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JiWeon Lee and Paul V. Nelson

Tomato `Marglobe' seed were sown on germination paper in enclosed plastic dishes in a growth room Ammonium was more toxic when applied as the single salt, ammonium sulfate, than when applied as part of a complete Hoagland solution. The lowest toxic ammonium levels were for the single salt 1.5 mM and for the complete solution 4.5 mM. Symptoms included reduced length of primary and particularly lateral roots, reduced numbers of root hairs, and chlorosis, distortion, and slower development of cotyledons. Tomato `Marglobe' seedlings were also grown in 288 cell plug trays in a substrate of 3 sphagnum peat moss and 1 perlite containing no N, P, or K but amended with dolomitic limestone to pH 6.0 They were fertilized every third watering with 4 mM NH4 + NO3, 0.4 mM PO4, and 1.2 mM K from 15 to 28 days after sowing and at double this concentration from 29 to 42 days. A zero leaching percentage was practiced. Ammoniacal-N comprised 25, 50, or 75% of total N. There were no effects of ammonium on root or shoot weights, height or appearance of plants through this period. Plant growth was limited throughout this period by N stress in accordance. with commercial practice. After 42 days N stress was alleviated by again doubling the nutrient solution concentration and applying it with every watering. Ammonium toxicity developed with symptoms of shorter plant height, general chlorosis of lower leaves, and necrosis of the base of lower leaves.

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JinSheng Huang and Paul V. Nelson

It is desirable to have a large root mass and compact shoot in the final stage of plug seedling production. Marigold `Discovery Orange' was grown for six weeks from sowing in a hydroponic system. Hoagland's all nitrate solution was used at 0.25X for the first three weeks and 0.5X for the final three weeks. P was applied continuously in the control and was eliminated for the first one or three weeks in the two stress treatments. Weekly mot and shoot dry weights indicated: a.) P stress caused an increase in root/shoot ratio with roots larger than in the control plants and b.) restoration of P after a P stress resulted in a rapid shift of root/shoot ratio back to the control level with final root and shoot weights less than in the control plants. A continuous marginal P stress or a stress near the end of seedling production is suggested. Tomato `Marglobe' was grown for five weeks and impatiens `Super Elfin White' for six weeks in a 3 sphagnum peat moss: 1 perlite substrate in 288 cell plug trays. Fertilizer was applied at every third watering at a zero leaching percentage. The control nutrient ratio (mM) was 5.4 NH4+ NO3: 0.5 PO4: 1.6 K while the low P treatments contained 0.15, 0.1, and 0.05 mM PO4 throughout the experiment. The root/shoot dry weight ratios increased in the low P treatments. Tomato plants at 0.15 and 0.1 mM P and impatiens plants at 0.15 mM P had larger roots than the control plants. A continuous stress at 0.15 mM PO4 appears promising.

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Janet F. M. Rippy and Paul V. Nelson

Measurement of substrate pH entails procurement of the substrate solution and measurement of the solution pH. Acid-base reactions are completed at the time of testing. Determination of substrate pH during development of a titration curve is more complex because it involves initially the reaction of a base with the substrate. Five factors that can influence the resulting pH values were investigated in this study and include amount of water added to substrate, method to procure substrate solution for pH determination, chemical form of base used, time allowed for acid-base reaction and the addition of CaSO4. Substrate in this study consisted of 3 sphagnum peatmoss: 1 perlite (by volume) amended with wetting agent. Dolomitic limestone (6 g·L-1 substrate) was added to substrate for the water amount and solution procurement method experiments. Except for the water amount experiment, deionized water was added by weight to achieve 95% container capacity. Dishes were incubated at 20 °C for specified times. To identify the minimal level of water necessary to ensure complete contact between base and substrate for neutralization, additions equivalent to 95%, 100%, 120%, and 150% container capacity were tested. The 95% level proved adequate. The saturated media extraction and pour-through bulk solution displacement methods for pH determination resulted in higher pH measurements in the incubated substrate than the squeeze bulk solution displacement method. This indicated that the former two methods diluted the soil solution. The squeeze method was deemed most effective. NaOH resulted in higher pH endpoints than Ca(OH)2. This was apparently due to a higher affinity of Ca2+ for peatmoss exchange sites. Since Ca2+ is the predominant cation associated with liming materials for soilless substrates, Ca(OH)2 is more appropriate for titration. From the tested incubation times of 0, 2, 4, 8, 24, 48, and 96 hours, the duration of 24 hours was found to be adequate to allow complete reaction of base with substrate acidity. The best procedure for determining pH in a substrate titration situation included a water level of 95% container capacity, Ca(OH)2 base, an incubation time of 24 hours and the squeeze solution displacement method. The additional CaSO4 was not necessary. Chemical names used: calcium sulfate (CaSO4), sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)2], calcium ion (Ca2+).

