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- Author or Editor: George Elliott x
Abstract
Rapid assays for urea hydrolysis and for the oxidation of NH4 + to NO2 – and NO3 – are required for extensive studies of these processes in potting media. Methods originally developed for use with field soils were evaluated and modified for use with soilless media. Effects of urea concentration, buffer pH, and incubation temperature on rates of urea hydrolysis were tested with two media. Response to these variable was less pronounced in soilless potting media than in field soils. Urea hydrolysis can be conveniently assayed in soilless media under the following conditions: 5 mol urea/m3 medium, Tris buffer pH 8.0, 37°C. Rates of NH4 + and NO2 – oxidation were estimated from the accumulation of NO2 – in samples incubated for 0 to 24 hr with 4 mol NH4 +/m3 medium as substrate in 1.0 mM phosphate buffer, pH 7.0, with or without 20 mol NaClO3/m3 medium. The rate of NO2 – accumulation in the presence of ClO3 –, an inhibitor of NO2 – oxidation, estimates NH4 + oxidation, and the slope of the difference in NO2 – between samples with and without ClO3 – estimates the rate of NO2 – oxidation. Although ClO3 – does not completely inhibit NO2 – oxidation in one of the two media tested, the method is useful for comparison of nitrification activity in potting media.
Water retention was measured in soilless potting media irrigated by capillary mat, flood and drain, drip or overhead sprinkler. Media were amended with wetting agent or hydrophilic polymeric gel. Pots 12 cm high with a volume of 465 cm3 were loose-filled to the top with media. Potted media were wetted overhead with 120 ml water, then pots were randomly assigned to irrigation treatments. Capillary mat irrigation was continuous; other irrigation treatments were applied daily. Water retention was measured by weighing. Irrigation was continued until no further retention was measured. Water retention was significantly affected by irrigation method and medium amendments. Irrigation method followed the order overhead >= drip > flood and drain >= mat. Hydrophilic gel increased water retention, but in contrast to previous results, wetting agent did not, nor was any interaction of gel and wetting agent observed. Retention of water at container capacity, measured in situ at the end of each experiment, was significantly larger than actual retention.
Water retention was measured in soilless potting media irrigated by capillary mat, flood and drain, drip or overhead sprinkler. Media were amended with wetting agent or hydrophilic polymeric gel. Pots 12 cm high with a volume of 465 cm3 were loose-filled to the top with media. Potted media were wetted overhead with 120 ml water, then pots were randomly assigned to irrigation treatments. Capillary mat irrigation was continuous; other irrigation treatments were applied daily. Water retention was measured by weighing. Irrigation was continued until no further retention was measured. Water retention was significantly affected by irrigation method and medium amendments. Irrigation method followed the order overhead >= drip > flood and drain >= mat. Hydrophilic gel increased water retention, but in contrast to previous results, wetting agent did not, nor was any interaction of gel and wetting agent observed. Retention of water at container capacity, measured in situ at the end of each experiment, was significantly larger than actual retention.
Addition of lime to increase pH is generally essential for soilless media based on acidic organic materials. Media pH may decrease over time as the result of addition of acidic fertilizers. The objectives of this research were: to characterize reactions of conventional or finely ground lime in soilless media; to compare resistance to acidification in soilless media amended with conventional or finely ground lime; and to evaluate, for media containing rockwool, an equation to predict H+ activity (HA) of binary mixtures from HA of components. Various soilless media were amended with each type of lime at rates from 1 to 16 kg m-3 and incubated 8 weeks at 20 C. Subsamples were removed and pH was measured in saturated slurries. Finely ground lime was about twice as effective as conventional lime in adjusting pH to 6.0 within 1 week after mixing and wetting. Neither initial nor final HA of unlimed peat-rockwool mixes could not be predicted from HA of components. Three media were amended with each type of lime, planted or left unplanted, and irrigated with fertilizer solution with and without 1.0 N H2SO4. Final pH of media amended with finely ground lime averaged 0.2 units lower than media amended with conventional lime. Final pH of unplanted media averaged 0.5 units lower than planted media. Final pH of media fertilized with solution containing H2SO4 averaged 0.6 units lower than without. Addition of H2SO4 to fertilizer stimulated growth of New Guinea Impatiens (Impatiens xhybrida).
