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James N. Smith and Michael R. Evans

Vegetative 6-cm Euphorbia pulcherrima `Freedom' cuttings were placed in black 200-ml bottles containing humic acid solutions, nutrient solutions, or deionized water. Humic acid solutions were prepared using Enersol SC (American Colloid, Arlington Heights, Ill.). Concentrations of 500, 750, and 1000 mg/L humic acid were compared to solutions containing mineral element concentrations equivalent to those contained in humic acid solutions. After 4 weeks, 88%, 75%, and 88% of cuttings had rooted in the 500, 750, and 1000 mg/L humic acid solutions, respectively. Cuttings placed in nutrient controls or deionized water failed to form roots after 4 weeks. Average root fresh mass was 175, 80, and 72 mg for cuttings placed in 500, 750, and 1000 mg/L humic acid solution, respectively. Average number of roots formed per cutting ranged from 21 in the 500-mg/L solution to 6 in the 1000-mg/L solution. Average lengths ranged from 26 mm in the 500-mg/L to 12 in the 1000-mg/L solution. As humic acid concentration increased, average root fresh mass, average number of roots, and the length of the longest root significantly decreased.

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Jack A. Hartwigsen and Michael R. Evans

Seed of Pelargonium ×hortorum L.H. Bailey `Freckles' (geranium) and Tagetes patula L. `Bonanza' (marigold) were soaked for 12, 24, or 48 h in solutions containing 0 (deionized water), 5000, 10,000, or 15,000 mg·L-1 humic acid (HA) or nutrient controls (NC) containing similar levels of nutrients prior to planting. Soaking in deionized water (DI) and NC treatments had no significant effect on root fresh weight. However, several of the HA treatments increased root fresh weight of marigold seedlings, and all increased geranium root fresh weight. Percentage of germination and shoot fresh weight were not significantly affected by treatment. Seed of Cucumis sativus L. `Salad Bush' (cucumber), Cucurbita pepo L. `Golden Summer Crookneck' (squash), `Freckles' geranium and `Bonanza' marigold were sown into 15-cell plug trays (5 mL volume), and the substrate was drenched with DI, 2500 or 5000 mg·L-1 HA, or 2500 or 5000 mg·L-1 NC. DI and NC treatments did not affect root fresh weight. However, cucumber, squash, and marigold seedlings germinated in substrate drenched with 2500 and 5000 mg·L-1 HA and geranium seedlings germinated in substrate drenched with 2500 mg·L-1 HA had significantly higher root fresh weight than did seedlings from all other treatments. Percentage of germination and shoot fresh weight were not significantly affected by treatment. `Salad Bush' cucumber and `Golden Summer Crookneck' squash seedlings germinated on germination towels soaked with 2500 or 5000 mg·L-1 HA, had significantly higher root fresh weight than did seedlings germinated on towels soaked with DI or NC solutions. Treatment with HA did not affect shoot fresh weight or the number of lateral roots. However, HA treatment increased the total length of lateral roots. The increase in lateral root growth occurred primarily in lateral roots developing from the lower hypocotyl.

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Jack A. Hartwigsen and Michael R. Evans

Seed of Cucumis sativus and Pelargonium ×hortorum were imbibed for 24 hours in solutions containing 0 (deionized water), 2500, 5000, 10,000, and 20,000 ppm humic acid. Additional treatments included seed which were imbibed in nutrient solutions corresponding to the nutrient content of each humic acid solution as well as an untreated dry control. Percent germination was reduced for geranium seed imbibed in 20,000 ppm humic acid and for cucumber seed imbibed in either 20,000 ppm humic acid or the corresponding nutrient control. Root fresh weights for untreated and water imbibed geranium seed were 0.05 g. Humic acid treatment increased root fresh weights to a maximum of 0.14 g at 5000 and 10,000 ppm. Shoot fresh weights for geranium were 0.12 and 0.10 g for untreated and water imbibed seed, respectively. Humic acid treatment increased shoot fresh weight to a maximum of 0.18 at 2500 ppm. Root fresh weights for cucumber were 0.16 and 0.18 g for untreated and water imbibed seeds, respectively. Humic acid treatment increased root fresh weight to a maximum of 0.33 g at 10,000 ppm. Shoot fresh weights for cucumber were 0.31 and 0.38 g for untreated and water imbibed seed, respectively. Humic acid treatment increased shoot fresh weight to a maximum of 0.43 at 10,000 ppm.

