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United States Department of Agriculture

Agricultural Research Service

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Location: Application Technology Research

2012 Annual Report

1a. Objectives (from AD-416):
Development of a method for stabilization of pH in container substrate during crop production. This will be done by 1) quantify the rate of release of acidity from the plant system throughout crop time, 2) Determine the extent of change in substrate titration properties during the 11 week crop chrysanthemum, 3) Determine how to draw an unaltered soil solution sample that is representative of the pH situation throughout the soil ball, 4) Establish a titration method for testing substrate, 5) Test the capacity of a stable, reproducible dairy compost to set initial substrate pH and to buffer it against later change, 6) Establish the optimum proportion of compost in a peat moss:perlite base substrate for establishing and holding the target pH, 7) Profile the pH buffering capacity of compost during an 11 week crop of chrysanthemums, 8) Evaluate the impact of compost on substrate physical properties, 9) Explore methods for lowering the bulk density of compost-containing substrates, 10) Lower the bulk density of compost by composting for a shorter time, just sufficient for complete nitrification of NH4+ to NO3- to avoid a later pH drop if nitrification were to occur, 11) Test the use of light weight components along with compost to compensate for weight, and 12) Measure the impact of compost on nutrient supply to establish a compensating fertility program.

1b. Approach (from AD-416):
We plan to use a physiologically neutral fertilizer, water with zero alkalinity (deionized water), and water soluble liming materials devoid of residual lime. The net effect will be pH depression caused by plant system respiration and chemical acidity of the fertilizer. The test crop will be potted chrysanthemum because it has a suitably long production time of 11 weeks, is available year-around, and is one of the most important potted crops world-wide. A substrate of 3 sphagnum peat moss and 1 perlite will be used. Nutrient solution will be applied with each irrigation. Acid release will be determined periodically by extrapolating between the initial and final pH points for a given period on a substrate pH titration curve for that period. Substrate will be titrated quarterly during the crop. This will enable us to determine if the initial pre-plant titration curve is adequate for measuring acid release throughout the crop or whether subsequent curves are needed in addition to the initial curve. It will also yield the subsequent curves in the event they are required. The pour-through extraction procedure will be used as a point of reference for a close examination of the efficacy of Rhizon porous plastic vacuum extractors. Parameters to be considered include: length of time for water sorption prior to titration, the balancing cation on the titrating base (Na vs. Ca), mixing time after each addition of base, and advisability of adjusting substrate from pH 3 to 11 and back to 3 prior to titration to open up exchange sites in the organic material.

3. Progress Report:
The major challenge for the culture of crops grown in containers of soilless root substrate, such as greenhouse ornamentals, greenhouse vegetables, forestry seedlings, and nursery landscape crops, is the maintenance of pH throughout production. Substrate pH controls availability of all plant mineral nutrients. Five factors are known to influence substrate pH: residual limestone (Factor 1), alkalinity in irrigation water (Factor 2), the potential acidity/basicity of fertilizer used to grow the plant (Factor 3), the chemistry of the fertilizer used (Factor 4), and the effects of the plant system (Factor 5). A working knowledge of the effects of residual limestone (Factor 1) and alkalinity in irrigation water (Factor 2) currently exists. Our project focuses on how to better quantify the effects of the remaining three factors. In order to quantify the three remaining factors it is necessary to develop titration curves for the root substrates under study. The titration curve offers a rapid tool for quantifying the amount of acidity that is gained or lost under the influence of the tested factors that cause pH shift during cultivation. The current, alternative procedure for measuring acidity shift is the substrate incubation method which requires up to one week to run and provides an undesirably limited number of data points for research purposes. In an earlier phase of this project, titration curves were generated using automatic titrators. However, through subsequent test runs it was obvious that there were additional parameters that could be changed to improve the procedure. During the past year we evaluated these parameters to ensure that the final titration curve was reproducible, reduced hysteresis, and produced a curve that was as close to actual incubation data as possible. The eight parameters tested included titrating at fast, moderate, and slow rates (maximum 7.00, 2.00, and 0.33 mL acid/base per minute, respectively); fast rate with slowed addition near upper and lower pH endpoints; changing the forward titration endpoints from pH 11 to pH 8 for one or both cycles; and titrating at a moderate rate after shaking for 2 hours or 24 hours at approximately pH 11. Reducing the speed of acid and base titrant addition improved the titration outcome, but it was possible to achieve the same result with a great savings in time by using the fast addition combined with proportional slowing of the addition as the two pH end points of 11 and 3 were approached. Results were further improved by shaking the mixture for two hours when the first pH 11 end point was reached. Shaking for 24 hours instead of only 2 at this point added little improvement. The final procedure calls for hydrating 1.5 grams of equivalent dry peat moss in 50 mL of de-ionized water by raising it from its ambient pH of around 3.5 to 11, holding it at pH 11 while stirring for two hours, then lowering the pH to 3. Titration then proceeds by addition of 0.1 M sodium hydroxide base to raise the pH to 11 followed by 0.1 M hydrochloric acid to lower pH back to 3. The rate of addition of titrants is 7 mL per minute except at the pH 11 and 3 end points where the rate is lowered in proportion to how close one is to the pH end points. The results of this work benefit two audiences. The first is the scientific community who can use the titration technique as a method for precisely measuring acidity changes in root substrate as a consequence of experimental treatments. The second audience is comprised of larger greenhouses and nurseries who mix their own root substrate and the substrate formulation companies who supply substrate to container plant production firms. This will give them a rapid method for assessing the limestone requirements of successive batches of root substrate as they are being prepared for commercial use. This project relates to sub-objective 3a of the parent project: Evaluate existing and alternative growth medium components to determine the potential benefit of individual components on production efficiency, transportability, and shelf life of ornamental crop. It assists with milestones at 60 months: Complete comparative studies of promising substrates that enhance media growth characteristics.

4. Accomplishments

Last Modified: 06/21/2017
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