SOIL / chemistry / how plants get nutrients

The chemistry of the soil is based on how acidic the water in the soil is, the nutrient elements present in the soil, and how well the soil can hold these nutrients and make them available to the plants.

Based on their requirements by plants, the elements present in the soil are classified as:

Macronutrients
These are nutrients that the plant requires in high quantities to sustain growth and development.
Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Potassium, Sodium, Magnesium, and Calcium.

Micronutrients
These are the nutrients that are required by the plant in very very small quantities but are crucial for plant functions nonetheless. The range between deficiency and excess of these micronutrients/trace elements in the soil is very small. Also, if they exceed trace levels, they can be toxic to plants.
Sulphur, Iron, Boron, Copper, Zinc, Manganese, Selenium, Cobalt, Chlorine, Molybdenum and Iodine.

Out of the total concentration of these nutrients in the soil, plants can take up only a fraction, which is dissolved in water and in a form that the plants can assimilate.

Soil acidity (pH)
Acidity is the measure of total concentration of hydrogen ions (H+) in a aqueous medium. The acidity is measured on a pH scale starting at 1 (highly acidic) and going upto 14 (highly basic), the neutral (neither acidic, nor basic) being at 7.

Soil ph is the measure of the acidity of the moisture in the soil and is believed to be the foundation of all soil chemistry and nutrient reaction. Most plants prefer neutral (pH=7) or slightly acidic soils (ph<7).  

The soil pH fluctuates based on the decomposing organic matter content of the soil and its CEC.

 CEC
The soil particles and the organic matter in the soil carry a negative electrical charge (anions) while the nutrients present in the soil carry a positive charge (cations).

Common nutrient cations: calcium (Ca++), magnesium (Mg++), potassium (K+), ammonium (NH4+), hydrogen (H+) and sodium (Na+)

The difference is charges allows the soil and organic matter to hold the nutrients by an electrostatic force. The capacity of the soil to hold these nutrient cations is termed as Cation Exchange Capacity (CEC) and is measured in milliequivalents (me) per 100 gms of soil.

The plant roots absorb these nutrient cations by exchanging them for other ions like H+ (hydrogen cation) with the soil. If the soil has a higher CEC, these hydrogen ions will again be exchanged by nutrient cations. However, if the CEC is low, H+ ion concentration can start building up increasing the acidity of the soil. Thus a soil with high CEC is considered to have a good buffer against acidity.

A low or high pH can too result in a high CEC (from excess Hydrogen cations if acidic and Sodium cations if basic), thus preventing the soil particles to bind with the available nutrient cations.

Good cation exchange requires very small particles with a large surface area to hold electrically-charged ions. Clay particles being small, bind a lot of nutrients and thus have a high CEC. Sand and silt being larger, have a negligible CEC. Organic matter, because of its colloidal structure, can have CEC much higher than clays.

Many soil microorganisms also carry a negative charge which enables them to attract the nutrient cations and move freely about the negatively charged soil particles.

A soil test done in a lab will give the concentration of available nutrients, pH and CEC. Though these values can be useful as references if one understands the chemistry of the soil, they do not decide the health and fertility of the soil. 

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