When speaking with cultivation professionals we’re often asked about the various specifications 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?
Michigan State University defines Water Holding Capacity (WHC) as “the amount of water that a given soil can hold for crop use”. For field-grown crops and lower quality soilless media this is determined by measuring multiple samples.
How Is WHC Measured?
Here we’ll explain how RIOCOCO uses a standard horticultural research practice to determine the Water Holding Capacity of our coco coir.
Unlike other coco coir suppliers, we publish our WHC numbers. As an illustrative example, lets look at the published specs of a Riococo 250 grow bag:
- Dry Weight: 2.05 kg (2050g)
- Saturated Weight: 11.603 kg (11603g)
- Dry Volume: 4176 cm3 (4.176l)
- Saturated Volume: 18000 cm3 (18l)
Two variables are needed to calculate WHC: Saturated Weight and Dry Weight. Our quality control team ensures the average Dry Weight of 1 Gal PCMs is 395 grams. We then fully saturate and expand the coco coir grow bag before allowing them 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
- 11.603kg – 2.05kg = 9.553 kg
- 1kg = 1l
- WHC = 9.553l
The Importance of WHC
The Riococo team has spent two decades working with commercial fruit and vegetable growers who demand precise coco coir specifications for maximizing the yield of their crops. When you want 2% shots for quick vegetative growth, you know 9.553l * .02 = 191 ml (1 min 55 sec. for a slab with 3x 0.5 gph drip emitters). Does the irrigation manager need 10% shots for generative growth? With Riococo, take that WHC of 9553 ml, move the decimal a space to the left, and set the irrigation controller to provide 955 ml (9 min 33 sec for a slab with 3x 0.5 gph drip emitters).