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Dharmalingam S. Pitchay and Paul V. Nelson

It is a common practice in greenhouses to apply fertilizers with a high proportion of N in the NO3 form to achieve short, compact shoots and a moderate (25% or greater) proportion of NH4 or urea for large shoots. However, this practice is not substantiated in the scientific literature. Two experiments were conducted in a greenhouse to assess effects of N form on development. In the first, Petunia hybrida `Mid-night Dreams' was treated with five ratios of NH4:NO3 or urea:NO3 in a factorial arrangement with three concentrations of N (50-low, 100-adequate, and 200-high mg/L at each irrigation). In the second experiment six species of bedding plants were treated in a factorial arrangement of five ratios of NH4:NO3 and two pH levels (acceptably low, 5.4-5.8, and unacceptably low, 4.6-5.2). In all comparisons, height and dry weight of shoots grown with 100% NO3 were equal or larger than the plants grown with combinations of N. There was a general trend for plants to be shorter and lighter at higher NH4 or urea proportions. These results refute the hypothesis that shoot size is under the control of N form. Depth of green foliar color correlated positively with proportion of NH4 or urea. Reputed NH4 toxicity symptoms of chlorosis, necrosis, and curling of older leaves occurred only at adversely low pH levels below 5.2 in experiment 2. Resistance of plants to this disorder under conditions of pH levels in the range of 5.4 to 5.8, high N application rates, and applications of 100% NH4 indicates that bedding plants during commercial production are fairly resistant to this disorder.

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William C. Fonteno and Paul V. Nelson

Loose rockwool had a total porosity similar to peatmoss (92%, by volume) but with water retention capabilities similar to sand. Root media formulations containing loose rockwool were tested with seven plant species for plant response and nutrient uptake. The volume percent formulation, 20 rockwool : 10 peatmoss : 20 vermiculite : 45 pine bark : 5 perlite, was superior to formulations containing 10% or 30% rockwool. Plant response in this rockwool medium in bedding plant flats was superior to that in two high-performing commercial media for impatiens (Impatiens sultanii Hook), marigold (Tagetes patula L.), and petunia (Petunia hybrida Vilm) and equal to one commercial medium for tomato (Lycopersicon esculentum Mill.). However, response of chrysanthemum (Chrysanthemum × morifolium Ramat.), geranium (Pelargonium × hortorum Bailey), and poinsettia (Euphorbia pulcherrima Willd. ex Kl.) in 1.58-liter pots was inferior to both commercial media in one-half of the trials. Differential plant responses in the root media treatments did not relate directly to differences found to occur in plant nutrient composition. The high initial pH level of rockwool necessitated reduced application of limestone and increased application of calcium sulfate to offset Ca deficiency.

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Amy J. Compton and Paul V. Nelson

Many plug seedling growers complain about the inadequacy of substrate testing as a measure of nutritional status because results are too variable. We conducted two experiments to test a model system of sampling substrate at a set time after fertilization. Petunias (Petunia×hybrida Hort. ex Vilm. var. multiflora `Primetime White') were grown in 288-cell plug trays. Six fertilizer regimes were used consisting of a factorial arrangement of three fertilizer cycles (at each, every other, and every third irrigation) and two leaching fractions (0% and 20%). Fertilizer or water was applied at 0900 HR daily, and then 24 hours later in Expt. 1, and 1 hour later in Expt. 2, substrate solutions were sampled and analyzed. Samples taken after waterings were used to assess the dilution and leaching effects of water on substrate nutrient concentrations. In Expt. 2, additional substrate samples were taken at various hours after fertilizing to test the effect of plant depletion of the substrate. Substrate nutrient concentration curves constructed from data drawn at a fixed time after fertilizations, but not after waterings, were logical and could be interpreted. When data from a fixed time after fertilizations and waterings were plotted together, the curves could not be interpreted. Data from samples taken at various hours after fertilization in Expt. 2 revealed large reductions in concentrations, often after only 4 hours. Overall, leaching and dilution effects from watering in combination with the increased time span from fertilizing to sampling resulted in nutrient concentrations that could not be interpreted. Substrate testing can be effective for plug seedling production, but samples need to be taken 1 to 2 hours after fertilizations.

