Base Nutrients

The base nutrients (cations ) described in this section include potassium (K), calcium (Ca), and magnesium (Mg).

FORMS AND FUNCTIONS OF BASE CATIONS

Forms of the Base Cations

K: K⁺
Ca: Ca²⁺
Mg: Mg²⁺

Functions of the Base Cations

Potassium

Involved in many enzymatic reactions
Functions in the synthesis of the energy compounds
Required for translocation of carbohydrates within the plant
Involved in regulating gas exchange and water relations during transpiration

Calcium

An important component of the cell wall
Influences the permeability of the cell wall
Involved in cell growth
Participates in the translocation of carbohydrates and nutrients within the plant

Magnesium

A necessary component of chlorophyll, which is the site of photosynthesis
Involved in protein synthesis
Involved in the transfer of energy within the plant

CYCLING

There are no organic forms of potassium, calcium, and magnesium. Instead, these nutrients exist only in their cationic form (K+, Ca2+, Mg2+). Notice that potassium only has a single positive charge, while calcium and magnesium have two positive charges. We refer to potassium as a monovalent cation and calcium and magnesium as divalent cations.

Potassium:

There are four cycles that are associated with potassium.

First, potassium exists as a component of several soil minerals.
Secondly, potassium can be captured within the structure of expanding clay minerals (i.e. montmorillonite), which is referred to as potassium fixation.
Third, potassium is held onto the cation exchange capacity.
Finally, the soil solution contains potassium that is readily available for plant uptake.

Mineral K

Most of the potassium found in the soil exists as a mineral, such as a feldspar and mica. The transfer of mineral potassium to other states in the potassium cycle is a very slow process. Essentially, mineral K is not available for plant uptake during a single growing season.

Captured K

Potassium can also be trapped within the structure of expanding clay minerals. This captured K is referred to as “fixed K.” “Fixed K” should not be confused with nitrogen “fixation.” When nitrogen fixation occurs, plant-available nitrogen increases. In contrast, fixed K is not presently available for plant uptake. Although “fixed K” can slowly become available with time, it is generally unavailable within one crop’s growing season. K-fixation should not always be viewed as a loss, however, since it conserves potassium for future crop seasons.

Exchangeable K

The exchangeable fraction includes potassium that is retained by the cation exchange capacity of soil particles. Exchangeable potassium is in equilibrium with the soil solution potassium and may rapidly replenish the soil solution as potassium is removed.

Solution K

This fraction represents the potassium that can be directly removed from the soil solution by plants.

Fates of potassium in the soil solution

Plant uptake is just one possible fate of potassium in the soil solution. Potassium is a mobile nutrient in the soil and maybe:

lost to leaching
retained by soil particles
precipitated as secondary minerals

Factors affecting K availability:

Amount of K-bearing minerals in the soil: Soils that are inherently high in potassium minerals generally have greater potassium availability than soils that are inherently low in potassium.
Type of clay minerals in the soil: Since highly weathered clay soils typically have a low cation exchange capacity, exchangeable potassium may be limited in these soils.
Soil moisture: Potassium moves through the soil largely by diffusion. Diffusion occurs more rapidly at adequate moisture levels. On the other hand, too much moisture will result in potassium leaching. Since leaching is a source of potassium loss, it should be minimized.
Soil Temperature: Warm temperatures quicken the release of potassium from K-bearing minerals. And so, mineral K and “fixed” K become available more quickly at higher temperatures.
Aeration: Adequate oxygen is required by plants to take up potassium.
Soil pH:
- Under acidic conditions, aluminum and manganese toxicities may cause poor root development, which hinders potassium uptake. When acidic soils are limed, exchangeable K increases due to increases in the cation exchange capacity.
- If there are excessive amounts of calcium and magnesium, the potassium saturation on the cation exchange capacity is reduced by increased competition with calcium and magnesium.

Calcium:

There are three components of the calcium cycle. They are:

Calcium precipitation
Exchangeable calcium
Solution calcium

Ca-bearing minerals

Various minerals in the earth provide natural sources of calcium. Among these are the common liming agents, calcite, and dolomite.

Exchangeable Ca

Calcium is the dominant cation on the cation exchange capacity in most soils. It can readily desorb and replenish soil solution as needed for plant uptake.

Soil solution Ca

Calcium in the soil solution is readily available for plant uptake.

Fates of calcium in the soil solution

Like potassium, plant uptake is only one of the possible fates of calcium in soil solution. Since calcium is a very mobile nutrient in the soil, it may be:

lost to leaching
retained by soil particles
precipitated as secondary minerals

Factors determining calcium availability:

Total calcium supply: Soils that have a low cation exchange capacity are typically low in calcium.
- Soils that tend to have a low cation exchange capacity are heavily leached, highly weathered soils and/or coarse-textured soils.
Soil pH: Acidic soils tend to be low in calcium due to high aluminum saturation.
Type of soil: Moderately weathered soils typically have greater amounts of available calcium as compared to highly weathered soil.
Calcium saturation: If the cation exchange capacity contains less than 25% calcium, it is recommended that calcium should be applied to the soil.

Magnesium

The magnesium cycle is very similar to the calcium cycle. Like calcium, magnesium can be contained by:

Magnesium bearing minerals
The cation exchange capacity
Soil solution

There are a variety of primary and secondary minerals that contain magnesium. Magnesium becomes available when these minerals dissolve, or weather. After release, magnesium is held by the cation exchange capacity of the soil particles or resides in the soil solution. Magnesium in the soil solution may precipitate into secondary minerals, be taken up by plants, or be leached from the soil.

Factors determining availability

Similarly to calcium, magnesium is limited in soils that are:

Inherently low in magnesium-containing minerals
Acidic
Highly leached
Limed with non-magnesium-containing material
Contain excessive amounts of other cations, such as potassium, calcium, and ammonium, which compete with magnesium and reduces its presence on the cation exchange capacity
With a Ca: Mg ratio greater than 10:1 to 15:1, magnesium will likely be deficient.