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Rockland to Portland Trade Route

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Soil [Extra Quality]

Soils are dynamic and diverse natural systems that lie at the interface between earth, air, water, and life. They are critical ecosystem service providers for the sustenance of humanity. The improved conservation and management of soils is among the great challenges and opportunities we face in the 21st century.



You may be surprised to hear "dirt" described as "big". However, in the late 1800's soil scientists began to recognize that soils are natural bodies with size, form, and history (Figure 4). Just like a water body has water, fish, plants, and other parts, a soil body is an integrated system containing soil, rocks, roots, animals, and other parts. And just like other bodies, soil systems provide integrated functions that are greater than the sum of their parts.

One is usually able to distinguish different layers within soils, called soil horizons (Figure 5). These horizons interact with each other, and therefore cannot be considered as independent, although they can be very different from each other. There is great complexity and diversity in soil horizons, but in general the surface horizons are dynamic and rich in life and organic matter. Below the surface horizons, one often finds more stable horizons that are formed through a diverse suite of soil formation processes, such as bright white horizons formed through the removal of clays or deep-red, low-fertility horizons formed through millions of years of weathering (Figure 6). Below these horizons, soils transition into layers that are only partially affected by soil formation and ultimately into unaltered layers of parent material. Figure 5A soil from Alaska showing distinct horizons resulting from both soil formation processes and periodic deposits of volcanic ash.Courtesy of U.S. Department of Agriculture. Figure 6A highly weathered soil that is red due to the high content of iron oxide minerals.Courtesy of U.S. Department of Agriculture. The lateral extent of a soil can be difficult to define because adjacent soils can have sharp to gradual transitions. Soil bodies can be conceptualized and mapped at different scales, for example for an individual property or an entire watershed. The characterization and delineation of soil bodies forms the basis of most soil mapping systems (Figure 7).

Soils are the primary provider of nutrients and water for much of the plant life on earth. There are 18 elements considered essential for plant growth, most of which are made available to plants through root uptake from soils (Brady & Weil 2007). Soils retain nutrients by several mechanisms. Most nutrients are dissolved in soil water as either positively or negatively charged ions; soil particles are also charged and thereby are able to electrically hold these ions. Soils also hold nutrients by retaining the soil water itself.

salinization - A build up of salts in soils to the point that they destroy the soil's physical and chemical properties and plants are not able to take up water due to the high salt concentration; often associated with improper irrigation.

soil - 1. A material composed of minerals, living organisms, soil organic matter, gas, and water. 2. A body composed of soil and other parts such as rocks, roots, and animals that has size, form, and history and provides integrated functions that are greater than the sum of its parts.

Soil health is defined as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans. Healthy soil gives us clean air and water, bountiful crops and forests, productive grazing lands, diverse wildlife, and beautiful landscapes. Soil does all this by performing five essential functions:

NRCS is also strongly rooted in soil, as the agency was born as the Soil Conservation Service in 1935 during the Dust Bowl, a time of eroded topsoil and fierce dust storms. Since then, conserving and studying soil is at the forefront of what we do. More on NRCS history.

When soil issues arise, they can have long-term and costly impacts to soil health and production goals. If your land has soil issues, you can explore further with the Conservation Concerns Tool on

Solarization leaves no chemical residues and is a simple method appropriate for the home gardener and small- or large-scale farmers. Solarization is primarily used as a broad-spectrum pest control technique, but it may also improve soil health by increasing the availability of nitrogen and other nutrients to growing plants and by beneficially altering the soil microbiome.

The effect of solarization is greatest at the surface of the soil and decreases at deeper soil depths. The maximum temperature of soil solarized in the field is usually from 108 to 140F at a depth of 2 inches and from 90 to 99F at 18 inches. Control of soil pests is usually best for organisms found in the upper 6 inches of earth.

Soil solarization also speeds up the breakdown of organic material in the soil, often resulting in the added benefit of releasing soluble nutrients such as nitrogen (from nitrate and ammonium), calcium, magnesium, potassium, and fulvic acid, making them more available to plants.

