Asked March 02, 2017, 9:40 PM EST
Washington County Oregon
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Thank you for contacting Ask an Expert in regards to amending clay soil.
“My garden soil seems to be very clayey and for the past three years, all the plants have had poor growth. Does adding organic compost facilitate aeration to the soil? How can I loosen up my soil? Does adding organic mulch like bark dust provides nutrients and aeration to the soil too?”
Your first statement is an excellent observation on your part and your next questions are very good and quite typical in regards to clay soils, so you are not alone. To answer your questions I say, yes; let’s talk and explore; and yes.
Even though, clay has a terrible reputation among gardeners all is not to despair, in actuality clay soils, at least red clay soils which are common to our region are usually richer in nutrients than sandy soils are. “Of the three major soil components — sand, silt, and clay — clay has the highest nutrient content,” says Garn Wallace, president of Wallace Labs, a soil-testing firm in El Segundo, California. Clay soils have a tendency to hold water longer which can be an advantage. Clay soils retain minerals in forms that are readily available to plants and that aren’t water soluble, so rain and irrigation water are less likely to leach them away. The trouble is that clay lacks good porosity ("Soil porosity" refers to the amount of pore, or open space between soil particles. Pore spaces may be formed due to the movement of roots, worms, and insects; expanding gasses trapped within these spaces by groundwater; and/or the dissolution of the soil parent material1.). Its fine-textured particles tend to clump tightly together. Air, water, roots and seedlings can have difficulty moving through it, so crop yields may be lower.
Two notes of warning:
! Always be careful that you do not spade or rototill clay soils when they are wet. Doing so will severely compact the soil and create clods that are very difficult to break apart. Organic matter should be added each year for several years for best results. Since it will be difficult to improve the soil after the plants are added it is recommended that when you are preparing a planting bed for perennial flowers and shrubs, you should incorporate an extra thick layer of organic matter into the soil.
!! After the organic matter has been thoroughly mixed into the soil, a 1-2 inch layer of coarse builder’s sand may be spread on the soil and spade it in. However, do not use sand unless the organic matter has been added first. If you were to do so the combination of clay and sand mixed together can create a cement-like element that would be much worse than the soil you started out with! But, if properly added, sand can significantly improve the drainage of water through the soil by adding an additional porosity factor. Also note, that you will need to add a large quantity of coarse sand (1 part sand:2 parts clay) to make a difference.
Furthermore, sand lacks many of the benefits which organic matter does provide, such as beneficial microbial activity and nutrients for your plants to uptake.
Increasing the soil’s organic matter is the foremost important step toward improving it. Organic matter is also an open invitation for porosity-improving earthworms. By adding organic compost to the clay soil you will aerate the soil, thus improving it. There are many organic soil amendments for improving clay soil, but you will want to use compost or materials that compost quickly. Examples include well-rotted manure, leaf mold (shredded leaves) and green plant material. When we speak of well-rotted manure we want manure that is no longer sharply odorous, but rather smells of earth, a sweet-smelling humus. If you are having a landscaping materials company deliver your compost be sure to have a grand sniff BEFORE allowing them to drop it off. If it does not meet to your satisfaction do not accept the delivery and search for another vendor. If you are lucky enough to live near a farm or horse facility many will give you well-aged manure free for the taking.
Since compost breaks down rather quickly you will want to incorporate it well into your soil. Because clay soil can become compacted easily, place about 3 to 4 inches of your chosen amendment on top of the clay soil and work it gently down into the soil about 6 to 8 inches. In the first season or two after adding organic material to the soil, you will want to take care when watering. The heavy, slower draining soil surrounding your flower or vegetable bed can prevent drainage and water could build up in the bed. Cover the areas of clay soil with slower composting organic materials such as bark, sawdust or ground wood chips. Use these organic materials for mulch, and, as they break down, they will work themselves into the soil below. Do not manually work in these larger and slower composting materials into the soil itself. This could potentially cause harm to the plants you plan to grow in that space. Just let nature take its course and allow them to work in naturally over time.
Not only will this help improve drainage and lighten the soil, it will provide nutrients for the beneficial soil microorganisms which in turn, will also help improve the soil. Before planting in spring, add compost and aged manure. A 2- to 3-inch layer worked into the soil to shovel depth is a good amount. Throughout the growing season, mulch with organic materials like grass clippings, shredded leaves, or additional compost. Since soil microorganisms literally "eat" organic matter, make a habit of continually adding it to your soil. In fact, adding a bit of compost bi-monthly shouldn’t hurt at all. Just remember not to go overboard and smother your plants – allow the layer of organic material on the soil to begin composting before adding an additional layer.
Amending clay soil isn’t an easy task, nor a quick one – it takes time so don't expect overnight results. However, by utilizing the above advice you should notice improvements each year. It may take several years to have the luxurious soil you are pining for and you will be the envy of the neighborhood.
For further reading about “Improving Clay Soils” Fine Gardening magazine has published an outstanding article with photos which you can read from here. Should you need further assistance you can either ask a new question and one of my colleagues will respond or if you prefer you may reply back to this message to correspond with me. Again, thank you for contacting Cooperative Extension Ask an Expert.