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FAQs: Plants & Soils

Topics listed pertain to what plants need from soils to grow and what symptoms may reveal soil problems

Do certain plants like certain soils?
The answer to this question is, yes. And the effort to better understand the cultural practices (water, maintenance, pest control) and site requirements (light exposure, soil type, pH, drainage) to grow a particular plant, will pay off. Cultural information for most plants is easily obtained and usually well known by experienced nursery/garden center personnel. The site requirements like light exposure are also fairly well known for most plants. However, soil conditions may vary considerably in a residential landscape. Matching plants to soils is not always easy. Soil pH is one place to begin in matching plants to soils. The soil's acidity and alkalinity is measured in pH units. More acidic soils in the pH range of 5.0 to 6.0 are preferred by certain plants like potato, blueberry, rhododendrons/azaleas, camellia, holly etc. and they are more productive and hardier when the acidity is corrected. Likewise, there are plants that prefer or tolerate more alkaline soils. Fortunately, most plants fall in between and do well in slightly acid soils (pH 6.3 to 7.0). The point is to know if you have a plant with special soil requirements so that soil preparation, including what amendments are added, is likely to favor it's growth.

Soil type is another variable factor. In one spot you might have a very nice silt loam that is fairly well-drained and overall, productive for a vegetable garden. Then, not but twenty feet away, you could run into a pocket of silty clay or clay loam that is poorly drained and compacted, so only the weeds do well. If you take on the task of improving soils by adding amendments like sand, expanded clay, diatomaceous earth, compost or sphagnum peat moss, then the effort will pay off in the long haul. It might take several seasons to get it to the point where the soil remains friable, well aerated and easy to work. The addition of organic matter and other materials does not give you an instant healthy soil. Together with the action of rain, soil microbes and earthworms as well as, some turning, mixing and time, the soil will begin to take shape.

For many of our cultivated plants, there is no substitute for good soil, however, some plants are better adapted to lean or poorer soils. If it seems impossible to improve the soil to any great degree through amending and mixing, the other approach is to leave the soil alone and try to match the plant to the present soil type. The use of native plants (plants that are indigenous to your area and climate) is a current trend in gardening and one that should be considered from the perspective of the soil. Native plants are often found in relatively unproductive soils, able to withstand drought, heat and cold as well as poor nutrient supply. In contrast, when native plants are grown in very fertile soils, they tend grow so well that the plant is hard to recognize compared to the wild form. Prairie grasses are a good example and these sometimes end up growing very tall only to fall over at a certain point because of a strong wind or rain storm. Plants that grow too well because of a highly fertile soil should be moved to a leaner soil where soil moisture fluctuates, nutrients are not prevalent and the root system must struggle to take hold. A simple soil test will tell you how fertile the soil is and the particle size analysis will help you to know what soil texture (clay, silt loam, etc.) is present. This will assist you in knowing what to do to make it either more or less productive. Plants grown in the right soil with the right level of fertility will make all the difference in the growth pattern and performance.

How does soil acidity affect plant growth?
When you measure the soil pH, this evaluation is aimed at determining whether the soil is acidic or alkaline. Acidic soils have a pH less than 7.0 on a logarithmic scale ranging between 1.0 and 14.0. Anything less that 7.0 is acidic and likewise, anything greater than 7.0 is alkaline. Plant growth is generally best when the soil is in a pH range of 6.3 to 7.0, but the values are relative to the method used to measure the soil pH. Different methods include using water, a buffer solution or a salt solution to mix with the soil. The pH reading is affected by these media and the results need to be evaluated on the basis of what method was employed and how this relates to the plant/crop you want to grow.

What gardeners are most interested in is whether the soil is going to require an amendment to bring the pH back into the acceptable range for the given plant/crop in question. Some plants are tolerant of a wide range of pH values and do quite well at both ends of the pH range. In other cases, certain plants like potato, blueberries, rhododendrons and others are more productive in acid soils where the pH range is in the area of 5.0 to 5.9. If the soil pH is higher than this, then we commonly use a soil acidifier like sulfur or ammonium sulfate to move the pH over time back down into the acceptable range. It may take several applications to do this depending upon the degree of change needed and the material you are going to use. Typically, if the soil pH is too high or too low, an annual application is necessary to stabilize the soil through the growing season. If the pH is too acidic for the plant/crop in question, then a liming materials is needed to raise the pH. Lime applications are not required unless the pH is too acidic. "Sweetening" the soil with lime as a regular gardening practice will throw the nutrient availability of other essential elements out of balance if it is done without regard to knowing the need for lime and the amounts to apply. Only by doing a soil pH test can this be determined accurately.