Washington State University Extension

Garden Tips

Potting Soil RSS feed


GARDEN TIPS – Written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Published March 20, 2016

When talking about growing plants in containers I usually emphasize how important it is to use a quality potting mix. Admittedly, this is vague and does not help whey trying to decide what potting mix to buy when you get to the garden store.

Back in the 1950s, the selection of bagged potting mixes was not difficult because there were none. Most gardeners planted their annual flowers in flower beds or regular garden soil was used for planting in pots. However, the problem with using soil straight from the garden was that it typically did not provide adequate drainage and aeration for good plant growth.

In the 1960s universities, like Cornell University, researched what materials worked better for growing plants in pots than plain soil. Their research was prompted by a nursery industry that was finding it difficult to find good topsoil for growing potted plants. They needed a readily available substrate that would enable them grow quality plants, one that was disease and weed seed free, was relatively lightweight, provided good drainage and adequate nutrients, and did not contain residual herbicides.

Cornell’s solution to the problem was a soilless potting mix called the Cornell Peat-lite Mix. Their basic peat-lite mix was 50 per cent, by volume, sphagnum peat moss and 50 per cent horticultural grade vermiculite. Their B Mix was a 50:50 mix of peat moss and horticultural perlite. Fertilizers were added to the mixes to provide nutrients for growth. At about the same time, the University of California developed their basic UC Mix that contained sand and peat moss in equal proportions.

Potting mixes have changed for a variety of reasons since the 60s. One reason is the expense of obtaining sphagnum peat moss and environmental concerns over the destruction of peat bogs in Canada and elsewhere. Compost, softwood conifer bark, composted manure, and coconut coir (made from coconut husks) have all been used to replace some or all of the peat moss in soilless mixes. Horticultural vermiculite has also fallen out of favor because it will compact if not handled gently, losing its ability to provide aeration and drainage. There also have been concerns about using vermiculite because its ore naturally contains 2 to 3 per cent asbestos fibers.

My preference in potting mixes is one as close to Cornell’s B Mix as possible, but this is difficult to find. As already noted, many companies substitute other materials for the peat moss component. This substitution works out well if the substituted material is fairly stable. Coconut coir and composted pine and fir bark all decompose slowly and serve as adequate peat moss substitutes.

Potting mixes that get a thumbs down from me are those that are predominantly plant-based compost or containing inferior components. These mixes are usually the lower priced potting mixes. While they may be dark and crumbly, they often do not drain well or provide adequate aeration. Stay away from mixes that contain sedge peat, soil, stones, and discernable pieces of sticks and twigs and ones lacking perlite or vermiculite for drainage. Do not use products labeled “garden soil.” These are garden soil and are not intended for use in containers.

Finally, remember that adage of “you get what you pay for.” Look for the recommended ingredients on the bags of potting mix and potting soils and invest in “a good quality potting mix” for your container gardens.


written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published- MAY 9, 2014


I grow all my annual flowers in big pots on my back patio. At last count, there were ten of them and replacing the potting mix in them every year would bankrupt me. Instead of buying new potting mix each spring, I “refresh” and reuse the old.

I start by digging out all the dead roots and stems of last year’s plants, if I didn’t remove them in the fall. Potting mixes tends to compact over the course of the growing season, so next I use a trowel and garden knife to break apart residual roots and loosen the mix to a depth of at least eight inches. Along with loosening the mix each spring, I also add some controlled-release fertilizer and work it into those top eight inches.

After refreshing the mix, I add some new potting mix if the level in the pot has declined due to decomposition or from removing the old plant roots intermingled with mix. After several years, I may replace the old mix in the top half of the pot with new because it is not draining well due to the break down of organic matter over time. When I remove old mix, I don’t throw it away. Instead, I mix it into my sandy garden soil.

I recommend investing in a quality potting mix when starting a new container. I prefer a mix that consists of peat moss or coconut coir, perlite or pumice, earthworm castings, and some compost. I also like the ones that contain controlled-release fertilizer that the label indicates will last for several months.

