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What Do 'Compost', Topsoil', 'Humus', 'Composted Soil' and Other 'Dirty Words' Really Mean?
What Do 'Compost', Topsoil', 'Humus', 'Composted Soil' and Other 'Dirty Words' Really Mean?

Q. Hi, Mike! You always talk about using compost for mulch and improving lawns. My local garden center doesn't have compost, but they say their bagged humus has compost in it, and I'll get the same results. Is this true? Thanks!
    ----Andrea; in Hatboro PA
Mike: Can I use "biosolid" compost on my veggie garden? I have used it on flowers and trees with great success. Or should I stick with the compost made from horse manure and fall leaves I get for free near the stables in Fairmount Park? Thanks,
    ---Deirdre in Philadelphia
What is your opinion on the use of mushroom compost in flowerbeds?
    ---Daisy in St. Mary's County; MD
Some people have told me to use composted soil "straight up" for raised bed planting. Others have said l/3 composted soil, l/3 topsoil, and l/3 sand. What do you think?
    ----Erik in Milwaukee
A. I think it's time to create a 'soil lexicon'! This isn't just an exercise in semantics; these terms can be thrown around pretty carelessly at some garden centers, perhaps leading the occasional gardener to spend money on something they really didn't want. So take this (alphabetical order) list with you and make sure you know what you're buying…

BioSolids: A compost-like product made from the sludge at water treatment plants. Very controversial. Technically, human waste makes a fine fertilizer when completely composted, but idiots pour things like used motor oil down their drains, criminals use the sewer systems for illegal disposal of toxic waste, and traces of the antibiotics and other pharmaceutical medications people are using in record numbers are showing up in these 'biosolid' composts as well as in our tap water. Biosolids are not allowed for use in certified organic agriculture, I would not personally use them in my landscape (even on ornamentals), and I do not recommend them. If you need more compost than you can make yourself, look for straight yard waste compost or something like that fall leaf and horse manure mix Deirdre is getting in Philadelphia's Fairmount Park, which I like a lot and do use personally. (See this Previous Question of the week for more details on acquiring the right kind of bulk compost.)

Compost: The rich black soil-like result of decomposed leaves and other plant materials. Compost is high in organic matter content and is the perfect plant food, soil amendment and disease fighter. You want 'finished compost', where few to none of the original ingredients are discernable. A couple of twigs here and there are fine, but lots of wood chips are not. "Leaf compost" should look like compost, not leaves. Unfinished compost should be piled up and left to finish before use.

Composted manure: The rich black soil-like result of decomposed animal poop, pee and the bedding (straw, wood shavings, sawdust or shredded newspaper) used to make the stalls or coops easier to clean. Completely composted manure (also called 'aged' or 'well rotted') looks like compost, but is not compost per se. Composted manures are very high in organic matter, but may be too nitrogen rich to use alone on flowering plants. This is especially true of horse and poultry manures, which often grow enormous plants with few fruits and flowers. Composted manures are best used as specific fertilizers, not as a basic soil amendment. Raw manure should never be used. If you have access to raw barnyard manure, just mix it into a 'regular' compost pile, where it will enhance the finished product greatly; or pile it up and allow it to compost on its own. Never use dog or cat waste in any form; it just isn't safe. (See this PREVIOUS QUESTION OF THE WEEK for more details on using manures wisely.)

Composted soil: Although this term is thrown around quite a bit, it is a misnomer. Soil is already 'done' and cannot be composted. If someone offers you "composted soil", ask if they meant to say compost or topsoil.

Humus*: Latin for 'soil', this generally refers to components of soil that are rich in organic matter, whether from added compost or the natural decomposition of plant material. The top layers of forest soils are rich in humus. In commerce it is an incredibly slippery term that may refer to compost, harvested forest soil, or to nothing at all. "Rich in humus" means the material contains some organic matter, but maybe a lot of inert filler too. Completely finished compost made from mixed yard waste is virtually 100% humus.

Mulch: Anything placed on the surface of the soil to retain moisture and prevent weeds. Mulch does NOT have to be made from wood or bark; and although heavily marketed, such mulches are very problematic. Compost, shredded leaves, pine straw and seed and hull mulches (pecan, cocoa shell, etc.) are vastly superior to wood, bark and root mulches. Don't use rubber mulch—it stinks in the summer and I have concerns over the chemicals used in the production of the tires that are chipped up to make this stuff. (See this PREVIOUS QUESTION OF THE WEEK for more details on mulches and mulching.)

Mushroom Soil: A very specific material used as the growing medium for mushrooms; also called "Mushroom compost", "mushroom substrate", "spent mushroom soil", etc. This mixture of barnyard manures, straw, ground-up corncobs and other ingredients is in a very raw state when used to grow mushrooms; and "fresh mushroom soil" is still very raw, very warm and very odiferous. Allow it to compost until the heat and smell have greatly lessened and it becomes 'aged mushroom soil', a powerful fertilizer. Combine it 50/50 with true compost or dark-colored topsoil for use as a bulk soil amendment. Note: Some mushroom soils may be alkaline; if the pH tests high, add some milled peat moss to bring it down to neutral (a pH of 7) or slightly below. Don't use mushroom soil around acid-loving plants, like blueberries, azaleas and rhododendrons.

Soil: The basic term for what's already there; a.k.a. your 'dirt'. Most un-amended soils contain lots of sand or clay, and their organic matter content is generally very low. And that organic matter content is still low in improved soils compared to the levels in compost alone. A bag that says "garden soil" means nothing more than it probably won't contain hammers, marbles or grilled cheese sandwiches.

Topsoil: Technically this refers to the uppermost 15 to 30 centimeters of a soil; that is, 'the top of the soil'. In the woods, this would be the most humus rich portion; and organic farms depend on deep, humus-rich topsoil for fertility and productivity. In commerce it is a virtually meaningless term. It is not compost in any way. If you really want 'topsoil'; that is, the most humus-rich portion of a harvested soil, look for material that's dark in color. If its tan, it may be worse clay than you already have. Screened topsoil is better than unscreened; the rocks will have been removed and the particles will be smaller, which is good.

* Not to be confused with hummus, which you can eat with falafel. You would not want humus on your falafel. Oy!

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