Soils can be contaminated with chemicals—either naturally or by human activity—to such an extent that crops are adversely affected. In this section we’ll start with a discussion of problems of saline and sodic (alkaline) soils, normally found in arid and semiarid regions. Then we’ll discuss other types of chemical contamination.
Sodic and Saline Soils
Special soil problems are found in arid and semiarid regions, including soils that are high in salts, called saline soils, and those that have excessive sodium (Na+), called sodic soils. Sometimes these go together and the result is a saline-sodic soil. Saline soils usually have good soil tilth, but plants can’t get the water they need because the high salt levels in the soil inhibit water uptake. Sodic soils tend to have very poor physical structure because the high sodium levels cause clays to disperse, leading aggregates to break apart. As aggregates break down, these soils become difficult to work with and very inhospitable for plants because of compaction and greatly reduced aeration.
Electrical conductivity of a soil extract is greater than 4 ds/m, enough to harm sensitive crops.
Sodium occupies more than 15% of the cation exchange capacity (CEC). Soil structure can significantly deteriorate in some soils at even lower levels of sodium
Aggregates of sodic soils disperse when they are saturated, and the solids then settle as individual particles and make the soil very dense (figure 6.13). When a sodic soil is fine textured, such poor structure develops— the consistency and appearance of a wet sodic clay soil are something like that of chocolate pudding—that there are serious problems with drainage, seedling emergence, and root development. A soil like that must be remediated before growing crops. Also, the ionic strength of the cations in the soil can affect aggregate stability. Some believe that soils with high magnesium-over-calcium ratios tend to have weaker aggregates and would benefit from calcium applications, but that is not supported by research, except in unusual situations.
Saline and sodic soils are commonly found in the semiarid and arid regions of the western U.S., with pockets of saline soils found near the coastline. They are common in similar climate zones in many countries around the world.
Although some soils are naturally saline or sodic or both, there are a number of ways that surface soils may become contaminated with salts and sodium. When irrigation water containing significant salt content is used—without applying extra water to leach out the salts—accumulation of salts can create a saline soil. Also, routine use of irrigation water with high sodium levels relative to calcium and magnesium will create a sodic soil over time. Over-irrigating, which often occurs with conventional flood or furrow irrigation, can create salinity problems in the topsoil by raising water tables to within 2–3 feet of the surface. Shallow groundwater can then move by capillary action to the surface, where the water evaporates and the salts remain. Sometimes the extra moisture accumulated during a fallow year in semiarid regions causes field seeps, in which salty water high in sodium comes to the surface, leading to the development of saline and sodic patches.
Other Types of Chemical Contamination
Soils can become contaminated with all sorts of chemicals—from oil, gasoline, or pesticides to a variety of industrial chemicals and mining wastes. This contamination may occur through unintended spills, although in the past waste materials of these types were frequently disposed of by dumping on soils. In urban areas it is common to find lead-contaminated soils as a result of the use of lead-based paint for decades. Lead, as well as other contaminants, frequently makes creating an urban garden a real challenge. Frequently, new topsoil is brought in, mixed with a large quantity of compost, and placed in raised beds so that plant roots grow above the contaminated soil. Agricultural soils that have a history of applications of sewage sludge (bio-solids is the current term) may have received significant quantities of heavy metals such as cadmium, zinc, and chromium, as well as antibiotics and pharmaceutical drugs contained in the sludge.
There are a number of ways to remediate chemically contaminated soils. Sometimes adding manure or other organic amendments and growing crops stimulates soil organisms to break down organic chemicals into less harmless forms. Some plants are especially good at taking up certain metals from soil and are sometimes used to clean contaminated soil—but they then must be disposed of carefully.
SALT PRESENCE IN ALL SOILS
Salts of calcium, magnesium, potassium, and other cations—along with the common negatively charged anions chloride, nitrate, sulfate, and phosphate—are found in all soils. However, in soils in humid and sub-humid (drier than humid, where most crops can be grown without irrigation) climates—with from 1–2 to well over 7 inches of water percolating beneath the root zone every year—salts don’t usually accumulate to levels where they can be harmful to plants. Even when high rates of fertilizers are used, salts usually become a problem only when you place large amounts in direct contact with seeds or growing plants. Salt problems frequently occur in greenhouse potting mixes because growers regularly irrigate their greenhouse plants with water containing fertilizers and may not add enough water to leach the accumulating salts out of the pot