Horticultural Growing Mediums and Soil Amendments
Soil: A mixture of inorganic and organic materials that are in solid, gaseous and
aqueous states. It not only provides water and nutrients, but provides a base for
plant anchorage.
Characteristics:

Soil is characterized by its pH (potential hydrogen) which is measured
by how acidic or alkaline it is. The pH is given a number ranging from 0
– 14 (7 being neutral).

Acidic soil has a low pH and alkaline soil has a high pH

Lome: desired soil composition which is a relative even mixture of
sand, silt, and clay.

Cation/Anion Exchange Capacity: ability of soil particles to absorb and
store cations/anions. Abbreviated as CEC/AEC.
o Clay: has high cation exchange capacity
o Most soils have no AEC
Inorganic Components:
Sand: Is heavy, has low water-holding capacity so is good for drainage, and has
low cation exchange capacity.
Perlite: Non- buffering or nutrient holding medium. It is a light popped volcanic
rock with no cation exchange capacity. Good for aeration and drainage. It is the
Styrofoam type substance found in most potting soils.
Vermiculite: An expanded mica mineral. A high buffering aggregate with high
water-holding and cation exchange capacity.
Pumice: Crushed volcanic rock with low cation exchange and water-holding
capacity; good for drainage.
Organic Components:
Peatmoss: Many different kinds but most common and best to use is sphagnum.
It is decomposed organic matter that has a low pH and has high water-holding
and cation exchange capacity.
Coconut Coir: Shredded coconut fibers. Has high water-holding capacity, some
cation exchange capacity, and is often used in hydroponics.
Cottonseed Meal: good organic source of the three major macro-nutrients
(Nitrogen, Phosphorous, and Potassium). Is slow release and lowers the pH of
the soil.
Mulch: often composed of chopped/shredded trees or crop residue such as rice
hull or dry foliage. Has high Carbon content and is great for prevention of water
loss and weed growth.
*Soil in every region varies greatly and will need different amending and possible
fertilization. The commercial fertilizers offered by most nurseries contain the most
necessary of macro-nutrients; Nitrogen, Phosphorous (as numerous phosphate
compounds), and Potassium (as Potash). If your soil type and quality is unknown you
should, at the very least, get an at home test for pH and macro-nutrient levels. For
troublesome soils and a more comprehensive test; send a soil sample into your local
soil testing lab.
*It is important to not overuse fertilizer, especially when it contains Phosphorous.
Large amounts are damaging to the environment and unless it is needed you
should get a phosphate free fertilizer. When possible ALWAYS use organic
fertilizers.
For more information on fertilizers click here: http://www.tfi.org/
*Note: Many different companies offer soilless growing mediums for horticultural and
greenhouse use. This is a list of the major soil media and not completely
comprehensive. For more information on other soil amendments and possible growing
media click here: http://www.americansoil.org/index.html
This webpage was designed by Alex Hill, for Dr. Chiwon Lee, PLSC 211.