Home > Great Big Greenhouse Blog > June 2014 > DISHIN' THE DIRT--POTTING SOIL


Let’s talk about dirt…soil, that is.  The soil, or more properly, the medium that houseplants are growing in varies greatly by grower, species, container, and personal preference.  This potting medium, commonly called potting soil, serves several important purposes:  it gives the plant a place to be, anchored by the roots; it is a water reservoir; it contains minerals that are essential to the plants’ growth; and it must provide aeration to the roots.  Now there are plants that do not grow in soil, but rather in water or up on rocky cliffs or tree limbs, where they are adapted to meeting these basic needs in other ways.  But most houseplants are confined to containers in which the appropriate potting medium supplies these necessary functions.

Houseplants will spend their lives in a relatively small volume of medium, with a limited amount of nutrient, and a texture and water retention property very different from that of their native habitats.  Thus, it is important to get the medium right for the long-term health of the plant.  To move water downward and sideways through the medium, soil has to have capillary tubes formed by the spaces between soil particles (pores) in the soil.  Air and nutrients are also moved throughout the root ball by capillary action.  The texture of the potting medium (the relative proportion of soil particle sizes) must provide these pores which also allow for roots to grow into the medium.  A good, light, well-draining potting soil will have good texture. 

Top soil tends to be too heavy to use as a good, well-draining indoor potting medium.  Commercial potting mixes have been amended to provide the light, fluffy, well-draining texture that is needed for healthy container plant growth.   Good potting soils are made up of several ingredients, including inorganic soil particles (rock, clay, sand, or silt) and minerals, and organic matter. 

INORGANIC PARTICLES:  Clay or silt soils pack down over time, leaving no spaces for air, and while these kinds of heavy soils retain water and provide needed minerals, they drain poorly.  Therefore, we need to amend soil to increase aeration, improve drainage, and prevent soil compaction.  Just adding coarse sand provides spaces between particles for air and water, but it also adds weight which can be a problem for containers that may be moved about seasonally.

Inorganic soil amendments include perlite, vermiculite, horticultural charcoal, and a wide array of minerals.  Perlite is volcanic glass that, when heated at extreme temperatures, expands into a white, light-weight particle that holds moisture in the tiny cavities on its surface, while its irregular shape provides spaces for both air and water to move throughout the football.

Vermiculite is the name for a group of silicates that, when expanded by heat, hold moisture and nutrients (such as potassium, calcium and magnesium), and lighten and aerate the soil.

Horticultural charcoal is a cheap grade of natural charcoal that is used primarily for soil aeration.

While the above inorganic soil amendments are poor in nutrient value, minerals such as copper, iron, manganese, phosphorus, zinc, etc., are essential to plant health.  These minerals are found naturally in adequate amounts in soil and are also available in fertilizers.   

Posted: 6/11/2014 by Margot | with 0 comment(s)
Filed under: MargotGunn, TheGreatIndoors
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