Difference Between Compost and Top Soil

Question from Karen:

What is the difference between compost and top soil and do you need both when amending the soil in your yard?

Answer from Pat:

When you purchase something called “top soil” this should be the mainly composed of the stuff that is the top layer of earth that is made up of pulverized rock, microbes, water, air and humus and is found on top of the ground all over the world and is the stuff plants grow in. In your garden it is the soil that is in your flower beds and all over the ground under your lawns, trees, shrubs or whatever you are growing and is what they put their roots into. Soil goes down to bedrock, top soil is the top layer of it. When companies sell top soil they used to sell river bottom silt that they have mixed with composted amendments (such as manure or aged woody products sometimes mixed with some nitrogen to make it rot and perhaps other nutrients as well.) What I have just described is what top soil that you purchase SHOULD be. However, I have just purchased 2 cubic yards of “top soil” with which to fill a raised bed in my garden and I am distressed to find that it has very little actual soil in it and it is fluffy stuff that is mostly woody product that I hope is fully rotted but may not be. I think what is happening is that companies are running out of river bottom silt or other forms of top soil to mix with amendments. I have heard of top soil being sold that is mostly nothing but sand.

Compost on the other hand should be fully rotted organic material that has enough nitrogen in it so that it will rot even more in the soil and thus, through microbial action, releasing nitrogen to plant roots. If you own a compost pile or have learned how to make compost in a bin or other structure, you know how to make compost yourself by layering or mixing about half “nitrogenous” waste, such as grass clippings, green leaves, and wet kitchen stuff like papaya peels, with about half dry “carbonaceous” waste like dry leaves and woody stems of plants that you have chopped up. Then you keep it damp and toss and turn it. It gets hot and rots and when it cools down or if you have a slow compost pile, slowly ages, and once you have something that smells good and you can’t tell what went into it, that’s compost. You can also purchase compost bagged or trucked. Or you can add aged manure into your soil.

When amending a flowerbed or vegetable bed prior to planting you add compost (and also fertilizer). No you do not have to add top soil. You already have top soil. It is what you are walking on in your garden and that is what you are amending by adding more organic matter (compost) to it. When you add the organic matter called compost and combine it into soil, it rots further and becomes what we call “humus”. You cannot buy humus in a bag or a truck, even if it says so on the package. Humus is what compost becomes after it has become an integral part of soil. We should all try to increase the humus content of our soil by adding compost into it. We can also mulch our soil by putting mulch on top to cut down on weeds and maintain moisture in the ground. Mulch is not composted and not rotted. It must never be dug into the ground or it will subtract nitrogen from the soil in order to rot, thus killing plants or turning leaves yellow. Mulch eventually rots and only then will it become part of the soil. Once it has rotted it’s fine to dig into the ground or one can simply add more mulch on top.

Did you attend any of my lectures on soil this year? I gave a talk on soil at the Master Gardener Seminar in San Diego in March 2014 and also at the San Diego Rose Society in March 2014. If you know a large group that would be interested in this subject, I will be giving lectures again in September. The talk I give is called “Our Poor Pitiful Soil and How to Fix It.,” and it covers the whole subject of soil in detail.

Comments

  1. Thanks for the explanation regarding top soil. I’ve seen the bags of Top Soil at the nursery and always think “I should be buying these to improve my soil.” Now I see that compost is what’s needed.

    • Thank you for your feedback. I am glad you get the point. I deplore the current practice of mislabeling bags of soil amendment by manufacturers. It’s very confusing for gardeners. Make sure the bags don’t include sand and don’t include raw, un-composted wood shavings. Both these are detrimental to soil—sand because it changes soil structure and shavings because they rob nitrogen from the soil in order to rot.

  2. Mary Harrington

    Thank you Pat. I’m about to buy a load of something from one of the landscape supply yards and didn’t know if it should be compost or their veggie/flower mix – I think after reading this I’ll just go with compost. I have clay soil in Van Nuys.

    • When landscape supply companies say something is “veggie-flower mix” that usually means it’s designed for filling a raised bed and usually it drains much too fast. If you want something to improve clay soil by digging it into the ground, compost is best. Stable manure also works if you apply it to the top of the ground in fall, allow plentiful rain to fall onto it and then dig into the ground in spring. If no rain, then it contains too much salt for clay soils. If you were able to do this for your whole yard year after year you would eventually have amazing soil. I’ve seen it happen. Another great thing to do is to plant a winter cover crop such as crimson clover in September or October, let it grow all winter and in spring turn it over into the soil. You would be amazed at what this does to improve clay soil. In summer a good cover crop to use is Alfalfa. Plant in spring and dig into the ground in fall.

  3. My husband and I are trying to get our yard together so I appreciate you pointing out the difference between compost and top soil. I like how you point out that top soil acts as a layer of earth to support your plants and other materials. I imagine that in order to have it give the support that is needed, it will need to be made of high quality material. I’ll be sure to keep these things in mind!

    • Thank you for your comment. Top soil is supposed to be what the name suggests: Arable earth from the top layer of our planet. Unfortunately what is now sold under that name is often not the real thing but a man-made concoction of sand mixed with ground wood or bark. So try to be careful where you purchase top soil and get the real thing.

  4. Hi Pat. I am a just retired and shifted to a rural area in Victoria Australia. I am pulling down a shed that is at least 50 years old and ripping up the concrete floor. I want to grow vegetables in this spot. I have access to unlimited amounts of manure from my local horse stable. This manure contains both straw and sawdust. Is this enough to build a new garden bed, or will I need to also use topsoil. Is there anything else I need to add to the soil that has not seen any sun for over 50 years. I look forward to any advice that you or your readers could give me.

    • I have personal experience with a similar situation. Remove every bit of concrete. It is highly alkaline and can kill plants simply by being too alkaline. After removing all the concrete and gravel. (Make sure there is no gravel left since this can destroy proper drainage), then pile manure here with sawdust, straw and all. After aging and going through winter rains for a couple of years, you will be able to grow wonderful vegetable crops here, even without adding any minerals. I grew the best tomatoes in similar manure that was aged 2 years in that spot. Eventually you might need to add fertilizer, but maybe not at first. In my case, the manure pile was roughly 2 feet tall and was surrounded by a raised bed of planks that were two feet wide. Under that was the native soil.

      Additionally, in centuries past in some parts of Europe people sometimes gardened on top of old manure heaps that were flattened out on top for the purpose, with paths between, so they were like raised beds without sides, but these old gardens did have year-round rains.

  5. Just found your site. A miracle…getting ready to amend my soil. Going to check the PH…an go from there. Can I “work”some peat moss in now?

    • Glad you have purchased on of my books. They are all helpful but that sounds like an old one because it recommends peat moss. Today peat moss is no longer recommended as a soil amendment since it is a non-sustainable resource. I recommend you use aged manure or compost, either you own home made or purchased by bag or truck load.

Leave a Reply