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Josh B. Henry, Ingram McCall, Brian Jackson and Brian E. Whipker

A series of experiments investigated the effects of increasing phosphate–phosphorus (P) concentrations on the growth and development of four horticultural species. In experiment 1, petunia [Petunia atkinsiana (Sweet) D. Don ex W.H. Baxter] plants were grown using eight P concentrations, and we found that the upper bound for plant growth was at 8.72–9.08 mg·L−1 P, whereas concentrations ≤2.5 mg·L−1 P caused P deficiency symptoms. Experiment 2 investigated P growth response in two cultivars each of New Guinea impatiens (Impatiens hawkeri W. Bull) and vinca [Catharanthus roseus (L.) G. Don]. Growth for these plants was maximized with 6.43–12.42 mg·L−1 P. In experiment 3, ornamental peppers (Capsicum annuum L. ‘Tango Red’) were given an initial concentration of P for 6 weeks and then switched to 0 mg·L−1 P to observe whether plants could be supplied with sufficient levels of P, and finished without P to keep them compact. Plants switched to restricted P began developing P deficiency symptoms within 3 weeks; however, restricting P successfully limited plant growth. These experiments indicated that current P fertilization regimens exceed the P requirements of these bedding plants, and depending on species, concentrations of 5–15 mg·L−1 P maximize growth.

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Brian E. Jackson, Robert D. Wright and Mark M. Alley

The objective of this study was to compare substrate solution nitrogen (N) availability, N immobilization, and nutrient leaching in a pine tree substrate (PTS), peat-lite (PL), and aged pine bark (PB) over time under greenhouse conditions. Pine tree substrate was produced from loblolly pine logs (Pinus taeda L.) that were chipped and hammer-milled to a desired particle size. Substrates used in this study were PTS ground through a 2.38-mm hammer mill screen, PL, and aged PB. A short-term (28-d) N immobilization study was conducted on substrates fertilized with 150 or 300 mg·L−1 NO3-N. Substrates were incubated for 4 days after fertilizing and NO3-N levels were determined initially and at the end of the incubation. A second medium-term study (10-week) was also conducted to evaluate the amount of N immobilized in each substrate when fertilized with 100, 200, 300, or 400 mg·L−1 N. In addition to determining the amounts of N immobilized, substrate carbon dioxide (CO2) efflux (μmol CO2/m−2·s−1) was also measured as an assessment of microbial activity, which can be an indication of N immobilization. A leaching study on all three substrates was also conducted to determine the amount of nitrate nitrogen (NO3-N), phosphorus, and potassium leached over 14 weeks under greenhouse conditions. Nitrogen immobilization was highest in PTS followed by PB and PL in both the short- and medium-term studies. Nitrogen immobilization increased as fertilizer rate increased from 100 mg·L−1 N to 200 mg·L−1 N in PL and from 100 mg·L−1 N to 300 mg·L−1 N for PB and PTS followed by a reduction or no further increase in immobilization when fertilizer rates increased beyond these levels. Nitrogen immobilization was generally highest in all substrates 2 weeks after potting, after which immobilization tended to decrease over the course of several weeks with less of a decrease for PTS compared with PL and PB. Substrate CO2 efflux levels were highest in PTS followed by PB and PL at each measurement in both the short- and medium-term studies. Patterns of substrate CO2 efflux levels (estimate of microbial populations/activity) at both fertilizer rates and over time were positively correlated to N immobilization occurrence during the studies. Nitrate leaching over 14 weeks was lower in PTS than in PB or PL through 14 weeks. This work provides evidence of increased microbial activity and N immobilization in PTS compared with PB and PL. Increased N immobilization in PTS explains the lower nutrient (primarily N) levels observed in PTS during crop production and justifies the additional fertilizer required for comparable plant growth to PL and PB. This work also provides evidence of less NO3-N leaching in PTS compared with PL or PB during greenhouse crop production despite the higher fertilizer rates required for optimal plant growth in PTS.

