Soil test results and using the fertilizer calculator


It’s planting time!   Here’s a refresher on using the fertilizer calculator.

My test results for the high tunnel told me I needed to add all of the major nutrients-N, P, and K as well as lime.

N is nitrogen which primarily promotes healthy leaves. P is phosphorus for good roots, and K is potassium for healthy fruit.  If you are wondering WHY they use K: the chemical symbol K comes from kalium, the Medieval Latin for potash, which may have derived from the arabic word qali, meaning alkali.

My test recommended 90 pounds (#) per acre of N, and 75# of P and 150# K.  I use Harmony fertilizer with an analysis of 5-4-3.  That means it contains 5% nitrogen, 4% phosphorus and 3% potassium. Or to put it another way, each pound of Harmony contains .05# N, .04# P and .03# K.  I solved for Nitrogen first.  So 90 goes in the 1st box and 5 goes in the 2nd box and I put in my bed dimensions of 3 X 55 feet.  After clicking the “show result button”, it showed I need to add 6.82 #  of Harmony per bed or .66 oz per square foot or 41.32# per 1,000 square foot or 1800# per acre to get the N required.

You can find the Calculator here.
Now I need to figure out how much P and K  6.82 # of Harmony will supply.  The formula is rate per acre X 100 divided by % nutrient per pound = pounds of fertilizer per acre.  In our case we don’t know the rate per acre but we know the other numbers. % nutrient is 4 for P and 3 for K and pounds of fertilizer per acre is 1800. Here’s the equation: ? x 100/ 4 =1800 lbs per acre.  To solve the equation for P, multiply 4 X 1800 and divide by 100.  The result is 72# P, I needed 75#, close enough.

To solve for K multiply 3 X 1800/100=54.  The soil test called for 150# per acre.  Next, subtract 54 from 150, leaving 96# K per acre I need to still add.  Back to calculator, I put 96 in the pounds per acre box.  I have potassium sulfate which is 0-0-50. So I put 50 in the % nutrient box, keep my bed measurements and the result is .41#  of additional potassium sulfate per bed in addition to the 6.82 lbs per bed of Harmony.

If you haven’t taken soil samples in a while, you can read about taking a soil sample here.

One thought on “Soil test results and using the fertilizer calculator”

Comments are closed.