Soil Water Absorbency
Irrigation Flow Rate Factor
we reflect upon a soil's
Field Capacity [FC]
The percent of water a soil will HOLD
before reaching saturation (see below).
Effective Water Capacity [EWC]
A measure of the ability to ACCEPT water.
which is also the "air space" in the soil.
Wilting Point [WP] for a soil
is a low moisture point where the plant can no longer
utilize this remaining Moisture
These Soil-Water factors have been thoroughly
studied and published for each composition type.
As moisture drops from Capacity towards Wilting,
this mid-way point is known as "Available Moisture"
The MORE absorbent a soil is, the easier for the plants to thrive. Absorbency is essentially the difference between Maximum water held and minimum moisture before wilting. The greater this difference, the longer possible between irrigations. Absorbent soil is like having a large water tank for the plants.
Superior absorbency requires lower WILTING % levels while having higher FIELD CAPACITY levels. Clay has good water holding, but also high wilting levels (due to low capacity for air) - so CLAY is rated low in absorbency.
The purpose for providing an "Absorbency Rating: is to allow comparison bewtween the most absorbent soil types, and help with soil amendment decisions. Most soil amendments are very high in Absorbency (near 100%).
Higher Absorbency Ratings
Absorbency rating is calculated by multiplying exponential factors of the Effective Water Capacity [EWC] and the Field Water holding Capacity [FC] of that soil.
An example; Sand will "allow" a lot of water to be added, due to the large proportion of air space to soil particles, but Sand hasn't the ability to "hold onto that water". So; Sand winds up with only a 16% absorbency. Clay does not allow much water to be added, due to the small proportion of air space to soil particle, yet it does hold onto water well. So; Clay has an even lower Absorbency rating of 11%. Loam, a good composition mix, holds water well, and has excellent pore space or [EWC] so is rated near 70%. Add some organic amendments like a planting mix to any of these and the absorbency is improved.
Using a soil with a higher Absorbency rating,
allows you to apply more water to the pot during each irrigation.
Consider; if (using sand) it's going
to run through the soil quickly,
On the other hand (using clayey soil)
it basically sits on top of the soil
The amount of water that can be applied
in this case will be much less.
An Exception: When irrigating a slow draining soil,
and using a very long run time,
with very few emitters, more water can be applied.
Here the water is being given extra time to be absorbed.
Drought tolerant plants should have soil with higher EWC, even though they use less water. Heavy water use plants require both good EWC and Absorbency. Both of these two factors are very important when deciding how often to reapply moisture.
Due to these factors, the calculator specifies the different quantities of water. If watering a new plant with dry soil, then use the quantity of water specified for that.
Irrigation Flow Rate Factor
The speed at which the irrigation water applied
measured in Gallons per Hour
per Square Foot (gph/sqft)
has a major impact on the wetting
and moisturizing pattern within the soil.
The movement of moisture downward through
the soil is limited by that particular
soil's Hydraulic Condutivity [HCR] rating.
When water is applied in excess of that speed,
there is no place for it to go but sideways.
Water does move sideways to a degree anyway
from osmotic pressure, but this is smaller than
the force of gravity on the water.
When the HCR has been exceeded
and sideways movement is excellerated
The resulting moisture pattern will look similar
to those shown in the graphic above.
This results in wasted water, and encouraging
the growth of weeds.
Therefore Matching Irrigation Application flow
to the HCR of the soil is essential.
This is the primary benefit to utilizing
the Irrigation Matching Calcultaors found in the
Irrigation & Soil Matching Calculators
How many Minutes versus Gallons?
Remember "how many minutes" will need to vary with the number and flow rate of the emitters you use. Most emitters are rated in gallons or fractions of a gallon per hour. We program the controller in run-time minutes, yet what we are concerned with is gallons of water applied. To effectively get the water to penetrate deeply, we need to allow enough time to avoid water spreading to areas not in need of water.
The complexity of this is easily delt with through the use of the "I&S-M Pro-I" calculators. Relying on guess work for scheduling irrigation controllers and clocks is an expensive idea. Both in the health of the plants involved, and the amount of water effectively used or wasted.