Soil is a subterranean ecosystem that supports a highly diverse complement of organisms symbiotic with, and essential to, aboveground plants and animals that either directly or indirectly supply energy in the form of carbonaceous materials that are the products of photosynthesis.
In terms of ability to grow trees, the most important aspects of soil to consider are: texture and structure, soil reaction (pH) and natural fertility. The best means of assessing these factors is by observation and testing. Another source is the USDA Natural Resource Conservation Service (formerly the Soil Conservation Service), which may also have useful information about local soil types.
The texture of a soil refers to the relative proportions of sand, silt and clay, while structure refers to the aggregation of particles in the soil. Indirectly, soil texture has a tremendous influence on natural fertility because it implies something about the soil’s nature, especially how it will react to fertilization. For example, coarse-textured soils are well-drained and nutrient-poor. In general, coarse soils are fairly inert and lack a favorable chemistry for nutrient retention. Nutrients applied to coarse soils tend to wash right through when it rains. However, trees growing on coarse soils tend to respond readily to fertilization, even though the effect is short-lived. Thus, a successful fertilization strategy on coarse soils is to apply supplements lightly, but often.
Mineral nutrients, the stuff plants use, exist in soil either as salts or, when dissociated by water, as charged ions. It is these positively or negatively charged nutrient ions that roots use. If a soil is not able to provide adequate sites for these free-floating ions, holding them for use by plants, they are quickly lost to leaching. The measure of a soil’s ability to hold ions in a form available to trees is called cation exchange capacity (CEC). Fine-textured soils, composed of silt and clay, are not as well-drained, but tend to be more fertile because they have a higher capacity for good fertility, as measured by CEC.
Soil particles tend to have a predominantly negative charge. As a result, fine-textured soils have more negatively charged sites and a higher overall CEC than coarse soils. One of the reasons for this is that fine-textured soils, and clays in particular, have many times more particle surface area than do coarse soils. Surface area translates directly into nutrient-holding capacity. A good soil is a reasonable trade-off between drainage and nutrient-holding capacity. Unfortunately, in landscapes surrounding houses, nutrient-holding capacity is often the victim of good drainage.
The structure of a soil refers to the way particles are aggregated. It is more a description of the physical characteristics of the soil than its chemistry. Soils with good structure usually have higher proportions of silt and clay to sand. They are also soils that have a good supply of organic matter and an adequate supply of calcium.
In clay-based soils, good structure improves aeration and drainage. In coarse, sandy soils, good structure improves water-holding capacity. In clay-based and sandy soils, organic matter will dramatically improve soil structure while also increasing both CED and moisture retention. Thus, an easy way to improve soil structure and chemistry in landscapes is through the application of organic matter.
Probably the most common measure of soil chemistry is pH or the relative acidity or basicity of a soil. The term pH refers to a solution’s potential hydrogen. It is the logarithm (base 10) of the reciprocal of the hydrogen ion (H+) concentration. Since pH is actually an index that is based on large numbers, a logarithm is used to reduce the distance between values on the scale. When using a base 10 log, each number on the index is larger or smaller by a factor of 10. Thus, the higher the H+ ion concentration, the lower the pH and the more acidic the soil: pH = log 1/H+
Ions are rarely without a counterbalancing mate that carries an opposite charge. This is especially important to remember, because the fertilizers we use to improve soil chemistry are salts of mated ions. In the case of hydrogen, the mate is the negatively charged hydroxyl ion (OH). When H+ and OH combine, they form water (HOH or H2O). Pure water is a perfect match of mates the number of H+ ions (actually ions that carry a positive charge are called cations, and those with negative charges are called anions) equals the number of OH anions and is neutral. When there are more H+ cations than OH anions, the solution is acidic; when the reverse is true, the solution is said to be basic.
Soil pH is expressed on a logarithmic scale as a value between 1 and 14; and since the value is a reciprocal (that is, 1 over the actual number), the lower the number the higher the acidity by a factor of 10. A value of 7 is considered neutral—neither acidic nor basic (pure water)—and the higher the number, the more basic the soil. So, for example, a soil with a pH of 5 is 10 times more acidic than a soil with a pH of 6, and 100 times more acidic than a soil with a pH of 7.
An accurate measure of soil reaction, or pH, is indispensable to landscape managers who want to use fertilizers. Soil pH affects the availability of most nutrient ions by causing those with positive charges to leach under acidic conditions, while negatively charged anions tend to be locked up when a soil is too basic. Also, different tree species have different pH requirements.
In an acid soil, the negatively charged exchange sites on soil particles are covered with tenaciously held cations of iron (Fe++) and aluminum (Al+++). Thus, nutrient cations, like potassium (K+) and magnesium (Mg++)—without mates on soil particles—leach through the soil. Under basic soil conditions, strong, free-floating cations of Al+++ and Fe++ combine with nutrient anions, locking them up and making them unavailable to plants. For these reasons, soil pH is a delicate balance that has a tremendous bearing on the health and productivity of trees in the landscape. In general, most conifers prefer acid to slightly acid conditions (a pH of 5-6 by the soil-in-water method), while most hardwoods prefer slightly acid to slightly basic conditions.
Soils in humid areas of the country tend to be acidic and may need to have the pH raised, while soils in more arid regions tend to be basic and may need to have the pH lowered. This is a general rule, however, since soil reaction is more a function of the parent material of the soil and its history of land use in a particular area than climate.
The only way to know pH is by direct measurement of soil slurry in pure water. The most inexpensive test kits are the colorimetric tests. The user mixes a test soil with a reactive solution, then the resulting color is compared to a chart where color is associated with pH. These colorimetric tests are perfectly adequate for most applications. Other field-testing alternatives are considerably more expensive for a level of accuracy that is beyond what most users require. Regardless of the testing method you choose to use, subsequent measurements are consistent only when compared to similar testing methods. If you use the colorimetric system, then stick with it.
Soil pH is a measure of acid intensity. It does not reveal anything about the amount of acid in a soil. Consider the analogy of two vessels, one large and the other small, filled with water of the same temperature. To change the temperature in the vessels by an equal amount, it would require more energy to do so in the larger than the smaller vessel. Temperature in this analogy is the same as pH. The amount of energy required to change temperature is analogous to what is known as the buffer capacity of the soil.
Buffer capacity is a measure of a soil’s ability to resist a change in pH. As a general rule, the more finely textured a soil (e.g. clay) the higher its buffer capacity. A sandy soil of the same pH, with a much lower buffer capacity, will require considerably less base or acid-forming agents to affect a change in pH than would a clay soil. In many respects, the concept of buffer capacity is almost exactly the same as CEC.
Soils vary tremendously in fertility. Some sites require little or no additions to improve the overall nutrient status of trees. As a general rule, if the turf below trees in the landscape is healthy, trees are probably doing well without any additional fertilizers. If foliage begins to show symptoms that may be tied to deficiencies, and you’ve eliminated the possibility of insect and disease depredations, consider a modest application of nitrogenous fertilizers before the middle of the growing season. Nitrogen is one of the only nutrients that trees consistently respond to, but late-season applications can delay hardening off, especially in northern climates.
The author is a professor and extension forester with the Rubenstein School of Environment and Natural Resources at the University of Vermont.