- Author: Jane Callier
Following Napa's 6.0 earthquake of August 24, everybody has a lot to sweep up. Many houseplants were on the floor, their broken pots and soil mix scattered. My plant casualties were stuffed into plastic food containers to hold them until cleanup of more immediate damage was completed.
The easiest way to obtain good potting soil is to buy some your nursery recommends. Use caution when purchasing soil from box stores. You tend to get what you pay for when purchasing soil.
Another way to get quality potting soil is to make your own mix. Common ingredients are readily available. Use a mixture of half inorganic and half organic, mixing selected inorganic materials together first. Separately mix selected organic materials together. Then blend the inorganic and organic materials. Fertilizer is added last and mixed thoroughly into the final blend. It is easiest to prepare these mixes on a hard surface, such as the concrete floor of a garage. Once prepared, container mixes can be stored dry in a cool place for several years. Be sure to label your storage bin. Following are ingredients to use, and you can read more about soil mix at ucanr.edu/sites/ucmgnapa/files/153364.pdf
INORGANIC INGREDIENTS, 50% of total volume. Use any one or a combination of the following:
- Perlite – Silicaceous material mined from lava flows. Furnace heat expands particles to sponge like kernels, very light, sterile, holds 3-4 times its weight in water.
- Builders' Sand – 0.25 to 1.0 mm in diameter. Heavy. A source of micronutrients in the clay fraction.
- Vermiculite – “expanded mica” mined from deposits found in Montana and North Carolina. A hydrated magnesium aluminum iron silicate. Furnace heat expands minerals. Lightweight, neutral in acidity, it can absorb large quantities of water. Use USA sources only.
- Pumice – Volcanic origin may be white, gray, black, or red, crushed and screened to size.
- Dialoam – Manufactured by firing diatomaceous earth.
- Expanded Shale – Made by firing a type of shale rock at high temperatures.
ORGANIC PART: 50% of total volume, Use any one or a combination of the following:
- Peatmoss, Canadian Sphagnum – High moisture holding capacity (10 times its dry weight). High acidity (3.8-4.5 pH). About 1% nitrogen. Use only low ash content pure sphagnum peat.
- Compost – Use locally available ‘cold' or well aged compost.
- Rice hulls – Slow to decompose, contain some potassium, light in weight.
- Ground fir bark – lasts longer than sawdust.
- Grape pomace – blended with other organic materials, contains potassium.
- Nitrolized redwood or cedar sawdust. All sawdusts require the addition of nitrogen. Except for redwood or cedar sawdust, most break down too fast for long-term value. Course sawdust will last longer than fine. Freshly milled sawdust must be leached before using.
- Coir – ground coconut husks.
FERTILIZER: University researchers found that 4 to 6 pounds of coated, slow release 16-16-16 fertilizer mixed into one cubic yard of soil mix gave superior growth from container plants. The balanced formula N-P-K fertilizer, with equal numbers, performed better than other coated fertilizers in container trials. Fertilizer added to the mix should be considered ‘starter' and must be supplemented with topical applications after plants are established. Other additives are not necessary or recommended. Animal manure is too salty and is not recommended for use in container mixes.
You should renew your container mix after two or three year's service. Replace the old mix in the container with fresh, unused mix. The used mix from your container can be added to your garden bed.
Remember, that when you move a plant from one pot to another, you want a pot that is bigger, but not TOO big. Usually one to two inches larger is enough. DO NOT put gravel in the bottom of pots. It doesn't help with drainage, and simply takes up space that the roots should be using. Make sure you have more potting soil and if your plant has a large root system, you might need your clippers. Telltale signs that a plant needs repotting include soil that dries out quickly or has become degraded, roots are tightly packed within a pot or protruding from drainage holes, and water sitting on the soil surface too long after watering. The best time to repot most plants is when they're actively growing, in the spring or summer. However, plants can usually handle repotting whenever the situation warrants it.
Examine the roots for signs of rot or disease and remove them. If the plant is root bound and you would like to keep it in the same container, use a clean blade and cut the roots back by one to two inches on all sides. Then replant in the same container with fresh soil mix. If the plant's root ball appears compacted, cut it vertically with a sharp knife to encourage roots to move out into the new soil. Remember, you are not damaging the plant; you are renewing it by promoting good nutrient absorption and air circulation. Check out more tips about potting at ucanr.edu/sites/ucmgnapa/files/153364.pdf
To keep soil from leaking out the bottom of the pot, cover its drainage hole(s) with a paper towel, coffee filter, mesh screen, or pot shard, but avoid sealing the hole.
To repot a small plant that's easy to lift, put a few inches of moist soil in the pot and tamp it down lightly. Place the plant in the pot, centering it. The goal is to get the top of the root ball to sit about an inch below the rim of the pot. Fill the space around the root ball with soil, leaving room at the top so the pot can hold enough water with each watering to thoroughly moisten the soil.
Credits:Dean Donaldson, Farm Advisor and John Hoffman, UCCE Master Gardener, December 2010Denise Levine, UCCE Master GardenerMarin County Master Gardeners, original article by James D. KramerAdditional Reading:Plant Indoors: Their Care and Feeding, UC ANR Leaflet 2941, 1977.Soil, Physical Environment and How it Affects Plant Growth, UC ANR Leaflet 2280, 1978.Organic Soil Amendments and Fertilizers, UC ANR Publication #21505, 1992.The U.C. System for Producing Healthy Container-Grown Plants, Manual 23, 1957.Cornell Peat-Lite Mixes for Commercial Plant Growing, Cornell Ext. Bull. 1104, 1967.Pests of Landscape Trees and Shrubs, UC ANR Publication #3359, 1994.Selecting and Growing House Plants, USDA Home and Garden Bulletin No. 82, 1963.Indoor Landscaping with Living Foliage Plants, Texas A&M University, Publication B-1118, 1972.UC Master Gardeners of Napa County provide free home gardening advice. Visit, call or complete the Plant Problem Diagnosis Sheet for assistance.
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1710 Soscol Avenue, Suite 4, Napa
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