Avoiding groundwater contamination from agricultural activities is possible only if the processes that control deep percolation are understood. The source of contaminant movement to groundwater is typically through preferential flow, processes by which the bulk soil is bypassed by some part of the infiltrating water. Three mechanisms give rise to preferential flow: fingered flow, funnel flow, and macropore flow. Fingered flow occurs in coarse-textured soils and can be minimized by starting with an initially well-wetted profile. Funnel flow is likely in layered soil profiles of silt or coarser-textured soil, in which avoiding slow overirrigation is critical. Macropore flow is observed in all structured soils in which maintaining irrigation rates well below the saturated conductivity of the soil is essential. These prescriptions are quite different than conventional recommendations, which fail to consider groundwater protection.
Larry Parsons and Brian Boman
Best management practices (BMPs) started in Florida citrus (Citrus spp.) in the 1990s and have evolved to play a major role in production practices today. One of the earliest BMPs in Florida arose from concerns over nitrate-nitrogen concentrations in some surficial groundwater aquifers exceeding the 10 mg·L-1 drinking water standard. This occurred in an area of well-drained sandy soils known as the Central Florida Ridge that extends north and south through the central part of the Florida peninsula. State agencies could have used a strictly regulatory approach and restricted how much nitrogen growers could apply. Instead of setting arbitrary regulations, the agencies promoted a scientific-based BMP approach. A nitrogen BMP for Central Florida Ridge citrus was established, and research is now validating the earlier groundwater work on more grower field sites. The purpose of this BMP was to minimize the risk of leaching nitrates from fertilizer into the groundwater. Several important aspects of the BMP involve: 1) limiting the amount of nitrogen fertilizer applied at any one time, 2) increasing the frequency of fertilizer applications, 3) reducing fertilizer applications during the summer rainy season, and 4) managing irrigation to reduce leaching below the root zone. Since this Central Florida Ridge nitrogen BMP was established, major BMP actions to improve water quality and reduce the quantity of runoff water have taken place in the Indian River production area of Florida's east coast. BMPs continue to be set up in other parts of the state for a variety of plant and animal agricultural practices. In some cases, cost-share funds have been provided to help implement BMPs. With voluntary BMPs, growers have scientifically based guidelines, a waiver of liability, and an avoidance of arbitrary regulations.
Yusuf N. Tamimi, Dennis T. Matsuyama, Kimberly D. Ison-Takata, and Richard T. Nakano
Pollution of the environment, especially groundwater, may be reduced by proper fertilizer management, based in part on crop removal. The weights and concentrations of nutrients in tissue components of cut-flower roses (Rosa hybrida L.) were determined to assist in developing a fertilizer management system that sustains a high level of production but also is environmentally friendly. Harvested flower stalks of the cv. Royalty were cut to 45-cm length, and sectioned into 15-cm units, from which blossom, leaf, and stem components were separated, weighed, and analyzed for nutrients. The flower represented 28.5%, leaves 46.0%, and stem 25.5% of the total weight of the stalk. Upper leaves had the highest levels (g·kg-1) of N (29.3), Ca (21.8), and Mg (3.0), and (mg·kg-1) Fe (74) and Mn (71). The flower was highest in K (18.4 g·kg-1), P (3.0 g·kg-1), Zn (29 mg·kg-1), and B (23 mg·kg-1). Annual removal of nutrients by 45-cm flower stalks totaled: 256.2, 187.5, 116.3, 30.0, 26.0, and 21.1 kg·ha-1 of N, K, Ca, P, Mg, and S, respectively. Micronutrients removed per annum totaled 700, 470, 260, 200, and 190 g·ha-1 of Fe, Mn, Zn, Cu, and B, respectively. Assuming 50% recovery of applied N and 80% of K, a total annual application of N at 512 kg·ha-1 and K at 234 kg·ha-1 may replace the amounts removed. However, actual rates of N and K, as well as other nutrients applied, should be adjusted based on soil and tissue analysis results. Removal of nutrients will be greater if stalks harvested are >45 cm in length, which may necessitate additional nutrient application, depending on soil conditions.
