What is soil? What is potting mix? What is Compost? When to use each and why.
Soil contains 5 ingredients: Mineral particles, water, air, living creatures, and dead organic matter. What plants need to get from soil is: minerals (from dead organic matter and sometimes from the mineral particles), water, oxygen (because every living cell respires, using the air’s oxygen and releasing its carbon dioxide), and sometimes symbiotic relationships with living creatures (such as mycorrhizae—which rarely need to be purchased).
The mineral particles in soil range from tiny clay particles to the larger sand particles, with the in-between-sized silt particles. The best for plant growth is a mix of the three, with more sand than the other two, such as sandy loam, though other mineral particle proportions can be improved by adding organic matter.
The nutrients plants need to get from organic matter are largely nitrogen, phosphorous, and potassium, which are released from dead organic matter as it is digested by soil creatures, including soil-dwelling bacteria earthworms, fungi, and actinomycetes. Plants don’t need the complex molecules of organic substances, just the simple elements.
In nature, organic matter enters soil when plants or animals die or when animals urinate or leave manure. Most organic matter falls on the surface, then earthworms and other creatures carry it underground. Burrowing animals also produce some organic matter when they die or defecate underground, though animal burrows are also relatively near to the surface. Nutrients from this organic matter seep deeper with rainfall or artificial watering and are also left behind in deeper soil when plant roots die there. In nature, plants expect a gradient between the amount of nutrients from organic matter and the amount of oxygen, from greater amounts at the surface to lower amounts in deep soil. (There are fewer living creatures in deep soil, simply because there is less oxygen, so if organic matter is down there, nothing will digest it to provide plants with nutrients.)
Organic matter helps the soil hold onto more water longer and still have air spaces. All organic matter provides some nitrogen, phosphorus, potassium, and trace minerals. Not all organic matter is rich in nitrogen, which is the nutrient most likely to be low in local soils. For growing vegetables, we want to add more nitrogen than, say a for a shade tree or an ornamental shrub, because we want vegetables to grow big and grow fast. Some organic materials are so low in nitrogen that soil creatures will not be able to digest them unless they also use some nitrogen from other materials in the soil, such as nitrogen fertilizers we have added. Low-nitrogen organic materials include sawdust and shredded bark. These are high in complex plant carbohydrates, such as cellulose or lignin, for which we use the shorthand “carbon.” Others are high in nitrogen. If an organic material does contain plenty of nitrogen, we consider the organic matter to be fertilizer, however we need to be careful not to use too much of high nitrogen materials, since they can burn plant roots.
When we consider organic matter for adding to soil, we speak of the carbon/nitrogen or C/N ratio. Ideally we want to dig in some organic matter that is about 30/1. This will improve the water and air capacities of the soil as well as providing nutrients. The C/N ratio of sawdust or shredded bark is in the range of 200/1 to 700/1. That of garden wastes is 25/1 to 80/1, depending on whether it is fresh and green or dry and fibrous. That of kitchen scraps is about 17/1 and fresh animal manure is also quite nitrogen-rich. A good way to balance these widely varying materials is to compost them—mix them up and let them rot together for a while. Ideally, we’d mix materials to aim for a finished C/N ratio of 30/1. (We call mixtures of aged organic matter compost no matter what the end C/N ratio might be, but what went into it will determine how high the nitrogen content of the finished product can be. When a compost is made of only garden waste, which is modest in nitrogen content, you probably will need to dig another source of nitrogen into your soil as well.)
If you want to increase the organic matter in your soil, buy materials that are labeled “amendment” or “compost” and dig a layer into the soil surface along with any fertilizer you might be using. (You can also spread organic materials on the soil surface as a mulch. For vegetables you want to use s small-particled mulch, or something that will decay relatively fast, not big bark chunks that must be raked aside before you can replant.)