I mentioned earlier adding fertilizer to potting mix that is being reused. This is necessary because last year’s plants probably used up most of the available nutrients and whatever they did not use was likely lost through leaching with the frequent watering necessitated by hot weather. The addition of fertilizer to reused potting mix is important for the good growth of the annuals, flower or vegetables, planted in containers. Just imagine the fertilizer needs of a vigorous growing sweet potato vine, trailing petunia, or tomato vine!

I prefer controlled-release or “time-release” fertilizers for use in my containers. They are more expensive than traditional water soluble granular fertilizers, but I like the convenience of not needing to reapply them frequently during the season. When I select a controlled-release fertilizer, I look for one that is a balanced fertilizer, one that contains nitrogen (N), phosphorus (P), and potassium (K). The percentage of these by total weight is indicated somewhere on the product label in that order: N, P, and K. Because the amount of nutrients vary with different types and brands of fertilizer, I follow the recommendation on the label for the amount to apply to a particular size pot.

Product labels also indicate the length of time that the nutrients should last. However, because hot summer and early fall weather in our region dictates frequent watering, it may not last that long. You should consider applying the same fertilizer again in mid-summer. If not, you can use a water soluble liquid or crystallized fertilizer to add some nutrients later in the season if the plants are showing signs of nutrient deficiency, such as yellowing leaves or poor growth.

Anxious to get started, I readied by containers over a month ago. Now I am anxiously waiting for consistently warm and calmer weather before I get started. Maybe this weekend? We’ll see.

Published: 5/9/2014 11:43 AM


written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published- JULY 4, 2014


Today’s homeowners have smaller yards and less space for vegetable gardening. Many have opted to use raised beds for growing vegetables as a way to maximize space and minimize garden maintenance.

‘Square-foot’ gardening, popular with some gardeners, is a raised bed system for optimizing garden production promoted by Mel Bartholemew. His system includes using a potting mix that he calls Mel’s Mix. It contains compost, peat moss, and coarse vermiculite. It can be very pricey, especially if you are filling a number of beds.

Gardeners wanting to grow in raised beds do not need to invest in an expensive soil or potting mix. They can use their own soil in low-sided raised beds. Set up the beds and then take the native soil in the pathways around the beds and mix it with some good quality compost (no more than 10 per cent by volume) and place it in the beds.

If there is not enough soil to fill the beds, you will need to bring in soil. True topsoil is natural surface soil scraped up and transported to a site. Topsoil in many regions is more desirable than the subsoil (the soil layer beneath the topsoil) because natural processes have created a crumbly soil structure that is conducive to good plant growth. However, digging and transporting topsoil elsewhere generally destroys this crumbly structure and nullifies its benefits.

Dr. Craig Cogger, WSU Extension Soil Specialist, recommends sandy landscaping fill as a compromise but notes it will not hold much water and will dry out quickly. (True topsoil in our region generally lacks the crumbly structure found in areas with more rainfall.)

Sandy landscaping fill is sandy soil mixed with organic matter (OM). Probably much of what is sold commercially as topsoil in our region is basically sandy landscaping fill. If you decide to purchase sandy landscaping fill or ‘topsoil’ for your raised beds, ask where the soil came from and what it contains.

Buy your soil or fill from a reputable company. Not all soils sold as ‘topsoil’ should be used in raised bed gardens. They can contain broken glass, too many rocks, wood waste, and other debris. Inspect the topsoil before your buy it and before you accept delivery. You also do not want soil that may have come from an area that was treated with long-term residual herbicides or other chemicals.

Ask if the company has had the topsoil tested or knows how much OM it contains. If the topsoil or landscape fill already contains 10 per cent or more OM by volume (5 per cent by weight), you do not need to add compost or other OM. Dr. Linda Chalker-Scott, WSU Extension Horticulturist, indicates that 10 per cent OM is adequate from a fertility perspective. Adding substantially more OM to the soil contributes to high nutrient levels that can lead to plant health problems. Chalker-Scott recommends before adding organic amendments to the soil in your beds, have it tested to determine the OM content and nutrient levels.

