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United States Department of Agriculture

Agricultural Research Service

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Location: Application Technology Research

2009 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
This Specific Cooperative Project focuses on sustainable methods of stabilizing pH in container crop production. Agricultural limestone and compost, specifically daily cow manure compost, were selected as the standard and sustainable materials to control pH. It was found that additions of 5 to 30% dairy cow manure compost (DMC) resulted in initial substrate pH levels of 4.7 to 7.3. Although pH declined during plant production, the decline was similar in the agricultural limestone and the DMC treatments. Thus, pH buffering capacity of DMC was similar to the limestone. The difference in shrinkage of substrate from initial irrigation to end of crop across the treatments was 2.9 mm in Experiment 1 and 1.8 mm in Experiment 2. Shrinkage did not relate to addition of DMC and was of little commercial significance. Initial substrate measurements included dry bulk density, total porosity, container capacity, air space at container capacity, and available water between container capacity and 1.5 MPa. Substrate bulk density increased from 0.1 to 0.23 g per cubic cm with DMC increases from 0 to 30%. Total porosity decreased with increased DMC while air space was unaffected. Container capacity and available water was lower in the DMC substrate but did not significantly differ across the DMC addition rates. According to end of crop tissue analysis, DMC resulted in higher leaf concentrations of potassium, sulfur, copper, iron, and manganese and lower, but adequate, calcium and magnesium concentrations. Maximum plant growth (dry weight) occurred with 15% DMC in Experiment 1 and with 10% DMC in Experiment 2. All limestone and a portion of peat moss were effectively replaced with DMC. Additionally, a titration-based method of determining calcium carbonate (a common horticultural lime) requirements of peat-based substrates was developed. The method accurately predicts CaCO3 needs in a short period of time for commonly desired pH set points, reducing the need for long-term incubation protocols. Progress of this cooperative project was monitored through monthly or weekly electronic (email) communication, shared participation in industry-related educational courses, shared authorship of publications, and twice annual face-to-face communication at national meetings.

4. Accomplishments

Last Modified: 10/16/2017
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