In the Weeds • August 03, 2021
Testing Field Capacity?
Speaker 1: Field Capacity, what is it? Field capacity represents the maximum amount of solution that a medium could hold before you achieve runoff, all right? So it's the max amount of water that any medium can hold before you achieve runoff. That's the general idea of what field capacity is. In rock wall, you tend to have a higher field capacity, depending upon which substrate you choose. So slabs, they tend to be between 70 and 80%, Hugo Blocks tend to be between 50 and 65%, most coco is between 40 and [00:00:30] 55%. Some coco, if they have more pith in it, then you're going to have a higher field capacity. I've seen some blocks, that are all pure pith, have 65, 70%, versus like maybe Rio coco or Dutch Plantin, having a field capacity of about 56%, 55%.
So in this scenario, we're going to be looking at a two gallon cocoa bag, and what this all represents. So two gallons of coco [00:01:00] equals 7.57 liters of substrate. So to take liters and turn it into milliliters, we're going to multiply it by a thousand. So 1,000 x 7.57 = 7,570 milliliters. In this scenario that we're reading, a water content of 56% at max saturation, that's going to be 7,570 milliliters x 56% = 4,239.2 milliliters. So in this scenario, this medium [00:01:30] is holding a max field capacity of 56%.
So how do you check this? If you took 7,570 milliliters of water, and you poured it into that two gallon bag, you would then see the difference of 7,579 is 4,239. That would be what ran out the bottom, all right? So that's an easy way to check and see what your field capacity is. Is pouring the max, the same volume of liquid that [00:02:00] the substrate is, and whatever it didn't hold is the difference in what its field capacity is, versus what you actually apply to that medium. So that's just a roundabout way of checking this. If you don't have a substrate sensor, you literally could take your medium, saturated it in water, and then see how much it held versus what's there. And key to note, if you do this in a bucket, and the substrate's sitting in the bucket, you have to hold it above that bucket for a period of time, to watch those drips come out until it stops dripping. Once again, that represents field capacity. [00:02:30] It means it can't hold any more water, and what has ran out is it's leaching.
Now, what does a shot size represent? A shot size of 3% of a two gallon medium means 3% x 7,570 milliliters = 227.1 milliliters. So a 3% shot size of a two gallon substrate is 227 milliliters. So your shot size is directly reflected of your [00:03:00] medium size. So depending upon which medium you have, and how much of a volume of medium that is, your 3% will differ according to that number. Now, why do people talk about giving 3% versus a 6%, or 3% versus 10%? It all has to do with what the capillary action is of your medium, and making sure that you're not creating channel. So what we recommend is, that we recommend only applying the irrigation event no more frequent [00:03:30] than every 15 minutes. We find that if you apply it much more frequent than that, you can get what is called channeling.
Now channeling means that in your medium, you applied too large of an irrigation, and the water... or too frequent of irrigations... and the water, [00:04:00] instead of being absorbed by the medium, starts to pull down and out, because the greatest acting force on watering and your medium is gravity. Now water has cohesive and adhesive principles. Cohesion means it sticks together, adhesion means it sticks to other things. I'm sure you've learned this from other lessons of mine. So what this means is, if you apply too large a volume of water at one point in time, gravity is going to take hold of it, pull it down and out. If you're applying small irrigations and timing them properly, [00:04:30] the adhesive properties, the capillary action of your medium, will take hold of the water and allow it to act as a conveyor belt, slowly moving water down through the medium, and wicking it out to the sides. So the idea of applying small, slow irrigation events, is that the capillary action in the medium will allow it to take hold [00:05:00] and move it down and out from gravity. So the idea of that is to always manage it.
Now, a lot of people that have bought sensors recently are asking, "Well, why am I getting a 100% water content?" It's because you're placing the sensor too low on the medium. We put a guide, because with all mediums, the top 1/3 of the medium, from here to here, is drier than from here to here. And the bottom part, from here to here, [00:05:30] is always the most saturated. So this is the highest saturation, this is middle saturation, this is low saturation. So placing the sensor in the correct spot will represent what's actually happening in the medium, because as I said, the top third of the medium is drier than the bottom two thirds, and the bottom third of the medium is always going to be most saturated. So somewhere in this middle area, bottom to middle area, is where you're going to want the sensor, and we're always trying to figure [00:06:00] out where that is. We're dialing in, we're doing our own trials, but it's really on you guys to figure out where is that going to be the most accurate reading, according to what the field capacity is of the medium. So when you hit that saturation point, and you start to achieve leaching, that's usually representing that you hit field capacity, because you're now getting run off dirt.
So what do you do with this 3% number, knowing that shot size? Well, if you have a sensor, and you measure that the water content of your medium before you begin your irrigating [00:06:30] in the day is 40%, and your target is 56%, you can take 56 minus 40, giving you 16. Then you take 16% and divide it by 3%, because that's what you know your shot size to be, and that equals 5.3. So in order to re-saturate your medium, you're going to need to give six irrigation events at minimum of 15 minutes apart. Now, how do these numbers take effect if you wait too long? Well, if you wait too long and your medium's drying back 3% [00:07:00] per hour, and you give one irrigation every hour, well then we already know that you're never going to raise your water content, because by the time you applied your second irrigation event, you've already dried back 3%. So it's important to know what your dry back time is.
How do you figure that out? You take a reading at full saturation, when your medium is starting to achieve runoff, and you find out what that reading is. Let's say it's 56%. So your graph then here starts at 56%, [00:07:30] then one hour later, you take another reading, and you read what it is. So if it reads 53%, then that means that one hour dry back equals 3%. So in this scenario, as long as you're applying the irrigation once every 15 minutes, that means that you would be getting six irrigation events in, [00:08:00] in this one hour time period... sorry, four irrigation events in. So that means that you'd only see an irrigation... If you did one irrigation every 15 minutes, you would only see an increase of 9%. Because remember, in that 3% time, you dried back... I mean, in that one hour time, you dried back 3%. So in this scenario, if you're trying to re-saturate this medium in a one hour period of time, and you're drying back 3% per hour, [00:08:30] then we already know that you're going to need to apply a larger irrigation event if you want to re-saturate up to that 56% in a one hour period of time.
In that case of it, it would end up being... so it'd be 50, 16% + 3 = 19, and then 19... let's divide it by 4, [00:09:00] because we want to get four irrigation events to re-saturate, so that's one every 15 minutes. So divide this by four, and then you're going to end up with... It's basically going to be a 5% shot size. So if you applied four 5% shots in a one hour period of time, starting at 40%, you would get back up to your 56%. So if you guys followed how I did that, [00:09:30] what I did is I said, "Hey, if my time of dry back typically in one hour is 3%, then if I want to re-saturate the medium from 40% to 56% in a one hour period of time, there's a delta of 16%, plus the 3% that we dry back per hour, giving us 19%." And I divided that by four irrigations, because we're going to apply one irrigation every 15 minutes, so that means in one hour you [00:10:00] could apply four irrigation events. So 19 divided by 4 equals 5, so that means that if I applied four 5% shots, I would take my medium from 40% to 56%. And that's kind of a roundabout way of doing calculations on your irrigation events, and the frequency that you want to do it. Now, one thing that I-