where to get fertilizer

Asked June 25, 2017, 5:22 PM EDT

I got your results from my soil test and the recommended application is 20-10-20. Where do I get this blend?

Ramsey County Minnesota

3 Responses

To start with, you probably should not be fertilizing your lawn now. Wait until early autumn to put down fertilizer. You risk burning or otherwise damaging your lawn by fertilizing in the hot part of the year:

Unfortunately, the soil testing lab can't predict the formulations produced by the various fertilizer companies. To complicate matters further, state and local laws now forbid lawn fertilizers from including phosphorus, which promotes algae growth in our lakes. Since you have a test showing a P need, you can bypass that law, but still, the lawn fertilizers reflect the law and generally don't have a P component.

But there are workarounds, which require a little arithmetic. One of the exceptions to the no-P laws is starting a lawn. So a good source of P for lawns is in "starter" fertilizers. For example, Scotts makes a 24-25-4 product. When you combine this with a more typical lawn fertilizer (for example, Scotts makes a 32-0-4 product) in the right proportion, you will get your N and P taken care of. To get the K, muriate of potash (0-0-60) is a good choice. Of course, you can choose other blends, but you'll probably need a "starter" product to get the P and you'll probably have to use an almost-all K to get the K.

To do the arithmetic, you should use the pounds of N, P and K required for the season for your lawn (instead of the 20-10-20 ratio). These numbers are prominently displayed on your test report (and will be in the same ratio as your 20-10-20):

Remember that these numbers are how much is required for an entire season, not a single application. So you will have to divide this by the number of fertilizer applications.

I'd first compute how much "starter" fertilizer you need to satisfy the P requirement. Then fill in with the standard lawn fertilizer to complete the N. Finally, complete the K with an all-K fertilizer.

Next year, get another soil test done. P and K are not depleted as fast as N, so it might be that you can use a more standard lawn fertilizer next season.

Ok Dennis, I'm still a little confused. I'm getting ready to due my second fertilization for the fall.

Here is my supplies as you advised on.

I purchased a 8 Lb, bag of " High Yield" 0-0-60 & 20# of Scotts 21-22-4 & 12.5 # of Scotts 32-0-4.

Approximately 6,500 sq /ft of lawn. And I have a Scotts Broadcast Spreader.

Please Instruct me on how to apply,'Thanks

As I said in my first message, it is better to work with the number of pounds of each component that you need. This information should be on your test report.

I'll assume 1 pound of nitrogen per 1000 square feet. This is a common recommendation, but yours might be 2 pounds of N per 1000 square feet. In any case, you should not exceed 1 pound of N per 1000 square feet in any single application, so I'll assume you want to put down 1 pound of N this fall.

Skipping, for the moment, the calculations, your lawn needs about 15 pounds of the 21-22-4 product (about 3/4 of the 20 pound bag), 10 pounds of the 32-0-4 product (about 4/5 of the 12.5 pound bag), and 9 pounds of the 0-0-60 (use the entire 8 pound bag).

As for spreader rates, I don't know. Read the spreader instructions or simply set the spreader at a very low setting, then make multiple passes until you've put down the amount of fertilizer you want. You get better coverage that way anyway.

Into the weeds: here are the calculations I skipped.

Since the ratio recommended is 20-10-20 (or 2-1-2), you'll need 0.5 pound of P and 1 pound of K to go with the 1 pound of N.

Start with the "starter" fertilizer (21-22-4) and address the P deficiency. One pound of that gives you 0.22 pounds P. To get the amount you'd need to achieve 0.5 pounds P, divide 0.22 into 0.5, or about 2.27 pounds. Since this product is 21% N, you will also get about 0.5 pounds N and since it is 4% K, you will get about 0.1 pounds of K.

Next deal with the N requirement since you are still missing 0.5 pounds of N. Use the 32-0-4 product. Each pound of this product contains 0.32 pounds N. Divide the 0.32 into the 0.5 to get the number of pounds of this product you need, or about 1.5 pounds of this product. Since this product is 4% K, you will also pick up 0.06 pounds K.

Finally address the remaining K deficiency with the 0-0-60 product. You picked up 0.1 pounds K from the first calculation and 0.06 pounds of K from the second calculation. You need 1 pound total. That leaves a 0.84 pound shortfall. Each pound of this product is 0.6 pounds K. Dividing 0.6 into 0.84 gives 1.4 pounds.

Summarizing, 2.27 pounds of 21-22-4, 1.5 pounds of 32-0-4, and 1.4 pounds of 0-0-60 will give you 1 pound of N, 0.5 pounds of P and 1 pound of K. (The calculations are all approximate, but this isn't rocket science.)

Since your yard is 6500 square feet, and these amounts are for 1000 square feet, you'd multiply by 6.5 to get about 15 pounds of the 21-22-4, 10 pounds of the 32-0-4 and 9 pounds of the 0-0-60.