So I have a total of 13.2 cubic feet of this kind of old potting soil Happy Frog® Potting Soil .

The potting soil has been depleted of all its nitrogen, phosphorus, and potash. I want to replenish those nutrients into this potting soil so that all 13.2 cubic feet has adequate amounts of those nutrients for growing some thriving flowers. I plan on adding some slow-release 10-10-10 all purpose fertilizer. My question is how many cubic feet of this fertilizer should I add?

 

The 10-10-10 description means 10 pounds of each nutrient in 100 pounds of fertilizer.

What analysis do you want to get to and what is the analysis now?

 

I tested the soil using this home kit Amazon.com: Luster Leaf 1601 Rapitest Soil Test Kit: Patio, Lawn & Garden and the results are nitrogen, phosphorus, and potassium all "depleted" (lowest result possible with the test kit, virtually non-existent). The pH is normal about 7.

Not really sure at this point what soil analysis result I want to get by adding supplements. Will be planting some different types of flowers in planters outside this coming spring. One particular flower for example happens to be a bleeding heart and some instructions I've read is that I might want to add some 10-10-10 fertilizer slow release fertilizer to the soil. But at what ratio is what I can't understand. How much of such fertilizer per cubic foot of soil? Each of my planters has soil to a depth of six inches and the volume of soil for each is 1.32 cubic feet.

Edit: Since posting I decided to read some reviews of that home test kit I'm using. Based on these reviews I agree having the soil professionally tested probably through the extension service, despite having to pay the fee and wait for results, would be the way to go instead of trying to rely on this home test kit. I'm fairly certain I probably already wasted my time and money bothering with it. Amazon.com: Customer Reviews: Luster Leaf 1601 Rapitest Soil Test Kit

 

You may be over thinking it. I wouldnt think of it in a cubic yard scenario but in a square foot situation. The nutrients will leach down with every rain or watering so over time it will penetrate downward. Slow release can be applied at twice the rate of a quick release produce without burn potential. Here is an example of how we figure rates on a putting green. If we want to put out 1/2# of a 21-0-0 quick release we take the .21% per pound and divide 1 by .21 = 4.76#. That means for every 4.76# (5 is close enough) of bagged product we apply we are getting 1# of actual nitrogen. With that said divide the 4.76(or 5 rounded off) by 2 and apply 2.5# to every 1000 sq. ft. to get the half pound/1000sg.ft.
Also works for any of the nutrients listed on bag. Remember that the first number is Nitrogen (for growth and color), the 2nd number is Phosphorous (for root and bloom), and the third is Potassium (for hardiness and strength).

 

Okay kerry thanks for that response and example given. I'll try to use that as baseline guide to do my figuring.