Evaluation of an Organic Copolymer Fertilizer Additive on Phosphorus Starter Fertilizer Response by Corn

Fertilizer additive products have recently been developed with the intention of reducing phosphate fixation and improving phosphorus plant availability. We conducted two experiments at multiple North Carolina locations from 2007-2009 to evaluate the effects of an organic copolymer phosphorus fertilizer additive, AVAIL Phosphorus Fertilizer Enhancer (Specialty Fertilizer Products, Leawood, KS), on corn (Zea maize L.) nutrient uptake, growth, and yield. Treatments included a combination of diammonium phosphate (DAP, [(NH ) HPO ]) P fertilizer rates with and without AVAIL. Grain yields did not differ across fertilizer treatments or across low, medium, or very high initial soil test phosphorus. Grain P concentration differed among treatments in only 2 of 16 site-years, where the N-only treatment had less tissue P than the treatments including P with or without AVAIL. Also, Nonly plots occasionally had shorter plants compared with DAP and DAP + AVAIL. Treating DAP with AVAIL did not consistently affect corn plant growth parameters in the Piedmont and Mountain Regions of North Carolina, and using treated DAP did not offer a consistent agronomic benefit over DAPor N-only fertilization. Introduction New synthetic organic copolymer phosphorus fertilizer additives have been recently developed to combat P-limited crop productivity by reducing phosphate fixation in soil. These products do not supply nutrients and cannot be evaluated based on nutrient content. Some manufacturers claim that these products enhance cation exchange capacity (CEC), moisture-holding capacity, and soil organism populations, and may also stimulate plant root growth and development (4). However, Crozier et al. (4) report that only a small (0.01 to 0.12 meq/100 g) change in CEC could be expected under typical recommendation rates for many of these products and that an increase of CEC in the root zone or carry-over CEC changes for the subsequent crop is unlikely. Similarly, Jones et al. (8) reported that labeled humic acid rates may not significantly increase organic acid concentrations in the soil, based on their greenhouse wheat study where humic acid coatings on monoammonium phosphate (MAP) did not increase P solubility, availability, or uptake, nor did it increase spring wheat grain yields on Montana calcareous and noncalcareous soils. These two studies imply that in order to substantially increase CEC or soil humate, rates much greater than those recommended on product labeling may be needed. Currently, one organic copolymer phosphorus fertilizer additive being marketed throughout much of the USA is AVAIL Phosphorus Fertilizer Enhancer. The AVAIL product is available for use with either granular or liquid phosphate fertilizers and consists of long chained, high cation exchange capacity maleic-itaconic copolymers (17,18). In dry form, AVAIL is designed to be coated 4 4 4 22 March 2013 Crop Management onto granular phosphate fertilizers and is reported to surround P fertilizer in a water-soluble ‘shield’ that expands to block the elements that tie-up P in soil (e.g., Ca, Mg, Fe, and Al) (17,18). Research results from investigations on the effects of AVAIL have been quite variable. A one-year trial in Ohio did not find a corn yield difference among plots treated with or without AVAIL, though soil sampling indicated that additional P was not required to increase corn yield (11). Thus, starter P with or without AVAIL was not likely to increase yields in that field. McGrath and Binford (12) conducted a 3-year corn trial in the Delmarva area and found that none of their eight site-years showed an early plant growth or yield response on AVAIL treated plots. However, early plant growth was increased in all years and grain yield increased during two site-years when a P starter fertilizer was applied. Ward (19) investigated corn growth and yield response to AVAIL over eight site-years in Kansas during the 2008-2009 growing seasons. While all sites had initial Mehlich-3 extractable P (M3P) of ≤ 15 mg/kg, where a P response could be expected, only one of the eight site-years showed a significant yield response to P fertilizer addition, and there were no significant responses in V4 plant biomass, R1 ear leaf P concentration, grain yield, moisture, test weight, or grain P concentration as a result of the AVAIL-treated P fertilizer treatments. A 3-year corn trial in southern Minnesota on low and medium-high Olsenextractable P soil also found inconsistent yield results when comparing MAP and DAP with AVAIL-treated MAP and DAP; a yield response to broadcastapplied AVAIL-treated DAP over untreated DAP was observed in 1 of 2 years, and no response was seen with AVAIL-treated MAP compared with untreated MAP (15). In contrast, Gordon (7) reported increases in both corn grain and soybean (Glycine max L.) yields averaged over 3-years in north-central Kansas for AVAIL-treated MAP compared with untreated MAP on a