A few posts back I talked about how I found it useful to compare a 30 day running average growth ratio to a 30 day running average nitrogen genki. This would tell me how my grass was growing and how much nitrogen I had applied compared to the standard level which I have determined to be about 2/g nitrogen per square meter per month at a 100% growth potential.
After thinking and using that tool for a few months now, I wondered about the other nutrients. Last year I had issues with the grass not growing as expected and it turned out to be a phosphorus deficiency probably made worse by regular phosphite applications.
The growth ratio model showed that the grass was growing too slow and in reality it probably was as we suffered from issues caused by slow growing turf such as Anthracnose! The more experience I get with the growth ratio model, the more valuable it becomes as a tool to help me keep the growth rate where I want it and plant health issues to the minimum. Most issues I have had with the greens can be clearly seen when you look at how the grass is actually growing compared to how it should be growing.
I had soil tests and thought I had applied sufficient fertilizer but what I think may have happened was that as the roots shortened in the summer heat, the phosphorus in the surface layers of the soil were used up. The deeper rooted bentgrass was healthy and happy but the predominantly poa surfaces were suffering until phosphorus was applied.
This got me thinking about what it would look like to include a genki level for phosphorus and potassium as well. We know that potassium in cool season turfgrass is found at a ratio of about 0.5 compared to nitrogen and phosphorus is around 0.13. With this I can compare my applications to the standard amount.
If the nitrogen genki is above 1 but growth is lower than expected, I can look to see where the other macronutrients are in respect to their standard amount. If growth is higher than expected I can stop applying nutrients altogether until the growth ratio returns to where I want it to be.
I currently look at 30 day running averages for everything but think that an annual running average might be appropriate for something like phosphorus which is used in a much smaller quantity than the other macros and is relatively immobile in the soil.
There are times when soil nutrient status won't impact growth rates and this is when either moisture or frost come into play. Without sufficient moisture grass won't grow and when there's frost, we don't get much growth either.
I wonder if something like this could replace soil testing for the high use macronutrient content in the soil? Soil testing provides a general snapshot but this provides an up the the day idea of how the grass is growing and how my macronutrient applications might be affecting the growth rates.
It could also potentially provide a more precise tool for managing bentgrass vs poa annua too. In my experience, the deeper rooted bentgrass has more access to nutrients and has always had a much higher growth rate than the poa. I hypothesize it my last post that it seems that fertilizer applications might be the best way to manage bentgrass over poa and this might be another tool that I can use.
As bentgrass populations rise the growth rates on those greens tends to go up. Again, this is probably due to the deeper roots having access to more nutrients in the soil and the tendency of bentgrass to grow more for a given amount of nutrient inputs. These two things combine to leave the greens with more bentgrass growing much quicker than those with mostly poa annua. Micah Woods hypothesized that the most sustainable grass might be the one with the highest growth rate for a given amount of inputs. I agree.
As more and more bentgrass populates the putting greens I eventually need to start managing for bentgrass and not poa annua. Currently I need to keep both species happy so avoid a loss of turf quality. It's a fine balance and maybe this tool can help me do better with less guesswork. The practicality of managing greens to that extreme might prove to be a bit too ambitious but without the data you can't even begin to dream about doing anything like this.
Let's forget about that for a minute and look at the issues I had with growth in 2019 and how the genki level for the big 3 macronutients looked. For those sharp data people out there a few issues become glaringly obvious with models like this especially in the months with little to no growth. A light fertilizer application can throw that ratio wildly high as the ideal amount is close to zero and the closer to zero the ideal amount of fertilizer is, the more any amount of fertilizer applied will throw the model off towards infinite. You can see this impact on the chart below. Everything has this curved shape to it. This is kind of the reverse of the growth potential curve for my climate. Anyway.
Holy Shit! When you're not used to looking at something like this is looks like a rat's nest! That's ok and we will filter out some of the data to look at it individually. I've filtered the dates for the growing part of the season. You can still see how high some of the genki levels will go in the shoulder season.
It might look super inconsistent like this and it is! It will be interesting to see how increased awareness will impact my fertilizer applications going forward. Also if you want to criticize me, first show me your macronutrient and growth ratio chart. It might look like I have no clue what I'm doing and to be honest I really don't. The deeper you look into fertilizer use the more you will realize how much guesswork is involved. I'm just trying to reduce that guesswork.
The thick dark blue line is the growth ratio and as you can see, the growth rates in 2019 were less than I wanted them to be. Greens were fast but also anthracnose!
Here's the same chart but just with the N Genki 30 day and Growth ratio.
