Soil science is FULL of acronyms! We will discuss 5 of them in detail in this blog article. Nicely ordered, clearly explained and richely illustrated.There will be 2 further parts to be released in the coming weeks.
Let's start with a definition:
"An acronym is a pronounceable word formed from the first letter (or first few letters) of each word in a phrase or title. The newly combined letters create a new word that becomes a part of everyday language."
Soil science is FULL of acronyms! We'll discuss 11 of them, arranged in alphabetical order:
Part 1:
1. CEC
2. EC
3. FC
4. HC
5. NPK
Part 2:
6. OM
7. PAW
8. pF
9. pH
10. WRC / WHC
11. WP
Part 3:
12. HC
What is pH?
pH is a measure of the acidity or basicity (alkalinity) of a soil.
The pH scale is logarithmic and inversely symbolizes the concentration of hydrogen ions (H+) in a solution. The pH scale ranges from 1 to 14.
• A pH of 7 is neutral. This means that the amount of hydrogen ions in the solution is equal to the amount of hydroxide ions (OH-). For example, water has a pH of 7 because when water breaks up, the split is equitable into one hydrogen ion for every hydroxide ion.
• A pH lower than 7 is acidic. The solution contains more hydrogen ions than hydroxide ions (H+ > OH-). Or put differently: if a molecule releases hydrogen ions in water, it is an acid. The more hydrogen ions it releases, the stronger the acid, and the lower the pH value.
• A pH higher than 7 is basic or alkaline. The solution contains more hydroxide ions than hydrogen ions (OH- > H+). These OH- ions will combine with H+ to create water (H20). Because hydrogen ions are used, the number of hydrogen ions in the solution decreases, making the solution less acidic and therefore more basic. So, the more hydroxide ions a molecule releases, the more basic it is.
The table below shows you the pH of some common substances and may help you to understand the pH scale.
The pH Scale
The soil pH is very important for plant growth:
• Some plants prefer either acid or alkaline conditions. Most lawns and turf grasses for example, prefer a slightly acidic pH between 6.0 and 7.0.
• Certain diseases tend to thrive when the soil is either too alkaline or too acidic.
• The pH strongly affects the availability of nutrients in the soil.
The majority of food crops prefer a neutral or slightly acidic soil, because the solubility of most nutrients necessary for healthy plant growth is highest at pH 6.3-6.8. Some plants however prefer more acidic (e.g., potatoes, strawberries) or alkaline (brassicas) conditions.
When the pH falls drops below 5.5, most major plant-nutrient minerals (including nitrogen (N), phosphorus (P), potassium (K), sulphur (S), magnesium (Mg), and calcium (Ca)) and some micronutrients become insoluble and hence unavailable for uptake by plant roots:
• macro elements: these elements are needed in substantial quantities to promote healthy plant growth;
• microelements: these elements are important to plant growth in very small amounts
Many positively charged nutrients (cations, such as zinc (Zn2+), aluminium (Al3+), iron (Fe2+), copper (Cu2+), cobalt (Co2+), and manganese (Mn2+)) are soluble and available for uptake by plants below pH 5.0, although their availability can be excessive and thus toxic in more acidic conditions. In more alkaline conditions they are less available, and symptoms of nutrient deficiency may occur.
The table below visually illustrates how soil pH affects availability of plant nutrients.
How soil pG affects availability of plant nutrients
CEC
The CEC or Cation Exchange Capacity is the capacity of the soil to store exchangeable cations.
In a soil, clay mineral and organic matter components have negatively charged sites on their surfaces. Also plant roots have an overall negative charge.
Just like a magnet, these negatively charged sites will attract positively charged ions (cations) by electrostatic force. Some of these cations are critical for plant growth:
• magnesium (Mg2)+,
• potassium (K+),
• ammonium (NH4+) and
• calcium (Ca2+).
In general terms, soils with a high CEC are more fertile, because they can retain more of these cations. Cations in the soil compete with one another for a spot on the cation exchange capacity. However, some cations are attracted and held more strongly than other cations.
Soil Solution
CEC is expressed in meq/100g. Soil CEC typically increases as clay content and organic matter increases.
The relationship between soil texture and CEC
The relationship between soil texture and CEC
The CEC of a soil can be increased by mixing soil amendments with a high CEC. For example, the CEC value of TerraCottem exceeds 150 meq/100g. This is due to the high CEC of its carrier materials and especially its super absorbant polymers. These are crosslinked polymer chains with lots of negative charged areas inside its chemical structure:
Polymer Chains and Crosslink
There is a direct relation between the pH and the CEC of a soil. The CEC is lowest at soil pHs of 3.5 to 4.0 and increases as the pH is increased. Because CEC may vary considerably with soil pH, it is a common practice to measure a soil's CEC at a pH of 7.0. Remark (see figure below): at low pH, some positive charges may also occur on specific soil mineral surfaces. These may retain anions (negatively charged ions) such as chloride (Cl-) and sulphate (SO42-).
The soil cations can be divided into two groups:
1. "Base cations": ammonium (NH4+), calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+) (*)
2. "Acid cations": aluminium (Al3+) and hydrogen (H+)
The words "base" and "acid" refer to the cation’s influence on soil pH. A soil with a lot of acid cations held by soil particles will have a low pH. On the other hand, a highly alkaline soil predominately consists of base cations.
(*) Unlike ammonium, calcium, magnesium and potassium, sodium is not an essential element for all plants. Soils that contain high levels of sodium can develop salinity and sodicity problems.
NPK
NPK is short for nitrogen (N), phosphorus (P) and potassium (K). These are followed by 3 numbers, for example 20-8-5 and represent the percentage of these components in the package.
More precisely:
• N is the percentage of elemental nitrogen by weight in the fertiliser;
• P is the percentage by weight of phosphorus pentoxide P2O5 in a fertiliser. P2O5 consists of 56.4% elemental oxygen and 43.6% elemental phosphorus by weight. Therefore, the elemental phosphorus percentage of a fertilizer is 0.436 times its P value.
• K is the percentage by weight of potassium oxide K2O. K2O consists of 17% oxygen and 83% elemental potassium by weight. Therefore, the elemental potassium percentage is 0.83 times the K value.
This is a good reminder of describing the purpose of each element:
• Up = nitrogen (N). Nitrogen helps the above ground plant growth. It is largely responsible for the green leafy growth of foliage and lush green lawns.
o For that reason, fertilizers used for the maintenance of lawns will frequently have a high first number, for example 20-5-8.
• Down = phosphorus (P). Phosphorus , the middle number, is largely responsible for root growth and flower and fruit development.
o For example, a fertiliser used for flower production will have a relative high middle number.
o The same is true for starter-type fertilisers for your lawn.
• All Around = potassium (K). Potassium is important for overall plant health. It helps build strong plants cells As a result, the plants will have a higher resistance against heat / cold, pests and diseases.
o For example, winterizer fertilisers will have a high component of potassium.
Find out more about the below on our blog part 2 (coming soon)!
1. OM
2. PAW
3. pF
4. pH
5. WRC/WHC
6. WP
