Saturday, 23 November 2013

Growth Potential Fertility: After 2 years

GP finally = 0!!
Growth Potential Fertility (GP) is still a relatively new concept in turfgrass management. For the past 2 years I have been using it as the main method to determine rates and timing of fertilizer applications. If you haven't already read this excellent article by Dr. Micah Woods, I highly suggest you do otherwise this post won't make much sense.

I currently use GP to fertilize greens, tees, and fairways. My main motivation to use GP fertility is to reduce the incidence of disease and to reduce or eliminate any excess or waste in my operation. That is the main purpose of GP, to only apply as much nutrients as the plant needs at the correct times.

I have had quite a few questions how I use GP in practice. Here is a brief overview of how I use GP on each area of my golf course.

For Greens I apply simple soluble source nutrients weekly applied through my sprayer. Every Monday I check the forecast and from this I get the average temperature for the following week. This number is never 100% accurate but it's usually pretty close. I plug that number into the equation and it spits out the nitrogen rates needed for that week. I can then base the rest of my nutrient requirements off of the nitrogen. I apply in this manner from the beginning of March through to the end of October. I go every week because frequent light applications of urea are more efficient than infrequent heavy applications. The rest of the season is cold enough that a light granular app in mid October will carry the plants through the winter.

For tees and fairways I don't have the luxury of applying fertilizer to them every week. On tees I still use blended granular fertilizers with non-water soluble source N. What I use is the average monthly temperature for my area for the past 30 years. Again this number is usually nowhere near actual field conditions but it gives me a pretty good general idea of what to expect. I plug the monthly rate into the calculation and apply fertilizer to my tees once a month. For fairways I switched over to urea as my main nutrient source this season. I was going out every 3 weeks to spray wetting agents so I might as well apply my fertilizer at the same time to save time. I took the monthly GP N rate and multiplied it my 75% and that gave me my 3 week rate. For tees I applied fertilizer as long as the rates were high enough to be physically possible with a granular product. This turned out to be from April till Oct.

Now the above outline is pretty basic. I have tweaked my program over the past few years to better suit my site's specific needs. I have made changes to the GP calculation and have strayed from GP all together in some instances.


For my putting greens I have changed the optimum cool season turf temperature to 18C instead of 20C. This is because my putting greens aren't creeping bentgrass, they are annual bluegrass. It is well known that Poa is less tolerant of high temperatures and can require more fertilizer than other turf species. The adjustment to the equation gives me slightly higher rates of N but doesn't increase the total monthly maximum rate. 3.5gN/m/month is plenty in my opinion. I have always based my fertility program to balance plant health and incidence of disease. I don't want to apply more fertilizer if it is going to mean I have to apply more pesticides. For my grass and climate anything less than 200kg N/ha/ year will result in less M.nivale. To get a good number for your site I would suggest making a spreadsheet similar to the one below and play around with the number to give you a total annual N rate that your are comfortable with. This will give you a good place to start and you can fine tune from there.

My GP fertility calculator annual rates. This is what I use for long range
planning, not for weekly applications
You might have noticed the Civitas/PGR adjustment on the top right. What that is is a way for me to tweak rates based on Primo Maxx and Civitas applications. What I have done is run at 80% when I am applying Primo or Civitas as they seem to reduce the nitrogen requirements.
This is my weekly fert calculator. Plug in the temps and it spits out my tank mix.

Last season I had some greens that were severely damaged over the winter months. I also had some major drainage issues that I wasn't able to address until this fall. These greens required more fertilizer to keep healthy. My healthy greens only required 180Kg N/ha where my unhealthy greens required almost 200kgN/ha to aid in the constant recovery. If you commit to GP don't be afraid to stray from the plan. The really cool thing about GP, though, is that it gives you a really good way to compare fertilizer applications year to year. A 200% GP rate can be as small as 4kg N/ha/month difference in March or as big as a 35 kg N/ha/month difference in July! By keeping track of the GP and how you deviated from the GP you gain a very valuable tool to understand what you are seeing out on the course. If you aren't using GP yet it can be a valuable tool to compare your current fertilizer practices with GP and see how far off you are. Maybe it will help you make some correlations between fertilizer applications and plant health, or seeming lack of it. Here is another great articlefrom Micah Woods showing how reduce potassium inputs in the fall can result in a lower incidence of Grey Snow Mold. The GP fertility method lends itself perfectly to these observations as well.


