Soil assessment basics

Potentilla Gold Drop

A good soil assessment is a critical component to a successful garden. The soil condition will ultimately determine which plants will thrive and which plants will merely survive in your garden. We do not recommend you attempt to add a new layer of topsoil to build your soil structure because you would end up in a constant battle against nature to maintain the conditions you establish for your garden. A better approach is to understand what you already have in place and focus on improving it over time. Selecting the plants native to your area combined with a focus on improving your soil will result in a low maintenance successful garden.

Soil texture is determined by the amounts of sand, silt, and clay. Sandy soil has large and hard particles which provides for easy digging and free drainage. It will dry out extremely fast and will warm quickly in the spring. It is subject to a larger temperature fluctuation through the day because it absorbs and releases heat quickly in the morning and evening respectively. The free draining characteristic does not provide much stability for nutrients and allows them to drain away easily. Silty soil is not as course as sandy soil and consequently retains moisture much more readily. With a silty soil type you will need to guard against compaction. The soil can layer easily which can cause pockets of moisture to develop which can impact plant health in a variety of ways. Soil high in clay content will retain water and is full of nutrients. It is difficult to work and can become compacted very easily. If you let a soil high in clay content dry out then it can become almost impossible to cultivate. Medium loam contains approximately fifty percent sand with the other half being a mix of clay and silt. It is the best gardening soil because it has a good soil structure and retains water and nutrients.

Generally soil structure is comprised of topsoil, subsoil, and parent rock. You should be able to see a color difference between your topsoil and subsoil. The top soil should be dark due to the nutrients and organic matter which is maintains. Subsoil should be much lighter because it does not carry the nutrients and organic matter found in the topsoil. If you cannot distinguish between the two in your garden then it may be an indication that your topsoil is nutrient deprived. You never want to turn the subsoil into the top soil. One other issue related to soil structure is to identify a soil pan if you have one. This is the common name for a layer of compacted soil which prevents air and water from moving to an area below. This is what happens as described above when a layer of silty soil gets compacted. Typically it needs to be broken up through double-digging which is a process my wife has discussed in a previous article.

Soil pH is a characteristic most people do not monitor in backyard gardening. pH is a measure of acidity and alkalinity and is an important characteristic of your soil. pH can range from 0 to 14 with neutral being 7. Lower numbers indicate an acidic soil and higher numbers indicate an alkaline soil. Most plants desire a neutral or slightly acidic soil in a range of 5.5 and 7.5. Adjusting soil pH can be an ongoing maintenance activity so it is best not to battle too hard and just select plants suited for your garden. You local nursery can help you pick hardy plants suited for different pH bases.