Steven A. Weinbaum, R. Scott Johnson, and Theodore M. DeJong
Over-fertilization (i.e., the application of fertilizer nitrogen (N) in excess of the tree or vine capacity to use it for optimum productivity) is associated with high levels of residual nitrate in the soil, which potentially contribute to groundwater and atmospheric pollution as a result of leaching, denitrification, etc. Overfert-ilization also may adversely affect productivity and fruit quality because of both direct (i.e., N) and indirect (i.e., shading) effects on flowering, fruit set, and fruit growth resulting from vegetative vigor. Pathological and physiological disorders as well as susceptibility to disease and insect pests also are influenced by the rate of applied N. Over-fertilization appears to be more serious in orchard crops than in many other crop species. The perennial growth habit of deciduous trees and vines is associated with an increased likelihood of fertilizer N application (and losses) during the dormant period. The large woody biomass increases the difficulty in assessing the kinetics and magnitude of annual N requirement. In mature trees, the N content of the harvested fruit appears to represent a large percentage of annual N uptake. Overfertilization is supported by a) the lack of integration of fertilizer and irrigation management, b) failure to consider nonfertilizer sources of plant-available N in the accounting of fertilizer needs, c) failure to conduct annual diagnosis of the N status, and d) the insensitivity of leaf analysis to over-fertilization. The diversity of orchard sites (with climatic, soil type, and management variables) precludes the general applicability of specific fertilization recommendations. The lack of regulatory and economic penalties encourage excessive application of fertilizer N, and it appears unlikely that the majority of growers will embrace recommended fertilizer management strategies voluntarily.
Gregory S. Hendricks, Sanjay Shukla, Kent E. Cushman, Thomas A. Obreza, Fritz M. Roka, Kenneth M. Portier, and Eugene J. McAvoy
concentrations in the root zone. Therefore, water and nutrients have to be managed simultaneously to optimize yield and minimize nutrient losses to groundwater. Irrigation management practices used by watermelon growers vary from simple to complex. Some growers
Ian A. Merwin, John A. Ray, Tammo S. Steenhuis, and Jan Boll
Commercial apple (Malus domestica Borkh.) orchards in the northeastern United States receive heavy pesticide inputs and are often located on well-drained soils near surface and groundwater resources. Nonpoint-source water pollution by agrichemicals has been monitored in agronomic crop systems and simulated using computer models and laboratory soil columns, but inadequately studied at field scale in orchards. We monitored the concentrations of agrichemical tracers, nitrate-N, and benomyl fungicide in water samples from two apple orchards under mowed sodgrass (Mowed-Sod), shredded bark mulch (Bark-Mulch), preemergence residual herbicides (Resid-Herb), and postemergence herbicide (Post-Herb) groundcover management systems (GMSs). In one orchard, we evaluated subsurface spatial patterns and flow rates of a weakly adsorbed blue dye (pesticide analog) and potassium bromide (nitrate analog) under trees after six years of Post-Herb and Mowed-Sod treatments. Nitrate and pesticide tracers leached more rapidly and in higher concentrations under Post-Herb treatments, apparently via preferential macropore flowpaths such as root channels, soil cracks, and macrofauna burrows. At another orchard, we monitored subsurface leaching and surface runoff of benomyl and nitrate-N on a whole-field scale. Peak concentrations of benomyl (up to 29 mg·liter-1) and nitrates (up to 20 mg·liter-1) were observed in subsoil leachate under Resid-Herb plots during 1993. In 1994, nitrate concentrations were greater in leachate from all GMSs, with upper ranges from 48 to 66 mg·liter-1, while benomyl concentrations were lower in all GMSs compared with the previous summer. In surface water runoff during 1993, the highest benomyl concentrations (387 mg·liter-1) and most frequent outflows occurred in Resid-Herb plots. During 1994, benomyl runoff was more frequent in both herbicide GMSs, with concentrations up to 61 mg·liter-1 observed in the Post-Herb plots. Weather patterns, irrigation intensity, differing soil conditions under each GMS, and the turfgrass/clover drive lanes affected the relative frequency and concentrations of benomyl and nitrate leaching and runoff. Preferential bypass flow appeared to be a major subsurface leaching pathway, and erosion sediment an important factor in surface movement of these agrichemicals. Our studies suggest that nitrate-N and benomyl fungicide may be more prone to leaching or runoff from orchard soils under some herbicide GMSs in comparison with mowed sodgrass or biomass mulch systems.
Catherine S.M. Ku and David R. Hershey
Single-pinched poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch `V-14 Glory') received 210 mg·L-1 constant N fertigation from Hoagland solution with N sources of 100% NO3-N or 60% NO3-N : 40% NH4-N, P concentrations of 7.8 or 23 mg·L-1, and leaching fractions (LFs) of 0, 0.2, or 0.4. The P fertigation rates did not significantly affect plant growth measurements and N leaching. Shoot dry masses and leaf and bract areas of plants fertigated with 60% NO3-N were 11% to 26% greater than those fertigated with 100% NO3-N. Shoot dry mass at the 0 LF was 27% smaller than those at the 0.4 LF. The total amount of N applied via fertigation was 1.7 g at the 0 LF and 3.3 g at the 0.4 LF. Leachate N concentration ranged from 170 to 850 mg·L-1. Nitrogen recovery was 74% to 91%, and the percentage of fertigation N recovered in leachate ranged from 51% at the 0.2 LF to 74% at the 0.4 LF. With a 0.4 LF and 210 mg·L-1 N fertigation, 15% to 22% of the recovered N was found in the shoots, and 68% to 75% was found in the leachate. Even with a 0.2 LF, >50% of the N recovered was found in the leachate. Premium marketable quality poinsettia were produced with N at 210 mg·L-1 from 60% NO3-N : 40% NH4-N fertigation solution at the 0.4 LF. To reduce N leaching to the environment, good marketable quality poinsettias could be grown at a LF of ≤0.2 with 210 mg·L-1 N fertigation if quality irrigation water is available and if a small reduction in growth is acceptable.
