When speaking with growers we’re often asked about the various specification metrics used to describe our coco coir grow bags. One of the most important, and the basis for further measurements, is water holding capacity (WHC). By the end of this article growers will have a working understanding of what WHC is, how it’s measured, and it’s importance in developing an irrigation strategy.
What is Water Holding Capacity?
Legacy cannabis cultivators commonly are unaware of this term, though it is commonplace in commercial agriculture circles. Michigan State University defines WHC as “the amount of water that a given soil can hold for crop use”. When I was staring out this was measured by lifting a full pot, then lifting it again every day or so until it felt light. While it sounds rudimentary the principal of this practice ultimately is based on the pots water holding capacity!
How Is WHC Measured?
In the garden, lifting pots to determine their weight, I was using the same measurements (albeit imprecisely) to determine the water needs of the plants as are used to determine the water holding capacity. Here we’ll explain how RIOCOCO uses a standard horticultural research practice to determine the Water holding capacity of our coco coir.
This one is easy, Riococo is already doing the work. Unlike other coco coir suppliers, we publish our WHC numbers. As an example, lets look at the published specs of a Riococo PCM 1 gallon grow bag:
- Dry Weight: 395 g
- Saturated Weight: 2135g
- Dry Volume: 675 cm3
- Saturated Volume: 3375 cm3
To calculate WHC, all we need are two numbers: Saturated Weight and Dry Weight. Dry weight is easy, our quality control team ensures the average weight of 1 Gal PCMs is 395 grams. We then fully saturate and expand the coco coir grow bag before allowing it to drain for 1 hour. Ensuring the bag to field capacity, we weigh it again, determining the Saturated Weight: 2135 grams. WHC is calculated by finding the difference between these two known variables:
- (Saturated Weight) – (Dry Weight) = WHC
- 2135g – 395g = 1740g
- 1g = 1mL
- WHC = 1740mL
The Importance of WHC
When I first figured out how to measure WHC, growers were just starting to realize the benefits of micro irrigation. An old white paper, dug up from the bowels of google scholar, explained how tomato growers were using shots as small as 2% WHC to encourage vegetative growth! This was a game changer, and I promptly purchased a water-resistant kitchen scale to find the water holding capacity of the circa-2014 bagged coco.
The Riococo team spent two decades working with commercial fruit and vegetable growers who demand precise coco coir specifications for maximizing the yield of their low-margin (compared to this industry) crops. When growers in our fast-evolving industry develop irrigation systems and SOPs, Riococos published WHC is a welcome known quantity in a market of unknowns. When you want 2% shots for quick vegetative growth or to add bulk to that vista of colas, you know 1740ml * .02 = 34.8 ml (just over 60 seconds for a 0.5 gph drip emitter). Does the irrigation manager need fat 10% shots to avoid the stretch and keep those internodes close? With Riococo, take that WHC of 1740 ml, move the decimal a space to the left, and set the irrigation controller to provide 174 ml (~5min 15sec for a 1/2 gph emitter).