NC1186: Water Management and Quality for Ornamental Crop Production and Health
Statement of Issues and Justification
The ornamental plant industry ranks 5th (>$14.6 billion) in US agriculture commodities and is in the top 5 commodities for 26 states (USDA, 2004). Water issues, specifically irrigation scheduling, surface water management, salinity and runoff water quality are topics of major concern to ornamental producers. Drought, urban competition for water resources, and increasing legislation at state and county levels increase the need for ornamental producers to manage water more effectively and/or use alternative water sources that are often of inferior quality. Regardless of the area of the United States in which an operation is located, challenges exist regarding sufficient quantities of quality water sources. Legislation regarding water use and/or quality has been implemented in at least 8 states. Most field producers of nursery stock use irrigation at some point during the growing season. Many field producers use low-volume irrigation and some use such systems to deliver soluble fertilizers. While supplemental irrigation is beneficial in field production it is essential for container production. Container substrates need to be well drained and container volume limits the amount of available water, resulting in frequent irrigation and high water use. Almost all greenhouse crops are produced in containers. Over 75% of nursery crops in 17 of the major nursery producing states were grown in containers (USDA, 2007) and thus require irrigation. In Florida, container nurseries annually apply 56 to 120 inches per year in addition to the 40 to 50 inches of average annual rainfall. Container nurseries in Alabama were estimated to have used 30,000 to 40,000 acre-feet of water in 1985 (Fare et al., 1992) and container nursery production in Alabama has almost tripled since 1987 (USDA 1994, 2004). Amount of water applied, method of application, and irrigation frequency for Georgia nurseries has been summarized (Garber et al., 2002). Frequent irrigation along with high fertilizer and pesticide use can lead to significant losses of agricultural chemicals in runoff water that transports them to containment ponds and/or off-site into groundwater or surface water (Briggs et al., 1998, 2002; Cabrera, 2003; Camper et al., 1994). Irrigation water management is a key component in the nutrient management of ornamental crop production and in reducing the impact of runoff water on local water (Tyler et al., 1996; Lea-Cox et al., 2001; Ross et al., 2002). Recycling water includes another set of issues for growers, primarily in the form of disease and salinity management. Emerging constraints on water use and quality means that the ornamental industry needs to find ways to manage water without detracting from production schedules and crop quality. Water conservation and quality are top priority issues in agriculture. Research and extension projects that are designed to address these issues are needed in ornamental production (Ogg and Keith, 2002). Precision water management and resource efficiency were rated at the top of the issue/need/concern list developed at the joint USDA, ARS, NASA and NSF workshop Engineering Solutions for Specialty Crop Challenges (USDA, 2007). Furthermore, the United States Environmental Protection Agency (EPA) is enforcing federal legislation requiring states to implement Total Maximum Daily Load (TMDL) programs for watersheds (Lea-Cox and Ross, 2001). Since few states have the expertise to integrate all these issues we need a national multi-disciplinary approach to water management. There are five interrelated areas relevant to this project: 1. Source water management and quality, 2. Irrigation management, 3. Runoff water management and quality, 4. Substrate and nutrition management, and 5. Pathogens and crop health management.1.1 Source Water Management and Quality There are four primary water sources available for ornamental production: groundwater, surface water, reclaimed water, and recycled tailwater from runoff. All of these water sources can have quality issues that require management before they can be used for plant production. Additionally in some areas of the country, due to increased demand for freshwater, agriculture is being forced to use alternative water sources, such as reclaimed or recycled water, which may have quality management issues. Plant producers therefore must develop new technologies to remove harmful contaminants when necessary, modify horticultural practices, and/or develop or choose crops which are more tolerant of lower quality water. Many areas of the country have groundwater supplies that are being contaminated by saltwater intrusion as a result of removing groundwater faster than it is being recharged. This problem is occurring in both coastal areas where seawater is intruding and other inland areas with saline deep aquifers (Atkinson, 1986, Barlow, 2005). Groundwater is also being contaminated by infiltration of contaminants from nearby industrial, urban, and agricultural operations. In other regions of the U.S., groundwater is impaired by natural geological features, making water alkaline, sodic, and/or saline. Surface water is even more vulnerable to contamination since it has no protective