Clues about how we can manage for or against poa annua with soil testing by depth

I have had so much fun over the past 4 years learning more about how poa grows and how I can either make it better or worse to maintain quality putting surfaces while trying to promote bentgrass as much as I can. I am constantly told that it's impossible to grow anything other than poa in my climate but what I have found is that through careful record keeping and soil testing, we have the tools to keep poa angry for almost 12 months of the year.

When I left my previous course the greens were predominantly creeping bentgrass and the grass grew almost exactly as expected. When I arrived at my current course the greens were mostly poa annua and I struggled to get the growth that I expected for the amount of nitrogen I was applying. The chart below shows the drastically low clipping yield that we experience during my first year at the new job.

This was despite applying more nitrogen than I have in years.

The previous superintendent warned me that these greens needed more phosphorus than the MLSN would otherwise suggest and it turns out, he was right. In 2019 I did notice a growth response to the addition of phosphorus despite our soils being above the MLSN.

Wait a second, that must mean that the MLSN is wrong!

I don't think it is wrong, I just think that there are limitations to any standardized testing. When we test our soils, we test to a depth of 10cm. This gives us a soil nutrient level that is the average amount over the range of that soil depth. In reality, soil nutrients aren't used evenly through the depth of the soil. As I have learned, most of our poa roots are found in the top 2cm. My OM246 tests below show that most of the Total Organic Matter (TOM) is found in the top 2cm.

This suggests that most of the nutrient and water use will occur in the top 2 cm of the rootzone. If we are testing our soils as an average over 10cm but our roots are only 2cm or 20% of that depth, does that mean we have access to all the nutrients in that 10cm rootzone?

At some times of the year, our poa does have roots that are 10cm deep but when conditions get stressful, the roots will shrink back to shallower depths. In 2021 we had temperatures that were hotter than death valley (42C) for a few days. During this time we noticed a massive reduction in our root depth but we were able to keep our grass healthy with a higher soil moisture level (because the size of the rootzone we could access water from had diminished) and more frequent applications of all the macronutrients. Even though I want to promote bentgrass, I still need the poa to hang in especially in areas that are still predominantly poa. I want a slow transition to ensure that our playing conditions stay good through the transition. Dead grass doesn't help the situation.

This winter I tested our soils the standard way to 10 cm but also did some testing at 0-5cm and 5-10cm. Below is the 10cm total testing where you can see that our soil test results show the nutrients well above the MLSN guidelines (red dashed line). I shared in an earlier post about how we manipulated soil K levels down and this may have helped our winter disease situation.

The results by depth are below and they are exactly how you would expect them to be knowing that most of our roots are in the top 5cm.

Phosphorus is found in much lower quantities near the surface. This is because this is where phosphorus is used by our shallow rooted poa annua. Phosphorus is relatively immobile in the soil so below where our roots are able to access the phosphorus there will be little change in the soil test levels. The deficiencies I was experience in 2019 were resolved by adding phosphorus containing fertilizer and we are now just barely above the MLSN guidelines at the depth of our poa roots and the poa is performing better than expected. We can forget about applying phosphorus to keep our bentgrass happy and can just supply as much as we remove through clippings to keep the poa barely happy and slightly less competitive. This is something that I can easily track through my fertilizer records combined with clipvol measurements.

Potassium is the opposite. Potassium is much more mobile in the soil so if it finds it way below where the plant can use the nutrient, it will leach. We apply potassium to the surface in much higher quantities than we do for phosphorus so naturally we have the most K near the surface. Potassium is also used in much higher quantities than phosphorus so if we are spoon feeding K, there should be less nutrient that leaches through the rootzone as it is mostly all used up. You can see this below where we have remove an estimated 0.08 of K in our clippings since the last application of K was made 58 days ago at a rate of 0.09 g K/m^2. This shows me that I am removing what I apply and should probably think about applying more K with my next fertilizer application.

I think this is exactly why people will generally say that poa needs more phosphorus than other grasses. It doesn't actually need more of the nutrient, it just has access to a smaller reserve of the nutrient in the soil so it needs more to be applied as fertilizer for a given amount of growth where you remove the nutrient from the system.

It's the same for water. A 10cm rootzone at 20%vmc has 20 liters per square meter of water available for the plants to use. A 2cm rootzone only has 4 liters of water available at 20%Vmc. This is why it's super important to understand how deep your roots are when you are setting your moisture targets for the day.

Now some might think that my philosphy of not worrying about root depth is horrible and I should try and use products and practice to try and drive root growth of my poa annua. I'll explain why I think this is a bad idea.

I think that it doesn't matter how deep your roots are. You can spend a lot of blood, sweat and tears trying to drive your roots deeper but in the end you need to manage for the roots you have. As the season progresses, your root depths will change and you will also have to change how you irrigation and fertilize your grass. If you are aware of this, you can easily manage it and keep your poa healthy even during extreme weather patterns. If you just hope for deep roots but neglect to manage for the actual conditions, your grass quality will suffer.

For someone like me, I will use this to my advantage because Agrostis stolonifera has much deeper roots and more access to soil moisture and nutrients. So my question is this. Does bentgrass and fescue use less fertilier and water, or do we just need to apply less because they have access to a larger soil reserve of the nutrients and moisture. I've seen that it might take a lot longer for these grasses to need these inputs, but eventually, the soil reserves will be depleted and you will need to apply water and fertilizer in similar quantities to that which poa needs.

For nutrients like phosphorus, we can limit its application to put stress on poa annua. Additional nitrogen or phosphite applications without phosphorus will further put a phosphorus deficiency stress on the poa while allowing the desirable bentgrass or fescue to compete much better as they still have access to the nutrient reserves deeper in the soil. If we deplete the deeper reserves, we might want to thing about ways that we can inject phosphorus below where the poa annua can easily access it. 

I've seen many instances where the desirable bentgrass or fescue cannot compete with the poa annua because there are few nutrients found down deep which is the place where these grasses obtain their primary physiological advantage.

In theory, we can do the same with moisture management. Instead of taking readings with a single tine depth, we can take readings down to the depth of our desirable species' root depth and compare it to a reading from poa's root depth. We want sufficient moisture on a deeper profile but want to reduce the surface water to put stress on poa. This complete opposite is true if you are growing poa as the primary species.

By taking a more data driven approach to moisture management, we should be able to water deep and infrequent but with purpose to allow our desirable grasses to have ample access to water while limiting that water available to poa annua. You could, of course, just measure deep and keep it low and assume that there will be less at the surface because this is where most of the water will be used. Regardless, I think it would be cool to try and measure the difference in soil moisture at various depths to see if this could help you better manage for your desirable species.

I've seen lots of success growing bentgrass or fescue over the years but I've seen even more failure. I think this is because we are generally guessing more than we need to. You can't just water and fertilize less because this risks supplying insufficient nutrient and moisture to the desirable species as well as the poa annua. We need to keep the desirable species competitive and the poa annua dormant or slightly less competitive.

I am excited to continue to play with this phenominon in the coming years to see if we can continue to maximize the amount of bentgrass that is apparently impossible to grow in my climate. I want to reduce the amount of luck that is involved with this process and be deliberate to try and find more success with less risk of killing all my poa at once.

Even in areas with a high disruption, we can keep bentgrass competitive with poa annua if we know how soil nutrients and water change with the depth of the soil.

Bentgrass is dominating on certain fairway with low phosphorus and water management.