soil with a 22 ppm of Bray-1 P. Therefore, the objective of our study was to evaluate corn plant growth and grain yield response to starter P fertilizers applied with and without AVAIL on sites ranging in initial soil test P (STP). Field Studies Study 1. Research was conducted in 2007 and 2008 at seven sites on three research stations in North Carolina: Piedmont Research Station, Salisbury (Salisbury A, B, and C); Mountain Research Station, Waynesville (Waynesville A and B); and Mountain Horticultural Crops Research Station (MHCRS), Mills River (Mills River A and B), and at one cooperating farmer site in Buncombe County (Buncombe), representing 14 site-years. Corn was grown on low (13-30 mg P/kg), medium (31-60 mg P/kg), high (61-120 mg P/kg), and very high (>120 mg P/kg) STP and varying soil textures (Table 1). A representative 0-20 cm soil sample from each plot was analyzed using Mehlich-3 extractant to determine STP before treatment. Treatments were arranged in a randomized complete block design (RCBD) with four replications and included DAP (18-460) at 15 kg P/ha and 13 kg N/ha, DAP plus AVAIL (treated by supplier) at 15 kg P/ha and 13 kg N/ha, or ammonium nitrate (AN [NH4NO3]) only at 224 kg N/ha at planting. Both years, starter P fertilizer was surface applied in a 10-cm band over the row at planting. All DAP and DAP + AVAIL plots received an additional 211 kg N/ha as AN surface-broadcast at planting, while the N-only plots received AN surface-broadcast at planting. Corn was planted in no-till plots 7.6-m long by 3.7-m wide (4 rows, 0.91-m row spacing) at Salisbury and Buncombe. The sites at Mills River and Waynesville were 9.1-m long by 3.7-m wide (4 rows, 0.91-m row spacing) and were managed with conventional tillage (fall moldboard plow followed by two disk passes in subsequent spring). 22 March 2013 Crop Management Table 1. Site locations, year, soil series, and pre-study soil chemical characteristics (0-20 cm) for each of the 16 site-years. Corn plant height has often been used as an indicator of early season growth, corn total biomass, and grain yield (1,6) Thus, average plant height in each plot was measured 3 weeks after emergence; corn plants were measured from the ground to the top of the whorl. Tassel percentage was measured 8 to 10 weeks after planting. Corn grain yield was determined by hand-harvesting and weighing ears from the center 3.05 m of the center two rows of each plot and grain moisture was measured with a grain moisture meter. Grain samples were dried at 40°C for 48 h then ground and analyzed for N and P concentration using a Perkin-Elmer CHN Elemental Analyzer (Model II), and Pregl and Dumas analysis (5). The 2007-2008 growing season rainfall was less than 30-year average growing-season rainfall at each location. The growing seasons both years were exceptionally dry, as the entire state of North Carolina experienced drought conditions throughout that time (13) (Table 2). The average monthly temperatures during the 2007 and 2008 seasons at Mills River, Buncombe, and Waynesville were warmer than average, except in July at Waynesville when temperatures were cooler (Table 2). Salisbury temperatures were cooler than average except for July which was warmer. Analysis of variance for yield, N and P accumulation, plant height, and tassel percentage was performed using SAS PROC MIXED (16). Data were analyzed by location with treatment and rates as fixed effects. Tests of fixed effects were computed using ddmf = kl option of the MODEL statement. Differences in measured variables due to treatments were considered significant at P ≤ 0.05. Location Year Region Soil series HM (g/kg) pH P K (mg/kg) Mills River A 2007 Mountain Hayesville loam 7.6 6.0 72 199 2008 7.1 6.3 136 188 Mills River B 2007 Mountain Statler fine sandy loam 5.8 5.9 194 198 2008 4.6 5.8 121 148 Salisbury A 2008 Piedmont Hiwassee clay 3.0 5.6 57 145 Salisbury B 2007 Piedmont Chewaclo loam 3.8 5.5 110 77 2008 3.5 6.4 166 93 Salisbury C 2007 Piedmont Mecklenburg loam 5.5 6.9 385 409 2008 2.7 7.2 387 327 Waynesville A 2007 Mountain Cullowhee-Nikwasi complex 12.6 5.7 136 133 2008 11.7 5.5 141 110 Waynesville B 2007 Mountain Braddock clay loam 30.0 6.1 174 437 2008 27.0 6.2 159 385 Buncombe 2008 Mountain French loam 6.5 6.1 14 41 MHCRS 2008 Mountain Dillard loam 24.0 6.3 45 159 2009 Comus fine sandy loam 32.8 5.4 29 117 22 March 2013 Crop Management Table 2. Precipitation and temperature data for the 16 site-years, recorded by the State Climate Office of North Carolina. * Source: State Climate Office of North Carolina (www.nc-climate.ncsu.edu). Study 2. Research was conducted in 2008 and 2009 at the MHCRS. Soil test P was determined before planting using procedures as described for Study 1 (Table 1). Corn was grown on low and medium STP soils using conventional tillage (fall moldboard plow followed by spring disking). Average initial STP at Mills River was 35 mg/kg, and ranged from 16.2 to 58.4 mg/kg during the 2 years and plots (data not shown). Eighty-six percent of the plots tes