When speaking with growers we’re often asked about the various specifications of 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, VWC is a common term 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).
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 a RIOCOCO 250 grow bag, and the WHC calculated in a previous post:
- Dry Weight: 2.05 kg (2050g)
- Saturated Weight: 11.603 kg (11603g)
- Dry Volume: 4176 cm3 (4.176l)
- Saturated Volume: 18000 cm3 (18l)
- WHC = 9.553l
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 = 9.553l). We then measure the volume of a fully saturated and expanded coco coir grow bag in liters (18l) WHC is calculated by finding the quotient of these two known variables:
- WHC / (Saturated Volume) = VWC
- 9.553l / 18l = 0.5307
- 0.5307 * 100 = 53.1%
- VWC = 53.1%
This means that when fully saturated, the media in RIOCOCO 250 grow bags are ~53% water.
The Importance of VWC
VWC is integral to crop steering. 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.
Precision irrigation practices are key to achieving repeatable yields. Repeatable yields are the key to success in any operation. RIOCOCO has spent two decades developing and improving our coco coir grow bags for horticultural professionals. With inflation driving diminishing prices, do you have room for the time and costs which are part and parcel of using an inconsistent growing media?