In the spring we were recovering a few greens that died due to civitas applications too late into the winter. Don't apply civitas in the winter unless it's warm and sunny.
The growth ratio was pretty good until June when growth rates typically pick up. As soon as the heat arrived, the turf growth didn't keep up to expectations. It's important to remember that this is a growth ratio, it isn't total yield. The growth rate stayed rather constant or slightly down but it should have been much higher.
I've seen this before with nutrient deficiency issues. A neighboring course had growth stall after spring aerification and the more nitrogen they applied the worse things got. As they tried to increase growth (and the weather warmed up) they also increased the demand for the other soil nutrients and thus made the deficiency worse. It turned out that they were low in calcium and a week after a calcium application the greens were as good as ever despite what some people said (2 weeks to live my ass). This is exactly what happened to us last year. As soon as the demand for nutrients went up, the deficiency got worse. The total yield and growth ratio for June was much lower than we wanted and we paid the price.
For the entire year we were applying much more nitrogen than we needed to try and drive growth rates when in reality we were probably making things worse.
Now let's look at K Genki.
For the most part, the K genki is high all year long. In August when the anthracnose started to get bad I started applying higher N and K as this is recommended in most of the literature. In reality these nutrients only have an impact on anthracnose if they are deficient. They weren't deficient. I haven't read anywhere that low phosphorus can make anthracnose worse. In reality I don't think it matters. To someone that measures growth like I do it's not about any nutrient in particular, it's about growth plain and simple. If the grass is growing at an adequate rate it probably won't have many issues. If your grass is growing too slow and it's poa annua, watch out!
Now, let's look at the P genki below. I've also included a weekly running average growth ratio to see the impacts of P on the growth rate in more detail.
Again, crazy high genki in the shoulder seasons. Ignore that probably.
For most of the season the genki level for P was zero. I wasn't applying much P because my soil test showed that I had enough. The issue I think was with the soil testing depth. The depth was down to 10cm but the roots at this time were barely 5cm due to a hot dry summer and poa being the shitty grass that it is. The roots issue probably compounded itself because the growth rates were too low. Low shoot growth rate and probably a low root growth rate too.
Mid August I applied a bit of P. You can see a big jump in the 7 day growth ratio. I could also tell a big difference in the turf appearance but the growth ratio started declining again. Another P app in early September and BAM, another big jump in the growth ratio. After that the ratio's trend towards infinity but it was safe to say I have finally figured it out.
I was also having trouble determining how much P was enough. With the genki P you can easily see how much is enough. That first application I made in August was only 25% of the monthly P requirements. Once I applied more than a month's worth we finally saw good sustained growth.
The growth tapers off through October but this is due to other factors like raising the height of cut and less frequent mowing after aerification etc.
You also notice that the genki goes up even after application dates and this is because the nutrient demands are decreasing each day as it gets cooler so what was a month's amount of fertilizer in early October, is a year's worth of fertilizer at the end of October. I don't know enough about modeling to fix these issues yet. Maybe one day I'll learn and improve this.
Fast forward to 2020.
Here's the N genki and monthly growth ratio.
Right away you can see that the growth ratio is much better if a little high. I wasn't going to risk things this year and I wasn't sure how all this really worked so I was playing it safe.
In the spring the N genki and growth ratio closely mirror each other. On an adequately fertilized turfgrass soil N generally is the limiting nutrient so you can see that seems to be the case for the Spring of 2020.
When we get into August the typical mineralization event occurs which releases nutrients from the soil organic matter. This is something I've observed over many years at my last golf course property. I was relieved to see it again this year. N genki has remained low this year as we have had more than sufficient growth.
This shows me that if you growth ratio is low and N genki is high, you need to look elsewhere for the cause of the low growth assuming you aren't using crazy growth regulators or lack of water to slow growth. I do use Primo Maxx on a 200 GDD interval and this is worked into the growth model I have made.
What about the K genki?
I was going to make damn sure that K wasn't the limiting nutrient this year as can be seen above. I wasn't using this tool until early October 2020 so I wasn't aware of how my K applications looked compared to growth.
In September you notice that the K genki and growth ratio seem to be similar. The N genki was around 0.5 at this time but K was even lower. Eventually the growth ratio went down but started trending upward after a mid September K application. As we were getting great growth due to mineralization I assume there was more than enough free nitrogen in the soil. Perhaps K was starting to become a limiting nutrient here because there may have been a growth response. K is relatively free in the soil and is used in relatively high quantities. We were removing lots of nutrients from clippings in August and any K that was mineralized may have been lost or something. There's no way to know this for sure but I can speculate.
Now that growth is slowing for the Fall, you can see the K genki trend upward.
Let's look at the P genki