A very big part of any IPM program is turfgrass nutrition and arguably the biggest part of turfgrass nutrition is nitrogen. Since starting out in the turf industry I have always struggled with how much nitrogen I needed to apply to keep my turf healthy. What even is healthy turf?
|Is this turf healthy?|
I have always felt that nitrogen was the biggest part of turf nutrition but I had no real way of easily knowing what the turf required. The other required plant nutrients were easier for me to work with as soil tests painted a pretty clear picture of what I had in my soils. Nitrogen on the other hand is a bit trickier to test for and I always struggled to know exactly how much nitrogen I should be applying to keep my turf "healthy." Nitrogen is one of those things that you can apply to your turf and see immediate results a day later. There isn't much else out there that give the immediate and obvious results that nitrogen does (maybe iron) so this was my main focus when trying to combat turf diseases the past few years.
There is a lot of data out there about nitrogen rates and their effects on plant disease. Fusarium, for example, is much more prevalent when more than 1.8kg of nitrogen is applied to 100m2 per season. This was a great starting point for my nitrogen fertility program as fusarium was the most destructive and prevalent turf disease on my course. This number was a great starting point but it really didn't help me much. How much and how often? How did I know how much my specific plants needed this week vs next week.
It wasn't until recently that I came across the GP (growth potential) nitrogen use calculations which made it easy to calculate how much nitrogen your turf needed based on the climate for your specific site. AT LAST! A straightforward way for me to determine my turf nitrogen needs. I switched over to this way of fertilizing this past summer. At first I based my rates on monthly average temperatures but in talking with Dr. Micah Woods from the Asian Turfgrass Center he urged me to base my calculations on the next week's forecast. I thought "why not?" and made the switch. After all it only added about 2 minutes of extra work for me to determine this over my previous methods. This was a way for me to really fine tune my nitrogen fertility rates. This is all great but how does this tie into my IPM program and how do I practically use nitrogen to control disease?
What I am thinking is this. If we look at the "disease triangle" we can easily see that in order for there to be infection there needs to be the pathogen, the susceptible host and the favorable conditions for infection. The pathogens are always there, deal with it. We don't always have control over the environments but we can monitor it and forecast disease threats. In my climate my disease threats are mainly fungal in the forms of Fusarium Patch, Dollar Spot, and now Yellow Patch. When you look into these disease and the cultural practices that offer the most impact nitrogen fertility often comes up. This makes sense as it is the nutrient that has the greatest affect on plant health. Generally Fusarium is most damaging under high N fertility, Dollar spot and Yellow patch are most damaging under low N fertility. The problem with the way I used to fertilize is that I was guessing what the plant needed and inadvertently making these disease issues worse by doing so.
I was always taught that cool season turf sees increased growth in the spring and fall and during the warm summer months it saw a decrease in growth rate due to the warm temperatures. Well this was very true if you were managing cool season turf in California! Turns out this growth rate model doesn't apply to everywhere because, you guessed it, not all sites are the same. So what research was telling me is that in the spring I was to apply higher rates of N to kick start growth and to match the supposed high growth rates. This was during a time when the temperatures were the most favorable for fusarium attack. So I'm applying lots of N when lots of N promotes the most prevalent disease? This doesn't make sense. But again I was going off of what I was taught and attempting to use nitrogen to produce a "healthy" turfgrass plant. At least my intentions were good.
Again my problems were worsened in the summer. Research told me that I was to reduce my N rates in the summer as the turf growth rates were slower and didn't require as much. Again, the most prevalent disease in the summer temperatures in my climate was dollar spot. I was dealing my turf a double whammy on this one. Deficient nitrogen applications and increased dollar spot activity! No wonder we got smoked every summer!
|Look! I found some dollar spot! This was the extent of the damage|
on my putting greens this summer!
|The fairways had a "little" dollar spot!|
This fall I made plans to combat fusarium but none of those included anything to do with nitrogen. When I look back, the biggest difference to my maintenance practices this year was nitrogen rates. This year I had virtually no issues with fusarium patch. A traditional pesticide application was made in late September but my knock out trials showed that this was unnecessary. Ask any of the other superintendents around here and you will hear about the "Fall from hell" as far a fusarium is concerned. Not here it wasn't.
An interesting aside here about disease and shade. This year I broke my putting green soil tests into sunny and shady greens. Not surprisingly the sunny greens had significantly less nutrients in the soil due to the higher plant metabolism. The shady greens tested at almost double the amount of nitrogen as the sunny greens. Could this be a contributing factor in the disease development? Not only did I have more fusarium in shady spots but I also noticed less yellow patch and dollar spot on the shady greens. Yellow patch and Dollar spot were always the worst on my most sunny putting surfaces! I would really like to see a study comparing plots in shade and full sun with the same rates of N applied to both or rates applied based on the specific growing environments of each site for disease pressure. Would there be a difference? My greens in the sun have been running lean on nitrogen and my greens in the shade have excess nitrogen. Nitrogen is one of those things that if you apply more the plant will use more (to a point) so I wonder if this is having any effect on the disease?
|G1689 are the sunny greens. Cool huh! Highlighted are deficiencies according to MLSN! That's right I got a calcium deficiency hahaha.|
|My GP fertilizer calculator takes seconds to use|
A good analogy is a carb on an engine. The throttle determines how much gas enters the engine. The throttle is the temperature and the gas is the nitrogen. In turf we cannot control the throttle so we must supply the right amount of fuel or nitrogen. We can tweak the carb to either run rich on fuel or lean on fuel. Depending on the air pressure we need to adjust this setting. In the world of turf, air pressure represents disease pressure. As the disease pressure changes we need to adjust the amount of fuel ever so slightly to make sure that the engine performs accordingly for that specific condition. In turf, just like the engine, there is no point in adding more fuel than the engine can use. Doing so only makes the engine perform poorly. In this case more does not equal better. All too often I hear of turf manager giving a good heavy N application to get things growing quick. Sure this works to a point but if the temps aren't there you are wasting your time and probably creating a bigger problem with the lush succulent conditions. Fusarium outbreak following aeration and heavy N app sound familiar? Wouldn't it make sense to "harden" the plant leading up to aeration to reduce turf injury then increase N rates following all the wear and tear? Maybe I'm wrong and I'm living in a fantasy world but this is how I am starting to see things.
I know a lot of you will laugh and dismiss my rantings as nonsense but the fact is that for the most part we are guessing how much n our turf needs and how much n we supply to our turf has a very big impact on all aspects of the plant health. By spending more time and effort getting the N rates just right I think that we will be able to produce better turf for less money with little to no pesticides. Of course there will be times when outside intervention is necessary. By focusing on more complex "issues" and completely ignoring something as "simple" and important as nitrogen fertility I think you are creating more problems than you are solving. Part of a good IPM program is making sound management decisions based on science and fact and reducing any guesswork there might be. Let me know what you think.