Field Capacity 101

This is especially true in an indoor or greenhouse facility where humans must stand in for Mother Nature to create a nurturing environment for their plants to thrive. For cultivators on a mission to improve their yields using crop steering techniques, one major component of the job is adjusting the irrigation in response to vegetative and generative cues. And that requires regularly monitoring water field capacity.

What is Field Capacity?

Field capacity is the point after an irrigation event (or shot) at which a grow medium can no longer hold any more water. Once a substrate has achieved maximum water holding capacity, adding more water will cause it to seep as leachate (or runoff) from the bottom of the container. Typically measured as a percentage, field capacity lets the cultivator know the amount of water needed to ensure optimal growth and nutrition for their plants.

It’s not unusual for growers to conflate field capacity with saturation and Volumetric Water Content (VWC), both of which are related to irrigation – but they aren’t the same. Saturation occurs when water completely fills the spaces between the soil particles. Optimal field capacity, on the other hand, is achieved when the spaces between the soil particles contain both air and water. And while the field capacity refers to the amount of water in the substrate prior to runoff, VWC speaks to the amount of water a substrate is holding relative to its total volume. When the VWC in the soil is too low, the plant’s roots are unable to take up water as a result.

How Does Field Capacity Factor Into Crop Steering?

Crop steering is a plant growth management practice that manipulates light, climate, and irrigation in an indoor or greenhouse environment to encourage plants to grow a certain way.

Knowing how much water a grow medium can take before runoff happens is crucial when crop steering, because the process requires using specific irrigation tactics to get the plant in a state of water stress during key moments of its growth cycle.

When a plant is producing leaves, stalks, and shoots in its vegetative stage, water needs to be as easily absorbed as possible—and it takes high irrigation frequency to get the job done. Once a plant reaches its generative stage and is preparing to flower, however, targeted stressors must be used to keep the focus on reproduction—low irrigation frequency makes this possible. Unless cultivators take the time to test field capacity, trying to figure out the size and frequency of each shot becomes a guessing game.

At the end of the day, field capacity is a metric that growers will need to reference time and time again through the different phases of plant growth – which is why it’s important that it be measured and recorded accurately right from the start.

How to Test Field Capacity

Natural variations in the substrate may alarm growers who are unprepared or don’t understand it. Not only does field capacity vary between different substrates, but variations can also occur between brands as well as batches.

Rockwool, for example, tends to maintain a consistent field capacity from substrate to substrate, with some variation likely between vendors. Coco, on the other hand, is minimally processed and blended from natural ingredients – ​​typically coconut husks, pith, and chips – so achieving uniformity when hydrating can be difficult since variations are likely everywhere, from its weight to the way it’s packed by the vendor. And cultivators who use perlite mix or their own substrate blend will experience higher variances since these are the least standardized of all.

To be sure they have the most accurate measurement and are able to account for any variations, it’s important that growers test the field capacity. In fact, getting in the habit of testing field capacity anytime there’s a significant change – e.g., when setting up a new grow, bringing in a new substrate, or there’s turnover on the grow team – goes a long way toward setting up your cultivators for success.

Using a sensor like the TEROS 12 is the best, most reliable way for growers to not only test field capacity, but also monitor what’s going on with their grow media at all times. The more growers stay informed about the water content and electrical conductivity (EC) in their substrates, the better equipped they’ll be to respond to the generative and vegetative cues that lead to bigger buds and better yields with every harvest. Here’s how to test field capacity using a substrate sensor like the TEROS 12.

Materials needed for testing:

• Pre-dehydrated medium (e.g., a 2.2 gallon Dutch Plantin coco bag). If using soil, bake in an oven to dehydrate (and prepare for the aroma)
• A scale
• A measuring cup
• A tote or container to hydrate the substrate
• A substrate sensor (e.g., TEROS 12)
• A notepad and pen

Instructions for Testing Field Capacity:

1. Weigh the pre-dehydrated substrate. In this example, our substrate weighs 899 grams
2. Hydrate with the total volume of the substrate. In this example, a 2.2 gallon coco bag will need 2.2 gallons of water
3. After the substrate is fully hydrated, wait 20-60 minutes
4. Weigh the substrate and take note of the weight. In this example, our hydrated weight comes out to 4,729 grams
5. Subtract the dry weight from the hydrated weight. In this example: hydrated weight of 4,729 - dry weight of 899 = 3,830 grams (NOTE: 1ml = 1gram)

Divide the difference from dry to wet by the total volume of the substrate. In this example: 2.2 gallons = total volume of 8,327 ml. Hydrated-dry weight (3,830 grams) ÷ by total volume (8327) = field capacity of 46%

There’s a throughline between intelligent water management and better yields. For growers who crop steer their plants, understanding water capacity enables them to respond to vegetative and generative cues – and their respective shot needs – consistently and reliably with every harvest.

We built the AROYA cannabis production platform combining hardware and software to give cultivators greater insight into all aspects of their cultivation – including field capacity. By tracking and monitoring their grow at every stage, cultivators are better able to spot and respond to trends in real time.

For a deeper dive into field capacity, read "Plant available water: How do I determine field capacity and permanent wilting point?" by ADDIUM Inc.'s own Dr. Gaylon Campbell, and our Instagram crash course featuring Ramsey Nubani.

Reference:

Warrick, A. W., H. M. Van Es, R. R. Allmaras, Oscar Kempthorne, J. J. De Gruijter, S. R. Yates, O. Wendroth et al. "Methods of Soil Analysis: Part 4 Physical Methods, 5.4." (Article link)

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