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Eovaldo Hernández

Most of the studies on the effect of humic acids on micronutrient uptake by plants has been carried out in nutrient solutions. Commercial companies have tried, without adequate experimental support, to extend the conclusions of these studies to the production of vegetables in agricultural soils. The effect of humic acids on micronutrient uptake by plants has been attributed' to (a) the improved supply of micronutrients to the soil solution caused by a higher rate of release from soil minerals (probably via chelate formation by humic acids) and (b) the improved uptake of micronutrients as consequence of the larger root system promoted by hormonal compounds in the humic acids. In soils with limiting concentration of micronutrients (such as some calcareous soils) and low content of organic matter, chelation of micronutrients by added commercial humic acids might increase their availability to plants. However, in agricultural soils with and adequate content of organic matter, no significant effect of commercial humic acids on micronutrient uptake by plants can be detected.

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Jack A. Hartwigsen and Michael R. Evans

Cucumis sativus (cucumber), Pelargonium × hortorum (geranium), Tagetes patula (marigold), and Cucurbita pepo (squash) seed were sown into plug cells (5 ml volume) filled with a germination substrate containing peat, vermiculite, and perlite. After the seed were sown, the substrate was saturated with solution containing 0 (deionized water) 2500, or 5000 mg/L humic acid (HA). Additional treatments included seed which were sown into the substrate and saturated with nutrient solutions corresponding to the nutrient concentration of each humic acid solution. Seed were placed in a growth chamber and maintained at 22°C and under a 12-h photoperiod with a PPF of 275 μmol·m–2·s–1. After 10 d for cucumber and squash and 14 d for marigold and geranium, plants were harvested and root and shoot fresh mass recorded. Shoot fresh mass was not significantly affected by treatment for any of the species tested. Except for squash, root fresh mass was significantly increased by humic acid treatments. For cucumber, root fresh mass ranged from 0.24 g in deionized water to 0.34 g in 2500 and 5000 mg/L HA. Geranium root fresh mass ranged from 0.03 g in deionized water and 5000 mg/L HA to 0.05 g in 2500 mg/L HA. Marigold root fresh mass ranged from 0.02 g in deionized water to 0.03 g in 2500 and 5000 mg/L HA. Root fresh mass for nutrient controls were similar to those for deionized water.

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John G. Schluckebrier and Chris A. Martin

Rooted cuttings of crepe myrtle (Lagerstroemia indica L. × L. fauriei `Muskogee') were transplanted into 3.8-L black polyethylene containers filled with a bark-based rooting substrate and exposed for 2 months during Summer 1995 to either of three container shielding treatments: containers shielded from insolation (container shielded inside a whitewashed 11.4-L black polyethylene container), containers exposed to insolation, or containers shielded for 1 month then exposed for 1 month. Mean highest temperature in the western quadrant of rooting substrate of exposed containers was 16°C higher than for those in shielded containers. Containers exposed for 2 months had reduced root and shoot growth and increased leaf N compared with the other two treatments. Crape myrtle plants were next transplanted into 27.0-L polybags, transferred into a temperature-controlled glasshouse, and fertigated to container capacity every 3 days with humic acid extract at concentrations of 0, 50, 150, or 300 μl·L1 for 2 additional months. Effects of the container shielding treatments for all growth parameters remained evident until the end of the experiment. Shoot and root extension growth of plants previously in containers shielded for 2 months and containers exposed for 2 months, responded in a quadratic fashion to humic acid extract concentration levels.

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M.R. Evans and G. Li

The annual bedding plants `Dazzler Rose Star' impatiens (Impatiens wallerana), `Cooler Blush' vinca (Catharanthus roseus), `Orbit Cardinal' geranium (Pelargonium × hotorum), `Janie Bright Yellow' marigold (Tagetes patula) and `Bingo Azure' pansy (Viola tricolor) were grown on germination papers treated with deionized water (DI), 2500 or 5000 mg·L-1 (ppm) humic acid (HA) or nutrient control (NC) solutions. Seedlings grown on HA-treated germination papers had higher dry root weights than those grown on DI or NC-treated germination papers. Except for impatiens, seedlings germinated on HA-treated germination papers had higher lateral root numbers and higher total lateral root lengths than those grown on DI and NC-treated germination papers. Impatiens grown on NC-treated germination papers had higher lateral root numbers than those grown on DI or HA-treated germination papers. Overall, lateral root numbers for impatiens were higher for seedlings germinated on HA-treated papers than DI or NC-treated papers and highest lateral root numbers occurred on those impatiens germinated on papers treated with 5000 mg·L-1 HA. Except for geranium, seedlings grown in HA-amended sphagnum-peat-based substrates had similar dry root and dry shoot weights as those grown in unamended substrates. Geranium seedlings grown in HA-amended sphagnum peat-based substrates had significantly higher dry root weights than those grown in unamended substrates. However, dry shoot weights of geranium grown in HA-amended sphagnum peat-based substrates were similar to those grown in unamended substrates.