Catherine S.M. Ku and David R. Hershey
Poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch `V-14 Glory') grown as single-pinched plants and received constant fertigation of Hoagland solution with N at 210 mg·L-1 of 100% NO3-N or 60% NO3-N : 40% NH4-N; P at 7.8 and 23 mg·L-1; and leaching fractions (LFs) of 0, 0.2, or 0.4. The P at 23 mg·L-1 used in this study was about half the P concentration typically provided from a 20N-4.4P-16.6K fertilizer at 200 mg·L-1 N fertigation. The total P applied via fertigation ranged from 51 mg at the 0 LF to 360 mg at the 0.4 LF. The leachate P concentration ranged from 0.2 to 46 mg·L-1. With P at 7.8 mg·L-1, the percentage of total P recovered in the leachate was 6% to 7%. At 23 mg·L-1 P fertigation, however, the total P recovered in the leachate with 60% NO3-N treatment was 2-times greater than with 100% NO3-N treatment. This result is attributed to a lower substrate pH, which resulted from NH4-N uptake and nitrification processes with 60% NO3-N fertigation. The P concentration in the recently matured leaves with 7.8 mg·L-1 P fertigation was in the normal range of 0.3% to 0.6%. Fertigation P can be reduced by up to 80% and still be sufficient for producing quality poinsettias. Reducing the fertigation P concentration is beneficial because it reduces P leaching, reduces fertilizer costs, and reduces luxury consumption.
Catherine S.M. Ku and David R. Hershey
Geraniums (Pelargonium × hortorum L.H. Bailey `Yours Truly') were grown in a glasshouse from 15 Mar. to 9 May as single pinched plants in a growing medium with a bulk volume of 1.3 liters per 15cm diameter standard plastic pot. Plants received constant fertigation with N at 300 mg·liter-1 from 20N-4.4P-16.6K with leaching fractions (LFs) of ≈ 0, 0.1, 0.2, and 0.4. The LF is the volume of solution leached from the container divided by the volume of solution applied to the container. There were 24 irrigations during the study. Plants with LFs of 0.2 and 0.4 had 46% larger leaf area, 40% more shoot fresh mass, and 37% more shoot dry mass than plants with LFs of 0 and 0.1. By week 5, the leachate electrical conductivity (EC) at 25C for LFs of 0.1,0.2, and 0.4 had increased from ≈ 3 dS·m-1 initially to 12, 8, and 4 dS·m-1, respectively. At harvest, the EC of a saturated medium extract (ECe) was 7, 4, 3, and 2 dS·m-1 for LFs of 0, 0.1, 0.2, and 0.4, respectively. At harvest, medium EC, with LFs of 0.1, 0.2, and 0.4 was 47% 68%, and 60% less in the lower two-thirds of the pot than in the upper third. With a LF of 0, the medium EC, was `not lower in the bottom of the pot. With fertigation N at 300 mg·liter-1, minimizing the LF substantially reduced growth of container-produced geraniums. In addition to specifying LF, the number of container capacities leached per week, termed the leaching intensity (LI), should be calculated for container leaching studies. In two studies, the LFs may be the same yet the LIs can be very different.
Catherine S.M. Ku and David R. Hershey
Poinsettias (Euphorbia pulcherrima Willd. ex Klotzsch `V-14 Glory') were grown in a greenhouse for 70 days in 1.3 liters of medium (13 cm deep in 15-cm pots) with a leaching fraction (LF) of ≈ 0, 0.1, 0.2, or 0.4. Plants were fertigated with 300 mg N/liter from 20 N-4.4P-16.6K. The electrical conductivity (EC) of the fertigation solution was 2.1 dS·m-1. The leachate EC increased from 2 dS·m-1 initially to plateaus of ≈ 6, 9, and 15 dS·m-1 for LFs of 0.4, 0.2, and 0.1, respectively. Poinsettia height, shoot fresh and dry mass, and leaf and bract areas were not significantly different among the LF treatments. Leachate pH decreased from 6.1 initially to 5.1 at the end, but there was no significant difference among the LF treatments. The EC of a saturated medium extract (ECe) was between 17% and 48% higher in the lower third of the medium than in the middle third. The difference was greater with a lower LF. The EC, was 8.9, 7.3, 5.2, and 3.4 dS·m-1 in the lower third of the pot for a LF of 0, 0.1, 0.2, and 0.4, respectively. Under conditions of this study, container poinsettias required no leaching.