Soil Properties Determine Availability of Nutrients to Plants

Soil is essential to plants, not only as a substrate, but also as a reservoir for water and essential minerals including nitrogen and phosphorus, as well as calcium, sulfur, potassium, and other ions whose cycles were not discussed. Each of these minerals comes to plants dissolved in soil water. It follows that the properties of soils, including particle size, amount of organic material, and pH, play a very important role in helping to determine the availability of water and minerals to plants, as well as the rapidity with which these materials move through the soil.

Most soils are a complex mixture of mineral particles, organic material, water, soluble chemical compounds, and air. By far the dominant components are the mineral particles, which are composed largely of silicon and aluminum compounds. They vary in size from tiny clay and silt particles to coarse sand grains. The proportions of clay, silt, and sand particles in any given soil determine many of its other characteristics. For example, very sandy soils, which contain less than 20 percent of silt and clay particles, have many air-filled spaces. Unfortunately, such soils are so porous and their particles have so little affinity for water that it rapidly drains through them and they are unsuitable for growth of many kinds of plants. As the percentage of clay particles increases, the water retention of the soil also increases until, in excessively clayey soils, the drainage is so poor and the water is held so tightly to the particles that the air spaces become filled with water. Few plants can grow in such waterlogged soil. Though different species of plants are adapted to different soil types, most do best in soils of the type known as loam, which contains fairly high percentages of particles from each size class. In loamy soils, drainage is good but not excessive and there is good aeration; the soil particles may be surrounded by a shell of water, but there are numerous air-filled spaces between them.

Loams usually also contain considerable amounts of organic material, often in the form of humus, which consists mostly of products from the decomposition of cellulose and lignin. Humus contributes to soil fertility. Since it usually has a rather porous spongy texture, it helps loosen clayey soils and increases the number of pore spaces, thus promoting drainage and aeration.

The proportion of sand, silt, and clay particles affects not only the physical structure of the soil and its aeration and water-holding capacity, but also the amounts and the availability of certain minerals used by plants. If, for example, water percolates downward through the soil very rapidly and in large quantities, it will tend to remove many important ions from the soil, carrying them deep into the underlying rock layers, where roots cannot reach them. Nitrate ions are especially susceptible to removal by water, as are sulfate, calcium, and potassium ions.

Chemical analyses that give the total amount of various ions present in soils can be misleading, since a certain proportion of these ions may not be readily available to the plants. A complex equilibrium generally exists between free ions in the soil water and ions adsorbed on the surface of colloidal clay or organic particles. The acidity of the soil can shift this equilibrium; acidity influences the solubility, and consequently the availability, of calcium, aluminum, iron, magnesium, phosphate, and some other ions, as well as the activity of soil organisms, many of which are inhibited by high acidity (see text and Acid Rain links below).

Acid Rain Induces Nutrient Imbalances in the Soil

In 1970 the United States Congress passed the Clean Air Act, which established air quality standards to protect the public health. While some industries installed scrubbers on their smokestacks to remove pollutants such as sulfur dioxide, most complied by simply building higher stacks to disperse the pollutants (using "dilution as the solution to pollution!"). Locally, air quality improved, but unfortunately the prevailing winds simply moved the problem farther away, and rainfall hundreds of miles away became more acidic. (Normal rainfall has a pH of approximately 5.6; rainfall in the Adirondack Mountains in New York State now averages ~4.2.) Acid rain occurs because the burning of fossil fuels, particularly the less expensive high-sulfur coal, releases sulfur and nitrogen oxides and other pollutants into the atmosphere. These pollutants then react with water to form acids. This is a particularly serious problem in Upstate New York where the prevailing winds bring the pollutants from the coal-fired industrial plants in the midwest to our area.

The acid precipitation is doing billions of dollars of damage to buildings each year (e.g., the Washington Monument and the Capitol Building are being eaten away by acid; see Fig. 34.26 below) and has acidified over 3,000 lakes and 23,000 streams across the country, killing fish and other aquatic life. The effect of such precipitation on forests, particularly coniferous forests, is becoming increasingly clear. Over time, the soil in some affected areas becomes acidified, which in turn influences the solubility of nutrient ions, such as calcium, iron, potassium, and magnesium ions, with the result that water removes these nutrients more rapidly from the soil into streams and lakes. Other ions, such as aluminum, become more available and are preferentially taken up by plant roots instead of calcium ions.

What effect does acid rain have on the soil?

Since the H⁺ ions have a stronger affinity for the negative soil particles than other ions (with the exception of Al³⁺), the H⁺ in acid rain displaces calcium, magnesium, potassium and sodium ions so these are not available to plants.

Calcium and magnesium ions in particular are necessary for proper plant growth. Compounding the problem is an increase in the level of nitrogen compounds in the soil, again the result of air pollution, in this case mostly from automobile exhausts. The result is a nutrient imbalance in the forests, with increased nitrogen compounds acting as fertilizers to stimulate growth, and deficiencies of calcium and magnesium ions suppressing growth. In this stressed state, the trees are weakened to the point where they are highly susceptible to damage from insects, fungal infections, and drought (Fig. 34.27 below). There is increasing evidence that the loss of soil fertility as a consequence of acid rain is distressingly widespread and represents a serious blow to our environment.

Links:

• How does acid rain affect soil?
• What is acid rain and what causes it?
• Acid rain and forests.
• Acid rain still endangers Adirondacks.^{(skim this)}
• Northeast Still Suffering from Acid Rain. ^{(skim this)}
• 2001 Acid Rain Data - note low readings in NY State.

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