Soil and Water Availability I

The medium you use, affects the water holding capacity which will then affect the rhizosphere and over all plant metabolism.. One of the main functions of soil is to store moisture and supply it to plants between watering.  Knowing a soil's texture will help you know what you need to do to improve your soil.  Good growers know that evaporation from the soil surface, transpiration by plants and water drainage by gravity combine to reduce soil moisture between water applications. If the water content becomes too low, plants become stressed. The moisture storage capacity of a soil provides a buffer which determines a plant’s capacity to grow at an optimim rate.
The number one reason plants do not do well or die indoors is that people overwater them. Yes you can love and care for your plants too much.
The problem with over watering is that there is no longer oxygen available to the roots. Roots need BOTH oxygen and water, too much of either will slow growth and limit yields

High H20 = low oxygen, root physiology slows and stops. You will notice leaf yellowing
Low H20 = lower rate of photosynthesis, leaf physiology, specifically photosynthesis will slow and stop as water becomes unavailable to plants.
The amount of water held in a soil is a function of the pore size (cross-sectional diameter) and pore space (total volume of all pores) A balance results due to pore space - big pores drain freely small pores retain H20, a good medium will have varying pore sizes.

Pores > 60 uM - gravity controls H20 in pores - GRAVITAIONAL H20
Very important in air exchange in soils promoted by sand perilite and small stones

Pores 0.1 uM to 60 uM primary source H20 for plants - CAPILLARY H20
Capillary forces hold H20 against gravity so they provide water to plants over lonter perionds Vermiculite,  small perilite particles and organic mater like coir or compost

Pores < 0.1 uM hygroscopic H20 unavailable to plants (held too tightly) Clay

1.Gravitational water is found in the macropores. This water moves through the soil due to the force of gravity. It moves rapidly out of well drained soil and is not considered to be available to plants after a few hours or a day.
Too much gravitational can cause plants to wilt and die because gravitational water occupies air space, which is necessary to supply oxygen to the roots. If you let your plants sit in water, the gravitational water can not drain.

2.Capillary water: Most, but not all, of this water is available for plant growth. Capillary water is held in pores that are small enough to hold water against gravity, but not so tightly that roots cannot absorb it.
Capillary water is held by cohesion (attraction of water molecules to each other) and adhesion (attraction of water molecule to the soil particle).

3. Hygroscopic water: This water forms very thin films around soil particles and is not available to the plant. The water is held so tightly by the soil that it cannot be taken up by roots. Clay will contain much more of this type of water than sands because of surface area differences.

The reason you want a variety of pores in your medium is because as the water is withdrawn, the larger pores drain first. The finer the pores, the more resistant they are to removal of water.  You want large pores to drain quickly so your roots have oxygen, but you want small pores to hold water against gravity and provide your plants water for photosynthesis.

Please read my next post on water availability in soil...

Good Growing,
Dr. E.R. Myers


hillbilly said...

is it safe to use soil and soilless mix as interchangeable terms with regard to indoor horticulture? by soilless mix, i mean commercially prepared peat-based mixes such as ProMix, b'cuzz hydromix, or sunshine mix. when using these types of peat-based soilless mixes, do you recommend an irrigation "routine" of feed, water, water, feed water, water, etc. ? would it perhaps be more effective to feed with each irrigation, but use a 1/4 or 1/2 stregnth nutrient solution? i believe i have had some toxicity issues with manganese - heavy leaching solved the problems. i mix the solution at 1200 or 1300 ppm, ph of 6.0 - 6.5 and check both these parameters with a hannah pen which is calibrated with proprietary reference solutions before each and every use. my irrigation water base come through my htg supply RO100 and i change the stage 1, 2 and 3 filters when the tds reaches 60 -70ppm.

. said...

No, soil is a complex mixture and usually has a biological aspect. Soilless mixes like coir are sterile. This means you have more control over the nutrients and what organisms you add too the mix. Like hydroponics this can mean better growth, but if you don't know what you are doing, it can lead to more problems. I always recomend soil mixes for new growers, until you know what nutrients your plants need. It is hard to go wrong with a good soil mix from HTGSupply.com. I use fox farm often myself.

hillbilly said...

hey Doc - have you ever experimented with any type of "wick" type feeding/irrigating? is so, did it take several set up attempts before it worked well? how many wicks should i be using in a 2 gallon container? i'm using 3 glasscloth wicks per container - this will be the fourth attempt i've made at this "wick" system and i'm about to give up 'cause i can't get it right.