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The most important part of a plant’s environment

by Timber Press on May 19, 2017

in Gardening, Natural History

Photo by the author.

Here’s the dirt: soil is the medium in which roots are anchored and from which nutrients, water, and oxygen are obtained.

Soil is a complex mixture of inorganic materials, derived from the erosion of rock, and organic matter or humus, the decomposed remains of plants and animals. The inorganic fraction is divided into three classes, defined by particle size (1 mm = 0.04 inch): Sand particles have a diameter between 0.02 and 2 mm, silt ranges from 0.002 to 0.02 mm, and clay particles are smaller than 0.002 mm. Mixtures of sand, silt, and clay are called loams; a sandy-loam, for example, contains proportionately more sand. In humus-loams various proportions of organic matter are mixed with the other components.

The proportions of each material determine the water-holding capacity of a particular soil. Water-holding capacity (or field capacity) is defined as the water content of a thoroughly wetted soil after surplus water has drained off by gravity. Sandy soils retain little water, whereas the addition of humus increases the water-holding capacity, the moisture being held in tiny spaces (capillary spaces) within and between the organic particles. Capillary water is the principal source of moisture for roots.

A special problem is posed by soils containing large proportions of clay, the particles of which bear electrical charges that attract water molecules. The bond between water and clay, comparable to the attraction between the opposite poles of two magnets, is a difficult union for roots to break. Consequently, much of the water in a clay soil is unavailable to plants.

Another problem with clay soils is their lack of porosity. Dense packing of the fine particles leaves little space through which gases can be exchanged between the soil and the atmosphere; yet it is essential that carbon dioxide and other gases escape from belowground and that oxygen penetrates to a plant’s roots. Water-logging, the saturation of spaces between soil particles thereby excluding air, is common in heavy, sticky clay. In contrast, sandy- and humus-loam soils have a loose, porous quality favoring drainage and the diffusion of gases.

Soils have complex chemical compositions that determine, in addition to mineral nutrient content, their relative acidity or alkalinity—the pH. The pH scale is a numerical series from 1 to 14, with 1 being most acid and 14 being most alkaline. A neutral condition is arbitrarily assigned a pH value of 7. Acidity decreases from pH 1 to 7; alkalinity increases from pH 7 to 14.

Most horticultural species grow favorably in soils at or close to neutrality. However, ferns, azalea, rhododendron, and camellia, for example, require an acidic pH between 4.5 and 5.5, whereas asparagus, spinach, and cacti and other succulent species prefer mildly alkaline soils, to about 7.5 on the pH scale. Hydrangea tolerates a wide pH range, but the flower colors indicate the soil pH—the flowers become blue in acidic soils, pink in alkaline.

Most often, soils are made more acid by the addition of sulfur or organic materials such as peat moss or sawdust; limestone (calcium carbonate) is widely used to increase the alkalinity. The mineral content of irrigation water may alter the soil pH, particularly when the water evaporates and residual chemicals accumulate in the soil, which is a frequent, acute problem in arid regions.

Normally, rain leaches excess minerals deep into the soil and helps to restore the pH to more favorable conditions. But in some areas, industrial emissions are contaminating rain and affecting soil chemistry. When the air pollutant sulfur dioxide combines with atmospheric moisture, it falls to earth as a mild sulfuric acid solution called acid rain, which has a devastating effect on species ranging from simple nitrogen-fixing bacteria to forest trees.

Increased soil acidity, whether from acid rain or natural sources, causes aluminum, manganese, and iron to be liberated from harmless, insoluble forms in the soil to concentrations that slowly poison cells. Furthermore, free aluminum and iron cause phosphates to precipitate and interfere with the uptake of calcium by roots, thereby adding the deficiency of those macronutrients to other problems of plants in acidic soils.

At the other end of the pH scale, toxic amounts of molybdenum are released in alkaline conditions. Alkalinity also makes phosphates and calcium unavailable to roots when they combine to form insoluble calcium phosphate, and manganese and iron bind into tight chemical complexes in alkaline conditions. Because the resultant iron deficiency is fatal to many species, gardeners who work with alkaline soils must supply the metal in chelated form. Chelates (soluble organic compounds to which iron is bound) make the element available to plants without toxic effects. The chelate is eventually broken down by microorganisms. Two commonly used chelating agents are known by the acronyms EDTA and EDDHA.

 

Brian Capon received a PhD in botany from the University of Chicago and was a professor of botany at California State University, Los Angeles for thirty years.

 

 

 

 

 

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