Planning forwards and measuring backwards
I use the turfgrass growth potential (GP) all the time. I make use of the GP for all kinds of things, like predicting sand topdressing amounts, comparing locations, predicting nutrient use, checking overseeding dates, estimating mowing requirements, and many more.1
Do you notice that those uses are all about prediction, estimation, comparison, and analysis? I’ve often explained that I’d make use of things like sand topdressing estimates or nutrient use estimates for planning purposes, and then I’d adjust based on the turf response.
I think of this as planning into the future, or looking forward, based on some predicted turf response, but then when the day comes for me to actually do the work—fertilizer application, say, or sand topdressing application—I don’t want to stick to the plan anymore. Instead, I want to adjust based on what has actually happened.
Example 1: nitrogen fertilizer
I may estimate monthly N fertilizer by multiplying GP by a max of 2 g N/m2 (0.4 lbs N/1,000 ft2) for a 10 year old creeping bentgrass green. When the time comes to make an N application, however, I would be adjusting based on whether the recent clipping volume is too little, just right, or too much.
Example 2: sand topdressing amounts
I may estimate sand topdressing requirements using a site-specific growth potential calculation. When the time comes to make sure I’ve applied all the sand needed for the year, however, I’d be looking at total organic material (OM246) test results and would adjust the sand topdressing amount based on those.
Example 3: phosphorus and potassium fertilizer
I may estimate P and K requirements by a GP-based method. When it comes to actually applying these elements, however, I’d be checking the soil test results and using MLSN calculations to find the exact rate to apply.
With all the examples above, you’ll notice that I’d look backwards to decide what to do. I’d look at recent clipping volume, at recent total organic material data, and at recent soil test data, to decide what to do. It seems obvious that this should be the way to do it, right? I’ve still seen the occasional bit of turfgrass, or heard questions here and there, that make me think some are managing turf with so much of a focus on predictions into the future, that they fail to make use of the data they have from the recent past. I like to plan by models, and then do based on reality.