over-layer of soil. Surface water supplies are becoming use-restricted. In various parts of the SE, SW and NE U.S., recent droughts have reduced surface water levels to historic record lows. In addition, environmental agencies are claiming more surface water to protect endangered flora and fauna along waterways. In California, the Sacramento-San Joaquin River Delta, once a primary source of fresh water for agriculture, now has use restrictions to protect delta smelt populations. In Georgia, there is a mandatory release of water from Lake Lanier to support endangered mussels and sturgeon on the Chattahoochee River, to support a nuclear plant in Alabama, and supply water for southern Alabama, southern Georgia and Florida. Reclaimed water, which is wastewater or sewage that has been treated with conventional wastewater treatment processes or other processes, is being used in various parts of the country to irrigate specific agricultural crops. While reclaimed water offers a water source that can be available when other water sources are limited, there are drawbacks. Agriculture uses a significant portion of reclaimed water, using 48% in CA and 19% in FL in 2002. While not universally available, reclaimed water is available in many urban areas of the U.S. Benefits of using reclaimed water include lower costs and access to a water source that may be less affected by drought. However, the drawbacks of reclaimed water are maintenance cost of a separate water system, potential contamination with salts, heavy metals, and chlorine that may be harmful to sensitive crops. In many areas of the country, ornamental producers have begun recycling the tailwater and stormwater runoff from their facilities. This process can save money, especially for larger operations, since fertilizers in the runoff are re-utilized. However, runoff water may contain pesticide residues. Agricultural chemicals have been found in runoff ranging from less than 0.1 % to 20% recovery of active ingredient (Briggs et al., 2002; Riley et al., 1994; Willis, 1982). Pesticides and nitrates were detected in nursery runoff water and in nursery retention basins (Keese et al., 1994; Warsaw et al., 2009a; Wilson et al., 1996). Phytotoxicity problems may result when recycled water contains either pesticides with medium to high water solubilities or one pesticide is extensively used and the recycled water is applied to plants sensitive to that pesticide (Bhandary et al., 1997). Herbicide contamination levels of 1 to 10 ppm for short durations were found to be detrimental to growth and quality of several ornamental crops (Bhandary et al., 1997; Fernandez et al., 1999). Other drawbacks of recycling includes the need for adequate infrastructure to collect, capture and treat irrigation water runoff. However, in areas such as California, recycling of tailwater has been practiced since the 1970s (Skimina, 1986).
1.2 Irrigation Management Ideally when irrigating with good quality water, only the amount of water used through evapotranspiration is replenished. Applying water in excess of evapotranspiration can lead to reduced growth and nutrient loss (Tyler et al., 1996; Warsaw et al., 2009a, 2009b). However, irrigation system uniformity and efficiency are always less than 100% and result in wasted water. Manual versus automated irrigation systems and how those systems are managed also affects water use efficiency. Water quality and quantity issues affect irrigation management decisions: when water with high soluble salts is used for irrigation, a high leaching fraction may be required to prevent the buildup of excess salts in the substrate. Conversely, alkaline water should be applied at the lowest possible rates to minimize effects on substrate pH. Either of these scenarios can lead to nutrient management problems, a longer production cycle and more water used over the extended cycle (Beeson, 2006). Over-application of water is both an inefficient use of water and the main cause of fertilizer runoff (Bilderback, 2002; McAvoy, 1994; Warsaw et al., 2009a). New state laws have been passed that regulate the amount of runoff from all forms of agriculture (Lea-Cox and Ross, 2001). Excessive irrigation may also result in anaerobic conditions in the root zone possibly resulting in: 1) a root systems more susceptible to pathogens (Powell and Lindquist, 1997), 2) dissemination of pathogens from production areas, potentially leading to contamination of irrigation ponds, 3) nutritional problems due to denitrification and effects on root physiology and soil/substrate pH, 4) leaching of water, nutrients, pesticides, and herbicides from production areas posing a threat to water quality, and 5) excessive stem elongation, reduced plant quality and increased shipping costs. Excessive irrigation also has a direct impact on production costs: when a municipal water source is used, growers are charged on a per volume basis, while growers using well water incur energy costs associated with pumping water. Further, leaching nutrients from containers requires that greater quantities of fertilizer be used. Applying water efficiently and using more sustainable irrigation techniques will reduce production costs, conserve water, and produce higher quality crops. More precise control over irrigation will also allow growers