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Chunhua Liu, R.J. Cooper and D.C. Bowman

Humic acids (HA) reportedly enhance the growth of numerous crops; however, little information is available as to their effects on turfgrasses. Experiments were conducted to evaluate the effect of a commercial preparation of HA on the photosynthesis, chlorophyll concentration, rooting, and nutrient content of `Crenshaw' creeping bentgrass (Agrostis stolonifera L.). Bentgrass plugs were grown hydroponically in one-quarter-strength Hoagland's nutrient solution containing HA at 0, 100, 200, or 400 mg·L-1 with measurements made weekly for 1 month. The photosynthetic rates of plants growing in 100 or 200 mg·L-1 rarely differed from that of the control, but 400 mg·L-1 significantly enhanced net photosynthesis on all four observation dates. Chlorophyll content was unaffected by HA rate on all observation dates. Root dehydrogenase (DH) activity and root mass regrowth were significantly increased by HA at 400 mg·L-1 on all dates. The 100 and 200 mg·L-1 rates increased root DH activity on two of four observation dates, but root regrowth was unaffected. At one or more of the rates used, HA increased tissue concentrations of Mg, Mn, and S and decreased those of Ca, Cu, and N, but had no influence on the concentrations of P, K, Fe, Mo, and Zn.

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Chunhua Liu and R.J. Cooper

Growth and mineral nutrient content of creeping bentgrass [Agrostis stolonifera (L.) var. palustris (Huds.) Farw.] in response to salinity and humic acid (HA) application were investigated, and the effects of HA application on salinity tolerance was evaluated. Bentgrass plugs were grown hydroponically in one-quarter-strength Hoagland's nutrient solution containing HA at 0 or 400 mg·L-1 with salinity levels of 0, 8.0, or 16.0 dS·m-1. Clipping dry weight (DW), tissue water content, and net photosynthesis (PN) were measured weekly for 1 month. Maximum root length, and root DW from 0 to 10 cm and >10 cm root zones were determined 31 days after treatment (DAT). The turfgrass plugs were mowed three times weekly, with clippings collected and dried for mineral nutrient analysis. Salinity was inversely related to clipping DW, tissue water content, PN, and maximum root length. Salinity had less effect on root growth than top growth. HA treatment did not affect tissue water content, PN, or root growth of salt-stressed turf. Salinity decreased uptake of N, P, K, Ca, and S; increased uptake of Mg, Mn, Mo, B, Cl, and Na; and had no influence on uptake of Fe, Cu, and Zn. Application of HA at 400 mg·L-1 during salinity stress neither increased uptake of the mineral nutrients inhibited by salinity, nor decreased uptake of nutrients which were excessive and toxic in the salinity solution. In general, application of HA did not improve salinity tolerance of creeping bentgrass.

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Gang Li and Michael R. Evans

Seedlings of Cucumis sativus (cucumber), Tagetes patula (marigold), Viola tricolor (pansy), Pelargonium × hortorum (geranium), and Impatiens wallerana (impatiens) were germinated on towels soaked with either deionized water, nutrient control solutions, or humic acid solutions. Root fresh weight and root dry weights were higher for all seedlings germinated on towels soaked with humic acid as compared to seedlings germinated on towels soaked with deionized water or nutrient control solutions. Lateral root number and total lateral root length were higher for cucumber, marigold, pansy, and geranium seedlings germinated on towels soaked with humic acid than those germinated on towels soaked with deionized water or nutrient control solutions. Root fresh and dry weights were higher for impatiens, Begonia semperflorens (begonia), marigold, and geranium seedlings germinated in a sphagnum peat: vermiculite (80:20, %v/v) substrate drenched with humic acid as compared to seedlings germinated in substrate drenched with deionized water or nutrient control solutions. Foliar sprays of humic acid also resulted in increased root fresh and dry weights while foliar application of nutrient control solutions either had no effect or reduced root fresh and dry weights.