Soil Properties and Amendments

Soil is the upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles. It includes organic (from living) material. Please don’t call it dirt! Plants have nutrient requirements from the soil. Some plants require rich soils while others prefer primarily silt, sand or clay soils. Right plant in the right place!

Soils are decomposed rocks and organic materials. Since rocks and organic materials vary from one place to another, so does the soil characteristics. Characteristics of soil include its pH (acid-alkaline) levels, amounts of organic materials, macro and micro nutrient composition, % of sand, silt and clay, porosity (ability to drain) and active microbial population. “Ideal soil” in this area is about 50% mineral and 50% open space for air and water. Note though that this ideal may not be ideal for every type of plant. Sand, silt and loam are all the same mineral but are distinguished by their size with sand being the largest particles and clay being the smallest particles.

The soil in our area of interest is a silt loam. Silt soils, comprised mainly of intermediate sized particles, are fertile, fairly well drained and hold more moisture than sandy soils, but are easily compacted. Loams are comprised of a mixture of clay, sand and silt that avoid the extremes of clay or sandy soils and are fertile, well-drained and easily worked. This combination has good nutrient soil and variable pH levels. It is great for plants…….unless it is too compact or contaminated with toxic substances such as lead and arsenic as much of the city soils are.

The soil texture determines its water holding capacity, nutrient holding capacity, erodibility, workability, root penetration and porosity/percolation rate (Percolation is the movement of water through soil, and the percolation rate is the speed at which that movement occurs.). Considering the extremes, primarily clay soils allow water to runoff without absorbing, locks up nutrients as roots have difficulty penetrating it and is anything but very workable. Sandy soil on the other hand, allows water to run right through taking nutrients with it while eroding with wind and water such as streams and rivers. Our goal with the parks and those residences that wish to participate is to slow the water runoff rate so that less water goes into our combined sewer system decreasing flooding. By knowing our soil, we can make the necessary changes such as adding soil with more absorbtion, adding organic matter, planting natives with roots that will penetrate the soil and create more air spaces to hold water, or including retention basins or water catchments into the designs.

Soil pH is an indication of the acidity or alkalinity of soil. Neutral pH levels (between 6.3-7) are fine for lawns and gardens. At this pH level the nutrients in the soil are readily available to most common garden plants. However, some plants require more acidic (lower than 6.3) or more alkaline (greater than 7) soils to release the specific nutrients they need. pH levels of soil can be approximated with litmus paper or measured with pH meters. Consider the hydrangea. They are not blue or pink as you buy them in the nursery. They become blue or pink when exposed to more acidic (blue) or more alkaline (pink) soils. If you buy a blue hydrangea and plant it in the more common alkaline soils of the area, the flower will turn pink over time. To maintain the blue color, the soil pH must be approximately 5, well below neutral pH of 6.3-7. Some plants will only grow and fruit in very acidic soils including raspberries and blueberries, azaleas, pines and spruces, ferns, magnolias and hollies. Other plants will only do well in more alkaline soils including arborvitae, astilbe, hollyhocks, clover, spirea and yews. Soil pH can be adjusted to suit the plants by adding sulfer (acid) or lime (alkaline) to the soil. Caution though, only small amounts over a longer period of time are needed. Test the soil before applying anything and follow package directions to the letter.

Soil pH can change from one yard to another depending upon how the yards are managed. As an example, my next door neighbor’s yard has a pH along her driveway of 5 while my yard is 5 where my acid loving plants are and 7 everywhere else. She was very frustrated she could not grow grass along her driveway. She had been using road salt which was very acidic to clear her driveway in winter snows. When the salt melted it ran along the edge of the driveway lowering the pH so grass could not grow. Once she corrected the pH level, her grass grew. To maintain that though, she needed to change her cultural practice of using road salt by switching to sand (pH 7) to melt the ice.

To make a soil “rich”:

• test your soil not only for pH but also for key nutrients (nitrogen, phosphporous and potassium) University of Missouri Extension will do this for about $20 The engineers did this for our parks. Soil report identified key needs in each area that we will address in the renovations.
• adjust pH if necessary as that will release key micronutrients that might be “locked”
• correct mutrient deficiencies preferably with time released fertilizer. Nitrogen (for green growth) quickly moves through soil so that is almost always needed, Phosphorous, preferably bone meal (necessary for trees, shrubs and young plants) does not move through soil well so may not be needed, Potassium (chemical symbol is K, is heavily consumed by fruit and vegetables) but can burn plants if too high. Note the capitalized NPK-fertilizer is identified by its relative amounts of the macronutrients Nitrogen, Phosphorous and Potassium (K). Balanced fertilizers are 8-8-8 or 10-10-10. Fertilizers for new growth will have a higher phosphorous level such as 10-20-10.
• add organic matter to improve soil structure, hold nutrients and moisture, and buffers temperature extremes. By applying a 2-4 inch layer on top and then working it 6-8 inches into the soil, the benefits of the mulch will reach the roots.
• correct for drainage. Consider adding drains, planting rain gardens or choose plants that prefer extremes of wet and dry.
• improve soil structure with organic matter. Avoid working in wet soil as that tends sto break down soil structure
• improve soil texture with sand, expanded clay or diatomaceous earth. Adding earth worms works well too.
• mulch wisely with added compost to about 2-4 inches. Avoid creating mulch volcanos-mulch is on the ground, not climbing up the tree trunk.
• use time release fertilizers which will reduce nutrient loss, increases uptake by the plant roots without risk of burning, and is easier on soil microbes. Avoid the fertilizer “step” programs as they apply what is not needed. Apply only what you need.
• match plants to soil (right plant right place!)