrient accumulation or depletion on an area basis for three specified land uses. The three land use categories are shown because the predominant agricultural enterprises of each district differ and require different descriptions for accurate assessment of nutrient distribution. In Districts 3, 6, and 9, row crops (Table 2) represent 79 to 96% of the area used for crop, hay, and pasture production. In contrast, the majority of land area in the western two-thirds of Arkansas is used for hay and pasture production. The net balance for nutrient distributions shown only for row-crop hectarage assumes that all excess or deficit nutrients are applied only to land used for row- crop production and provides a fairly accurate description for the eastern one-third of Arkansas. In general, the row-crop hectarage in Districts 3, 6, and 9 is sufficient to prevent accumulation of N and P in the soil and with current usage should not result in rapid soil depletion of these nutrients (Table 5). Although the net balance for K (row-crop ha)–1 is positive, it is not excessive and would probably maintain plant available soil K considering that some K is lost via surface runoff, erosion, and leaching below the root zone.
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In the six districts in central and western Arkansas, total cropland is the most appropriate category to evaluate net nutrient balance because row-crop hectarage is low and animal populations are high. Data show that K may be limiting forage and crop production in several districts in central and western Arkansas (Table 5). However, regardless of the land-use category, the net nutrient balances per unit of land for N and P were positive. Assuming that the district row-crop, hay, and pasture hectarage estimates are representative and nutrients are applied within each district, all animal-producing districts have excess N and P, which will increase soil N and P when applied exclusively to land used for agricultural purposes. This is especially important considering that most soils used for warm- and cool-season grass production in Arkansas already have adequate Mehlich 3–extractable P levels that do not require additional P fertilization for forage production (DeLong et al., 2003).
Arkansas soil-test data show that the median Mehlich 3–extractable P for established warm- and cool-season grasses increased by 2.5 mg P kg–1 yr–1 between 1995 and 2002 (Fig. 2a). The median Mehlich 3–extractable P concentration has not changed appreciably for soils used to produce row crops (Fig. 2b), which are grown primarily in Districts 3, 6, and 9 in eastern Arkansas. Mehlich 3–extractable K has remained relatively constant for all crops, showing an increase of only 0.50 to 0.55 mg K kg–1 yr–1 (data not shown). Thus, the median soil-test P and K concentrations determined by crop tend to support information from the nutrient distribution assessment.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Inorganic fertilizers are used almost exclusively as the nutrient fertilizer sources for row-crop production in eastern Arkansas. Organic nutrient sources are seldom applied to land used for row-crop production because animal manures are not readily available due to the great distance between animal and row-crop production. About 91 Mg of poultry litter are transported to eastern Arkansas each year (Kellogg et al., 2000) for amending precision-graded soils to help restore soil productivity (Miller et al., 1990).
Transporting manure outside the animal-producing areas to row-crop producing areas is one of the many potential solutions (Sims, 1997) proposed to alleviate the accumulation of soil P in areas of intensive animal production, such as in northwestern Arkansas. The low economic value of poultry litter, which represents the majority of organic nutrient sources produced in Arkansas, as a fertilizer nutrient source is believed to prohibit its transport to the primary row-crop production area. Bosch and Napit (1992) proposed the fertilizer value of poultry litter ranged from $22.10 to $31.42 Mg–1 for several crops in Virginia based on estimated litter application rates to meet crop N, P, and K fertilizer requirements. Based on litter removal (i.e., clean-out), storage, and transportation fees, Bosch and Napit (1992) concluded that litter could be transported from 127 to 262 km before the net worth of the inorganic fertilizer value of the litter was exceeded. Based on their data, transportation of broiler litter from western to eastern Arkansas would not be economically feasible. However, the less tangible, positive effects of poultry litter on soil quality in row-crop production areas, such as improving soil water holding capacity and lowering bulk density to potentially better seedling emergence (Brye et al., 2004), are not yet economically quantifiable, but more than likely add extra value to poultry litter.
If poultry litter transport across district boundaries is not considered, a use other than land application must be developed in the very near future to sustain the current level of poultry production and to a large part the economy of central and western Arkansas. In 2001, the poultry and egg industry accounted for nearly 54% of the total agricultural commodity cash receipts ($5.13 billion) and 78% of the livestock, poultry, and dairy meat animal cash receipts in Arkansas (Arkansas Agricultural Statistics Service, 2003). A recent lawsuit settlement between poultry integrators in northwestern Arkansas and the city of Tulsa, OK, limits or in some cases prohibits poultry growers in the Eucha–Spavinaw watershed in northwestern Arkansas from applying poultry litter or other P sources to pastureland because runoff from such areas is considered to further accelerate eutrophication of the city's source of drinking water (Davis, 2004).
Soil P determined by routine soil-test methods is correlated to the amount of dissolved P in runoff (Pote et al., 1996). The soluble P contained in surface-applied manures or inorganic P fertilizers may contribute much more to dissolved P in runoff than the more stable, less soluble soil P (Sauer et al., 2000; DeLaune and Moore, 2001). Transporting P and N contained in poultry litter out of critical watersheds is an important step toward decreasing nonpoint-source pollution in central and western Arkansas. The high to excessive soil-test P levels common to central and western Arkansas will eventually decline as additional P is withheld, but for some soils this process may take decades before supplemental P is needed to sustain forage production (McCollum, 1991). In the meantime, these soils will still need to be managed appropriately to reduce soil P contributions in runoff and to sustain high forage yields. The Natural Resources Conservation Service in Arkansas is now preparing P-based nutrient management plans to determine application rates of poultry litter that should help reduce P concentrations in runoff (J. Caudle, Arkansas NRCS, personal communication, 2003).
