Soils and Nutrients
Have you ever scooped up soil and wondered why some feel very loose while some feel densely packed? In this article, I will delve more into the types of soils and how they affect water and nutrients retention.
The diagram above shows us the different types of particles that make up soils.
Sand: largest and loosest particles that allow quick water drainage (albeit nutrients can be easily washed off)
Silt: fine particles that pack tightly together, allowing high water retention (but less than clay)
Clay: tiniest and densest particles, allowing highest retention of water (drains slowly). However, they can become hard and compacted when dry.
Hence, the most ideal type of soil for plants is loam, which comprises a mix of sand, silt and clay. Yet, a caveat is the diverse range of loam, from more sandy loam to more clayey loam.
Nutrients
Besides water drainage, the ability to retain nutrients is another vital consideration for the best soil for gardening.
Colloids are the main particles responsible for retaining nutrients. They are negatively-charged particles in the soil (clay and organic matter) that adsorb to positively-charged nutrients (cations like potassium, sodium, calcium, magnesium). However, colloids do not adsorb to negatively-charged nutrients (anions like phosphorus, nitrogen and chlorine). Soluble anions will move together with water in the soil, being easily leached away. Thus, it is crucial that growers avoid overwatering the soil and end up leaching away these vital nutrients.
Clay holds the most nutrients as they have the largest surface area (smallest particles), while sand holds the least nutrients (smallest surface area). A caveat here is that plants require more energy to absorb the nutrients from clay due to its high density that binds more tightly to the nutrients.
Soil acidity (pH) is instrumental because it affects the availability of nutrients in the soil. The main process which affects soil pH is Cation Exchange, a process by which certain cations preferably adsorb to the colloids (due to stronger bonds). The order of strength of adsorption (when all cations are in equal quantities) is Al>H>Ca>Mg>K>Na. When organic matter decomposes, H+ ions are generated; with their high adsorption rate to colloids, they can often displace other cations like Ca2+, Mg2+, Na+. Consequently, the soil becomes more acidic as the bases are displaced (and potentially washed away by water). This process is reversible if the basic ions are added in high enough concentrations (Le Chatlelier's Principle), making the soil less acidic.
Cation Exchange Capacity (CEC) is a measure of the total moles of cations that a soil can adsorb. High CEC soils have less cation mobility (since more cations are adsorbed to the colloids) than low CEC soils (where the excess soluble cations can move together with water in the soil).
Below is a diagram that shows the optimal pH for the uptake of essential macronutrients and micronutrients plants need:
Nitrogen to Magnesium are macronutrients, while Iron to Molybdenum are micronutrients.
pH 6.0 to 7.0 seems to be the best range for the absorption of these nutrients, barring Molybdenum. So, besides using loamy soil, do monitor the pH for optimal plant growth! You can purchase a pH test kit online, or you can even use simple household items. Add lime if your soil is too acidic, or add alumnium sulfate if your soil is too alkaline. Remember that they take some time to change the pH of the soil, so don't expect instant results!
References:
https://www.gardendesign.com/soil/
https://www.sciencedirect.com/topics/chemistry/soil-colloid
https://www.canr.msu.edu/news/anions_and_cations_in_plants_oh_my_but_why_do_we_care
https://www.almanac.com/plant-ph