Potassium Cycle Part 2

Last week’s post focused on some mineral sources of potassium. This week will continue with that theme and provide an overview of three other sources of potassium: plant residue, animal manure, and chemical fertilizers.

First, let’s look at another schematic map that shows sources and form of potassium in the soil.

Potassium is present in plant cells and is vital to the cell functioning properly.  So, when leaves fall off a plant onto the ground, root exteriors slough off, a plant dies due to inhospitable weather conditions, is tilled in, or plant residue is otherwise susceptible to the soil microbial community, it begins to decay.  Through the decomposition process, potassium ions are released into the soil solution.  The potassium in the soil solution is available for nutrient uptake by plant roots.  The amount of potassium in the soil solution is in equilibrium with exchangeable pools of K, referring to K ions bound to the soil.  Last week’s blog post gave a brief overview of potassium is the soil structure.

The potassium in animal manure and chemical fertilizers may be referred to as potash, or K₂O.  K₂O is potassium oxide.  This is an inorganic form of potassium and as such, becomes apart of the soil solution and is available for plant uptake.  Here are the equations to convert between potash and elemental potassium:

% K2O × 0.83 = % K
% K x 1.2 = % K2O

The potassium in chemical fertilizers is a bit trickier to explain.  The rating seen on the bag refers to  potash, (K₂O), when in fact it contains potassium chloride, KCl.  Potassium chloride contains 2/3 of potassium by weight as potash.

Next week’s blog post will introduce the sulfur cycle.

Potassium Cycle Part 1

Potassium (K) plays an important role in regulating enzymatic activity and is a key component in the mechanism of photosynthesis.  It is also influential in sugar transportation and water balance.  Here is a diagram showing the general flow of potassium:

The Potassium Cycle:

There are four sources of potassium in the soil: mineral potassium already in the soil, plant organic matter, animal manure, and chemical fertilizers.  In this post we will examine how mineral potassium interacts with the soil.

Mineral Potassium is found in crystalline structures of micas, feldspars, and clays.  Potassium’s chemical formulas looks like this: biotite (a form of mica): K(Mg,Fe²⁺₃)(Al,Fe³⁺)Si₃O₁₀(OH,F)₂ and K-Feldspar: K(AlSi₃)O_8.  Potassium can create strong bonds with soil particles.  These break down releasing potassium ions (K+) during the weathering process, periods of water saturation followed by expanding and contracting when the soil dries and oxidizes.

Here is another great diagram showing the breakdown of Potassium in the soil:

Next week we’ll look at the chemical composition of other sources of potassium and what happens after potassium becomes an ion in the soil solution.

3 Nutrients That Easily Leach

According to Building Soils for Better Crops, plants need 18 essential elements: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, molybdenum, nickle, zinc, manganese, boron, iron, copper, cobalt, and chlorine.

Example of Nutrients Required by Plants

Leaching is the process by which water drains nutrients through the soil.  The extent of leaching is dependent on moisture, aggregate stability, pH, and cation exchange capacity.  (Each of these will be a topic in further blog posts.)  In a  flooded soil we would expect to see loss of nitrate, potassium, and sulfur.

Nitrate is the dominant mineral form of nitrogen.  Nitrate has a negative charge, called an anion, and this does not bind with the negative charge of the soil. The nitrate repels from the soil like two opposite poles of a magnet.  This is why nitrate can move through the soil, or is mobilized.  Nitrogen is the single most determining factor is success of plant growth and yield.

Potassium easily leaches from leaves.  Potassium acts as a catalyst for enzymatic reactions and regulates plant water use.

Sulfate leaches easily.  Sulfur is necessary for amino acid production in plants.

The rest of this month will contain a weekly post focusing on the nitrogen, potassium, and sulfur cycles in the soil.