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Garden Experiences: My Potting Mix

Copyright © 1998, 2006, 2011 by David E. Ross

The following potting mix has several advantages:

Using a three-pound size empty coffee can as a measure, take one can of peat moss (packed down firmly) and one can of "washed" plaster sand. ("Washed" is a term describing the process of preparing the sand. At a lumber or construction materials yard, the sand is found to be already washed.) Mix thoroughly.

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For plants that have damaged roots, withhold all nutrients except bone meal. Plants with damaged roots include plants you have dug out of the ground, repotting plants that were in the same pot and thus had to have the old root ball trimmed, and nursery plants that do not quite fit the container. Nutrients — especially nitrogen — can promote root rot. After a plant is well established in its container and growing for about two months, then add the remaining nutrients and cover them with a trace of compost. The compost contains soil bacteria that make the nutrients soluble so they will travel through the mix to the roots. However, phosphorus (bone meal) does not dissolve and thus must be placed where roots will find it.

Also, azaleas, rhododendrons, camellias, heathers and heaths, and certain herbs prefer a "lean" soil without abundant nitrogen. For such plants, use only half the indicated amount of blood meal.

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Add a double handful of bone meal and a double handful of blood meal. Add a handful of compost. Add a half-handful each of iron sulfate and gypsum and a pinch each of zinc sulfate (optional) and Epsom salts.

For acid-loving plants (e.g.: camellias, azaleas), remove a double handful of sand when measuring and add a double handful of peat moss; also add a pinch of soil sulfur. In this case, the zinc sulfate is not optional (if you can find any).

For cactus and other succulents, reverse the above — removing peat moss and adding sand. Cut the iron sulfate in half, and omit any sulfur.

For larger amounts (e.g.: raised beds), just magnify the amounts while keeping the proportions. Thus, you might take a wheel barrow full of sand plus a wheel barrow full of peat moss. Similarly greater amounts of bone meal, blood meal, and compost are required. If the measurements are not quite right, no harm will result. Try to err on the light side when magnifying the amounts of iron sulfate, zinc sulfate, Epsom salts, and (for acid-loving plants) sulfur; you can always add more later if deficiency symptoms appear. Slightly excessive gypsum is not harmful; in a raised bed over native soil, it will leach into the soil and help plant roots grow deeper. However, even with gypsum, it is better to err on the light side, adding more later if you did not use enough at the start. The only nutrient that cannot be added later is phosphorus (bone meal), which will not leach through the mix or soil and thus must be in place before planting.

I buy peat moss in compressed bales. This tends to leave small lumps when the bale is broken apart. After measuring the peat moss, I run it through a 0.25-inch mesh, which eliminates the lumps. I find that partially mixing the peat moss with moist sand helps move the peat moss through the mesh.

Updated 10 August 2011

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