After talking with John Taberna at Western Laboratories, and reviewing their
Fertilizer Calculation tutorial for Atlantic Giants, I was able to figure out how much fertilizer I need to add.
The recommendations for my soil from Western Laboratories were as follows:
In addition, my pH was 6.2.
The first thing to tackle is the pH. Atlantic Giant pumpkins do best at a pH of 7.4 to 7.8. Western Laboratories has a
pH tutorial which discusses how much lime to add to the soil to raise the pH by certain amounts. However, John Taberna said that I do not want to try to raise the pH all the way from 6.2 to 7.4 this year. If I add that much lime all at once, it will inhibit the ability of the pumpkin plants to extract other nutrients they need from the soil. He recommended adding 20 pounds of lime, or 25 pounds of dolomite lime, this year, and adding the rest next year based on the pH at that time.
Once the pH is more basic, or alkaline, then it is possible to follow the
Fertilizer Calculation tutorial.
I more or less followed the tutorial and just plugged in my numbers from my soil report.
Fertilizer is named as follows: x-x-x, or Nitrogen-Phosphorus-Potassium (potash). The first number represents the percent of the fertilizer that is nitrogen, the second number represents the percent of the fertilizer that is phosphorus, and the third number represents the percent of the fertilizer that is potassium or potash (K2SO4)
For amending the soil, Phosphorus is first. It will be amended using 11-52-0 fertilizer. The recommendation from the lab on the soil report is to add 3.5 pounds of phosphorus per 1000 sq. ft. 11-52-0 is 52% phosphorus (that is what the 52 stands for), so divide 3.5 pounds by 0.52 to get 6.73 pounds of 11-52-0 needed per 1000 sq. ft. However, I have a 1200 sq. ft. plot, so divide 6.73 pounds by 1000 sq. ft. and multiply by 1200 sq. ft (or just multiply by 1.2) to get 8.1 pounds of 11-52-0 needed for my 1200 sq. ft. patch.
Nitrogen is next. It will be amended using 21-0-0-24, or ammonium sulfate (the fourth place in the fertilizer name, -24, is for sulfur). The lab recommended adding 3.5 pounds of nitrogen per 1000 sq ft. However, 11-52-0 (which we added above) has nitrogen in it too, 0.47 pounds of nitrogen per 1000 sq. ft. Subtract 0.47 from 3.5 to get 3.03 lbs of nitrogen per 1000 sq. ft. needed to amend. 21-0-0-24 is 21% nitrogen, so divide 3.03 pounds per 1000 sq. ft. by 0.21 to get 14.4 pounds of ammonium sulfate to add per 1000 sq ft. As before, multiple 14.4 pounds by 1.2 to get 17.3 pounds of 21-0-0-24 needed for my 1200 sq. ft. patch.
The only catch with nitrogen is not to add it all at once. The lab recommended adding 1/3 of the total, or 5.8 pounds of 21-0-0-24, once every 3 weeks for 3 total applications.
Magnesium is next. My calculation was easy, since the amount recommended by the lab to add was the same as that in their
Fertilizer Calculation tutorial example. The lab recommended 0.7 pounds of magnesium per 1000 sq. ft. KMag (also known as Sul-po-mag, or Potassium magnesium sulfate) will be used to amend the plot. KMag is 11% Magnesium, so divide 0.7 pounds per 1000 sq. ft. by 0.11 to get 6.4 pounds of KMag per 1000 sq. ft. needed. Multiply by 1.2 for my 1200 sq. ft. plot, and I will need to add 7.7 pounds of KMag.
Potassium, or Potash (K2SO4) is next. We just added KMag above, which at 6.4 pounds per 1000 sq. ft., means that we added 1.4 pounds per 1000 sq. ft. of potash. The lab recommended adding 7.3 pounds per 1000 sq. ft. potash, so subtracting 1.4 from 7.3 yields 5.9 pounds per 1000 sq. ft. of Potash that we still need to add to our soil. We will supplement with 0-0-50. Since it is 50% potash, divide 5.9 pounds per 1000 sq. ft. by 0.50 to get 11.8 pounds of 0-0-50 needed per 100 sq. ft., or multiplying by 1.2, 14.2 pounds of 0-0-50 needed for my 1200 sq. ft. plot.
Sulfate sulfur is last for the major nutrients. In short, because all of the fertilzers we added so far already have sulfur in them, the amount of sulfur added so far will already be higher than the amount recommended by the lab. However, the
Fertilizer Calculation tutorial in essence says that this is ok.