N, P & K in the Soil Plant Environment
Much of the management of soils hinges on the management of nitrogen (N) and phosphorus (P), and to a lesser extent, potassium (K). These three plant nutrients are consumed by plants in relatively large quantities, and are often limiting to plant growth. Thus they are a major item of commerce and of vital importance to the agriculture sector of the economy. What makes the management of nitrogen and phosphorus all the more critical is that they are among the most common non-point-source water pollutants.
1. Plant nutrients. Plants use N, P & K in large quantities. These plant nutrients, called the major plant nutrients, are commonly added as fertilizers. Nitrogen is by far the most widely applied plant nutrient. Plants also need the three secondary nutrients: calcium, magnesium and sulfur. These nutrients are used in moderately large amounts but are less likely to be limiting to plant growth, because their natural abundance is adequate for many crops. Plants also need at least seven micronutrients: boron, chlorine, molybdenum, copper, iron, manganese, and zinc. Some plants apparently either need or can benefit from other elements such as nickel, silicon, or sodium; but these are not considered to be generally essential elements for plant growth.
2. Sources of Soil Nitrogen. Ironically, 78% of the gas content in the atmosphere is N2, yet the soil is usually low in N. We add N to the soil by various means. One way is called industrial fixation. This can be summarized by the equation: N2 + CH4 ® NH3. This is the Haber process used to produce N fertilizer. The dinitrogen reactant is free, but the methane is a major expense. Not surprisingly, nitrogen fertilizer prices closely follow energy prices. Another source of soil N is lightning which produces some nitrate that falls in the rainwater. Symbiotic fixation is the most powerful natural way of adding N to soil. Plants in the Legume family (beans, peanuts, clover, etc.) produce nodules in their roots that house bacteria from the genus Rhizobium. The bacteria turn atmospheric N into ammonium, while the Legume feeds sugar to the bacteria. Alder trees have a similar relationship with an actinomycete, Frankia. Some free-living bacteria also fix atmospheric nitrogen and convert it to useful forms in the soil.
3. Nitrogen transformations. Soil nitrogen undergoes many changes. Those changes are illustrated by the following reactions:
|Immobilization: NH4+ & NO3- ® microbial biomass|
|Mineralization: organic-N ® NH4+|
|Nitrification: NH4+ ® NO3-|
|Denitrification: NO3- ® N2 + N2O|
Immobilization removes N from the pool of plant-available N, at least temporarily, while the N resides in microbial cells. As these cells or other organisms decompose they release N through mineralization, most of which is accomplished by heterotrophic bacteria. Nitrification is done by autotrophic bacteria that obtain energy by oxidizing ammonium. These bacteria are ubiquitous, so the process of nitrification is not just a possibility, it is a probability. Therefore ammonium has a low persistence in soils. Denitrification occurs mainly in wet soils. In the absence in O2, some bacteria will use nitrate as an electron acceptor in respiration. The end products are various nitrogen gases. In addition to these transformations, ammonium can be rather irreversibly adsorbed onto illite clay.
4. Nitrogen losses. Nitrogen is lost from the soil by various means. One has already by described, i.e., denitrification, in which nitrate exits as a gas. Another loss of nitrate occurs through leaching. This creates a serious pollution problem because of the toxicity of nitrate in drinking water. Ammonia, NH3, is lost directly to the atmosphere through the process of volatilization. Through soil erosion N can be lost, usually in the form of NH4+ held onto soil surfaces by cation exchange. Finally, N is lost from the soil by harvest. The species consumed by plants are NH4+ & NO3-.
5. Phosphorus. Nature has provided no way to replenish phosphorus removed from soil. For this reason, P is often deficient in the soil. It also has a low solubility, which helps keep it in the soil, but also keeps it relatively unavailable to plants. Phosphorus is used by plants and microbes as the orthophosphates:
The form of orthophosphate that prevails is regulated by soil pH, with the more protonated forms dominating in lower pH environments. Phosphate absorbs strongly to soil (as indicated in Figure 10.10 in the textbook). Phosphate is commonly added to soil as fertilizer or organic residues. It is lost from the soil by erosion and harvest. Soil erosion results in phosphate pollution, as P-rich soil particles accumulate in bodies of surface water.
6. Potassium. As with phosphorus, nature provides no way to replenish soil potassium. Fortunately, K is naturally abundant in soils, and is quite persistent because of its low solubility. However, plants use large amounts of K+. It is an exchangeable cation. Like ammonium it can absorb strongly to illite clay. Potassium is lost by erosion and harvest. Farmers often add K as a fertilizer. Unlike N or P, potassium is not a serious pollutant. It is rather easy to manage and inexpensive to purchase.
Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web sites.
The Potash & Phosphate Institute has a useful web site,
and they publish a journal, Better Crops with Plant Food.
Cornell University maintains a web site devoted to
A Canadian manure management web site.
Ohio State University Extension offers practical nitrogen
fertilizer advice based on what you just learned about nitrogen behavior.
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