Soil - More Than Just Dirt

Soil is the most overlooked factorwhen planting a tree. Trees are living organisms that require air and water much like we do. They rely on the soil to obtain the air, water and nutrients needed to thrive. Soil is a complex mix of mineral particles, air, water, and organic matter. The characteristics of the soil determine how hospitable it will be to the tree. Species that need light, fine soil should not be planted in rocky or clay type soils. Tolerance for acidity or alkalinity varies from species to species - it is an important factor in selection of the new tree.

Two important characteristics of soil are the soil type and the pH balance. Learning about these two factors and knowing what is present at your planting spot will aid you in making a good choice of tree to plant in that soil.

Types of soil can range from extremes of clay soil at one end to sand at the other. Plants need a balanced soil to allow air and water to circulate. If you have too sandy a soil, it drains too fast and the tree does not get enough moisture. Clay ' soils can stick together and smother the roots. The best soil for good planting is a nice loam, which falls somewhere in the middle. Unfortunately, you don't see much of that nice loam in Stillwater. The most common soil in Stillwater is clay. In new construction areas-such as new subdivisions-another problem is highly compacted subsoil, which is extremely difficult to grow trees in.

How do you know what kind of soil you have? Take a handful of soil from your garden and rub between fingers and thumb. Clay soil feels sticky and will roll into a ball that simply changes shape when pressed. It feels slick to the touch-squeeze a ball of it in your hand and it will ooze through your fingers in ribbons. Sand is coarse and gritty, while silt feels silky smooth. Limestone has a dry crumbly feel and a grayish-white color, while peat is black and moist.

Clay
Soils composed mainly of clay have very little pore space between particles. Water 'soaks in slowly and flows through slowly as well. Clay soils retain water longest and its heavy density tends to hold nutrients well - that is the good news. But, because drainage is slow, plants can become deficient in soil air if over watered. 'iy In wet weather (or when trees are over watered) clay soil prevents moisture from draining correctly, and the tree can literally "drown" because it cannot get enough air. Clay is slow to warm up in spring. Since clay does not absorb sunlight easily, clay is slower to bring warmth to plants when they need it. Tree roots can have difficulty reaching down through clay soil.

Clay can also pose significant problems in hot Oklahoma weather. Clay has a significant proportion of very tiny particles. Because the particles are so minute, they tend to stick together when wet. When clay soil dries out in hot weather, the particle groups separate. The soil then cracks, more moisture evaporates, and the hot, dry situation becomes even worse.

The clay soil in Stillwater is often a chief culprit of tree problems. There are ways to deal with clay soil. While you can amend the soil, it generally would have to be repeated over time - not a good option when dealing with a tree that may live longer than you do. The best way to deal with clay soil is to choose clay-tolerant. You can also help your tree along by using the planting instructions shown If page 17 and by proper watering methods.

pH balance. pH is a measure of relative acidity or alkalinity, based on a 14-point scale. A 7 would be neutral, higher numbers are alkaline and lower numbers are acidic. Knowing the pH of your planting site is important because extremes in pH can interfere with the ability ofthe tree to absorb nutrients. The soil's acidity alkalinity affects the availability of certain nutrients. If pH readings are significantly greater in either direction, key nutrients are tied up in the soil and not available to the roots. Fertilizers won't help because their nutrients, too, will by an over acidic or over alkaline soil.

For instance, one problem caused often by the alkaline soil in Stillwater is chlorosis. Many of our pines and several oaks including water oak and pin oak suffer from chlorosis. Symptoms of chlorosis show as yellow new growth. In milder cases, areas of yellow show up between the dark leaf veins. In severe cases, the entire leaf turns yellow and can cause severe growth mutations in the tree. Chlorosis usually indicates an iron, manganese or zinc deficiency. Usually this deficiency occurs in an alkaline soil that renders existing iron unavailable to the plants.

pH of your garden soil, you can do a fairly simple test with a kit from your local garden center, or you can request a more complete analysis from your county Extension office at 744-6432.

Plant trees adapted to your climate and soil type without attempting to significantly alter your soil. If your soil is distinctly acid or alkaline (as is much of Stillwater) you may be able to moderate the extremes with soil treatments. But the effect won't be permanent, and in time you'll need to repeat the treatment over and over. This is impractical among permanent tree plantings. Most trees will outgrow any "improved" soil within a few years.

The best way to deal with extremes of soil type and pH is to select trees that do well in those types of soil. A list oftrees that will do well in Stillwater is discussed on page 21.