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| Most of our greens have slowly transitioned towards bentgrass dominated swards in the past few years |
In a recent post I discussed how the nutrient content of the soils was different depending on the depth and outlined some ideas of how we could use this to our advantage. We can keep poa super happy with more consistency (it's known to be temperamental and this is probably why) or we can use it to suppress poa while promoting other turf species like bentgrass or fescue.
There are major differences in nutrient content at depth and it is useful to understand this but it is also cumbersome to test at various depths or tissue test every week like some might do. Instead, we can use our Supercharged fertilizer and clipvol records to help us understand how much nutrient we are removing from the turfgrass system and adjust our fertilizer applications to either under-fertilize or apply a sufficient amount of each nutrient to favor one turfgrass species over the other.
We can estimate how much nutrient quantity we remove in our clippings by multiplying our clipvol (ml/m^2) by 0.063. This is the approximate dry mass of grass per wet volume.
Generally, 4% of the dry mass is nitrogen. We we multiply the figure we get above by 0.04 to get the amount of nutrient removed in our clippings.
The ratio of nutrients found in turfgrass is related to nitrogen use so we can multiply the amount of nitrogen removed by 0.5 to get potassium removal or by 0.125 to get phosphorus removal. My spreadsheet does this automatically for me.
This is awesome because it allows us to estimate how much of each nutrient we are removing since the last fertilizer application and can help us understand if we need to apply more or less of that nutrient in our next application.
In my recent post about soil testing by depth, I suggested that we can use the understanding of where the phosphorus is located in our soils to manage our turfgrass with more precision and consistency. The table below outlines my most recent fertilizer applications with an estimate of nutrient use (GP use) and nutrient removal through clippings since the last application.
This is cool to me because instead of measuring something over an arbitrary date range we can measure it since we last applied that nutrient. Nutrients to respect date ranges and just because you applied something in June, doesn't mean that it will be used by the plant in June. This is especially true in the winter months where we often apply much more fertilizer than the plant is capable of using. I find it useful to compare usage since my last application and usage YTD which I do on a separate table to the one illustrated above.
In the case of using phosphorus to keep our poa healthy we can see from the table above that we have removed approximately 0.02g P/m^2 since our last fertilizer application which was made at approximately 0.13g P/m^2. If I was to fertilize my greens today, I would not need to include phosphorus as we haven't removed everything applied in the last application yet and our roots are relatively long during the cool winter months. I know that our phosphorus levels in our soils are adequate and above the MLSN so I only need to ensure that I apply as much phosphorus as I remove to keep my poa happy. I also know that we have less phosphorus contained in the part of the rootzone where the poa roots are able to access it so I need to be careful to supply a consistent amount of P relative to the amount that I am removing through clippings. I've included my latest soil test results below as a refresher.
If I wanted to put some selective phosphorus stress on my poa this spring I would continue to withhold phosphorus fertilizer as growth rates increased and would keep both my nitrogen and potassium fertilizer rates consistent with nutrient removal to ensure that my bentgrass remains competitive. As the demand for nutrients goes up, the soil levels where the poa has the most access will drop and we will eventually run into a deficiency situation for the shallow rooted poa annua.
If I wanted to make the P deficiency situation worse for the poa today where soil levels are adequate and nutrient demand is low, I could try applying phosphite which will make phosphorus deficiencies worse.
Eventually, as the deficiency for P in the upper rootzone worsens, we can increase our nitrogen rates to give our deeper rooted desirable species a larger competitive edge while also making the deficiency symptoms worse for the poa because as we increase nitrogen, we increase the demand for the other nutrients.
Cool.
If the poa starts to die quicker than you would like, apply a bit of P but only enough to get the poa through to your next fertilizer application date. This way, you can make a decision again at that time whether or not to make it easier or more difficult for your poa.
Dude.
I also find it interesting to not that the growth potential model is suggesting that the nutrient removal based on the weather is almost identical to the nutrient removal through our clippings. Cool!
While I don't think potassium is a good candidate for managing poa I think that this tool can also help us ensure that we don't put our bentgrass under a deficiency stress from a lack of potassium. This is because potassium is relatively mobile in the soils and will leach if we apply too much. Therefore, we need to ensure we apply a consistent supply relative to nutrient removal to keep our bentgrass happy. The data above shows that the deeper rooted bentgrass does have a larger access to nutrients, but there isn't a ton of K down deep compared to poa annua. Generally, if you need to apply K, you are also applying it equally to the poa as the bentgrass.
You can see from my fertilizer table above that we have removed all the potassium that we applied in our last fertilizer application. Between now and our next fertilizer application date, I can expect our soil levels to drop from nutrient removal from the turf but also from leaching. Therefore, I will closely watch my potassium levels to ensure that I apply as much as we remove while continuing to consider lower levels of K through the winter to reduce fusarium disease activity levels.
To me, tools like this feel like having a super power. It's not perfect but it's a lot better than guessing. I can make more informed decisions to achieve the outcome that I desire whether is is promoting bentgrass over poa or keeping my poa happy and conditions great.
