I had an amazingly successful winter but it all came crashing down in June. I again had a very successful July and August but again in early September I was hit hard with fusarium. June and September have always been tough months as they are transition months, they are warm but also usually wet. This creates the perfect storm for fungal attack. Without an effective organic contact pesticide I was out of luck.
It is pretty well known that high nitrogen generally means higher incidence of fusarium on putting greens. It wasn't until I came across the GP fertility method that I was able to more accurately determine the actual nitrogen needs of the plant. Basically what this helped me determine was nitrogen application timing, something that was more or less a mystery to me until last summer. I knew that the total amount of nitrogen applied was a factor in disease development but felt that the timing also played an important role.
In recent readings I came across something about nitrogen release from soil organic matter and this got me thinking about how I have been trying to use nitrogen to control M.nivale on my putting greens. All along I had been completely ignoring the amount of nitrogen released from OM (organic matter) in the soil. Obviously this would have a huge impact on plant health but I had no way of determining how much was being released at any given time. I actually still don't have a good way of knowing but I have come up with a way that I can very roughly estimate what might be happening in there. When I say rough I mean the worst dirt road in the deepest darkest corner of Siberia rough. Ugly rough.
So most if not all of the research I have used to determine nitrogen rates to discourage fusarium have not taken into account soil OM levels. It's probably safe to assume that OM levels in a lot of studies are rather low as the study greens are often not that old. Either way the amount of N released from the study root zones is probably similar to what I am experiencing but there is obviously no way to know. So using rates determined from studies is sill a good idea I just think that there is more to be known regarding nitrogen application timing and when OM nitrogen release occurs.
So that basis of my OM release estimation comes from a formula where;
amount of N released from OM /1000 sq ft per year = 1.7 * ln(OM%) + 1.8 per year
Larry from PACE Turf was kind enough to show me this formula but cautioned that it was a " VERY VERY ROUGH GUESS."
As I have about 1.7% OM in my putting green rootzones which gave me about 2.7 Lbs N/1000 sq ft or to put that in a measurement system that makes sense about 1.22 KG N per 100m2! HOLY SMOKES!! That's more nitrogen from OM that I apply using fertilizer but it doesn't at all mean that I should stop applying fertilizer. After all this OM release has been happening forever but it's just not something that I have quantified yet.
For IPM and specifically managing for M. nivale it's not just about total amount of N applied per season but I'm thinking it's more about the timing of those applications. As of now, I don't have a way of determining how much N is released from the soil OM levels at any given moment but I can use this rough seasonal estimation to make a further rough estimation based on a growth model I already use. You guessed it, GP (growth potential).
Now the release of N from soil OM is going to be dependant on a lot of things, not just temperature. To be fair, so is the growth of a plant. We got air temperature, soil temperature, humidity, soil type, soil air, soil moisture, amount of light the list goes on. The growth potential model is just a basic way of trying to understand the complex way nature works based on temperature, it's the best we got OK. If you really think about it though, most of the biological activity is based on temperature as we are pretty good at controlling all other environmental aspects of the growing environment.
So lets assume that the release of N from soil OM closely mimics the N requirements of a plant after all the N requirements are based on biological activity. The GP formula gives me theoretical monthly N requirements which can be added up for a total yearly N requirement of about 1.1Kg N/100m2 per season in my climate. I can then take the monthly requirements and divide them by the yearly requirement which gives me a month % of total. I can then reverse the formula using this percentage and the theoretical OM N release and this gives me the theoretical N release from OM per month based on temperature...... or something.
Now please before I go on, this is more than likely all a pile of garbage, I'm just speculating here, as usual.
So the above chart shows the amount of N added using GP (blue), the theoretical amount of N added from OM (red), and the total amount of N being added to the system (orange).
Now I should be able to use this data to compare nitrogen rates with disease pressure. Simply put the months of June and September are hell. May wasn't that bad and neither was October. The total N being added to the system in May is about 0.23 Kg N/100m2 and October sees about half of that. The total amount of N being applied to the system in June jumps to 0.4Kg N/100m2 or almost double. This could be a bid deal especially if the environmental conditions are still favourable for fusarium development.
|June 2013. Look familiar anyone?|
Since I'm being so half assed and guessy here I could just assume that the amount of N release is exactly the same as the GP theoretical N requirement. If disease pressure is high I better keep rates less than 0.23 Kg N/100m2 per month. They say rates above 2 Kg N/100m2 per season significantly increase fusarium activity so why can't I say that rates higher than 0.23 Kg N/100m2 per month or 0.06kg N/100m2 per week significantly increase disease activity. Half that number for amount of N actually added as fertilizer! Those are pretty low rates.
I could speculate further that the reason guys who use "biostimulant" products or amino acids for plant health is because they for once aren't apply too much nitrogen.
In the end I could be totally wrong about the relationship with N and fusarium but at this point it's the best I can come up with and something that I have had the most success with so far. I now have all winter to think about this and try to come up with more crazy theories and ideas of how to survive June and September without traditional pesticides. If you got a better way to determine N release from OM based on something measurable I would love to hear about it!