to expose their crops to mild water stress, potentially hardening them to a drought stress that the plants may experience in the retail environment or landscape (Eakes et al., 1991). Reducing irrigation has been studied as a method to reduce stem elongation and/or improve quality of bedding plants (van Iersel and Nemali, 2004; Barrett and Nell, 1991; Latimer, 1992), perennial plants (Burnett and van Iersel, 2008) and woody plants (Warsaw et al., 2009a; 2009b). This has the potential to reduce the need for growth retardant applications or shorten the production cycle. Recent advances in irrigation technology have made it easier for growers to reduce water use during production and to harden plants by limiting water without risking plant damage or loss due to excessive drought stress. Such technology can assure an adequate water supply to the crop, even if environmental conditions change rapidly (Nemali and van Iersel, 2006). Because water traditionally has been available in abundant quantities in many parts of the country, irrigation recommendations for ornamental plant production have not received much attention. Currently, reliable tools and information to precisely schedule irrigations for diverse crop species in different environments is the missing link to reduce water use, optimize water use efficiency and minimize irrigation nutrient and chemical runoff from ornamental plant production. To reduce nutrient runoff and address issues related to irrigation water availability, it is crucial that more efficient approaches to irrigation, ground and surface water, and nutrient best management practices be developed (Lea-Cox et al., 2001).
1.3 Runoff Water Management and Quality. Container production creates the largest challenge to managing runoff water. The volume of water used and amount of runoff generated is much less for field production but issues are similar and still can have a major impact on water resources. Runoff of rain and irrigation water is an important avenue for the movement of agrichemicals from production sites into nearby receiving water bodies (Bjorneberg et al., 2002; Keese et al., 1994; Latimer et al., 1996; Meisinger and Delgado, 2002). If subsurface flow occurs and it is largely shallow and in a lateral direction, receiving bodies may be nearby surface waters located within or near the production site. If the site is underlain by porous soils or shallow aquifers, subsurface flow may directly enter groundwater resources. However, if properly managed surface runoff can be channeled and contained on site for reuse (Skimina, 1986; Hasek et al., 1986) or treated to remove sediment and/or agrichemical pollutants (Huett et al., 2005). Assessment and management of runoff therefore plays a critical role in minimizing environmental impacts of ornamental plant production operations. Fertilizer recommendations for ornamental plants often exceed recommendations for other similar agricultural crops (Rose, 1999). While controlled-release fertilizers (CRFs) are commonly used in container production, the nature of substrates and irrigation practices often leads to nutrient leaching. Pesticides are commonly applied every two to three weeks and up to 80% of an applied material can miss the application target (Gilliam et al., 1992). The current best management practice recommendation for overhead irrigation is that 10-20% of the applied water should leach out of containers. Overhead irrigation creates runoff water which transports pesticides and fertilizers to containment ponds and/or off-site before they can undergo degradation or be bound to soils/substrates. The plant production surface at container nurseries or greenhouses is commonly covered with plastic, fabric, gravel, concrete or other low permeable material. All ground covers result in greater surface runoff amounts than bare ground because of reduced infiltration and sheet flow. The combination of these factors leads to significant levels of agricultural chemicals in runoff water. The environmental fate of the pesticides is complex. Willis (1982) cites nine factors as having an effect on the concentration of pesticides in runoff water. The three most important are rainfall intensity and duration, amount of time between pesticide application and rainfall, and properties of the pesticide. In many production systems the daily use of overhead irrigation in effect increases rainfall frequency and decreases the time interval between pesticide application and rainfall. Concern for the protection of water resources, and the detrimental reintroduction of pollutants onto the crop through recycling, dictates that techniques to reduce the movement and enhance remediation of agricultural chemicals in runoff waters be developed. Unfortunately, little is known about the current state of runoff management in the ornamental plant industries throughout the United States (Yeager et al., 1993). Runoff can be substantially reduced by proper irrigation management (Warsaw et al., 2009b). Capturing and treating or recycling are other options. Many larger greenhouses have addressed the issue of runoff by using closed irrigation systems. Although subirrigation systems can virtually eliminate runoff from greenhouses, they are cost-prohibitive for many, especially for nurseries and small greenhouses.
