# If test is perfect when do you add P and/or K?



## tommydearest (9 mo ago)

This spring, I got a soil test that said I needed some P and even more K. It gave me recommendation for how much to add this year. I did.

Now, let's say next spring the test says everything is optimal. How do you know how much P and/or K to add throughout next season? Do you just wait until a soil test shows something abnormal? Is there standard amount to add per season? We see so much advice on how much N you need during the year, but I don't really see much about the P and K unless soil test shows you need it.

Thanks!


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## corneliani (Apr 2, 2019)

Once your soil has the sufficient nutrient load you're pretty much on cruise control, aka maintenance mode. This can either be done using straight nitrogen (Urea or AMS) as your main fert - and interjected with something like a Starter fert at overseeding (to provide your maintenance P & K) - or with a N+K blend since that's easily found everywhere (like Scotts 32-0-10). But the idea is that you're replenishing whatever nutrients the turf uses up as you see fit.


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## tommydearest (9 mo ago)

corneliani said:


> Once your soil has the sufficient nutrient load you're pretty much on cruise control, aka maintenance mode. This can either be done using straight nitrogen (Urea or AMS) as your main fert - and interjected with something like a Starter fert at overseeding (to provide your maintenance P & K) - or with a N+K blend since that's easily found everywhere (like Scotts 32-0-10). But the idea is that you're replenishing whatever nutrients the turf uses up as you see fit.


So, just kind of play it by ear? There are so many NPK ratios in fertilizers out there. Just pick one and keep doing soil testing I guess?


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## corneliani (Apr 2, 2019)

tommydearest said:


> corneliani said:
> 
> 
> > Once your soil has the sufficient nutrient load you're pretty much on cruise control, aka maintenance mode. This can either be done using straight nitrogen (Urea or AMS) as your main fert - and interjected with something like a Starter fert at overseeding (to provide your maintenance P & K) - or with a N+K blend since that's easily found everywhere (like Scotts 32-0-10). But the idea is that you're replenishing whatever nutrients the turf uses up as you see fit.
> ...


No, not really what I meant. You already have all the info you need (assuming you had a soil test from a reputable lab done). The report is telling you that you don't have any *deficiencies* and that your soil nutrients are at a *sustainable* level. A bit like going to a financial planner and all of your savings needs (retirement, college savings, etc) are in order. Now you just need to fund your living expenses (Nitrogen?). Not sure how well this analogy is working but i'm digging it :lol:

As for your next 'checkup', plan for 2-3 years from now. Or test again in the spring to confirm your assumption and then set it on cruise control for 2-3 years.


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## tommydearest (9 mo ago)

corneliani said:


> As for your next 'checkup', plan for 2-3 years from now. Or test again in the spring to confirm your assumption and then set it on cruise control for 2-3 years.


I guess my question could then be narrowed down to, "How do I figure out cruise control settings?"

Is there some NPK ratio that's kind of standard for cruise control? You mentioned the 32-0-10 but then I'm left with no P, other than what I deposit in the toilet. :lol:

If I plan on putting out 4lbs/1msqft per season, is there a recommendation for how much P or K to use? How do we know how much of those nutrients our lawns use?

Thanks


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## Virginiagal (Apr 24, 2017)

For someone who never has a soil test a general recommendation is to get a fertilizer with a 4-1-2 ratio. But you have had a soil test. Your P is good. The K will gradually be used and need replenishing eventually but you're good for now. Phosphate is a problem for the environment when it gets washed into streams and a lot of places now prohibit phosphate fertilizer use unless you have a soil test saying you need it. Over application of phosphorus can discourage the mycorrhizae and we certainly want to keep them happy. Also, if you leave the clippings on the lawn, the nutrients go back to the soil. So if the soil test says you don't need phosphorus or potassium, just use nitrogen.


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## corneliani (Apr 2, 2019)

Virginiagal is correct, the general 'going blind' fert ratios are 4:1:2 (or even 3:1:2). There is no exact cruise-control fert outside of this though, so you use your soil test results to create that. In your case a 3 or 4:1:2 may be too much P, and like mentioned above may be illegal to apply without a documented deficiency - esp with your surrounding freshwater lakes (I grew up in the northern Detroit suburbs btw.. fun times!). But depending on your exact ppm levels you may be ok with going this route for another year(?). I'm assuming here since I didn't see your soil test but generally speaking it's better to apply nutrients gradually than all at once.