A schematic diagram and parts list is presented for a simple and inexpensive system for pulsed subirrigation (commerically referred to as ebb and flow or flood and drain). The system can be readily modified for flowing solution culture. It has proven useful in teaching and research applications. It can be assembled from readily available parts using hand tools.
Abstract
Ureolytic activity was evaluated in uncropped soilless potting media and in media that had been cropped for 4 to 12 weeks with subirrigation or drip fertigation systems in two sequential experiments. Space heating and root-zone heating treatments were compared in one of these experiments. Higher rates of urea hydrolysis tended to develop with drip fertigation than with subirrigation. Increased media temperatures during cropping were associated with increased urea hydrolysis rates. Ureolytic activity was not consistently affected by length of cropping. Interactions of media, fertigation system, and length of cropping indicate the complexity of the development of ureolytic activity in soilless potting media.
Abstract
In the article “Urea Hydrolysis in Potting Media”, by George C. Elliott (J. Amer. Soc. Hort. Sci. 111:862–866, November, 1986), Figs. 2 and 3 were reversed. The correct figures and legends are printed below.
Abstract
Urea hydrolysis and the fate of ammonium produced were evaluated in bark : peat : sand, peat : vermiculite, and 14 commercial potting media, in cropped and uncropped samples. Urea hydrolysis was much more rapid in pine bark : peat : sand than in peat : vermiculite. Significant variation in urea hydrolysis was observed among the commercial media. Urea hydrolysis was more rapid in cropped than in uncropped media. Substantial net N immobilization was observed in most media. Although nitrification was evident, there was generally very little net nitrate accumulation. Differences in urea hydrolysis and ammonium accumulation were of sufficient magnitude to warrant consideration in fertilizer management.
Water retention at effective water-holding capacity (EWHC) and container capacity (CCAP) were measured in four rockwool-peat potting media amended with a wetting agent and/or a hydrophilic gel in pots 12 cm tall containing 445 cm3 medium, and irrigated by capillary mat, flood-and-drain, trickle emitter, or overhead sprinkler. Water retention was measured by weighing. Irrigation was continued until EWHC (i.e., net water retention when no weight increase could be obtained by further irrigation) was reached. CCAP (i.e., net water retention following saturation and free drainage) was measured at the end of each experiment. Irrigation method and medium amendments significantly affected EWHC. Rank order of irrigation treatments was sprinkler ≥ trickle > flood and drain ≥ mat. Hydrophilic gel increased both EWHC and CCAP, while the wetting agent increased EWHC but decreased or had no effect on CCAP. Significant interactions of gel and wetting agent were observed in some media. EWHC was less than CCAP, and EWHC was better correlated with CCAP with trickle emitter and overhead sprinkler irrigation than with capillary mat and flood-and-drain irrigation.
The potential for N mineralization or immobilization in potting media containing compost was investigated in experiments using sunflower (Helianthus annuus `Sunrich Yellow') as a test crop with potting media formulated from 3 aged pine bark: 2 sphagnum peat or compost: 1 vermiculite (by volume). Cropped and uncropped media in 350-mL pots were fertilized by subirrigation with complete nutrient solutions containing N at 2, 4, 6, 8 or 12 mmol·L–1 as ammonium nitrate. In control medium without compost, sunflower fresh mass increased linearly with increasing N. In one compost medium (C), growth was stimulated and with N at 6 mmol·L–1 was equal to growth with N at 12 mmol·L–1 in the control medium. With another compost medium (A), growth was inhibited and did not respond to increasing N. In medium C, saturated media extracts (SME) obtained before and up to 12 days after transplanting had higher concentrations of NH4 and NO3 than the control medium. In medium A, NH4 concentrations were similar to controls, but NO3 concentrations were lower. Nitrite concentrations were less than 0.2 mmol·L–1 and were highest in medium C. Nitrogen derived from compost in medium C substantially reduced the soluble fertilizer N requirement for sunflowers, while N immobilization by compost in medium A was not overcome by increasing fertilizer N.