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Amy N. Wright and Robert D. Wright

Root growth following transplanting allows a plant to exploit water and nutrient resources in the soil backfill (landscape) or container substrate and thus is a critical factor for transplant survival. The Horhizotron, a horizontal root growth measurement instrument, has been developed and evaluated for use in measuring root growth under a variety of root environments. The design of the Horhizotron includes four wedge-shaped glass quadrants that extend away from a plant's root ball allowing measurement of roots as they grow out from the original root ball. The substrate in each quadrant can be modified in order to evaluate the effect of substrate or root environment on root growth. Materials used for construction were lightweight, durable, easy to assemble, and readily available from full service building supply stores. Units were suitable for use on a greenhouse bench or outdoors in contact with the ground. Horhizotrons provided a simple, nondestructive method to measure root growth over time under a wide range of rhizosphere conditions.

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Robert D. Wright and L. Eric Hinesley

Eastern redcedar (Juniperus virginiana L.) seedlings were grown in 1986 through 1988 in pine bark container media with various levels of dolomitic limestone and micronutrients. Supplemental micronutrients reduced shoot growth, especially in the absence of limestone, and root growth was greatest when neither limestone nor micronutrients were added. Including at least 3.0 kg limestone/m3 in the medium was beneficial, not only as a source of nutrients, but also as a buffer against potentially toxic effects of excess micronutrients.

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Ronald F. Walden and Robert D. Wright

Pine bark-filled containers periodically fertilized with a (NH4)2SO4 solution were heated from 21°C to one of 5 temperatures (28°, 34°, 40°, 46°, or 52°C) for a daily exposure duration of 1, 2, 4, 6, or 24 hours. Medium solution extracts were analyzed for NH4-N and NO3-N every 5 days for 20 days. Treatment temperature of at least 40°C and a daily exposure duration of 24 hours was necessary to inhibit nitrification, thereby increasing NH4-N concentration in the medium solution. Similar increase in NH4-N was found for a 2 hr/day exposure to 46°C, with further increases in NH4-N at longer exposure times. By day 10, the maximum level1 of NH4-N concentration in medium extracts was found after a 1 hr/day exposure to 52°C. Decreases in medium solution NO3-N concentration generally coincided with the increases in NH4-N. Results indicate that high container temperatures may increase the ratio of NH4-N to NO3-N in the medium solution of plants fertilized with predominantly ammoniacal N.

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Robert D. Wright, Mike Witt and Carol Leda

Ilex crenata `Helleri' rooted cuttings were container grown for one summer with Osmocote 18N-2.6P-7.6K which was reapplied at different times. Beginning in April and every month thereafter until October, five plants were harvested and separated into roots and shoots for dry weight and nitrogen (N) analysis. Plants that had received a reapplication of Osmocote in June or July were larger in October than those that received no supplemental fertilizer. If reapplication occurred in August or September, no influence on growth was realized. Plants that were not supplemented had lower percent tissue and medium solution N. These data demonstrate that tissue nutrient analysis as well as the medium nutrient levels extracted with the pour-through procedure can be used to determine when reapplication of a slow-release fertilizer is necessary to promote optimal growth.

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Ronald F. Walden and Robert D. Wright

Rooted cuttings of Ilex crenata Thunb. `Helleri' were grown for 12 weeks in pine bark with two root-zone temperature treatments (unheated or heated to 40C for 6 hours·day–1), two rates of limestone addition (0 or 6 kg·m–3), and three weekly N application rates (200, 400, or 600 mg·liter–1 as urea ammonium nitrate) in a factorial combination. Decreases in shoot and root dry weights due to root-zone heating (69% and 75%, respectively) or limestone addition (41% and 42%, respectively) were not influenced by N application rate. Effects of root-zone heating on medium solution characteristics, which differed in response to limestone addition, were similar for all N application levels. In unlimed pine bark at 400 mg N/liter, the pH and the NH4-N: NO3-N ratio were higher in the heated medium (5.5 and 1.15, respectively) than in the unheated medium (3.9 and 0.64, respectively) after 80 days, suggesting that 6 hours of daily exposure to 40C inhibited nitrification. The higher medium solution pH due to root-zone heating resulted in lower medium solution and shoot tissue Mn concentrations.

