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

effects of low phosphorus fertilization of floriculture species. MS Thesis, N.C. State Univ., Raleigh Krug, B.A. Whipker, B.E. McCall, I. Cleveland, B. 2010 Geranium leaf tissue nutrient sufficiency ranges by chronological age J. Plant Nutr. 33 3 339 350

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Josh B. Henry, Penelope Perkins-Veazie, Ingram McCall and Brian E. Whipker

anthocyanin coleus ( Solenostemon scutellarioides ) and maize ( Zea mays ) Funct. Plant Biol. 39 3 264 273 Henry, J.B. McCall, I. Jackson, B. Whipker, B.E. 2017 Growth response of herbaceous ornamentals to phosphorus fertilization HortScience 52 1362 1367

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John M. Smagula and M. Susan Erich.

Nutrition Surveys of commercial blueberry fields in 1987 and 1988 indicated leaf phosphorus levels were below the standard (0.125%) in most fields. To determine if phosphorus was limiting, liquid phosphorus (23% phosphoric acid) was applied preemergence at 0, 22.4, 44.8 67.2, or 88.6 kg/ha to 3 commercial blueberry fields identified in 1987 as very low (<.111%), 3 low (.111-.125%), and 3 adequate (>.125%) in leaf phosphorus. Phosphorus concentration in leaf tissue sampled in July 1989 increased linearly with increasing rates of phosphorus. Phosphorus application raised leaf phosphorus levels more in fields which had levels below 0.125%. Fields with higher phosphorus levels were also higher in leaf nitrogen, potassium, and calcium.

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P.R. Johnstone and T.K. Hartz*

Heavy P fertilization of vegetable crops in the Salinas Valley of California have increased soil P levels, with > 50 mg·kg-1 bicarbonate-extractable P (Pbc) now common. To evaluate the response of lettuce (Lactuca sativa L.) to P fertilization in fields with elevated soil P levels, 12 trials were conducted in commercial fields during 2002-2003. Pbc at the trial sites varied from 53-171 mg·kg-1. In each trial four replicate plots receiving the growers' P application were compared with paired plots in which no P was applied. Leaf P was monitored at cupping stage and at harvest. At harvest mean whole plant mass and % of marketable plants were recorded. The correlation of Pbc to bioavailable P (Pba) was evaluated using 30 representative Salinas Valley soils; Pbc varied among these soils from 15-177 mg·kg-1. Pba was estimated by P adsorption on an anion resin membrane during a 16 h incubation. The effect of temperature on P bioavailability in 6 of these soils was estimated by conducting the Pba incubation at 5, 15 and 25 °C. A significant increase in lettuce yield with P fertilization was achieved at only one trial site, a spring planting where Pbc was 54 mg kg-1 ; at all other sites, including 3 with Pbc < 60 mg kg-1, P application resulted in no agronomic benefit. P application resulted in only a marginal increase in plant P uptake. Pba was highly correlated with Pbc (r = 0.89). Pba increased approximately 40% across soils with each 10 °C increase in soil temperature.

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Brent K. Harbaugh, David A. DeVoll and R. Zalewski

Phosphorus is considered a major pollutant of lakes in central Florida, and growers producing crops in the Lake Okeechobee watershed are being challenged to reduce use of P fertilizer. Caladium (Caladium×hortulanum Birdsey) tubers are produced on organic soils within this area. This study was done to determine if current commercial P fertilization rates could be reduced or eliminated, since these organic soils have high levels of water extractable P (Pw). Two farms were selected with low (Farm A 19 lb/acre; 21 kg·ha-1) or high (Farm B 59 lb/acre; 66 kg·ha-1) preplant Pw levels. Production of caladium tubers with the standard grower P fertilization practice (Farm A = P at 39.2 lb/acre; 43.9 kg·ha-1, or Farm B = P at 15.9 lb/acre; 17.8 kg·ha-1) was compared to production with either one-half the standard grower rate of P or no P. The percentage of harvested tubers in each of five grades and the estimated harvested tuber value index were similar irrespective of the amount of P fertilizer used on either farm. These results indicate that P could be eliminated from the fertilization program for caladium tuber production on organic soils.

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P.R. Johnstone, T.K. Hartz, M.D. Cahn and M.R. Johnstone