This year was the first season I used GP on my tees. I was unsure how applying fertilizer with my new sprayer would work on my tees as they are very small and surrounded by steep slopes and rock walls so I chose to use a granular blend. For the most part this worked out pretty good. Because I wasn't able to apply rates as low with a granular as with a liquid. I wasn't able to apply the actual GP rates early and late in the season. I used a blend with a higher % of water insoluble nitrogen which seemed to work out OK.
These tees are hard to spray.
I was able to do a few test sprays on my tees this summer and it turns out that spraying them isn't that big of a deal. Next year I plan to use the sprayer early and late in the season as well as during Mid Aug-Sept to attempt to reduce the incidence of dollar spot. The worst outbreaks of dollar spot have always been where slow release nitrogen sources have been used. Urea packs a punch and makes a big difference. I won't be exclusively using the sprayer on tees, though, because I don't want to make that frequent of fert applications due to time.

Last year I noticed a green up response due to worm castings so this year I will apply a nitrogen punch at this time to help green up and out-compete weeds on the tees. I waited too long to apply fert to my tees this year and as a result there was an increase in clover and plantain. I am less concerned with disease on my tees in the spring so some extra nitrogen at this time shouldn't make a big difference to disease.


In the past I was never able to fertilize my fairways with a sprayer because we only had a pull-behind 200L sprayer and only had a 2wd utility vehicle to pull it. This just wouldn't work for my hilly course. This year I was lucky enough to get a 757L Toro workman spray system on a 4wd chassis. Instantly I was able to spray fairways.

In the past with granular fertilizer applications I was limited to the N release duration, and prill size for my application rates. With my mild season I wasn't able to get rates below 200kg N/ha/year which is quite high for fairways on native soil where the clipping are returned. I switched to a slower release N source last year to try and reduce my N inputs but had a way more dollar spot on my fairways. So much that in some areas I had almost catastrophic losses.

This year I took the advice of a few superintendent on Twitter who said they had been going out with straight urea every 2 or 3 weeks and hadn't noticed any difference compared to other slow release N sources. Of course N losses were my biggest concern using straight urea for longer durations. I trusted their experience and advice and can say that I wasn't disappointed. I chose not to apply any other nutrients as I had obviously been over applying them for the past 20 years!

Just like the greens I adjusted my GP fertility calculation to take into account that I was on native soils and was returning the clippings. I used 18C for an optimum temperature and then went out at 50%GP. This gave me an annual rate of only 80kgN/ha!

This was a significantly lower rate than I had ever applied and I was a little nervous. What I observed was nothing short of amazing. The fairways were more consistent, green, and disease free than they had ever been before. Of course there are other factors at play here but I was not disappointed in the least! The Urea seemed to last the 3 weeks except for a long hot dry spell in early August. Next year I will shorten my application intervals to 2 weeks in the summer. I will also apply an early season boost just like on my tees to help out-compete the weeds. I will also add in some iron sulphate in the wet season to try and help with disease and colour.

In the future I think there are still areas where I can improve. One of my biggest issues this summer was wet conditions and disease. These were combined with huge growth surges which I think were due to excess nitrogen in the soil from OM release. I think that regular (weekly) testing of available soil N would be a huge help to further fine tune the GP fertility for disease management but until a fast, easy and economical way to measure soil N is developed I am stuck guessing.

So that's about it for now. I have switched over the way I fertilize, have saved a ton of money, increased the quality of the course, and gained a better understanding and ability to compare inputs to the course year over year.

Global Soil Survey Fert Recommendation

I also want to give another quick plug for the MLSN and the Global Soil Survey. I have also been using the MLSN for the past 2 years and so far so good. The results from my soils survey this year concluded that according to the MLSN I should be able to get away with applying only nitrogen to my putting greens next year. I think I have solved the major issues with my greens so next year should be good fun. I am going to give it a whirl and see what happens. Stay tuned....