6.6: Chemical Contamination of Soil

Soils can be contaminated with chemicals, either naturally or by human activity, to such an extent that crops are adversely affected. Problems of saline and sodic (alkaline) soils are most found in arid and semiarid regions, or in soil affected by coastal flooding. Other types of contamination may derive from natural toxic chemicals or pollution.

Saline and Sodic 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 as the soil exerts an osmotic force that counters the plant’s own osmotic potential.

Note

Saline Soil

Electrical conductivity of a soil extract is greater than 4 ds/m, enough to harm sensitive crops.

Sodic Soil

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.

Sodic soils tend to have very poor physical structure because the high sodium levels cause clays to disperse, leading aggregates to break apart. Aggregates of sodic soils disperse when they are saturated, and the solids then settle as individual particles and make the soil very dense (Figure 6.16). These soils become difficult to work with and are very inhospitable for plants because of both compaction and greatly reduced aeration. When a sodic soil is fine textured its consistency and appearance are something like that of chocolate pudding. It causes serious problems with drainage, seedling emergence and root development. A soil like that must be remediated before growing crops.

Figure 6.16. A sodic soil in Tasmania, Australia, that lacks aggregation and has problems with waterlogging when wet and with hardsetting when dry. Photo by Richard Doyle.

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 has limited support from research except in unusual situations.

Saline and sodic soils are commonly found in the semiarid and arid regions of the western United States and in similar climate zones in many countries around the world. They are difficult to remediate. After major hurricanes, coastal flooding areas may also experience temporary saline-sodic conditions until the salts are washed out by rains.

Although some soils are naturally saline, 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 salinity. 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 be wicked up 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.

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 subhumid 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 also 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.

Other Types of Chemical Contamination

Soils can become contaminated with many sorts of chemicals from oil, gasoline or pesticides to a variety of industrial chemicals and mining wastes. This contamination may occur through unintended spills, but in the past, waste materials were often deliberately disposed of by dumping on fields. In urban areas it is common to find lead-contaminated soils as a result of the past use of lead-based gasoline and paint. Lead, as well as other contaminants, frequently makes creating an urban garden a real challenge. Often, 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, and the lead is made less available by organic chelates. Agricultural soils that have a history of applications of sewage sludge (biosolids is the current term) may have received significant quantities of heavy metals such as cadmium, zinc and chromium, as well as antibiotics, pharmaceutical drugs and an assortment of toxic organic chemicals contained in the sludge. Some phosphorus fertilizers contain cadmium that can build up in soils. Toxicity related to such contaminants may impact plants and humans, like itai-itai disease among Japanese rice growers in the 1950s.

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. For example, pesticides, organic wastes and oils can be naturally broken down in soils. Some plants are especially good at taking up certain metals from soil and can be used to clean contaminated soil (but they then must be disposed of carefully). Adding organic matter can also reduce the availability of heavy metals by forming chelates (Figure 2.5).

Urban Soils

Severe soil degradation can be observed in urban areas where soil is often intensively used, physically disturbed or contaminated by a wide variety of chemicals. In addition, urban ecosystems are challenged by a difficult microclimate (so-called heat islands). On the positive side, urban spaces are very valuable—many people use them—and there are therefore more financial resources available to invest in remediation. Also, urban areas concentrate organic materials that can be used for soil improvement, like food waste and street leaves turned into compost to help build soil organic matter, and urban tree branches that are chipped and used for mulch. For more on the special issues of growing plants on urban soils see Chapter 22.