#116 - Field Capacity My Ass!

I've titled this episode, Field Capacity My Ass, because honestly, using terms like field capacity can easily confuse the average person.

So I wanted to take 5 mins today and share my experience understanding field capacity, its importance, and some tip to help you better understand it.

#1 Let's define field capacity

Soil field capacity refers to the maximum amount of water that soil can hold against the force of gravity. It is the point at which the soil is fully absorbed with water and excess water has drained away. At field capacity, the soil is holding as much water as it can, and any additional water that is added to the soil will drain away due to the force of gravity.

Seeing this on a graph is the best way to understand this concept. The graph looks like a tall spike, then a slow curve down. It is the slow curve that illustrates field capacity.

Another way to describe field capacity is WET. Not too wet, not too dry, just right. It's Goldilocks. And you can measure it. It's a scientific measure. 

A second way to think about this is like a percolation test. Where you dig a hole, fill it up with water, and watch it drain over time. The faster it drains, the more sand is in the soil, and the slower it drains, the more clay is in the soil. Or - based on the drainage rate it can also describe the soil compaction and soil structure because the soil type is only one variable that will determine the drainage rate - and Field Capacity.

Ok - got it?

Field Capacity = perfectly Wet. Goldilocks wet.

Now, here is where it gets really fun, and where current manufacturers have kind of missed the mark - in my opinion.

In my opinion, they have missed the mark because they forget to think about the user.

Engineers know exactly what all of this means, and they know exactly how to set the sensors, and have almost 20 years of experience, and I know how to set sensors. However, the average knuckle dragger has no idea, and the first question they always ask, every time, is how to set the sensor.

Let's talk about this.

Field Capacity, allowable depletion, permanent wilting point, etc. can be confusing as hell.

Here is my tip: If field capacity equals Wet, and Wet equals Full, then Full must equal 100%.

The soil profile is a tank, and at field capacity, the tank is full. 100%

This is where manufacturers should set the calibration. Instead of displaying a percentage of volumetric soil moisture, they should allow the user to set the tank, and instead, display a % full.

How much water is in the soil tank.

Is it 100% full, 50% full, or empty.

It doesn't really matter what the actual volumetric soil moisture percentage is. It matters more, what the percentage is relative to field capacity.

let me give you an example.

I get a phone call and the customers say, "Hey Andy, my moisture sensor is reading 28.5%, what does that mean and how do I set the sensor?"

My response - I have no freaking idea.

Why?

because I need more information. I need to know what field capacity is before I can understand 28.5%

Is this making sense now?

Maybe 26% is field capacity, which would be 28.5% is saturation.

Or, maybe the field Capacity is 32%, which would mean the soil tank is 50% full.

Every moisture reading must be compared to the field capacity, of the same sensor. Not the field capacity of another sensor, only the field capacity of the same sensor.

for this reason, I always recommend pouring a bucket of water over the sensor, or enough to actually saturate the sensor, then wait for 24hrs, like a Percolation test, then taking a reading and seeing what it is - or running a report remotely and look at the data.

Write that number down. That is your Baseline - pun intended!!

My very general broad approach which works most of the time, is to take away 20% of the number and use that as depletion.

Now you know your tank size.

Let's pretend 30% = Field Capacity or Full

using my 20% depletion rule, dry would be what?

if 10% of 30 is 3%, then 20% of 30 is 6%.

Tank size = 6%

So your tank size becomes 24% dry, 30% wet.

That is what you're working with.

So, now that you understand, and if you don't listen to it again, I can explain how different soil types have different tank sizes.

Clay soils hold more water. So field capacity of clay soil will be higher. Let's say 33%

Sandy soil holds less water. so the Field Capacity of sandy soil will be lower, say 22%

if we use my general rule of 20% depletion, 20% of a bigger number is a bigger tank. So the more clay in the soil, the larger the tank size, and the more sand in the soil, the smaller the tank size.

And the smaller the tank, the more frequently it will need to be filled up.

The larger the tank, the less frequently it will need to be filled up.

If you have a 12-gallon gas tank, you'll fill it up more often than if you have a 20-gallon gas tank.

That's it. That's all there is to it, essentially. It can be so easy to way overthink this, to overcomplicate this, and to miss the point.

The point is that you must first identify Field Capacity, and that becomes your baaabaaba Baseline.

And everything becomes relative to that.

this is why, in my opinion, it is actually impossible to manually input the soil type into the ET Formula. Because it is essentially impossible to tell the soil type without sending it to a lab, and even the lab can't tell how compacted the soil is, or what the soil structure is.

You can only determine the soil type by measuring field capacity with a soil moisture sensor.

So wrapping this up. I believe that sensors will be the next revolution that we see in this industry, and as it relates to the soil, everything is relative to Field Capacity. Learn it. Understand it. Have a beer and think about it. Field Capacity, My Ass.