Potting mix, container mix, landscape mixes and similar products are often labeled “soil,” which they are not. They most often get their bulk from sawdust or shredded bark, that low-nitrogen stuff. (And plants don’t really need that much organic matter of any type. Three percent is usually plenty.) Potting mixes may also contain mineral particles that are inert or nearly so as far as the plants are concerned, such as perlite, pumice, or even pea gravel. They may contain small amounts of aged manure or worm castings, which is a good thing, but which does not make them adequate to use as garden soil. They are typically sterilized, so as purchased they do not contain any living creatures. This is fine if you are growing houseplants, but not ideal for garden soil. They may also contain chemical fertilizers or even pesticides—read the label carefully.
If you need to fill an empty garden bed or raise the soil level in your plot significantly, the best idea is to raise it using plain soil, such as sandy loam, then dig some compost and, if you like, other organic fertilizer, into the top 6-8 inches of the soil. (or you could stop adding soil at 8 inches from the top, dig a couple of inches of compost in, then add 8 more inches and dig in twice as much compost as you just did.) This imitates nature, in that the bed will have more organic material and higher fertility near the surface, less deeper in the soil. . (If you are making a Dearborn garden bed much deeper than it was, or just adding soil to raise its level in the bed, it would be best to take out the old soil, which someone has been amending and fertilizing for years, add the fresh sandy loam, and then put the old soil back on top, and then dig in some amendment and fertilizer.)
Potting mix or container mix are best used in a container 2 feet or less deep, the kind with one or a couple of holes for drainage. They are great for this use, water-retentive and providing good drainage. However, after a while, container mixes collapse. This is when the wise gardener will repot the plant in fresh mix. Potting or container mix are not designed for beds with bottoms open to the soil, or for any use that is over 2 feet deep. When a potting mix is used in a deep raised bed, a problem will ensue in a year or two when the stuff collapses. The plant roots will then lack air, and the bed will need lots of fertilizer to supplement the carbon-rich wood products that remain. The gardener may spend a lot of money adding more potting mix and fertilizer, or dig it out and start all over with fresh potting mix or, better idea, soil. There are so many names of products sold in bags now that it becomes confusing, but if you stick with potting mix for containers two feet or less deep, and, for garden beds open to the soil beneath them, use soil with organic matter (well-made compost, soil amendment, maybe some extra fertilizer) dug into the top 6-8 inches, you can't go wrong.
I explored the web site of Lyngsø Garden Products (in the SF Bay Area) which is where we have obtained the turkey manure-based compost for the past couple of years. They sell sandy loam, and have a very nice explanation of what it is and what it’s for. They also sell many other “soil” products. I noted that some of them were a lot cheaper than sandy loam—probably because they contain so much sawdust or shredded bark. They also sell Organic Diestel Structured Compost, the turkey-manure-based compost. It’s very pricy, and worth it I think. Lyngsø sells by the bag or will deliver as little as a cubic yard of material for a flat fee of $85. They are in San Carlos.
Now let us consider for a minute how volume of these materials is calculated. Soil, amendment, compost, etc. are sold by the cubic foot or cubic yard. Bags of soil, compost, etc. for outdoor use contain 1 or 2 cubic feet. Any more and you could not lift them. There are 27 cubic feet in a cubic yard (3’ x 3’ x 3’).
A bed that is 8 x 8 feet and 2 feet deep requires, as an example, 128 cubic feet, or about 4.74 cubic yards of soil to fill, or a bit less to give yourself headroom and room for some amendment, say about 4.25 cubic yards.
A final tip: When adding a material different from what is below it, it is always wise to add a layer a few inches thick first, and mix it with the layer below it, then add the rest. This avoids having an abrupt transition, which water may not readily pass through.
Sandy Loam is $62.00 a cubic yard, $3.50 /sk (sk, for some reason, means cubic foot bag)
Organic Diestel Structured Compost is $145.00 a cubic yard, $6.50 /sk
Note that the information about filling garden beds with soil, not potting mix is also explained in the soils chapter of my book Golden Gate Gardening,