Hydrophilic Gel Amendments to Sand Soil Can Increase Growth and Nitrogen Uptake Effi ciency of Citrus Seedlings

We tested the hypothesis that amendments of two hydrophilic gels to a sand soil would reduce N leaching losses and increase growth of citrus seedlings. Three-month-old seedlings of ̒ Swingle ̓citrumelo [Citrus paradisi Macf. xPoncirus trifoliata (L.) Raf.] were transplanted into containers of steam-sterilized Candler sand, amended with a linear acrylamide/acrylate copolymer (PAM), and/or a cross-linked copolymer agronomic gel (AGRO). Two rates of each amendment were applied either alone or together and were either mixed into dry sand prior to seedling transplant, used as a root-dip slurry at transplant or applied to the soil surface in a solution after transplant. Seedlings were grown in the greenhouse for 5 months and irrigated to container capacity with a dilute nutrient solution without leaching. Pots were leached every 2 weeks and total N losses from the soil were measured in the leachate. PAM amendments increased N retention in soil slightly but PAM had no effect on plant growth, water use, N uptake, or N leaching relative to unamended control plants. The AGRO amendments increased seedling growth, plant water use and uptake of N from 11% to 45% above that of the unamended control plants depending on application method. AGRO decreased N concentrations in the leachate to as low as 1 to 6 mg·L–1. Only 6% of the total applied N was leached from the AGRO treatments, which was about half that from the untreated control plants. There was no additional benefi t of using both amendments together or of an additional AGRO root dip treatment. The largest plants used the most water, required the most N and had the greatest N uptake effi ciency. AGRO amendments clearly enhanced seedling growth, increased their N uptake effi ciency, and reduced N losses from this sand soil. Materials and Methods Seedlings of ʻSwingle ̓citrumelo [Citrus paradisi Macf. xPoncirus trifoliata (L.) Raf.] that were 3 months old and ≈10 cm tall were selected for uniformity from the nursery. Roots were gently washed free from the organic soilless potting media and transplanted individually into 0.15-L containers of steam-sterilized Candler sand (Hyperthemic, uncoated typic Quartzipsamments) on 1 May 2001. Two commercial hydrogel materials were compared. One was a sodium-based linear copolymer of acrylate/polyacrylamide (PAM) and the other a potassium-based cross-linked copolymer agronomic gel (AGRO; Stockhausen, Greensboro, N.C). There were a total of 14 treatments: with and without combinations of soil amendments, with and without seedlings, plus a sand only (no plant, NP) no amendment treatment (Table 1). Different combinations of hydrogels were either uniformly incorporated as dry (DI) material into the sand prior to transplant, used as a root dip slurry during transplant, or applied as a liquid in a single 2-mL surface drench (SD) just after transplant. The hydrogels were applied at concentrations near the commercial recommended rates for transplanting in the nursery (Stockhausen, Technical Bulletin 103-1197). Pots with DI AGRO treatments were under fi lled with sand to allow for expansion as preliminary tests revealed that AGRO mixed into dry soil at 2.5 g·L–1 swelled the volume of the hydrated soil 28%. There were six replicate pots in each treatment. Plants were grown for 5 months (until 28 Sept.) in an unshaded greenhouse with maximum photosynthetic active radiation of 1500 μmol·m–2·s–1, natural photoperiod, maximum/minimum temperatures of 38 °C day/26 °C night and relative humidities that ranged from 40% to100%. After establishing treatments, all plants were well-irrigated for 2.5 weeks to allow recovery from transplant. On 19 May, all pots were watered to container capacity, allowed to drain and weighed. For ≈3 months thereafter, pots were reweighed every second day and their weight loss was replaced with a diluted commercial liquid fertilizer solution of 7N–2P–7K+ micronutrients, which contained 100 mg·L–1 of total N (from ammonium nitrate) with an electrical conductivity (EC) of 0.95 ms·cm–1. Thus, each treatment was fertigated three times per week to container capacity without leaching. On 23 Aug., all pots were irrigated with water (without leaching) alternating with fertigation to avoid any nutrient salt accumulation between leaching events. About every 2 weeks from 21 May until 28 Sept., all pots were leached with water until 10 to 25 mL of leachate was collected. These samples were frozen until analyzed for concentrations of NH4-N and NO3-N with a rapid fl ow analyzer (ALPKEM, 1986; 1989). Concentrations of NH4-N and NO3-N were combined and expressed as total N in mg·L–1. There were a total of nine sets of 84 leachate samples collected during the course of the experiment. To compare sample concentrations at equal volume, N concentrations were arithAn important goal of Florida citrus nursery tree production (Maust and Williamson, 1994) and in commercial fruit production in the fi eld (Syvertsen and Smith, 1996) is to maximize fertilizer N uptake so as to minimize leaching losses. The poor water holding capacity of the sand soils in central Florida and the high summer rainfall that normally exceeds evapotranspiration, combine to make N leaching losses a serious problem that has been associated with citrus production (Riotte, 1994). Current best management practices recommend multiple split applications of fertilizer to minimize residual soil N that is susceptible to leaching and avoiding fertilizer applications during the summer rainy season (Tucker et al., 1995). Since intense rain storms are possible at any time of the year, some leaching losses of N are unavoidable. The percentage of the applied N that is taken up, the N uptake effi ciency (NUE), is inversely related to the N status of trees (Smith, 1966). NUE can be increased by reducing N application rates (Lea-Cox and Syvertsen, 1996) or by reducing N losses through careful fertilizer and irrigation management (Syvertsen and Smith, 1996). Soil amendments of high organic compost (Turner et al., 1994) or of water absorbing polymers (hydrogel; James and Richards, 1986) can reduce leaching by increasing water holding capacity of sand soils and soilless potting media (Elliot, 1992). The time required for plants to wilt after irrigation can be lengthened by hydrogel amendments to soil (James and Richards, 1986). Although plant growth of roses was enhanced with a hydrogel amendment in pot culture, larger plants suffered more drought stress than the smaller unamended plants in the limited soil volume (Davies et al., 1987). Oak tree transplant survival was decreased by hydrogels (Ingram and Burbage, 1985) so hydrogels apparently can compete with roots for available water. Growth, nutrient uptake, and protein content of crop plants was increased by incorporating a hydrogel into sand media in pot culture (Magalhaes et al., 1987; Sayed et al., 1991). This implied that plant water relations and nutrient uptake were improved by the hydrogel amendment. Hydrogels can enhance ammonium N retention in sand (Henderson and Hensley, 1985), but the ability of roots to extract nutrients from hydrogel amended soils is not clear. The purpose of this study was to determine growth and N uptake of citrus seedlings in hydrogel-amended central Florida sand. We tested the hypothesis that soil amendments of two hydrophilic gels would increase growth and NUE thereby reducing N leaching losses. Received for publication 1 Nov. 2002. Accepted for publication 27 May 2003. This research was supported by the Florida Agricultural Experiment Station and approved for publication as Journal Series R-08845. This research was partially supported by a gift from Stockhausen, Inc. HORTSCIENCE 39(2):267–271. 2004.