Growing Native Tallgrasses and Wildflowers for Prairie Restoration

Part 2 - The Science of a Tallgrass Prairie

Soil Nutrient Availability

The General Nutrient Cycle

Like energy, nutrients cannot be created or destroyed, they are basic elements and can only be moved or transformed into inaccessible forms. Nutrients are cycled or moved slowly from one point to another, moved through various processes, creating a global nutrient cycle. Nutrient content in a particular site can be altered by geological processes such as volcanic, glacial, or erosional action or through industrial processes such as the mining of nutrients or industrial fixation of nitrogen. In Canada, the relatively recent glaciation has had an important impact on soils. Soils in Canada are young compared to those in other areas of the world. Therefore, they are less weathered and are richer in essential plant nutrients. The most productive soils on the earth are those that have had a relatively recent geological disturbance.

Plant Nutrients

Plants need two basic classes of nutrients to grow: major nutrients (macronutrients) and minor nutrients (micronutrients). Macronutrients are those nutrients that are absolutely crucial to the physiological development of plants, those nutrients that are needed in high concentrations. Micronutrients are the nutrients required in small concentrations. They are necessary for filling in the gaps, finishing minor, yet crucial, developmental processes. While micronutrients usually make up a small percentage of a plant's dry weight, they still provide invaluable and irreplaceable functions (Table 2.1).

The major plant nutrients are carbon (C), oxygen (O), hydrogen (H). nitrogen (N), phosphorous (P), and potassium (K). The carbon, oxygen, and hydrogen needs are available through the atmosphere. The availability of the other three nutrients can also strongly influence the growth rates of the plants. If the soil cannot provide an adequate supply of nitrogen, phosphorous and potassium, to sustain the plant communities, the vegetation will change.

The productivity of soils, when measured by the amount of plant growth, is a function of all of the factors of plant growth. This includes the physical state of the soils, the relationship between water and the soil, the appropriate soil pH and the nutrient status of the soil. A fertile soil that has a good supply of nutrients may not always be a productive soil; there are many other factors that may be limiting plant growth.

Table 2.1
Nutrient Availability

Nutrients in the soil are held by the mineral or organic elements in the soil. If these nutrients are in an accessible form for the uptake by the plants, the plant can remove the nutrient from the soil particle, and the ion moves by way of water in the pores between the particles. Water is the necessary transporting agent in this system. The ease of this removal of nutrients from their chemical bonds with the soil particles depends a lot on soil pH. As a soil becomes either more acidic or more basic nutrients get held up, or other reactions are favoured and the nutrients become inaccessible. Having a pH somewhere close to neutral is usually the optimum.

Nitrogen

Nitrogen is needed in plant development because it is essential in the production of proteins, nucleic acids and chlorophyll for photosynthesis. The cycling of nitrogen in soil is easily understood by looking at the nitrogen cycle (Figure 2.7). It is important to note that all nitrogen, regardless of where it comes from, will move through the cycle by the same processes (Table 2.2). The source of nitrogen is unimportant to plant uptake or other processes in the soil.

About 95% of the nitrogen in a soil is a component of organic matter or plant residue, and as such is unavailable for plant uptake. Only NH4+ (ammonium) and NO3- (nitrate) are taken up by plants. Plants that are deficient in nitrogen are easily identifiable as the plants undergo ‘chlorosis', and turn yellow.

Nitrogen becomes available when organic matter is decomposed; ammonium ions are produced by the mineralization of the decomposing materials. From there, some of the ammonium is taken up by the plants directly, but the majority of the ions are oxidized by other organisms to form NO2- (nitrite) and then to NO3- (nitrate) ions. This nitrate can then be taken up by the plants. However, if the nitrate ions are present in anaerobic conditions, they can be reduced to N2 or N2O which then get released into the atmosphere as gases.

Losses of nitrogen from the cycle occur only:

(i) when there is an export of plant or animal products out of the system, eg. harvested
(ii) when the volatilization of gaseous forms of nitrogen causes them to get released into the atmosphere and the cycling time increases
(iii) from the export of soil in the form of erosion of clay and organic matter on which there is exchangeable ammonium ions
(iv) from the leaching of nitrate into the groundwater, which then leaves the system

Phosphorous

Phosphorous is essential to plant development because it creates the compounds that store the energy needed by the plants (ATP and ADP). It is also used with nitrogen in the formation of the nucleic acids. The phosphorous content of soil depends on the parent material and the extent of weathering of the soil. The phosphorous cycle is similar to the nitrogen cycle, however there are not as many stages of transformation (Figure 2.8). The plant uptake of phosphorous, the return of the residue to the system and the mineralization process are all similar to the nitrogen cycle. Both cycles depend on conditions favouring biological activity. Both cycles use organisms to fix the nutrients.

Like all nutrients, there is no natural way to increase the phosphorous in a system, the only way is to artificially increase levels by adding a fertilizer. When importing organic residue (fertilizer) to increase phosphorous levels, the phosphorous cycle is simply moved from another part of the world, shifting the global nutrient balance. When a plant is deficient in phosphorous, the leaves start to turn a deep purple.

Unlike nitrogen, phosphorous cannot be lost from the soil, except by human removal. This does not mean that phosphorous deficiencies are rare. It may be that substantial amounts of the phosphorous are in less soluble forms and inaccessible to the plants. These less soluble forms may be transformed as more of the soluble forms are taken up by plants. Virtually all of the phosphorous used as fertilizer in Canada is imported from either the United States or North Africa, making those systems deficient.

Potassium

Potassium is essential to plant development as it regulates the opening and closing of the stomata, the structures within the plant which release the oxygen byproducts of photosynthesis. This can be seen as the breathing of the plant. Also, when combined with nitrogen potassium is key in the creation of some proteins. The potassium content of the soil is a direct reflection of the mineral composition of the bedrock underneath the soil. The potassium cycle is much simpler than either the phosphorous or the nitrogen cycles (Figure 2.9). Potassium can exist in plant residues, as K+ ions in solution, as an exchangeable ion on surfaces of clay and organic matter, as a non-exchangeable ion trapped in certain types of clay and as part of certain minerals such as feldspar or mica. Losses are due to the harvesting of plant material and the erosion of soil from a site. If there is a deficiency, it can only be amended by the addition of fertilizer. Deficiencies in potassium are not as easily detectable as the other two major nutrients. Potassium works against early maturation and ripening, so, if your plants are ripening too soon, or they are ripening late in the season or not at all, the level of potassium in the soil is unbalanced compared to the other nutrients.


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