This brings up another point and a valid one: soil nutrient extraction does not happen in a bubble as there are many variables involved. Your soil qualities (OM%, amount of clay vs sand, CEC value) the amount of rainfall/water, etc etc. These can affect your soil nutrient levels as much as the turf extraction itself. Hence why you test again in 2-3 yrs. just to keep your finger on the pulse of the soil. You have a LOT of wiggle room before you get to those minimum levels of sustainability though, esp with established turf.

The way I would do this if I were you is by opting for value. Find products that are widely available and thus has best price (and usually best fit for your area too, as suppliers know what sells). Here's what SiteOne Livonia has in stock, for reference, and would make sense for you. You can also go with their 24-0-11 if you bag your clippings. I would limit any Phosphorous inputs to seeding time since your seedbed will benefit from that timing, and keep it out of my ferts the remaining parts of the year.


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## Ridgerunner (May 16, 2017)

It would greatly help if you posted the soil test as a number of variables can influence the answer but basically:


> Do you just wait until a soil test shows something abnormal?


or shows a trend of change in nutrient levels.
After the initial soil test, adjust until you get P and K into the sufficiency range as measured/reflected by another soil test the next year.
Once a follow-up soil test shows P and K within sufficiency range then:
As ballpark rule of guidance, cool season turf will use about 0.3 lbs of P fertilizer and 0.6 to 0.9 lbs of K fertilizer for every pound of N applied. (known as maintenance fertilizing)
Re-test the next year and if P or K levels fell or increased by 20% or more, lower (cease if the measured amount is above sufficiency range) or increase the amounts of P and/or K you apply the next year.
For Melich and Ammonium Acetate testing: Every pound of P fertilizer equates to 9.5 ppm of elemental P and every one pound of K fertilizer equates to 18 ppm of elemental K. These are ballpark and due to turf, soil and climate variables, they aren't exact but are for guidance. Watching the changes between year to year soil tests will help you refine your applications for your turf/soil and region.


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## Green (Dec 24, 2017)

Coming in here...

Good topic and info. Similarly to P and K, I wonder about Calcium Carbonate application maintenance for those who started at a low pH like me, and are now at optimal pH. Do similar principles apply, and how far should you let pH and/or Calcium levels drop before trying to top off the levels? Feel free to reply to me with this post in quotes in the recent Mag-I-cal thread if you think doing so here takes away from this topic. https://thelawnforum.com/viewtopic.php?f=24&t=38425


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## Virginiagal (Apr 24, 2017)

You should only add lime when a soil test recommends it and even then use some judgment. Regular lime reacts slowly, over a period of 2-3 years. VA Tech asks when you applied lime and how much and gives a lime credit. If you applied in the last six months, it's a 75% credit. If in the last year, it's a 50% credit, in the last 18 months, it's a 25% credit. That's what they expect to still be working from your previous application. If the lab doesn't ask about previous lime applications, they are not taking previous applications into account when they make their recommendation and you should reduce the recommendation accordingly if you have applied lime in the last 18 months. Also, labs commonly use 6.5 as a target pH and make a lime recommendation even if you are above 6.0. If you are between 6.0 and 6.5, you are in a good zone for pH and don't have to do anything. Even 5.8 and 5.9 are good though that's getting low enough to want to do something to push it up. In any case, don't add any lime without a soil test. You don't want it going too high. Though lime has calcium and magnesium, which the grass will use, the rise in pH is because of the carbonate. If you were low and calcium or magnesium and did not need a pH adjustment, you would use another product like gypsum or epsom salt, not lime.


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## Green (Dec 24, 2017)

Virginiagal said:


> You should only add lime when a soil test recommends it and even then use some judgment. Regular lime reacts slowly, over a period of 2-3 years. VA Tech asks when you applied lime and how much and gives a lime credit. If you applied in the last six months, it's a 75% credit. If in the last year, it's a 50% credit, in the last 18 months, it's a 25% credit. That's what they expect to still be working from your previous application. If the lab doesn't ask about previous lime applications, they are not taking previous applications into account when they make their recommendation and you should reduce the recommendation accordingly if you have applied lime in the last 18 months. Also, labs commonly use 6.5 as a target pH and make a lime recommendation even if you are above 6.0. If you are between 6.0 and 6.5, you are in a good zone for pH and don't have to do anything. Even 5.8 and 5.9 are good though that's getting low enough to want to do something to push it up. In any case, don't add any lime without a soil test. You don't want it going too high. Though lime has calcium and magnesium, which the grass will use, the rise in pH is because of the carbonate. If you were low and calcium or magnesium and did not need a pH adjustment, you would use another product like gypsum or epsom salt, not lime.


