Water issues are an increasing concern for the ornamental container nursery industry. Growers rely on frequent irrigation and applications of controlled-release fertilizer (CRF) to produce saleable plants [1
]. These practices contribute to increase runoff of nitrogen and phosphorus, causing detrimental environmental effects such as contamination of local water resources, eutrophication, and death of aquatic species [2
]. Overirrigation may also lead to a faster release of CRF [4
] requiring additional fertilizer applications during the production cycle, at a significant cost for growers [7
]. Leaching fraction (LF) is one method of monitoring irrigation efficiency [8
], and is calculated by dividing the amount of water that leaches from a container by the total amount of irrigation applied:
In previous studies and nursery applications, irrigating based on a 0.15 to 0.2 target LF or monitoring substrate moisture has shown the potential to reduce the loss of nutrients through leaching and preserve CRF longevity [5
]. Owen et al. [11
] found that a target LF 0.1 to 0.2 reduced leachate volume by 64% and reduced dissolved reactive P concentration in leachate by 64% without influencing plant dry weight. Tyler et al. [12
] reported that a low LF of 0 to 0.2 decreased nitrate and phosphorus contents in effluent compared to a LF of 0.4 to 0.6. Prehn et al. [13
] reported that plants irrigated with a target LF of 0.2 had equivalent growth compared to those that were irrigated with an on-demand irrigation system, suggesting that plants of similar size could be produced with a significantly reduced LF. When determining the effects of substrate moisture on CRF release rates, there are conflicting reports in the literature. Kochba et al. [5
] reported that coated KNO3
release was essentially equal if the moisture content of the soil was greater than 50% of field capacity. This contrasts with results from Du et al. [14
], which demonstrated that rates of release for CRF were approximately 5 to 20% slower in a column of sand at field capacity compared to saturated sand or free water. Finally, Adams et al. [4
] reported that although there were no differences between CRF release in a moist solid substrate and pure water, the mass flow of water across the prill surface in fluctuating water potential environments may lead to faster exhaustion of the CRF.
The objective of this study differed from previous work in that it evaluated LF influence on the longevity of a CRF and how different LFs affect leachate nutrient content in a pine bark substrate.
2. Materials and Methods
This study was conducted in a greenhouse at Paterson Greenhouse Complex, Auburn University in Auburn, Alabama, USA (USDA Cold Hardiness Zone 8a). “Trade gallon” 2.72 L black plastic nursery containers were filled with 1200 g of 100% pine bark with a gravimetric water content of 37.3% amended with 336 g (rate of 2.97 kg/m3) of dolomitic lime to simulate a common nursery mix. The pots were fallow and contained no plants. SOAX® liquid wetting agent (Smithers Oasis, Kent, OH, USA) at 1200 ppm was applied to the substrate to help with surface wetting and minimize the effects of channeling.
Harrell’s 16-6-13 POLYON®
CRF (Harrell’s LLC, Lakeland, FL, USA) was applied at a rate of 6 g to every container. Fertilizer was weighed and encased in 11 cm square bags made from vinyl-coated charcoal fiberglass mesh that were heat sealed around the edges (Phiefer Inc., Tuscaloosa, AL, USA). Mesh bags were applied 2.5 cm below the substrate surface. Containers were irrigated to obtain six different target LFs: 0.5, 0.15, 0.25, 0.35, 0.45, and 0.55. Each container represented an experimental unit; there were four replications of each irrigation treatment for a total of 24 containers arranged in a completely randomized design. To determine initial irrigation application, containers were thoroughly watered in and drained for one hour. Containers were then weighed, left for two days, and weighed again to determine water loss due to evaporation, noted as “water loss”. Initial irrigation volumes were calculated by determining the amount of water needed to replace the evaporated water (water loss) and adding the amount needed to reach the target LF for each container (water loss × LF):
After the initial irrigation calculation, adjustments to irrigation volume were determined using the actual LF obtained from each irrigation. The equation used was adapted from Owen et al. [15
Data collection began on 12 August 2019 and took place over 10 weeks. Containers were irrigated by hand three times a week with a syringe. Water was distributed slowly and evenly over the surface of the substrate. During irrigation events, each fallow container was fitted into a 2.5 L leachate collection bucket. The containers fit snugly into the collection buckets, leaving adequate space between the container and bucket for leachate to collect. After irrigating, containers drained for 30 min. Leachate volume per container was then measured with a graduated cylinder and recorded. Leachate pH and electrical conductivity (EC) were measured using a HACH Pocket Pro + Multi 2 Tester (Hach Co., Loveland, CO, USA). A 15 mL aliquot of each leachate sample was placed in a sealed collection tube and refrigerated. Throughout each week, the three individual samples collected from each replication were combined, a total of one pooled 45 mL sample per container per week. Samples were kept in refrigeration at 3 °C during the collection week, after which the samples were frozen.
After 5 and 10 weeks, samples were thawed and sent to Quality Analytical Laboratories in Panama City, FL for a complete soilless media analysis. Leachate samples were analyzed for NO3-N and NH4-N (fertilizer did not contain urea) with a Lachat Quikchem® 8500 series flow injection analysis system (Hach Co., Loveland, CO, USA). Total phosphorus, potassium, SO4-S, calcium, magnesium and micronutrients (Fe, Mn, B, Cu, Zn, Mo, Na, Al, and Cl) were analyzed using a Thermo Scientific™ iCAP™ 7400 ICP-OES analyzer (Thermo Fisher Scientific™, Waltham, MA, USA).
At the end of the study, mesh bags were retrieved to determine nutrients remaining in the fertilizer prills. Bags were separated from the substrate and allowed to air-dry for 14 days. The prills from each recovered bag were weighed after which 100 prills were separated and weighed again. These 100 prills were ground using a mortar and pestle and mixed with 1 L of deionized water. The prill solution was stirred with a stir rod for 5 min before a 45 mL aliquot of the extractant was taken and sampled for pH and EC. The samples were frozen until analyzed using the same methods described above.
Initial fertilizer application was determined from an average of four analyses of 6 g of unused CRF. Fertilizer recovered from the mesh bags after the completion of the study were recorded as remaining fertilizer. Fertilizer loss was calculated as:
Total fertilizer leached (mg) was determined by multiplying the concentration of nutrients in the weekly leachate samples by weekly leachate volume and totaled over the 10 weeks. Fertilizer remaining in the substrate or lost to volatilization was calculated by subtracting fertilizer loss from the total fertilizer leached:
Data was analyzed in JMP® and SAS University Edition by SAS® (SAS Institute Inc., Cary, NC, USA) using a Tukey’s honestly significant difference (HSD) test for means comparison and general linear mixed models (GLIMMIX) for regression.