Wayne Schmidt's Potting Soil Comparison Page ..Five brands tested for germination rates and growing speed.

 
Every year I get an early start on the season by growing dozens of plants from seed for transplanting into the garden. Raised under fluorescent lighting (please see FLUORESCENT PLANT LIGHTING for my system,) they let me get a two-month head start. Growing the best plants requires that the best potting mix be used to ensure maximum germination and the fastest, healthiest growth. To determine which potting mix works the best I compared a high-end professional mix from Peaceful Valley Farm Supply against four commonly available brands.

From left to right:

Peaceful Valley Farm Supply Potting Soil

Cocoa-fiber peat, worm castings, bat guano, bone meal soybean meal, soft rock phosphate, greensand, fish meal, blood meal, langbenite (K-mag), bark, diatomaceous earth, perlite, kelp meal, and beneficial mycorrhizae fungi.

 
Ferry Morse Jiffy Mix

Sphagnum peat moss, perlite, lime

 
Miracle Grow Organic Choice Potting Mix

Composted bark, sphagnum peat moss, pasteurized poultry litter, organic wetting agent

 
Hyponex Potting Soil

Hypnum peat, forest products or compost, sand, perlite

 
SuperSoil Potting Soil

Fir bark, redwood bark, sphagnum peat moss, sand, potassium nitrate, triple superphosphate, calcium nitrate, ureaform, ferrous sulfate, dried poultry waste, feather meal, alfalfa meal, kelp meal

Appearance, texture and easy of potting:

For each potting soil I filled a 1-quart plastic pot level to the top with each mix and after placing it in a larger container, soaked it from the bottom with warm water until all of the mix was saturated. It was left in the water for 10 minutes to ensure every fiber had been hydrated then moved to a draining rack for excess water to drain out.

From left to right:
Peaceful Valley, Jiffy Mix, Miracle Grow, Hyponex, Supersoil

Peaceful Valley Farm Supply potting soil is extremely coarse, contains a very high amount of large irregular pieces of perlite and contains many pieces of what looks like long sections of fine plant roots. It scooped and wetted easily and did not shrink very much after its initial soaking.

 
Jiffy Mix This mixture of peat moss and very fine perlite wetted very easily but shrank 30-percent after its initial soak. I had to add more mix and resoak it to end up with the same amount of soil as the other mixes.

 
Miracle Grow This mix looked and behaved like pure peat moss. Even though it felt damp out of the bag it aggressively repelled water. I had to repeatedly stir it into the water to get it to hydrate. Extremely messy and frustrating to use. After draining it shrank even more than Jiffy Mix.

 
Hyponex By far the heaviest of the mixes, either dry or wet. It's a very dark mix containing both medium and fine particles with a very few bits of tiny perlite. The top of the mix in the bag was damp and wetted easily. The bottom was drier and repelled water a little. An extremely rich-looking mix when wet. Very little shrinkage.

 
SuperSoil A medium coarse mix that is a delight to use. It scoops very easily and wets the best of all the potting soils compared. Wet or dry it is the lightest of the mixes, which would be advantageous if it's going to be used in a very large pot that has to be moved around. It's extremely fast and easy to bottom water and drains quickly. Very little shrinkage. Although not quite as rich looking as the Hyponex potting soil, it still looks like it has an outstanding texture for seedling emergence and plant growth.

Water retention:

From left to right:
Peaceful Valley, Jiffy Mix, Miracle Grow, Hyponex, and SuperSoil

The picture above shows each of the potting mixes after they had drained for 12 hours. The Hyponex, fourth from the left, was by far the heaviest and as can be seen retained the most water. Jiffy Mix and Miracle grow appeared to retain equal amounts of water but Miracle Grow's slightly greater weight indicates that it won the tie. The lightest mix was Supersoil, suggesting it might require more frequent watering.

While high water retention may seem to be a good thing, plant roots also require oxygen to grow properly. Jiffy Mix, Miracle Grow and Hyponex ended up being so dense that this might be a problem.

Potting soil comparison methodology:

Six pots were filled with each brand of potting soil, soaked for 10 minutes to thoroughly hydrate them and then planted with a known number of tomato or muskmelon seeds. They were then covered with transparent plastic and placed in an artificially lighted and heated growing chamber until the seeds germinated, at which time the plastic was removed. All together 30 pots planted with 54 tomato and 15 muskmelon seeds distributed equally among the five brands of potting soil were prepared.

Each pot was watered once a week with a solution containing an organic fertilizer (Omega 6-6-6 by Peaceful Valley Farm Supply) and a B-1, boron, iron, manganese, and zinc supplement.

The potting soil brands were rotated through every position in the growing chamber to eliminate the chance that one brand might accidentally be located an a location that receives more light or fresh air.

 
Results:

Germination:

The following picture was taken after all the seeds that were gowing to germinate had germinated:

From left to right there are 6 pots each of Peaceful Valley Farm Supply,
Jiffy Mix, Miracle Grow Organic, Hyponex, and Supersoil potting soil.

Supersoil provided the fastest germination with plants emerging 12 hours ahead of Jiffy Mix, Hyponex and Peaceful Valley Farm Supply potting soils. Miracle Grow Organic potting soil was the slowest, a full day behind the middle group and a day and a half behind Supersoil Potting soil.