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Brian E. Jackson, Robert D. Wright and John R. Seiler

The objective of this study was to evaluate a pine tree substrate (PTS) for decomposition, changes in physical and chemical properties, and substrate carbon dioxide (CO2) efflux (microbial activity) during a long-term production cycle under outdoor nursery conditions. Substrates used in this study were PTS constructed using a 4.76-mm hammer mill screen and aged pine bark (PB). Plastic nursery containers were filled with each substrate and amended with either 4.2 or 8.4 kg·m−3 Osmocote Plus fertilizer and planted with Cotoneaster horizontalis or left fallow. Substrate solution chemical properties and nutrient concentrations were determined each month during the summers of 2006 and 2007 in addition to measuring substrate CO2 efflux (μmol CO2/m−2·s−1) as an assessment of microbial activity. Substrate breakdown (decomposition) was determined with particle size analysis and physical property determination on substrates at the conclusion of the study (70 weeks). Substrate solution pH was higher in PTS than in PB at both fertilizer rates in 2006, but pH levels decreased over time and were lower in PTS at both fertilizer rates in 2007. Substrate solution electrical conductivity levels, nitrate, phosphorus, and potassium concentrations were all generally higher in PB than in PTS at both fertilizer rates through both years. Pine tree substrate decomposition was higher when plants were present in the containers [evident by an increase in fine substrate particles (less than 0.5 mm) after 70 weeks], but breakdown was equal at both fertilizer rates. Shrinkage of PTS in the presence of plants was equal to the shrinkage observed in PB with plants, but shrinkage was higher in fallow PTS containers than PTS with plants. Substrate air apace (AS) was highest in PTS and container capacity (CC) was equal in PB and PTS at potting. Substrate AS decreased and CC increased in both substrates after 70 weeks but remained in acceptable ranges for container substrates. Substrate CO2 efflux rates were higher in PTS compared with PB at both fertilizer rates indicating higher microbial activity, thereby increasing the potential for nutrient immobilization and substrate breakdown. This work provides evidence that PTS decomposition is unaffected by fertilizer rate and that substrate shrinkage in containers with plants is similar to PB after two growing seasons (70 weeks), which addresses two major concerns about the use and performance of PTS for long-term nursery crop production. This work also shows that the higher microbial activity in PTS increases the potential of microbial nutrient immobilization, which is likely the reason for the lower substrate nutrient levels reported for PTS compared with PB over 70 weeks.

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Brian E. Jackson, Robert D. Wright and Michael C. Barnes

The use of freshly harvested and processed pine trees as a container substrate for greenhouse and nursery crop production is a relatively new concept, and fundamental knowledge of the construction of a pine tree substrate (PTS) for optimal physical properties is insufficient. Therefore, this research was conducted to determine the influence of mixing PTSs produced with different wood particle sizes and adding other amendments to PTS on substrate physical properties and plant growth compared with traditional substrates. Coarse pine wood chips produced from 15-year-old loblolly pine trees (Pinus taeda L.) were ground in a hammermill fitted with either a 4.76-mm screen or with no screen (PTS-NS) allowing a fine and a coarse particle PTS to be produced. Increasing proportions of the finer (4.76-mm) PTS to the coarser PTS (PTS-NS) resulted in increased container capacity (CC) and shoot growth of ‘Inca Gold’ marigold (Tagetes erecta L.). In another study, PTSs were manufactured in a hammermill fitted with different screen sizes: 4.76, 6.35, 9.54, or 15.8 mm as well as PTS-NS. After being hammermilled, each of the five PTSs was then amended (by mixing) with 10% sand (PTS-S), 25% peatmoss (PTS-PM), or left unamended. Pine tree substrates were also produced by adding 25% aged pine bark (PB) to pine wood chips before being ground in a hammermill with each of the five screen sizes mentioned (PTS-HPB). These five substrates were used unamended as well as amended with 10% sand after grinding (PTS-HPBS). Control treatments included peat-lite (PL) and 100% aged PB for a total of 27 substrates evaluated in this study. Container capacity and marigold growth increased as screen size decreased and with the additions of peatmoss (PTS-PM) or hammering with PB (PTS-HPB) to PTS. Container capacity for all substrates amended with peatmoss or PB was within the recommended range of 45% to 65% for container substrates, but only with the more finely ground PTS-4.76-mm resulted in marigold growth comparable to PL and PB. However, when the PTS-NS was amended by mixing in 25% peat or hammering with 25% PB, growth of marigold was equal to plants grown in PL or PB. In a third study, hammering PTS-NS with 25% PB followed by the addition of 10% sand increased dry weight of both azalea (Rhododendron ×hybrida ‘Girard Pleasant White’) and spirea (Spiraea nipponica Maxim. ‘Snowmound’) resulting in growth equal to plants grown in 100% PB. This work shows that amending coarsely ground PTS with finer particle PTS or with other materials (peatmoss, aged PB, or sand) can result in a substrate with comparable physical properties such as CC and plant growth compared with 100% PL or PB.