Plants often grow faster, with higher and better-quality yields, when grown following soil solarization. This may be attributed to improved disease and weed control, increased availability of nutrients, and greater proportions of beneficial microorganisms.

The degree to which various pests can be controlled is related to the intensity, depth, and duration of the elevated soil temperatures, as well as to the sensitivity to treatment of each pest species. Although some pests may be killed within a few days, 4 to 6 weeks of exposure to full sun during the summer is required to ensure control of many others.

Solarization controls many important soilborne fungal and bacterial plant pathogens, including those that cause Verticillium wilt, Fusarium wilt, Phytophthora root rot, Southern blight, damping-off, crown gall disease, tomato canker, potato scab, and many others. A few heat-tolerant fungi and bacteria, such as those causing melon decline and charcoal rot of many crops, are more difficult to control with solarization.

Soil solarization can be used to reduce soil populations of many species of nematodes. This is particularly useful for organic and home gardeners. However, soil solarization is not always as effective against nematodes as it is against fungal disease and weeds. This is because nematodes are relatively mobile and can move deeper in the soil profile to escape the heat, rapidly returning to recolonize soil and plant roots following solarization treatment. Furthermore, control of nematodes by solarization will be greatest in the upper 12 inches of the soil. Nematodes living deeper in the soil may survive solarization, later causing damage in plants with deep root systems.

Soil solarization controls many of the annual and perennial weeds present in California. While some weed species seeds or plant parts are very sensitive to solarization, others are moderately resistant and require optimum conditions for control; that is, good soil moisture, tight-fitting plastic tarps, and high solar radiation.

Solarization generally does not control perennial weeds as well as annual weeds because perennials often have deeply buried underground vegetative structures such as roots, corms, tubers, and rhizomes that may resprout. Rhizomes of bermudagrass and johnsongrass may be controlled by solarization if they are close to the soil surface.

Important among these beneficials are mycorrhizal fungi, and fungi and bacteria that parasitize plant pathogens and aid plant growth. The increased populations of these beneficials can make solarized soils more resistant to pathogens than nonsolarized soil.

Soil solarization is most effective in warm, sunny locations such as the Central Valley, desert valleys, and other inland areas of California. It has also been used successfully in the cooler coastal areas of California during periods of high temperature and no fog. Soil treatment by anaerobic soil disinfestation (ASD) may be done where soil heating is insufficient for solarization.

Highest soil temperatures occur when days are long, air temperatures are high, skies are clear, and wind is minimal. The soil heating effect is not as great on cloudy days. Wind will disperse the trapped heat and may loosen or damage the plastic sheets. Shady areas may not be effectively treated by solarization.

Solarization is most effective when done during the hottest weeks of the year. The best time for solarization of soil in inland California is from June to August, although good results may be obtained starting as early as late May or as late as early September in the southern California desert regions. July is the most reliable time to solarize, except for coastal areas, where persistent, warm, fog-free periods may not occur until August or September. Soil within most regions of California, except high elevation areas and some coastal valleys, can be reliably solarized if treatment is instituted during the period of late June through August.

Solarization can be done on flat areas or raised beds. Flat areas are easiest to solarize (prior to lawn reseeding, for example) and ensure more uniform solarization of the entire area. Raised beds are best formed prior to solarization so that tarps can be placed over preformed beds. This practice also minimizes disturbance of the soil after solarization, which may bring up viable weed seeds from deeper in the soil profile.

For best results, wet the soil to at least 12 inches deep. In larger areas, it is easiest to do this prior to laying the plastic, but in smaller areas it can be done after the plastic is applied using a garden or soaker hose or by laying drip tape under the tarp.

If wetting soil beforehand, place plastic covers over the site as soon as possible after the water has been applied to reduce evaporation. Unless the soil gets dry during the course of soil solarization, or you are aiming to do an ASD treatment, do not irrigate again, as this will lower the soil temperature and lengthen the time required for successful solarization. 041b061a72


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