Sixteen chemical elements are recognized as being essential for the growth of all plants. Five others, silicon, sodium, cobalt, vanadium, and nickel, have been recognized as necessary for the growth of some plant species. Although certain essential elements can exist in nature in a number of ionic forms, plants can use only specific ones. Nitrogen Other than carbon, hydrogen, and oxygen, nitrogen is the nutrient required by plants in the greatest quantity. The nitrogen concentration of plants ranges from about 0.5 to 5% on a dry weight basis. Since most plants have a rather high nitrogen requirement and most soils can't supply sufficient nitrogen to meet this demand, nitrogen normally must be supplemented from organic or inorganic fertilizer sources.

The ultimate source of all nitrogen in soils is the atmosphere, which is approximately 78% N2. Although this quantity represents an almost unlimited supply, nitrogen as N₂ is not directly available for uptake by most plants. Recall that legumes in symbiosis with particular species of the bacterial genus, Rhizobium, transform gaseous N₂ to plant available form, with capacities to fix N₂ ranging from about 40 to greater than 300 lbs N/acre/year. Nonsymbiotic fixation of N₂ by free living soil microorganisms also occurs but quantities are usually less than 10 lbs N/acre/year. Lightning discharge in thunderstorms can oxidize atmospheric N₂ to plant available nitrate, but quantities are generally less than 20 lbs N/acre/year. N₂ can also be transformed to plant available forms through the fertilizer manufacturing process. The chemical triple bond that exists between the two N atoms in N₂ is very strong. All the above processes that convert N₂ to plant useable forms, therefore, are highly energy intensive.

Nitrogen is an extremely reactive element with many of its possible transformations being depicted in the nitrogen cycle. The nitrogen cycle is the dynamic system in which nitrogen is transformed, or cycled, from one form to another. Total nitrogen in the soil-plant-water-atmosphere continuum is conserved, but amounts existing in various "pools", or forms, of nitrogen change with time and environmental conditions. The vast majority of soil nitrogen (approximately 95%) is found in soil organic matter. This organically-combined nitrogen is not immediately plant available, but can be converted to inorganic, available forms through the actions of soil microorganisms. This process, as previously discussed, is termed nitrogen mineralization. Prior to the production of inorganic nitrogen fertilizers, most nitrogen for plant growth was supplied through leguminous N₂ fixation and nitrogen mineralization of added animal manures and indigenous soil organic nitrogen. Sole reliance on nitrogen mineralization from soil organic matter normally is not sufficient for crop production and may result in soil deterioration associated with the loss of organic matter. Nitrogen supplementation, whether from organic or inorganic fertilizer sources, is normally necessary for crop production.

Nitrogen, primarily in the form of nitrate, may be lost from soils through leaching. Nitrate, being an anion, is repelled by the negatively-charged cation exchange sites in soils. Since the ion is not adsorbed and is highly water soluble, it will move downward in soils with percolating water. Nitrate leaching is most common in coarse, sandy soils receiving excess rainfall or irrigation. Nitrate losses may also be increased by applying nitrogen fertilizers, whether inorganic or organic, in excess of a crop's requirement. Nitrate leaching should be prevented not only from economic, but also from environmental and health standpoints. Ingestion of waters high in nitrate has been implicated in gastrointestinal problems in adults and methemoglobinemia ("blue baby syndrome") in infants, though confirmed cases are rare. Near-surface aquifers normally are the most susceptible. Nitrate contamination of waters is usually localized and can be decreased through proper management.

Denitrification is the bacterial reduction of nitrate under anaerobic conditions to N₂ or N₂O gases. Under anaerobic conditions, most nitrate in soils may be denitrified in a period of a few days. Some scientists theorize that atmospheric N2 is the result of denitrification over geologic time. Evidence also indicates that N₂O may be partially responsible for depletion of the protective ozone layer and is also a potent “greenhouse gas”. Thus, loss of nitrate through denitrification not only results in an economic loss of plant available nitrogen, but may also have other detrimental effects. Combustion of fossil fuels also produces nitrous oxides that may contribute to this effect.

Nitrogen is an essential ingredient for the production of sufficient food for an expanding world population. Proper nitrogen management can decrease the potential for negative environmental impacts.