Finally if the soil in your beds is distinctly different from the native soil beneath, it can impede drainage. Cogger recommends mixing the introduced soil with your native soil as you build the bed to create a textural gradient that will allow for better drainage. For more on raised beds consult WSU Extension Fact Sheet FS075E at

Published: 7/4/2014 11:40 AM


written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

Gardeners may be wondering about the numerous new products containing coconut coir being touted as”better than peat.” Being a huge fan of peat moss for use in potting mixes and for use as a soil amendment in our local soils, it’s hard for me to give up this “tried and true” organic matter and replace it with something else. However, there’s two reasons why gardeners like me should try to make this transition.

One reason not to use peat moss is that it has become quite expensive. I would suppose this is a result of higher gas prices and the distance from where it’s harvested in Canadian bogs to where it’s sold in the U.S. The higher cost of peat moss has made peat-based potting mixes prohibitively expensive for many gardeners.

The other reason for gardeners to turn away from using peat moss are environmental concerns about the harvesting of peat from bogs. Sphagnum peat moss, the superior type of peat moss for gardening, results from the slow decomposition of sphagnum moss and the build up over time of the decomposition products in the bogs. Bogs in Canada, Finland, Ireland, and Russia have been developing over thousands of years. Dr. Linda Chalker-Scott, WSU Extension Horticulturist says, “Peat moss is a non-renewable resource whose replacement takes centuries.” Chalker-Scott has expressed concern because peat lands are “bio-diverse ecosystems with important functions in water quality and carbon storage.”

Coconut coir is one material being suggested as a good replacement for peat moss. It’s a byproduct of the coconut processing industry. It’s made from the fibers found between the husk and shell of a mature ripe coconut. The brown fibers are high in lignin and are water-proof. “Coco” doormats and stuffing for automobile seats and mattresses are made from coconut coir fiber.

Researchers have found that coconut coir is a viable substitute for peat moss in potting mixes because:

1. It retains water as well or better than peat moss.

2. It drains as well or better than peat moss.

3. It doesn’t contain weed seeds or disease organisms.

4. Its pH is suitable for growing plants.

5. It breaks down more slowly than peat moss.

6. It’s easier to wet than peat moss, which is notorious for resisting wetting.

7. It’s a processing byproduct, making it a renewable resource.

This spring, I have discovered that some of our local garden centers and nurseries are carrying coconut coir. There are a number of different brands available in the area, most coming in compressed “bricks”. You just add water to the dry, compressed ‘bricks” and their volume expands almost exponentially. Voile’

The fineness of the coir seems to vary quite a bit. Some that I bought is quite coarse with big chunks of coir. Another type has long fibers, but not the coarse chunks. These should work well for amending soil in flower beds, but are not as good as finer textured coir dust for use in potting mixes. I’m hoping that the finer coir dust materials will soon be readily available too… at a lower cost than peat moss.

Another byproduct that may soon become a suitable substitute for peat moss in the horticulture and gardening industry is a different type of fiber. It’s a byproduct of dairy manure. A WSU research team is looking at the feasability of using anerobic digestion to change manure into three useful byproducts. One byproduct is methane for energy generation. A second byproduct is struvite . This is a crystallized solid that’s rich in plant nutrients, such as nitrogen and phosphorous. The third byproduct is a high quality fiber for use in potting mixes.

Research by WSU scientists has shown that the fiber is a viable alternative to peat moss in potting mixes for growing greenhouse plants. Researchers have concluded that after a “post-digestion” treatment the fiber byproduct is “equal to or superior to peat moss when used as a major component of greenhouse potting soil.” This manure bio-solid fiber is not yet available to home gardeners, but hopefully it will be before long. Just think… a manure byproduct can help us be more environmentally responsible gardeners. Wow!

It looks like these two peat moss substitutes will help wean me off of peat moss as soon as they become readily available and included in potting mixes on the garden store shelves. I’ll try to make the change, how about you?

Published: 5/19/2007 2:51 PM



Garden Tips, WSU Extension, Benton County, 5600-E West Canal Drive, Kennewick, WA 99336-1387, 509-735-3551, Contact Us

WSU Extension, Franklin County, 1016 North 4th Ave, Pasco, WA 99301-3706, 509-545-3511, Contact Us
© 2018 Washington State University | Accessibility | Policies | Copyright | Log in