The in your garden plays a huge role in determining how well plants grow. Different plants are adapted to different types of soils, and growing them in the wrong type of soil negatively impacts growth. Understanding the different properties of soil, and how they affect your plants, helps you select the best plants for your garden. It also serves as a guide for whether or not to amend the soil and how often to fertilize and water.
When gardeners talk about soil texture, they're referring to the size of particles in the soil. Particle sizes are categorized as clay (small particles), sand (large particles) or silt (particle size between clay and sand). If the soil is mostly composed of tiny particles, it is a clayey soil. If the soil is mostly composed of large particles, it is a sandy soil. Loamy soil has roughly equal parts sand, silt and clay.
Soil texture affects how well the soils retain water, how quickly the soil drains and the speed at which soils warm up in the sun. Clay soils do not drain well, hold high amounts of water and warm up slowly. Sandy soils drain very quickly, do not hold water well and warm up quickly. Loam and other soils that fall in between sand and clay have varying characteristics depending on the size of the particles.
Different types of plants do best in different soil textures. Citrus trees (Citrus spp, U.S. Department of Agriculture plant hardiness zones 9 through 11), for example, grow well in sandy soils because they need loose soil that drains well and warms up quickly. Clay soils would be too heavy for citrus trees, and hold too much water around their roots. Some plants, however, grow well in clay soils, especially if you loosen the surrounding soil before planting and add organic amendments to improve the soil structure. One example is the Tartarian Dogwood (Cornus alba, USDA zones 3 through 7), which adapts to most soil conditions.
Soil structure describes how the sand, silt and/or clay particles are arranged in the soil. It also refers to the pores, or spaces, in the soil, as well as the soil particles' ability to group together and form aggregates. The soil structure affects drainage, water-holding capacity, how much air is in the soil and how easy it is for roots to grow.
- Good garden soil typically has a "granular" structure, with several sizes of particles and aggregates and about 50 percent pore space.
- Heavy and compacted soils develop a "platey" or "massive" structure, which allows very little drainage and has few pores.
- Sandy soils have a "single grain" texture, which drains quickly but since the particles and pores are all about the same size it doesn't retain water well.
- Soils with "blocky" or "prismatic" structures have larger aggregates and pores to allow moderate drainage.
Plants typically grow best in soils that contain 40 to 45 percent loamy soil, 5 to 10 percent organic matter, and 50 percent pore space occupied by both air and water. Soils that have a platey or massive structure are hard for plant roots to penetrate and will stunt plant growth. Soils with a grainy texture don't hold water well, and plants that aren't drought-tolerant wilt easily in these soils.
You can improve soil structure for most garden plants by adding organic matter. Organic amendments improve the water-holding capacity of sandy soils and the drainage in clay soils. For most planting beds, apply by spreading a 1- to 2-inch layer of organic matter, like well-rotted compost, over the planting bed and working into the soil to a depth of 6 to 8 inches.
Keep in mind that some plants grow best in unamended soils. Blanket flower (Gaillardia x grandiflora, USDA zones 2 through 10), for example, needs well-drained soil with low fertility, and grows best in sandy soils without organic amendments. Other plants, like daylilies (Hemerocallis spp. USDA zones 3 through 10, depending on cultivar) thrive in a variety of soils, including sand or heavy clay.
The cation exchange capacity, or CEC, describes the soil's ability to hold positively charged ions. This is important for gardeners because CEC affects nutrient availability in the soil. The smaller the particle size, the higher the CEC of a particular soil. This means that clay soils hold nutrients longer than sandy soils. Gardeners can add larger amounts of fertilizer less frequently to clay soils because the nutrients will stay in the soil longer. In sandy soils, gardeners can fertilize more frequently using smaller amounts of fertilizer, since the nutrients leave the soil quickly. You can check the CEC of your soil with a soil test.
The pH scale describes the acid/alkaline reaction in soil. Soil pH directly affects plant growth because it helps determine the availability of nutrients. Soils with a pH of 7.0 are described as neutral. A pH between 0.0 and 6.9 is described as acidic, and a pH between 7.1 and 10 is described as alkaline. Most soils have a pH range between 4.0 and 8.0. You can check the pH of your soil with a soil test.
Most plants grow well in a pH range of 6.0 to 7.0, though some plants thrive in different pH conditions. Plants like Japanese barberry (Berberis thunbergii, USDA zones 4 through 9) and Sargent crabapple (Malus sargentii, USDA zones 4 through 8a) grow well in neutral or slightly acidic soil, and also tolerate alkaline soils. Other plants, like azalea bushes (Rhododendron spp., USDA zones 4 through 9, depending on the species), require acidic soil -- pH 4.5 to 5.5 -- to thrive and won't grow well in soils that are neutral or alkaline.