Best management practices will also be needed on soils in eastern Arkansas with low to medium soil-test P levels that will eventually receive P, regardless of whether the source is poultry litter or inorganic fertilizer. One advantage of applying poultry litter to land used for row-crop production rather than permanent pasture is that opportunities exist for mechanically incorporating the litter into the soil immediately after application. Soil incorporation may reduce P concentrations in runoff unless soil erosion is excessive and also reduce gaseous losses of N, which will improve the efficiency and value of poultry litter as a N fertilizer.
Kellogg et al. (2000) showed that soils in eastern Arkansas had the greatest capacity to assimilate N and P from animal manures primarily due to the large number of hectares that are used for row-crop production. They also showed that soils in western Arkansas had a greater assimilative capacity per unit of agricultural land area for P due to the greater removal of soil P from harvested forages compared with row crops. However, the use of nutrient management plans to prescribe soil nutrients will influence P soil inputs since manure, as well as inorganic fertilizer, application rates will be limited or sometimes prohibited due to soil-test P level.
In northwestern Arkansas, DeLaune and Moore (2001) reported that Mehlich-3 P increased 1 mg P kg–1 per 4 kg P ha–1 for a Captina silt loam (fine-silty, siliceous, active, mesic Typic Fragiudult) with an initial Mehlich-3 P concentration of 117 mg P kg–1. In contrast, the silt-loam soils used for rice and irrigated-soybean production in eastern Arkansas have much lower soil-test P and require about 5 kg P ha–1 to increase Mehlich-3 P by 1 mg P kg–1 (Slaton et al., 2003). However, the rate of increase is somewhat misleading because 25 to 50 kg P ha–1 yr–1 were needed to replace P removed by harvested crops and maintain the initial soil-test P. The alternating aerobic–anaerobic status of soils used for flood-irrigated rice production tends to retard increases in soil-test P by fixing P into forms that are less soluble and apparently not extracted by most routine soil-test methods even when relatively high P fertilizer rates are applied. Some evidence of this is shown in Fig. 2b. Flood-irrigated rice and irrigated soybean are usually grown in rotation on soils with the lowest soil-test P levels. However, rice is grown in rotation with corn and cotton less frequently or not at all on most farms and this fact is at least partially responsible for the higher soil-test P levels associated with soils used to produce these upland crops. These data, coupled with P removal by row crops, indicate that accumulation of soil P would be slower for soils in eastern Arkansas, which make these low-P soils well suited for land application of excess poultry litter from western Arkansas.
Assuming that 5 kg P ha–1 are required to increase soil-test P by 1 mg P kg–1, the 2.5 mg P kg–1 yr–1 increase observed between 1995 and 2002 for soils cropped to warm- and cool-season grasses indicates that 12.5 kg P ha–1 are applied in excess of the amount required to maintain soil-test P. This range corresponds closely to the amount of excess P applied to soils in some of the six districts located in the western two-thirds of Arkansas (Table 5). If the average row-crop yield removes 20 kg P ha–1 and poultry litter is applied to replace only the removed P (1400 kg poultry litter ha–1), approximately 2.6 million ha of soils with low to medium soil-test P are needed to distribute all the P from Arkansas poultry and turkey production each year.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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While nutrient balance data on either a county or district basis should not be used to make recommendations at the field or farm level, they can be very useful data for natural resource planners to address nonpoint-source pollution on a watershed or regional scale and develop appropriate total maximum daily loads (TMDLs) and nonpoint control strategies. For example, eastern Arkansas is a part of the drainage basin for the Gulf of Mexico where concerns have grown over the hypoxic zone. This study shows that excessive N is not being applied to the row-crop hectarage in the eastern one-third of Arkansas within the Mississippi flood plain. While this does not imply that cropping practices in eastern Arkansas are not contributing N to the gulf, it may well suggest that if N is being lost, it is not from excessive application, but perhaps from mismanagement after application. This type of information may assist planners with developing strategies that effectively address the issue while avoiding unnecessary economic hardships on agricultural producers.
The results from this assessment may help reinforce the thought that current nutrient application strategies in western Arkansas are not sustainable without the danger of creating and/or exacerbating water-quality issues from excessive nutrients. Transport of excessive N and P contained in poultry litter outside of the central and western Arkansas districts that have restricted land area available for nutrient application is needed if the current poultry production levels are to be maintained. If poultry litter is eventually transported to eastern Arkansas, the use of inorganic fertilizers will need to be reduced from current levels so that these soils are not eventually enriched to the point of excessive P content as observed in some areas of western Arkansas. While redistribution of nutrients, especially P, contained in poultry litter is needed to address environmental quality concerns in western Arkansas, best management practices are needed so that nonpoint-source pollution does not further degrade the surface and ground water resources of eastern Arkansas.
The literature contains volumes of information on manure-nutrient management, but few of these studies have been conducted on the soils and cropping systems common to eastern Arkansas. Thus, additional research and grower education concerning how to best manage these nutrients are required. Likewise, the export of poultry litter from western Arkansas will require the prescriptive use of inorganic N and K fertilizers to maintain the productivity of soils used for pasture and forage production in western Arkansas that were previously amended almost solely with poultry litter. Use of inorganic fertilizers on forage hectarage will also require comprehensive educational and research programs for both growers and fertilizer distributors.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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