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Ronald F. Walden and Robert D. Wright

Pine bark-filled containers periodically fertilized with NH4-N were heated from 21C to 28, 34, 40, 46, or 52C for daily exposures of 1, 2, 4, 6, or 24 hours over 20 days. Concentrations of NH4-N and NO3-N in medium solution extracts were determined every 5 days. Medium solution NH4-N concentration was higher at constant (24 hours) exposure to 40C than at lower temperatures or exposure times. There was a similar increase in NH4-N concentration for a 2-hour·day–1 exposure to 46C, with further increases in NH4-N for longer exposure times. By day 10, NH4-N concentration was highest after 1 hour·day–1 exposure to 52C. Decreases in medium solution NO3-N concentration generally coincided with the increases in NH4-N. These results indicate that container medium thermal periods, similar to those observed in nurseries of the southern United States, may inhibit nitrification, thereby influencing NH4-N: NO3-N ratios in the medium solution of plants fertilized with predominantly ammoniacal N sources.

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Robert D. Wright and Jake F. Browder

Due to uncertainties of future supplies of pine bark (PB) and peatmoss, ground Pinus taeda logs [pine chips (PC)] were compared to ground PB as a potential container substrate for japanese holly (Ilex crenata Thunb. `Chesapeake'), azalea (Rhododendron obtusum Planch. `Karen'), and marigold (Tagetes erecta Big. `Inca Gold'). Plants were potted in 2.8-L plastic containers 8 Apr. 2004 with either 100% PC, 100% PB, or 75% PC:25%PB (v/v), and glasshouse grown 8 weeks for marigold and 13 weeks for holly and azalea. Plant dry weights were higher for marigold grown in 100% PB compared to 100% PC but not different from plants grown in 75% PC:25% PB. Plant dry weights of azalea were higher in 100% pine bark than both substrates containing chips. There was no difference in shoot dry weight for japanese holly between the three substrates. Root dry weight was higher for 75% PC:25% PB than for 100% PB, but root weight of 100% PB and 100% PC was the same. The percent air space for the PC was higher than the PB substrate but container capacity and available water was not different for the three substrates. Substrate solution electrical conductivity (EC) for PC, was lower than that of PB, possibly due to greater leaching with the more porous PC and nutrient retention by the PC. These factors could account for the cases where larger plants developed with the PB substrate. Nutrient analysis of the substrate solution indicated that there are no toxic nutrient levels associated with PC. The pH of PC is also acceptable for plant culture. As well, there was no apparent shrinkage due to decomposition during the course of this short-term experiment. Pine chips, therefore, offer potential as a container substrate for greenhouse and nursery crops.

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D. Joseph Eakes, Robert D. Wright and John R. Seiler

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D. Joseph Eakes, Robert D. Wright and John R. Seiler

The influence of K nutrition (25, 75, 150, 300, 450, and 600 mg K/liter) and moisture stress conditioning (MSC) (exposing plants to four sublethal dry-down cycles) on leaf water relations, evapotranspiration, growth, and nutrient content was determined for salvia (Salvia splendens F. Sellow `Bonfire'). Potassium concentration and MSC had an interactive influence on osmotic potential at full (π100) and zero (π0) turgor. Differences in osmotic potential between MSC and non-MSC plants for π100 and π0 increased with increasing K concentration. Increasing K concentration and MSC resulted in active osmotic adjustment and, consequently, increased cellular turgor potentials. Foliar K content increased with increasing K concentration and MSC. High K concentrations and MSC both reduced plant evapotranspiration on a per-plant and per-unit-leaf-area basis. Greatest shoot dry weight occurred for plants grown with 300 mg K/liter and non-MSC. Total leaf area increased with increasing K concentration, but MSC had little effect.