Thanks. In most areas, I have a pH of around 6.2 now, so I will wait until it gets to 6.0 before even thinking about a small app of more lime in those areas. When it reaches that point, I'd like to find a nice slow-release Calcitic Lime to help hold things sterady...I've only used fast-acting Limes in the past, since pH started out in the mid to lower 5s 10 years ago.

Any suggestions for finding a slow-acting calcitic lime? Anything to look for in particular? Again, I only have experience with fast-acting Lime. Speaking of which, some have done tests comparing fast to slow (e.g. grassfactor) and found no appreciable difference. This in itself is confusing. I'm not sure if it was the testing used (buffering ability was I think the test used), or if there truly is no difference in speed necessarily, between fast acting and normal calcitic limes.

Finally, a lot of people don't realize that some fertilizers use lime products as fillers.


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## Virginiagal (Apr 24, 2017)

Is there a Site One near you? This is on the website.
https://www.siteone.com/en/018174-micro-pelleted-calcitic/p/4808

The speed of reaction depends on how finely ground the limestone is.


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## Green (Dec 24, 2017)

Virginiagal said:


> Is there a Site One near you? This is on the website.
> https://www.siteone.com/en/018174-micro-pelleted-calcitic/p/4808
> 
> The speed of reaction depends on how finely ground the limestone is.


Yes, but it appears they do not stock that particular Lime.

I did not know that speed depends only on the particle size for standard limes. Wouldn't the one above be a fairly fine-SGN product, since it's for golf fairways? Any other advice for evaluating calcitic lime products that are not marketed as fast-acting, enhanced, etc.? Again, I will definitely be looking for a slower acting product in the future. I was also under the (wrong?) assumption that most standard (cheap/inexpensive) limes are probably dolomitic, while the name-brand fast-acting products are the calcitic ones, and are premium products. (Maybe you can guess where that assumption came from.) There is very little info in the way of types of Lime online, unlike with fertilizers. It would be interesting to read some more comprehensive articles about it, if there are any. Everything I've internalized just says you need Lime of your pH is low and you pick the type based on whether you also have a substantial magnesium deficit or not. If you don't have a large magnesium deficit, you go with calcitic Lime because dolomitic carries the risk of increasing magnesium levels too much. And if your pH is low but calcium levels are too high, you're not a good candidate for any lime (screwed??). I've actually purposely kept Mg levels a touch on the low side of optimal because I don't want to risk having tight soil particles.

Anything you can add, and/or debunk from the above?


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## Virginiagal (Apr 24, 2017)

Some reading material for you:
https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/452/452-510/SPES-158.pdf

https://www.canr.msu.edu/news/should_i_use_dolomitic_or_calcitic_lime
That one has further sources linked that you can read.

And a piece I wrote a few years ago:
https://thelawnforum.com/viewtopic.php?t=1610

High calcium levels are not a problem as long as other nutrients are sufficient. Think of the people out West with high calcium levels anyway who have to use gypsum to correct certain issues. They have high calcium and are adding even more calcium.

I wouldn't stress too much about calcitic or dolomitic. Use what you can find. It seems supplies of this and that are not found as readily as they used to be.


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## robjak (Mar 31, 2020)

> Any suggestions for finding a slow-acting calcitic lime?


I can find slow-acting calcitic lime at my local true value and also home depot. Both carry calcitic but not dolomitic. I have to go to tractor supply to get dolomitic. Both are 4-5 dollars a bag.



> "most standard (cheap/inexpensive) limes are probably dolomitic, while the name-brand fast-acting products are the calcitic ones, and are premium products."


 Yes, this is not true.


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## robjak (Mar 31, 2020)

Both home depot and true value present online list limestone sources as dolomitic.


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## Ridgerunner (May 16, 2017)

Green said:


> Coming in here...
> 
> Good topic and info. Similarly to P and K, I wonder about Calcium Carbonate application maintenance for those who started at a low pH like me, and are now at optimal pH. Do similar principles apply, and how far should you let pH and/or Calcium levels drop before trying to top off the levels? Feel free to reply to me with this post in quotes in the recent Mag-I-cal thread if you think doing so here takes away from this topic. https://thelawnforum.com/viewtopic.php?f=24&t=38425


https://thelawnforum.com/viewtopic.php?f=24&t=39709&p=522441#p522441


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## Green (Dec 24, 2017)

Ridgerunner said:


> Green said:
> 
> 
> > Coming in here...
> ...


Wonderful. Thanks. Going to have to read through all that very carefully sometime.


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