Hyponex and Supersoil had the highest germination rates at 87-percent each. Peaceful Valley came in next at 83-percent, Jiffy Mix was fourth at 75-percent and Miracle Grow's Organic potting soil came in dead last at only 67-percent.

At this stage the plants are so young that they still haven't sprouted their first set of true leaves. As such I had assumed there wouldn't be any significant differences in how they were growing. I was wrong.

From left to right: Supersoil, Hyponex, Jiffy Mix and Peaceful Valley Farm Supply potting soil

While looking for which potting soil planted with muskmelons showed the best root development I was amazed that while none of the others showed a single root growing against the side of its container after the first week, the Peaceful Valley potting soil clearly showed an aggressive network of roots on all four sides of its pot. I attribute this to Peaceful Valley inoculating their potting soil with mycorrhizae fungi, which are well known to promote vigorous root growth.

Such rampant root development is good because it creates a much more substantial root base to nourish the plant. One extremely minor drawback is it that this rapid root growth could lead to the plant quickly becoming root bound. I don't consider this a serious issue because simply using a large pot takes care of it. I'd happily trade purchasing a little more of this top-quality potting soil and larger pots for plants with big, healthy roots that are many times the size of the next best option.

 
Watering:

I prefer watering potted plants by placing them in a container that's slightly larger than them and filling it with water so that it seeps into the potting soil very gently from the bottom. This prevents the formation of dry pockets, which can occur with top watering, disturbs the roots the least and avoids digging holes in the surface by water pouring in. For this reason I rate potting soils that quickly pull up water from the bottom higher than those that are so dense that they inhibit the inflow of water. In this regard Peaceful Valley Farm Supply's potting soil is the best with Supersoil coming in a close second. Hyponex was third and barely satisfactory. The extremely high peat component of both Jiffy Mix and Miracle Grow Organic potting soils render them so dense the bottom watering is a painfully slow process.

 
Growth Rates:

After 19 days I compared first how the melons grew, turning the pot in each case so that the melons displayed in the same orientation to make comparisons easier.

From left to right: Supersoil, Peaceful Valley, Jiffy Mix, Hyponex and Miracle Grow Organic potting soil. Supersoil is the clear winner. (I soaked the soil off the roots of the Supersoil and Peaceful Valley potting soils to compare root development. Although the Peaceful Valley soil was larger at the beginning, after 19 days the roots growing in Supersoil were a full 10-percent larger.)

Next I compared how the potting soils did in growing tomatoes. The variety in each row, top to bottom, was: Burpee's Big Beef, Sun Gold, a second row of Sun Gold, and finally Burpee's Supertasty.

The potting soils from left to right are Supersoil, Peaceful Valley, Miracle Grow Organic, Hyponex, and finally Jiffy Mix. (The missing pot in the second row of Sun Gold tomatoes in the Miracle Grow column was because none of the seeds in that pot germinated.)

The results here are not as obvious as with the melons. Although in this image the plants in the Peaceful Valley soil (second column from the left) look slightly larger than those in Supersoil, in person the plants grown in Supersoil were much more robust and had a healthier, darker shade of green to their leaves. If I were choosing which transplants to buy I'd take those growing in Supersoil.

 
Conclusions:

Supersoil's high germination rate and superior growth in both melon and tomato seedlings make it the preferred choice of the potting soils tested. When the additional factors of ease of handling, easy watering, fast drainage and low cost are also considered, it is the clear winner.

Peaceful Valley Farm Supply's potting soil came in a close second. But, the fact that it's four times the price of Supersoil after adding in the cost of shipping and not conveniently available makes it much less desirable. Another downside is that it contained weed seeds that germinated in the sample I tested. This was an annoyance that could be a serious problem for someone whose desired seedlings looked like the germinating weeds. How would he or she know which to remove?

They are many dozens of other potting mixes on the market that I wasn't able to test so this experiment can't be considered definitive. But, it strongly suggests that Supersoil potting soil is a superior product capable satisfying anyone's gardening needs. It's certainly the brand I'll be using in my garden from now on.
 

PRODUCT WARNING!!!

Gardener and Bloome Potting Soil

The year before the above comparison I purchased four bags of Gardener and Bloome's Rose Potting soil and used it to plant 77 pots with zinnia, tomato, melon, Canterbury bell and salvia seeds. The results were a complete disaster. Although I followed my regular routine for growing seedlings, a routine that's worked flawlessly for over 20 years, with this brand of potting soil I only got a 20-percent germination rate, the few plants that came up grew poorly, exhibited many disease symptoms and 30-percent of them died. Upon testing the soil I discovered that in spite of the package advertisements stating that the soil contained a wide spectrum of organic amendments such as bat guano, worm castings, kelp meal, alfalfa meal and so forth, the soil was completely lacking in nitrogen, potassium and phosphorous. Even after adding extra fertilizer the plants grew poorly. I've used many potting mixes over the years and this is clearly the worst. Perhaps I got a bad few bags out of what may otherwise have been a a good production run, but for myself I will never use this product again.

 
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