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Lesley A. Judd, Brian E. Jackson and William C. Fonteno

Container production of plants use substrates that are formulated to have adequate physical properties to sustain optimal plant growth; however, these properties can change over time as a result of substrate settling and root growth of the growing plant in the container. An apparatus (rhizometer) was developed that measures the changes caused by plant roots on physical properties of substrates during crop production in containers. The design of the rhizometer included a clear core, which allowed for observing and measuring a range of root system characteristics in situ, including total root length visible along the rhizometer. Physical properties of planted and fallow rhizometers were measured, and the effect of four species on substrate physical properties was determined. There was a general decrease in substrate total porosity and air space (AS) over time with both fallow and planted rhizometers as a result of both settling of the substrate and root growth into the substrate. Container capacity did not change over time with or without roots. Plants with large root systems such as Begonia ×hybrida acut. decreased AS over time, whereas plants of Rudbeckia hirta L. with a smaller root system did not have the same effect. Measured total root length was highly correlated to the total dry root mass of Tagetes erecta L. and Zinnia marylandica D.M. Spooner, Stimart & T. Boyle plants. This may allow tracing and measuring root lengths to be another (alternative) method to measure root systems. Planted rhizometers also allowed easy access for viewing the root system non-destructively, providing the ability to observe and measure root growth.

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Brian E. Jackson, Amy N. Wright and Jeff L. Sibley

In the southeastern United States, inconsistent pine bark (PB) supplies and overabundance of cotton gin by-products warrant investigation about the feasibility of replacing PB with cotton gin compost (CGC) for container horticultural plant production. Most research on the use of composted organic substrates for horticultural plant production has focused on shoot growth responses, so there is a need to document the effect of these substrates on root growth. In 2004, `Blitz' tomato (Lycopersicon esculentum), `Hot Country' lantana (Lantana camara `Hot Country'), and weeping fig (Ficus benjamina) were placed in Horhizotrons to evaluate root growth in 100% PB and three PB:CGC substrates containing, by volume, 60:40 PB:CGC, 40:60 PB:CGC, and 0:100 PB:CGC. Horhizotrons were placed in a greenhouse, and root growth in all substrates was measured for each cultivar. Physical properties (total porosity, water holding capacity, air space, and bulk density) and chemical properties (electrical conductivity and pH) were determined for all substrates. Physical properties of 100% PB were within recommended guidelines and were either within or above recommended ranges for all PB:CGC substrate blends. Chemical properties of all substrates were within or above recommended guidelines. Root growth of all species in substrates containing CGC was similar to or more enhanced than root growth in 100% PB.

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Jeb S. Fields, William C. Fonteno and Brian E. Jackson

Wettability is a major factor in determining whether a material can be effectively and efficiently used as a component in greenhouse substrates. Poor wettability can lead to poor plant growth and development as well as water use inefficiency. This research was designed to test the wettability and hydration efficiency of both traditional and alternative components of substrates under different initial moisture contents (MCs) and wetting agent levels. Peatmoss, perlite, coconut coir, pine bark, and two differently manufactured pine tree substrate components (pine wood chips and shredded pine wood) were tested at 50% and 25% initial MC (by weight). The objective of this research was to determine the effects of initial MC and wetting agent rates on the wettability and hydration efficiency of these substrate components. Each component received four wetting agent treatments: high (348 mL·m−3), medium (232 mL·m−3), low (116 mL·m−3), and none (0 mL·m−3). Hydration efficiency was influenced by initial MC, wetting agent rate, and inherent hydrophobic properties of the materials. Wetting agents did increase the hydration efficiencies of the substrate components, although not always enough to overcome all cases of hydrophobicity.

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Brian E. Jackson, Robert D. Wright and Nazim Gruda

This work was conducted to evaluate the effect of limestone additions to pine tree substrate (PTS) and PTS amended with peatmoss on pH and plant growth. ‘Inca Gold’ marigold (Tagetes erecta L.) and ‘Rocky Mountain White’ geranium (Pelargonium ×hortorum L.H. Bailey) were grown in three PTSs—100% PTS, PTS plus 25% peatmoss (v/v), and PTS plus 50% peatmoss (v/v)—made from freshly harvested loblolly pine trees (Pinus taeda L.) chipped and hammermilled through a 4.76-mm screen and a peatmoss/perlite (4:1 v/v; PL) control. Each substrate was amended with various rates of dolomitic limestone and used to grow marigolds in 10-cm square (l-L) plastic containers and geraniums in round 15-cm (1.25-L) plastic containers in a glasshouse. Regardless of limestone rate, pH was highest in 100% PTS and decreased with peat additions with PL having the lowest pH. As percent peat increased from 25% to 50%, more limestone was required to adjust pH to a particular level showing that PTS is more weakly buffered against pH change than peatmoss. Adding limestone did not increase the growth of marigold in 100% PTS, but additions of limestone did increase growth of marigold when grown in PTS containing peatmoss or in PL. Geranium growth was higher in PTS containing peatmoss (25% or 50%) and PL than in 100% PTS at all limestone rates. This research demonstrates that PTS produced from freshly harvested pine trees has an inherently higher pH than PL, and the additions of peatmoss to PTS require pH adjustment of the substrate for optimal plant growth.

