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 for crop steering is volumetric water content (VWC). By the end of this article growers will have a working understanding of what VWC is, how it is measured, and its importance when establishing yield-boosting crop steering practices.
What is Volumetric Water Content?
Much like WHC, many old-school cannabis growers are unaware of VWC, though it is commonplace in commercial ag and with researchers. Cornell University defines Volumetric Water Content as “the volume of water per unit volume of soil, expressed as a percentage of the volume” (Geohring et al. 11). When first starting out growing in coco coir, this concept of measurement was completely alien! We just watered the plants until we saw runoff, thinking nothing of using precise volumes of nutrient solution
How Is VWC Measured?
RIOCOCO uses a standard horticultural research practice to determine the Volumetric Water Content of our coco coir grow bags. Unlike other coco coir suppliers, we publish our VWC numbers. For this calculation we’ll use published specs of the Riococo PCM 1 gallon grow bag, and the WHC calculated in a previous post:
- Dry Weight = 395 g
- Saturated Weight = 2135g
- Dry Volume = 675 cm3
- Saturated Volume = 3375 cm3
- WHC = 1740 mL
To calculate VWC, we’ll use two variables: WC (Water Content) and Saturated Volume. For water content, we’ll use WHC, which was previously determined (Saturated Weight – Dry Weight = 1740mL). We then measure the volume of a fully saturated and expanded coco coir grow bag in cubic centimeters (3375 cm3) WHC is calculated by finding the quotient of these two known variables:
- WHC / (Saturated Volume) = VWC
- 1740 mL / 3375 cm3 = 0.5155 ml/cm3
- 0.5155 * 100 = 51.55%
- VWC = 51.6% ml/cm3
This means that when fully saturated, the media in RIOCOCO 1 Gal PCM grow bags are ~52% water.
The Importance of VWC
When I first figured out how VWC is measured cultivators were beginning to experiment with pushing dry backs as a part of crop steering practices in cannabis production. Smaller dry backs push plants towards vegetative growth, while larger dry backs push them towards generative growth. We can determine the size of these dry backs by finding the difference between the VWC after the last irrigation event of the day and the VWC before the first irrigation event of the next day.
For example, after the final irrigation event on Monday we’re seeing runoff from our grow bags. One can deduce that they’re fully saturated, and therefore have a VWC of 52%. On Tuesday, just before the first irrigation cycle, we take a measurement (either by weight or with a sensor) and determine the VWC to be 22%.
To see how we’re steering the crop, let’s do a bit of subtraction to find the dry back:
52% – 22% = 30%
30% is considered a rather large dry back. This is increasing the EC within the medium, decreasing the overall WC, and pushing the plants toward generative growth.
Implementing standardized irrigation practices is key to achieving repeatable yields. Repeatable yields are the key to success in any operation. RIOCOCO spent nearly two decades developing and improving our coco coir grow bags for horticultural professionals. With a flooded market and diminishing prices, do you have room for the time and costs which are part and parcel of using an inconsistent growing media?
RIOCOCO builds its business from the success of our clients. We’re here to stay, are you coming with us?