Soil sample results

Asked October 22, 2019, 6:36 PM EDT

I recieved the soil sammple ressults from the UofM Testing lab. They recommended a fertilizer fomula of 20/10/20 for my lawn. I'm covering 3500 sq/ft. I purchased Scotts starter fertilizer which has a 21/22/4 mixture. A 21.52 lb bag covers 5000 sq/ft. I also purchased a bag of Potash which has a 0/0/60 mixture. A 4 lb. bag covers 800 sq/ft. I also purchased a bag of Scotts fertilizer which has a 32/0/4 mixture. And a 12.5 Lb bag covers 5000 sq/ft. Can you tell me what I should put in my broadcast spreader to do my 3500 sq/ft ? Its a combination I can figure out.

Ramsey County Minnesota soil testing lawn fertilizer horticulture

1 Response

An important piece of information from the soil test report that you omitted is the recommendation for the number of pounds of N, P and K per 1000 square feet. These numbers are given directly above the ratio recommendation. See here:

I'll make an approximate calculation, based on the standard recommendation that N only be applied at the rate of 1 pound per 1000 square feet in any one application, and on the assumption that the N-P-K ratio be 20-10-20. So your 3500 square foot lawn would need 3.5 pounds of N, 1.75 pounds of P, and 3.5 pounds of K.

Since two of your fertilizers do not contain P, I'll use the Scotts Starter to satisfy the P requirement first. It is 22 percent P and you need 1.75 pounds P, so you will need about 8 pounds of this fertilizer (22% of 8 is about 1.75), or somewhat more than a third of the 21.5 pound bag you bought.

Of the two remaining fertilizers, one does not contain any N, so let's next satisfy the N requirement. The Scotts Starter has given you about 1.7 pounds of N (21% of 8). That leaves 1.8 pounds from the Scotts. It is 32% N, so you'd need about 5.6 pounds (32% of 5.6 is 1.8), or somewhat less than 1/2 the 12.5 pound bag you bought.

Finally, you will have to use the potash (60% K) to supply any K that is not in the first two fertilizers. First, let's compute how much K is in those two fertilizers. The Scotts Starter is contributing 4% of 8 pounds, or 0.32 pounds. The Scotts is contributing 4% of 5.6 pounds, or 0.22 pounds. Altogether you've got about 0.5 pounds already supplied. You need another 3 pounds. That means you need 5 pounds of potash (60% of 5 is 3), which is a bit more than the entire bag you've got. I wouldn't worry about the 1 pound shortfall - you can make it up in the next application.

After you make the application I recommended, you will have put down 1 pound N, 0.5 pounds P and about 0.7 pounds K per 1000 square feet. Any shortfalls that remain you should make up over the course of next year, with a spring application and a fall application (you could use what remains of the bags you bought).

Often, once the P and K deficiencies are addressed, no further P or K is needed. But in following seasons, you will have to apply N two or more times each year, at the rate of 1 pound per 1000 square feet. And I'd recommend another soil test in two or three years.