 |
| Understanding soil moisture math makes managing poa in extreme heat a lot easier. Temps were 42C when I took this pic last summer |
Before we could accurately measure the soil moisture we were left to essentially guess. We had to use our limited powers of observation to make judgement calls on how much moisture we needed to apply. We would irrigate at night, and top up any visibly dry areas with a hose. We would then have to go out again in the afternoon to top of the additional areas that had dried out over the day. The only real way to tell if something needed more water was to look and see if the grass was wilting.
It was more difficult to determine if we were over-watering. There's a big difference in the amount of water in the soil between 20% VMC and 35% VMC but it is difficult to tell this difference with your eyes.
Along came digital moisture meters and instantly we didn't have to guess. This made irrigating so much easier and precise. Now we would know exactly how much water was in the soil and whether we needed to apply more, or less water to areas across the golf course.
But I would bet that for many, irrigating didn't become easier. Most turf managers I know like to push things to the extreme. Now that we know exactly how much water we have in the soil, we can dry the greens out to the extreme. The problem with this is the lower you go with average VMC, the more hand watering will be required, and the less water will be available to get the grass through the day especially when the ET is high.
 |
| Digital moisture meters are supposed to make your life easier |
Naturally, when I first got a moisture meter, I tried going low too. This was pretty good for someone who lived on the golf course, had a relatively big crew, and could afford to baby the grass. The grass was better but I was spending just a much time as before.
Times got hard for all the reasons and I just didn't have the time to do all that hand watering. I figured out that if I adjusted my target average VMC for my greens from 20% to 25% or even 30% I could make it through the day without hand watering in the afternoon. I also drastically reduced the amount of hand watering that I needed to do in total because the dry spots were less dry. This was a basic observation that helped me in a basic way.
Now you're probably thinking "OMG hE iS tOtAlLy OvEr-WaTeRiNg!" Actually, no. Overwatering is applying more than the grass can use every day. This was maintaining the moisture level in the soil at a slightly higher level. The grass still uses the same amount of water each day regardless of what the VMC is (to a certain extent obviously). I just turned the irrigation on a bit sooner and irrigated just as much as I always would regardless of what the average soil VMC was.
This "turfhack" saved me a lot of time and heartache especially as my crew size shrunk.
I would put a lot of money betting that not a single golfer noticed the difference between the 20% VMC and the 30% VMC I was now maintaining.
Micah Woods recently shared a Turfhack that Bill Kreuser shared on Twitter to determine how much water you need in your greens to make it through the day.
I've been using Micah's version of this turfhack for years. It really helps me visualize how much water I need to apply and if you use metric, it makes it even easier. Need to raise the VMC by 5% in the top 10cm? Apply 5mm of water! Super easy.
The trouble with this is that it assumes that you have a 10cm rootzone. Normally this is approximately what I have with my poa greens but during the heat dome of 2021, our poa roots shrunk down to 1cm which is actually probably being generous. I'm pretty sure poa likes 42 Celsius less than me and my motto (not unlike poa) is "anything over 18C is silly." Either way, we used an adaptation of this "turfhack" to get through the heat without any supplemental afternoon hand watering and we didn't lose any grass on our greens.
Our fairways were a different story but I'll share more about that in another post sometime.
 |
| Greens are doing just fine. We didn't have enough water for fairways unfortunately as our water supply is dependant on rainfall which doesn't happen when there's also a drought. (funny how that works) |
Our greens had a 1cm rootzone so therefore had access to only 10% of the water that we would normally require with a 10cm rootzone except the ET was also through the roof! The reference ET was 6mm (0.25")! Multiplying this by the crop coefficient that we use which is 65% we get a water use of 3.9 mm or 3.9 liters of water per square meter. A 1cm rootzone has a volume of 10L per square meter. That means that we needed a VMC of 39% to have enough water to get through the day. Now that's WET! It's actually not that wet, it's just wet in the top 1cm. The rest of the soil was as dry as a popcorn fart! Now you're thinking ,"if you water deep, you roots grow deep." As Joe Gulotti would probably say, "not on poa annua at 108 F, Son!)
 |
| A shallow rootzone requires you to adjust your target moisture levels in order to ensure you have enough water to make it through the day. |
It's important to note that you need to adjust your tine depth on your moisture meter to the dept of the roots for this to work. The shortest tines we have are 3.8cm so we just put them in at an extreme angle to determine how much water was in the top 1cm.
At those temperatures and with such a shallow rootzone we actually had to apply supplemental irrigation in the afternoon for a few days only.
Bill's "Turfhack" is even easier although I would suggest you be a bit more honest with your root depth than he is in his example ;)
If we use his method for my situation last year we find that we can take our forecast reference ET of 0.25", multiply this by the crop coefficient of 0.65 and we get 0.1625. Divide this by a rootzone depth of 0.5" gets us 0.325. Add our wilting point which I'll say is 10% because I want a buffer between the daily water usage and the grass actually wilting and that gives us 42% VMC. That's almost exactly what I calculated.
 |
| Grees were doing great despite the extreme heat and lack of afternoon watering. |
Using this math and my moisture meter it actually made managing the soil moisture in our rootzone during the extreme heat pretty easy. Not only did we not lose any grass, we didn't have to spend much time babying the grass in the afternoons to get it through the day.
Sure, the VMC was very high in the top 1cm of the rootzone, but it drastically went down as the day progressed to something around 10% VMC before we fired the sprinkler back up during the night. The daily average VMC was about 25% or near damn perfect if you ask me.
This situation translates to non extreme weather as well. If you understand this math, you can set individual moisture targets that are essentially guaranteed to get you through the day. Hell, during normal hot weather, we only water every few days. We can identify the dry spots and give them enough water for a few days and still keep our soils much "drier" than we did before we had digital moisture meters. We maintain well above what will get us through one day and spend a lot less time hand watering. We still need to top up the dry spots, but can get a few days to a week between these labor intensive practices.
I get that there are some extreme situations like I had last year that require water to be applied multiple times a day, but if you are regularly having to water mid day you might try doing this calculation and adjusting your target soil VMC levels to a point that allows you to go home at 1pm and not worry about it. The target soil VMC in the morning will likely be slightly higher but I bet no one will notice because you are still doing a drastically better job than was even remotely possible a decade ago.
Remember, technology is supposed to make our lives easier. You're probably not hosting the US Open today so why try and have greens wilt mid day?