Phosphorus

Phosphorus in soil organic matter accounts for about 20 to 65% of the total phosphorus found in soils. Therefore, phosphorus mineralization from soil organic matter is an important source of available phosphorus for plant growth. Phosphorous ranks second to nitrogen as a limiting nutrient for plant growth. Although plant available forms of this element are anionic, phosphorus is immobile in soils with appreciable colloid content because it tends to be tightly bound to these tiny particles. Phosphorus may also form water insoluble compounds such as insoluble calcium phosphates in alkaline soils and insoluble iron and aluminum phosphates in acid soils. The concentration of phosphorus in soil solution is normally much less than one part per million (ppm), even in fertilized soils, and often is only hundredths of a ppm in unfertilized soils.

Phosphorus fertilizers are normally produced through acidification of the mineral, apatite, found in high concentrations in some sedimentary deposits. Organic phosphorus sources, such as manure, may also be used. Manures, however, usually contain relatively large quantities of phosphorus relative to nitrogen. Care must be taken with manure additions so that excess phosphorus doesn't result in deficiencies of other nutrients, such as zinc, or contribute to soluble phosphorus in runoff waters.

Soluble phosphorus can be lost in surface runoff waters, but is usually found adsorbed to soil particles transported by erosion. Phosphorus in runoff has been implicated in eutrophication (excessive algal growth) of lakes and streams.

Potassium

Potassium is required by plants in amounts second only to nitrogen. Unlike nitrogen and phosphorus, potassium is not organically combined in soil organic matter. Different potassium-containing minerals, such as micas and feldspars, therefore, are the principal sources of potassium in soils. Clay-sized micas weather more rapidly to release potassium than feldspars because of their much greater surface area. Soils that contain considerable micaceous clays may be able to supply all of a crop's potassium requirement without fertilization. Acid, weathered soils are those most likely to be deficient in available potassium.

Calcium and Magnesium

Calcium is the predominant exchangeable cation in soils, even in the majority of acid soils, followed by magnesium. This occurs because of the large number of minerals in soils that contain calcium and/or magnesium. Actual plant deficiencies of these elements are infrequent because problems associated with soil acidity, such as aluminum toxicity, become limiting first.

Sulfur

Approximately 85% of total soil sulfur is found in soil organic matter. Microbial mineralization of the soil organic fraction is an important source of available sulfur for plant growth. Reactions of sulfur in soils are very similar to those of nitrogen. Sulfides, such as pyrite and other reduced forms of sulfur, are commonly unearthed in metal and coal mining. Upon exposure to oxygen, sulfuric acid which is produced through chemical and biological oxidation can result in soil acidification and acid mine drainage. Soils may also receive sulfur through atmospheric deposition. Soils near large metropolitan areas may receive greater than 150 lbs S/acre/year from the combustion of fossil fuels. Volcanic eruptions can also emit large quantities of sulfur gases. Soils most commonly deficient in available sulfur are sandy, leached soils that are low in organic matter.

Micronutrients

Iron, zinc, manganese, copper, chlorine, boron, and molybdenum are classified as micronutrients. Micronutrients are plant essential elements that are required by plants in much smaller amounts than the other essential nutrients. Generally, less than 1 lb/acre of each micronutrient will be present in the aboveground portion of crops. This small quantity contrasts with the 200 lb/acre or more of nitrogen.

The total quantity of many micronutrients in soils doesn't necessarily relate to plant availability. Most soils, for example, will contain from 20,000 to 200,000 lbs total iron/acre to a depth of six inches, but may not be able to supply a crop with sufficient available iron for uptake of 1 lb/acre. Iron deficiency is usually associated with highly alkaline soils because iron solubility roughly decreases 1000-fold for each one unit increase in soil pH.

Zinc, manganese, and copper availabilities are also decreased by alkalinity and by high organic matter concentrations (> 10%). These three elements form very stable bonds with soil organic matter which decrease their availability.

Plant micronutrient deficiencies are becoming more widespread because of greater quantities required by higher yields and decreasing micronutrient impurities in fertilizers.

Lesson 9. Soil Management

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