1.4 Substrate and Nutrition Management Water runoff, fertilizer leaching and effects on recycling water systems are important considerations when selecting the components that formulate container substrates. Additionally, the use of inorganic and biological amendments including clay, pumice, mycorrhizae, and other biofungal amendments could be part of the formulations that could improve water and nutrient management of ornamental crops. With the increase in prices and availability of current substrate components, the interest within the ornamental industry to identify alternative materials that are suitable for formulating container substrates has also increased. Most of the alternative mixes can be used successfully to amend conventional substrates when used at proper ratios. However, an integrated approach to assess the physical and chemical properties of substrate mixes and their impact on plant health and growth and nutrient and water management is lacking. Pre-plant fertilization is achieved by incorporating water-soluble fertilizer, CRFs, and/or lime and microelements into the substrate (Nelson, 2008). CRFs are often applied when plants are initially potted. For established plants, top-dressing with CRFs during the spring or a combination of CRFs and water-soluble fertilizers is done. In some cases, particularly greenhouse and field production, water-soluble fertilizers serve as the primary fertilizer and are either applied each time the crop is irrigated or at regular intervals. Water-soluble fertilizers, by their nature, are more prone to leaching if mismanaged. Fertilizer concentrations are determined based on the N requirements of the crop. Many fertilizers are not well-balanced, containing considerable more P than required. Thus, by fertilizing based on N requirements, P is applied in excess. Substrate chemical and physical properties in combination with irrigation management and water quality affect leaching of nutrients regardless of management method and thus, the quality of runoff water. Excessive water applications result in substantial leaching of nutrients, and leaching of N and P can be of special concern. The susceptibility of N to leaching depends on the form of N that is applied. Due to the extremely low anion exchange capacity of many soilless substrates, nitrate is very susceptible to leaching, while ammonium (NH4+) leaches less readily due to the cation exchange capacity of the substrate. Although the use of NH4+ reduces the likelihood of N leaching, many greenhouse crops are susceptible to NH4+ toxicity, and a general guideline suggests not applying more that 40% of total N in the form of NH4+ or urea (Nelson, 2008).
1.5 Pathogens and Crop Health Management Plant pathogens in irrigation water were recognized early in the last century as a significant crop health issue (Bewley and Buddin, 1921). Plant pathogens threaten the sustainability and profitability of the ornamental plant industries as much as water shortages. Recycling irrigation conserves water but it may spread pathogens from a single point to an entire enterprise and from a single farm to other facilities sharing the same water resource (Hong et al., 2008c). This could result in severe losses of both crop and consumer confidence. At least 17 Phytophthora species, 26 of Pythium, 27 genera of fungi, 8 species of bacteria, 10 viruses, and 13 nematode species have been detected from water sources (Hong and Moorman, 2005). Among those pathogens are the sudden oak death (SOD) pathogen, and Ralstonia solanacearum, one of the USDA select agents under the Agricultural Bioterrorism Protection Act of 2002. Thus, there is an urgent need to assess the waterborne pathogen risk and develop mitigation strategies. Restrictions or timing of irrigation to conserve water may also decrease crop health due to coincidence of irrigation and ideal conditions for disease development. These issues have increased greatly in degree of impact and it will continue to be a problem with the increasing dependence on alternative irrigation water sources (Hong and Moorman, 2005).
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