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Brian E. Jackson, Robert D. Wright and Michael C. Barnes

‘Prestige’ poinsettias (Euphorbia pulcherrima Willd. Ex Klotzsch) were grown at different fertilizer rates in three pine tree substrates (PTS) made from loblolly pine trees (Pinus taeda L.) and a peat-based control. Pine tree substrates were produced from pine trees that were chipped and hammer-milled to a desired particle size. Substrates used in this study included peat-lite (PL), PTS produced with a 2.38-mm screen (PTS1), PTS produced with a 4.76-mm screen (PTS2), and PTS produced with a 4.76-mm screen and amended with 25% peatmoss (v/v) (PTS3). Initial and final substrate physical properties and substrate shrinkage were determined to evaluate changes over the production period. Poinsettias were grown in 1.7-L containers in the fall of 2007 and fertilized at each irrigation with 100, 200, 300, or 400 mg·L−1 nitrogen (N). Shoot dry weight and growth index were higher in PL at 100 mg·L−1 N but similar for all substrates at 300 mg·L−1 N. Bract length was generally the same or longer in all PTS-grown plants compared with plants grown in PL at each fertilizer rate. Postproduction time to wilting was the same for poinsettias grown in PL, PTS1, and PTS3. Initial and final air space was higher in all PTSs compared with PL and container capacity (CC) of PTS1 was equal to PL initially and at the end of the experiment. The initial and final CC of PTS2 was lower than PL. The incorporation of 25% peat (PTS3) increased shoot dry weight and bract length at lower fertilizer rates compared with 4.76 mm PTS alone (PTS2). Substrate shrinkage was not different between PL and PTS1 but greater than shrinkage with the coarser PTS2. This study demonstrates that poinsettia can be successfully grown in a PTS with small particles (2.38-mm screen) or a PTS with large particles (4.76-mm screen) when amended with 25% peatmoss, which results in physical properties (CC and air space) similar to those of PL.

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Michael R. Evans, Brian E. Jackson, Michael Popp and Sammy Sadaka

The use of biochar as a soil amendment has fostered much attention in recent years due to its potential of improving the chemical, physical, and biological properties of agricultural soils and/or soilless substrates. The objective of this study was to evaluate the chemical properties of feedstocks, common in the southeast United States, and their resulting biochar products (after being torrefied) and determine if the chemical properties were within suitable ranges for growers to use the biochar products as root substrate components. Poultry litter biochar produced at 400 °C for 2 hours had a higher total phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), chloride (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), sodium (Na), and zinc (Zn) concentration than biochar made using the same process with mixed hard wood species, miscanthus (Miscanthus capensis), cotton (Gossypium hirsutum) gin trash, switchgrass (Panicum virgatum), rice (Oryza sativa) hull, and pine (Pinus sp.) shavings feedstocks. The pH of the biochar products ranged from 4.6 for pine shaving biochar to 9.3 for miscanthus biochar. The electrical conductivity (EC) ranged from 0.1 dS·m−1 for mixed hardwood biochar to 30.3 dS·m−1 for poultry litter biochar. The cation exchange capacity (CEC) of the biochar products ranged from a low of 0.09 meq/g for mixed hardwood biochar to a high of 19.0 meq/g for poultry litter biochar. The water-extractable nitrate, P, K, Ca, Mg, sulfate, boron, Cl, Cu, Fe, Mo, Na, and Zn were higher in poultry litter biochar than in all of the other types of biochar. The high EC and mineral element concentration of the poultry litter biochar would prevent its use in root substrates except in very small amounts. In addition, the high degree of variability in chemical properties among all of the biochar products would require users to know the specific properties of any biochar product they used in a soil or soilless substrate. Modifications to traditional limestone additions and fertility programs may also need to be tested and monitored to compensate for the biochar pH and mineral nutrient availability. Users should be aware that biochar products made from different feedstocks can have very different chemical properties even if the same process was used to manufacture them.