Water Availability in Soil Mediums II

 I will continue to talk about water availability in soil mediums. Most soils have about 50% pore space. This may surprise you. It is the pore spaces that are important and various mediums will increase or decrease the pore space, meaning they will increase or decrease the amount of water a soil/medium can hold. Please see my posts on sand, perilite, vermiculte coir, coir and soil  and more to determine what you should be adding to your medium.

When soil is saturated it means, all the pores are full of water, but after a day, all gravitational water drains out, leaving the soil at what is called field capacity. Plants then draw water out of the small pores which hold the water against the force of gravity. The longer the soil goes without water the greater the difficulty the plants will have at getting water, until no more can be withdrawn. The soil is then at what is termed the wilting point and without water additions, plants die.  Like I tell my students, most things in Biology go by the 'Goldilocks Principle'  Not too much, not too little, but just the right amount is what is needed.  You don't want a saturated soil, you will get reduced growth and often have yellow leaves as a first sign you are aver watering.

The amount of soil water available to plants is governed by the depth of soil that roots can explore (the root zone) and as I said the nature of the soil material.  Taller containers will allow roots to grow deeper.  A wide short pot may have the same volume, but it won't hold as much water once the gravitational water is gone.   Because the total and available moisture storage capacities are linked to porosity, the particle sizes (texture) and the arrangement of particles (structure) are the critical factors. I will discuss these terms in the next post..

Good Growing,
Dr. E. R. Myers

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