What Is The Best NPK Ratio For Lawns & Grass?

As we get closer to lawn season this is the time that I put a lot of thought into the feeding schedule I think I want to follow for the year. I like to plan the whole year out as best as I can so that I can pick up the products I need and schedule my applications around holidays, vacations, and irrigation.

This year I’m going to be trying my best to use as many organic products as possible in my lawn but I also want to continue my work on building up the soil health and inch my soil’s pH lower over the course of the season which is going to force me to do a few things to the lawn that I wouldn’t do otherwise.

Today I want to talk about only one distinct aspect of the lawn this year – I want to talk about the ratio of N-P-K that I plan on applying to the lawn and how much of each macronutrient my grass type is going to be fed.

For people new to the lawn game NPK is the amount of Nitrogen, Phosphorus, and Potassium found in a bag of lawn food or fertilizer.

To overly summarize what these nutrients do I’ll say that Nitrogen is for leaf growth or top growth, phosphorus is for root development, and Potassium is for the grasses immune system, it’s overall health and vigor.

Excess Nitrogen can become a groundwater contaminant as can excess Phosphorus while excess Potassium doesn’t pose that risk if over applied.

When planning your fert schedule for the year you should think of your soil like a bank account. It stores nutrients the plant needs to grow.

If environmental conditions are good then the plant will use Nitrogen to grow… but to support that growth the root system of your grass will uptake both Phosphorus and Potassium to support the new top growth.

Assuming you have enough Phosphorus and Potassium bound to soil particles in the lawn then the root can tap the bank account to uptake what it needs.

In theory a lawn can continue to grow vigorously without any fertilizers applied until the bank account starts running low. If your lawn-soil’s “account balance” of NPK starts dropping too low then deficiencies start happening.

Basically if your grass is using less Nitrogen throughout the year then it will require less P or K to support the top growth it puts on.

For instance, let’s say hypothetically you don’t fertilize your lawn with Nitrogen in a given year. That grass will continue to grow but it’s speed and vigor will be less and it will use up some of the P and K that already exist in the lawn to support that growth. If you fertilize with P and K throughout the year to replace the P and K that was used up by the end of the year an analysis of the tissue of the grass plant (not the soil) will show that NPK contents are no different than they were at the beginning of the year however the ratio of N to P and K in the soil will have changed.

The PPM of N will be lower in the soil in comparison to the PPM of P & K.

On the other hand if you fertilize with N only or even a non-Phosphorus fertilizer that contains N and K without any P then by years end you will have been replacing your soil’s “account balance” of N and K while slowly using up the balance of P.

By year’s end your grasses’ physical tissues will still be in balance but your soil’s NPK ratio will be altered to have lower levels of P left for the grasses use as growth continues.

Basically everything will be fine when you don’t fertilize… until one of those macronutrients reaches a minimum threshold in the soil and gets too low.

If you run a soil test and see that you have adequate levels of N and P but low levels of K and then you fertilize with a heavy app of Nitrogen then the grass is going to try and uptake extra amounts of P and K from the soil to support that new growth… the problem however is that you may already have adequate amounts of P in the soil for the grass but with low levels of K then that new growth is going to be less healthy than it should be because your “account balance” of K is too low to start with.

The more you fertilize with N the more top growth you are going to get which means your lawn will require more available P and K to support the growth.

If on the other hand you fertilize heavily with P and/or K instead of N then this will not actually make the grass grow faster and it will also not make the grass any healthier either. Instead it will simply build your soil’s account balance of P and K so that it is available when the grass actually starts growing faster due to eventual Nitrogen feeding.

If you apply too much P or K you will get leaching because only so much of these nutrients can be held by your soil at any one time.

A soil’s Cation Exchange Capacity (CEC) is the numerical measurement of how big your account balance can get. Sandy soils typically have a lower CEC while soil’s rich in organic matter and/or clay have a high CEC. This however is a topic for another video.

The point here is that over applying P and K will build up into the soil and not benefit your grass. Really going overboard will result in leaching and waste until nitrogen pushes growth enough to start using up the excess P and K.

Most grass types do well when they are fed between 3-4 pounds of nitrogen per year although some grass types like fine fescues or Centipede for instance require a lot less, 1.5-2 pounds is plenty for those grass types.

In turfgrass studies conducted by University of Nebraska-Lincoln they found that a 25-5-10 feeding at 4 pounds of Nitrogen per year resulted in grass that was perfectly healthy after a few years time but the PPM of P and K in the soil declined indicating that the addition of P and K was enough to keep their respective account balances high enough but not enough to keep them stable. Given enough years at this 25-5-10 ratio eventually the P and K available balance would get too low to support healthy top growth.

You can find that study here.

This tells us that a 5-1-2 NPK ratio is good but it doesn’t quite supply enough P and K over the long term to support healthy top growth without eventually depleting the soil’s balance of P and K.

By increasing the ratio to 8-2-5 this adds enough P and K relative to N to support healthy top growth while not slowly depleting the soil’s balance of P or K.

When we look at a common product like Milo you’ll see a 6-2-0 ratio. You can clearly see that over time the soil’s going to eventually run low on K as stores are used up and excess P will slowly build up over time because N is not high enough to use up all the P.

After years have passed you’ll have P leaching out all over the place and causing problems with some of the other micronutrients that want to be in balance with it and your grass will not have the immune system to deal with stressors like heat, cold, drought, disease, wear, and tear.

If you run a soil test and find that your levels of P or K are high then a good way of fixing that problem is to apply a N only fertilizer to push growth and use up some of those excess macros nutrients in the soil.

If you find you are high in P then applying an N + K fertilizer only will push growth, replace K used to support growth, while also using up excess P in the soil.

Same goes for excess K, Pushing an N + P fert will use up those extra K nutrients and get them back into balance.

Once you are in balance then finding a way to consistently get an 8-2-5 ratio on the lawn annually will keep your grass growing healthy and will keep your account balances of P & K in the soil exactly where they need to be.

If you plan on fertilizing with 4 pounds of Nitrogen in a given year then that means you need to also apply 1 pound of Phosphorus spread out through the year and 2.5 pounds of Potassium.

If you use specialty products that include only one nutrient then in later feedings you can balance things out with other ferts that contain what you didn’t get on the first feeding.

For instance, this Spring I’ll be applying a small dose of Ammonium Sulfate fertilizer to my lawn for the small effect it will have on my soil pH. This product doesn’t contain P or K however so if I end up applying 0.4 pounds of nitrogen to the lawn on that feeding then I’ll have to plan on getting some extra P and K down later in the year to keep the ratios in line.

The liquid aeration products I use contain Potassium only so I’ll make up some of that on those apps and if I throw down a late Spring app of Milorganite which is heavy in Phosphorus in proportion to N and K then I’ll make some of that up there.

Each time you make an app do a quick calculation to find out how many pounds of N P and K you are applying and try to reach the 4-pounds, 1-pound, 2.5-pounds numbers by year’s end. All apps don’t have to be the same but by the end of the season if you are close to those poundage numbers then you are sure to have a healthy lawn with no deficiencies or surpluses.

In future videos I’m going to go deeper into soil testing and understanding those soil tests you do run. I’ll also be producing some extra videos where I go into greater depth on the correct balance for some of the micro nutrients relative to the macros. I hope you’ll stick around for those videos and give them a watch as they hit your subscription feed.

Thanks for watching and make sure to see the description below for relevant links including a link to the University study I referenced in this video… and if you haven’t seen my Spring lawn care guide then let me encourage you now to give it a look. It should be linked on the screen right now.

Thanks again!