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calculate potting media amendments

Calculating the amount of amendments to add to a potting medium should be done very carefully. Plants growing in containers live in a much different environment than those growing in the ground. The ratio of root occupation to soil is significantly higher in containers and thus, utilization of fertilizer nutrients is also much higher. We advise caution when fertilizing plants growing in container media. If you are going to use this form, we strongly urge you to follow these instructions closely. In most cases, correcting potting media imbalances is unnecessary and applications of CWO products (as directed) will provide ideal fertility, but if the analysis indicates very low levels of most nutrients, it may be advisable to increase the recommended CWO application rate by as much as 50 percent. Conversely, if the analysis report indicates very high levels of most nutrients (especially EC), it would be prudent to reduce the application rate. This form will be most useful if only one or two nutrients are below optimum levels.

  1. First, send a sample of your potting media to a lab and have it analysed. Make sure you ask the lab for potting media analysis…not soil analysis. Most labs will offer parameters on the report and let you know if the nutrient levels in your sample(s) fall inside or outside of an acceptable range. Typically, labs report the results of potting media analysis in parts per million (ppm) of the elemental nutrient. If the lab does not give parameters, you can compare your test results with These Ranges.
  2. Next, you’ll need to determine the density (i.e., weight per yd3) of the moistened media (moistened media should ball up when squeezed but no water should drip out).  Some labs can provide density data, usually in gr/cc. To convert gr/cc to gr/pint, multiply by 473.2. To DIY, measure a pint (2 cups) of the material and weigh it. If you only have a measuring cup, determine the net weight of one cup and double it. Fill your cup or pint container and tap it lightly on a solid surface to settle the material down to the desired level. Add more or remove some medium if necessary. Do not compress it by pushing down on the potting media. Get as accurate a weight measurement as you can. Enter the weight in ounces or grams (not both), in the Convert box to determine the weight in pounds. Note: grams will be more accurate than ounces.
  3. The weight (in pounds/pint, /ft3, and /yd3) will automatically populate the Bulk Weight box. Now you can determine the effect your amendments will have on your potting media.
  4. Determine the percentage concentration of the correcting element from your amending material (found on the package under “guaranteed analysis”), and enter it in the first field of the PPM Calculation box.
  5. Then you’ll need to calculate how much of the correcting amendment to add using the Pounds/yd3 box. Enter the ppm reported on the lab report and the desired ppm (a value within the acceptable range). The result will be expressed in pounds/yd3. Multiply it by 16 to convert to ounces (weight) or by 454 to convert to grams. It’s important to keep elemental levels within the recommended range. Amendments added to potting media must be mixed thoroughly to avoid pockets where excessive concentrations of nutrients might exist. Please understand that not all amendments are in the ideal consistency for mixing with potting media. The more coarse the material, the more difficult it is to mix evenly. In certain situations, it may be more appropriate to surface apply amendments after plants become established.
  6. To adjust pH, see the table at the bottom of this page.

pH adjustments
To lower pH 0.5 – 1.0 unit

Material Bench soils Potting soil Or
Finely ground sulfur ½-lb/100ft2 ¼-lb/yd3 ½-oz/2.5 bu (~3ft3)

To Raise pH 0.5 – 1.0 unit
Material Bench soils Potting soil Or
Ground limestone 5 lbs/100ft2 2.5 lbs/yd3 5 oz/bu (~3ft3)

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