- Potassium (K)
- Importance of Potassium
Of the major nutrients, this is the least understood element in terms of plant nutrition.
K is not found in the organic compounds of plants.
K is very mobile
and may be moved from one part of the plant to the other.
K is used in the plant metabolism and photosynthesis.
K is also important in developing plants that have strong stems and stalks. Good potassium levels will help reduce lodging of tall crops such as corn, wheat, sorghum etc.... Crops like these\ that get high rates of N and low K are more easily lodged than other crops.
vegetables from plants that did not get enough K are generally small,
irregularly shaped, poor in color, and may rot quickly in storage.
Most soils are relatively high in total K (30,000-45,000lbs)/AFS
- Forms of Soil K
1. Relatively Unavailable
- in mica and feldspar
- 90-98% of all K
2. Slowly available
- fixed in 2:1 clays
3. Readily available
Of the 3 macronutrients used in the greatest amount, K is the most abundant in the earths crust.
The K content of a given soil is related to the amount of k in the parent material, and to the degree of weathering that has occurred.
K occurs as the simple cation K+ throughout the soil system.
It is mobile and will be leached out of the soil easily. This is most important in sandy soils.
§ Losses of Potassium
A. Crop Removal
It may be common to have 100-125 lbs of K20 used by a 20 ton silage corn. This amount of K would be removed from the field. Most crops remove more K from the soil than any other nutrient except N. If you have heavy N for fertilization on hay and you cut and remove all of the forage, K removal may be greater than N.
This occurs because of luxury consumption., which occurs when heavy K fertilization has occurred.
Some plants will continue to take up K above the amount they actually need. Forage grasses and legumes are examples.
On sandy soils, K fertilizer will leach below the root zone. There are not enough exchange sites to hold all of the K and the other nutrients.
- If the pH is low the leaching will be increased even more. This is because the plant will not be at optimum uptake and will not be able to get the K as it leachs through the soil.
On soils that have a higher CEC and a greater clay content, this will not be a problem.
The reason that this occurs is that different cations will be attracted to the colloids in a certain order.
The size of the ion and the charge characteristics will detemine how well the cation is held on the colloid.
As a rule: Al> H > Ca> Mg> K> NH4> Na
The charge on the cation may be the biggest determining factor on how easily the cation will leach. This will hold for everything except H. It is so small that it is able to be held very tightly.
Because the K is held less tightly than many of the other cations it will leach out sooner.
- It is easier for K to be adsorbed on the colloid when it is competing with Ca and Mg than with Al, and H. This is why a well limed soil will hold this cation better. You have reached most of the H and Al through liming.
As with the other nutrients, any erosion will carry away nutrients. In these cases the readily available K may not be lost by slowly available sources may be removed.
- Cell wall component
- Available form is Ca2+
2. Ca feldspars
3. other liming materials
a. found in chlorophyll
b. available form in Mg2+
c. sources - biotite mica, dolomite, hornblende, pyroxene
- part of amino acids
- important in photosynthesis
SO2 + HOH
- organic matter
required in small amounts
- adequacy vs toxicity - this is a very narrow range
Typical surface soil analysis of humid region
1. igneous rocks
2. organic matter
3. complex with many micronutrients to form chelates
- Conditions favoring micronutrient deficiencies
1. highly leached acid sandy soils
2. muck soils
3. very high or very low pH
- intensively cropped