Decades of heavy phosphorus (P) fertilization of vegetable crops in the Salinas Valley of California has increased soil test P (STP) levels, with bicarbonate-extractable P (Pbc) values >50 mg·kg–1 now common. To evaluate the response of lettuce (Lactuca sativa L.) to P fertilization in fields with elevated STP levels, 12 trials were conducted in commercial fields during 2002–03. Initial Pbc at the trial sites varied from 53 to 171 mg·kg–1. In each trial, four replicate plots receiving the growers' P application were compared with paired plots in which no P was applied. Leaf P was monitored at midseason and at harvest. At harvest, mean whole and marketable plant mass and percent of marketable plants were recorded. A significant increase in lettuce yield with P fertilization was achieved at only one trial site, a spring planting with 54 mg·kg–1 Pbc; at all other sites, including three with Pbc <60 mg·kg–1, P application resulted in no significant yield increase. Phosphorus application resulted in only a marginal increase in plant P uptake; in the nonresponsive fields leaf P concentration of nonfertilized plots was in excess of established sufficiency levels. In a laboratory study, the correlation of Pbc to bioavailable P (Pba) was evaluated using 30 representative Salinas Valley soils; Pbc varied among these soils from 15 to 177 mg·kg–1. Pba was estimated by P adsorption on an anion resin membrane during a 16 hour incubation. The effect of temperature on P bioavailability in six of these soils was estimated by conducting the Pba incubation at 5, 15, and 25 °C. Pba was highly correlated with Pbc (r = 0.89), and increased about 40% across soils with each 10 °C increase in soil temperature. Therefore, Pbc was determined to be an accurate reflection of bioavailable P in these soils, although the addition of a temperature correction factor in setting threshold values is desirable.

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Thomas A. Obreza, Robert E. Rouse and Kelly T. Morgan

. 368 Spencer, W.F. 1963 Phosphorus fertilization of citrus Fla. Agr. Expt. Sta. Bul. 653 Systat Software, Inc. 2006 SigmaStat 3.5 user's manual Systat Software, Inc Point Richmond, CA Tucker

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Donglin Zhang, Renae E. Moran and Lois B. Stack

Scaevola aemula R.Br. (fanflower), an ornamental plant native to Australia, produces stunted growth when fertilized with high concentrations of P. To determine optimum P concentration, rooted cuttings were transplanted into 15 cm standard pots and grown with a water soluble fertilizer, where P concentrations were 0, 14.5, 29.0, 43.5, 58.0, 72.5, 87.0 mg·L-1 and all plants received 200 mg·L-1 N and 166 mg·L-1 K. Shoot growth and flowering data were taken every 21 days until the experiment was terminated after 84 days. Shoot length, number and dry weight, and leaf size were reduced significantly at P concentrations higher than 14.5 mg·L-1 with severe reduction at P levels higher than 43.5 mg·L-1. Number of flowers per plant was not affected by P concentrations in the range of 0 to 43.5 mg·L-1, but decreased significantly at P levels higher than 43.5 mg·L-1. Medium pH decreased with increase in P rate due to the acidic nature of the P fertilizer. When P was applied in every irrigation, the optimum concentration was 14.5 mg·L-1 or less. P greater than 43.5 mg·L-1 was detrimental to vegetative growth and flowering, possibly due to above optimum P or to medium acidification.

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Ricardo González-Ponce, Esther G. López-de-Sá and César Plaza

Struvite (MgNH4PO4·6H2O) production is widely studied as a way to remove phosphorus (P) from wastewater and generate a potentially marketable P fertilizer, but its effects on crops have yet to be researched more thoroughly. This study was conducted to evaluate struvite recovered by the Spanish Research Council (CSIC) pilot process (STR) as a source of P for lettuce (Lactuca sativa L.) by comparing its effectiveness with that of single superphosphate (SUP), a common P fertilizer derived from phosphate rock. In a greenhouse pot experiment, a P-deficient loamy sand soil was amended with either SUP or STR at P rates of 0, 4, 8, 12, 16, and 20 mg·kg−1. Nitrogen and potassium were uniformly supplied to all treatments. The response of lettuce head fresh weight and P uptake to P rate exhibited statistically significant quadratic relationships for both SUP and STR. With respect to SUP, STR was significantly more effective in increasing lettuce yield and P uptake, probably because of the larger amount of magnesium (Mg) incorporated with this material and a synergistic effect on P uptake. This work supports previous findings based on other test crops in suggesting that STR can be a P source attractive to the fertilizer market with additional agronomic and environmental benefits such as providing available Mg and nitrogen, helping attenuate consumption of phosphate rock, and reducing release of P by discharge of treated wastewaters to surface and groundwater systems.

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George Hochmuth, Pete Weingartner, Chad Hutchinson, Austin Tilton and Dwight Jesseman

Phosphorus (P) fertilization studies were conducted on four commercial farms and at the University of Florida Institute of Food and Agricultural Sciences Hastings Research and Education Center in Hastings. All sites were in the potato (Solanum tuberosum) production area of northeastern Florida. Preplant Mehlich-1 soil test P was very low at one commercial site and very high at the other four sites. The yield of marketable size A tubers, the desired tuber category, did not respond to P fertilization from 0 to 66 lb/acre (74.0 kg·ha-1) of P at any site. The average yield across all sites was 324 cwt/acre [16.2 ton/acre (36.3 t·ha-1)]. Leaf-P concentration at midseason did not respond to P fertilization. Leaf-P concentration averaged 0.38%, which was sufficient for potato. Potato tuber specific gravity averaged 1.075 and responded slightly to P fertilization only at one site.