To reduce the carbon-to-nitrogen (C:N) ratio, pine tree substrate (PTS) and other wood-based substrates can be precharged with urea so that growers do not have to add extra nitrogen (N) during crop production to compensate for immobilization. However, the impact of urea hydrolysis from this addition on the substrate solution has not been documented for wood-based substrates. The objectives of these experiments were to determine how urea hydrolysis in PTS impacts substrate solution and how hydrolysis is affected by urea and lime rates. In Expt. 1, 16-month-old pine chips (from loblolly pine trees, Pinus taeda L.) were milled to make PTS and PTS was amended with 0 or 1.0 kg·m−3 dolomitic limestone in factorial combination with urea-N rates of 0, 0.5, 1.0, 1.5, or 2.0 mg·g−1 dry weight. Urea hydrolysis was quantified by the detection of NH4-N in the substrate solution at 0, 48, 96, and 144 hours after urea addition. Substrate pH and electrical conductivity (EC) values were also measured. In Expt. 2, non-limed PTS was treated with the same urea rates as described; NH4-N and pH were measured at 24 and 48 hours after urea addition. Substrate solutions were incubated with jackbean urease to determine the remaining urea-N amount after 144 hours in Expt. 1 and after 24 and 48 hours in Expt. 2. In Expt. 1, NH4-N increased from 0 to 48 hours for the 0 and 1.0-kg·m−3 lime treatments and for all urea-N rates (except for the 0 rate); NH4-N did not increase thereafter. As urea-N rate increased, the amount of NH4-N increased and more N was detected for the limed PTS than in the non-limed PTS. Initial substrate pH values for the 0 and 1.0-kg·m−3 lime treatments were 4.5 and 5.6, respectively, and peaked 48 hours after urea application; pH values were higher in the limed PTS than for the non-limed PTS. At the highest urea-N rate and after 48 hours (Expt. 1), the PTS pH value increased 3.1 units to 7.6 for the non-limed PTS and the value increased 2.3 units to 7.9 for limed PTS. In Expt. 2 the increase in PTS pH values was approximately half of the Expt. 1 pH increases. Samples treated with urease derived from jackbean had less than 2% of the initial urea amount after 144 hours in Expt. 1 and after 48 hours in Expt. 2. However, less than 13% of the total amount of urea-N added to PTS was detected as NH4-N in the non-limed treatment after 144 hours in Expt. 1 (for all urea rates); detected amounts for the 1.0-kg·m−3 lime treatment ranged from 15.5% to 18.3%. Five percent or less of the total amount of urea-N added to PTS was detected as NH4-N in non-limed PTS after 48 hours in Expt. 2 (for all urea rates). The large amount of unrecovered NH4-N is likely explained by microbial N consumption. Using pH increase as an indication of urea hydrolysis, we found that an initial pH of 4.5 or higher (Expt. 1) resulted in twice the urea hydrolysis as an initial pH of 4.2 (Expt. 2). Initial substrate pH had a major impact on the amount of pH increase and substrate pH status and our findings suggest that the urea precharge rate should be based on the initial pH of the substrate.
Alexander X. Niemiera, Linda L. Taylor and Jacob H. Shreckhise
Jacob H. Shreckhise, James S. Owen Jr. and Alex X. Niemiera
Phosphorus (P) uptake efficiency (PUE; percent of applied P absorbed by roots) for containerized crops is ≈27% to 62%. Reducing P fertilization may increase PUE without decreasing growth and may reduce P leaching from containers, thus mitigating the environmental impact of containerized production while potentially reducing fertilizer input costs for growers. The objective of this study was to determine the minimum P application concentration and the resulting substrate pore-water (i.e., solution residing within and between substrate particles) P concentration that maintains maximal growth of three containerized woody plant taxa grown in pine bark substrate. Hydrangea paniculata Sieb. ‘Limelight’ (hydrangea), Ilex crenata Thunb. ‘Helleri’ (holly), and Rhododendron L. ‘Karen’ (azalea) were potted in pine bark substrate amended with dolomite and micronutrients and grown for 81 d in an open-wall greenhouse. Plants received either one of five constant liquid-feed treatments with varying P concentrations [80 mg·L−1 nitrogen (N), 50 mg·L−1 potassium (K), and 0.5, 1.0, 2.0, 4.0, or 6.0 mg·L−1 P] or a single application of controlled-release fertilizer (CRF; control) at experiment initiation. Calculated lowest P fertilizer concentration that sustained maximal shoot dry weight (SDW) in hydrangea and azalea was 4.7 and 2.9 mg·L−1, respectively, and holly SDW was the same across all liquid fertilizer treatments. In all three taxa, CRF-fertilized plants achieved <50% of maximal SDW observed in liquid-fertilized plants. Hydrangea root dry weight (RDW) nearly doubled as fertilizer P increased from 0.5 to 2.0 mg·L−1 P, but higher P concentrations did not further increase RDW. Holly RDW was unaffected by liquid P treatment. Pore-water P concentrations of treatments that sustained maximal SDW of hydrangea and azalea were as low as 0.6 and 2.2 mg·L−1 P, respectively. Our findings suggest that when using constant liquid feed, applied P levels more accurately predict plant growth responses than pore-water P levels.
Jacob H. Shreckhise, James S. Owen Jr., Matthew J. Eick, Alexander X. Niemiera, James E. Altland and Brian E. Jackson
Soilless substrates are routinely amended with dolomite and sulfate-based micronutrients to improve fertility, but the effect of these amendments on phosphorous (P) in substrate pore-water during containerized crop production is poorly understood. The objectives of this research were as follows: compare the effects of dolomite and sulfate-based micronutrient amendments on total P (TP), total dissolved P (TDP), orthophosphate P (OP), and particulate P (PP; TP − TDP) concentrations in pour-through extracts; to model saturated solid phases in substrate pore-water using Visual MINTEQ; and to assess the effects of dolomite and micronutrient amendments on growth and subsequent P uptake efficiency (PUE) of Lagerstroemia L. ‘Natchez’ (crape myrtle) potted in pine bark. Containerized crape myrtle were grown in a greenhouse for 93 days in a 100% pine bark substrate containing a polymer-coated 19N–2.6P–10.8K controlled-release fertilizer (CRF) and one of four substrate amendment treatments: no dolomite or micronutrients (control), 2.97 kg·m−3 dolomite (FL); 0.89 kg·m−3 micronutrients (FM); or both dolomite and micronutrients (FLM). Pour-through extracts were collected approximately weekly and fractioned to measure pore-water TP, TDP, and OP and to calculate PP. Particulate P concentrations in pour-through extracts were generally unaffected by amendments. Relative to the control, amending pine bark with FLM reduced water-extractable OP, TDP, and TP concentrations by ≈56%, had no effect on P uptake efficiency, and resulted in 34% higher total dry weight (TDW) of crape myrtle. The FM substrate had effects similar to those of FLM on plant TDW and PUE, and FM reduced pore-water OP, TDP, and TP concentrations by 32% to 36% compared with the control. Crape myrtle grown in FL had 28% lower TDW but pour-through OP, TDP, and TP concentrations were similar to those of the control. Chemical conditions in FLM were favorable for precipitation of manganese hydrogen phosphate (MnHPO4), which may have contributed to lower water-extractable P concentrations in this treatment. This research suggests that amending pine bark substrate with dolomite and a sulfate-based micronutrient fertilizer should be considered a best management practice for nursery crop production.