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Kimberly A. Williams and Paul V. Nelson

Soilless container medium components such as peatmoss and perlite have almost no capacity to retain PO4-P, and preplant amendments of triple superphosphate (TSP) are readily leached. Al amendments were tested to reduce P losses from these media. Al2(SO4)3 solutions at rates of 320 and 960 μg Al/cc were applied to a 70 peat: 30 perlite medium and dried at 70C. Adsorption isotherms were created at 25C for the Al2(SO4)3-amended media and an unamended control using solutions of Ca(H2PO4)2 at concentrations of P ranging from 0 to 500 μg·ml–1. Isotherms showed that P retention increased as Al concentration increased. In a greenhouse study, Dendranthema ×grandiflorum `Sunny Mandalay' was grown in these media with 100 g P/m3 from TSP incorporated into the mixes before planting. PO4-P, soluble Al, and pH were determined on unaltered medium solutions collected throughout the cropping cycle and foliar analyses were determined on tissue collected at mid- and end-crop. The highest rate of Al was excessive and resulted in low pH and soluble Al levels in the medium solution early and in the cropping cycle, which were detrimental to plant growth. When Al was applied at 320 μg/cc, soluble Al levels in medium solution were not significantly higher than in the unamended control, PO4-P leached from TSP was reduced, and sufficient PO4-P was released throughout the cropping cycle to result in optimal plant growth.

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Kimberly A. Williams and Paul V. Nelson

Soilless container root media have little capacity to retain P, and preplant amendments of triple superphosphate (TSP) and water-soluble fertilizer (WSF) P applications are readily leached from them. A soilless medium modified with Al2(SO4)3 was tested to reduce such P losses. Aluminum sulfate solutions were applied to a 70 sphagnum peat: 30 perlite (v/v) medium to result in 0.32, 0.96, and 1.92 kg Al/m3 and dried at 70C. Adsorption isotherms (25C, 0 to 500 mg P/liter) showed that P retention increased as the rate of Al addition increased. In a greenhouse study, plants of Dendranthema ×grandiflorum (Ramat.) Kitamura `Sunny Mandalay' were grown in Al-modified media and an unmodified medium in factorial combination with P from preplant amendment of 0.1 kg TSP-P/m3, or P applied at each watering as WSF at rates of 5.5 or 21.8 mg P/liter. The two highest rates of Al were excessive and resulted in low pH and excessive soluble Al levels in the root medium solution early in the cropping cycle, which were detrimental to plant growth. When the root medium was modified with 0.32 kg Al/m3, soluble Al levels in medium solution were not significantly different than in the unmodified control. TSP-P that leached was substantially reduced by the addition of Al, yet sufficient P was released throughout the cropping cycle for adequate plant growth. Plants grown in Al-modified medium with 0.1 kg TSP-P/m3 did not differ from control plants in unmodified medium + 0.27 kg TSP-P/m3 and were larger than plants grown in unmodified medium + 0.1 kg TSP-P/m3. Aluminum modification of the root medium substantially reduced P leaching when used with WSF containing P. In addition, growth of plants in unmodified medium fertilized with 5.5 vs. 21.8 mg P/liter was similar.

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Carl E. Niedziela Jr. and Paul V. Nelson

A new tube method for determining physical properties in container substrates was compared to an existing system. While both offer the advantages of undisturbed substrate and measurement of properties without altering the geometry of the substrate in the container, the tube method is easier to conduct. Both methods proved equally effective for determining air-tilled porosity, container capacity, total porosity, bulk density, and particle density.

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Kimberly A. Williams and Paul V. Nelson

Many soilless root media have limited ability to retain nutrients. Zeolites are minerals that have substantial nutrient buffering capacity and can be precharged with K, and possibly PO4, and combined with soilless media to provide these nutrients during crop production. The zeolite clinoptilolite was precharged with K and PO4 at two rates that were estimated from sorption isotherms to result in equilibrium root medium solution concentrations of P at >1 μg·ml–1 (low rate) and K at 125 μg·ml–1 (high rate). Precharged clinoptilolite was mixed with a 7 sphagnum peat: 3 perlite root medium to comprise 20% (v/v) and evaluated as the sole source of K and PO4 during production of Dendranthema ×grandiflorum (Ramat.) Kitamura `Sunny Mandalay'. Phosphate, K, Na, and pH were determined on unaltered bulk medium solutions collected over the course of the cropping cycle, and foliar analyses were determined on tissue collected at mid- and end of crop. Plants that relied on K release from precharged clinoptilolite at the low and high rates and received a N/P/-K fertilizer produced growth and tissue K concentrations that were not significantly different than the control which received a complete fertilizer. Plants that relied on PO4 release of precharged clinoptilolite did not result in growth or tissue P levels similar to those of the complete control. Phosphate levels in the root medium solution were adequate only during the first month of the cropping cycle, but PO4 release should be taken into consideration when developing a fertilization program using precharged clinoptilolite to provide other nutrients. Using precharged clinoptilolite at the low rate reduced K losses through leaching to 26% of the amount leached from control plants receiving K at 176 mg·L–1 at each watering.