Today I’ll teach you a very important lesson on fertilizers, which will let you figure out how much of each compound to use, so that you can get the perfect fertilizer mix at the least cost.
The goal is to obtain and use NPK in the ratio of 110-60-110. This is called by some fertilizer companies “the preferred horticultural mix”, and this particular set of numbers is the amount of actual fertilizer needed to grow a crop on one acre of ground – in pounds. For hectares the numbers are 125-68-125 in KG’s. Don’t worry about that for now, unless you are actually planting acres or hectares. The ratios are what is important.
No matter what fertilizers are available, so long as you have all three of the major nutrients, you can figure out and then buy and mix just the amounts you need of each compound to get those ratios. And of course in doing so you also want to buy the compounds that will end up costing you the least money.
1) Make a spreadsheet with the following headings:
Fertilizer – %’s Bag Weight Cost/bag “To Purchase” Cost N(125) P (68) K (125)
2) List the fertilizers that are available to you, with their weights and costs in the proper columns.
3)Calculate how much of each fertilizer you will need to buy to get the amount in brackets by that nutrient, by dividing the bracketed number by the % of fertilizer in the mix – such as 125/.46 = 272kg for Urea – 46-0-0.
4) Calculate the cost for that much urea and put that in the Cost column.
5) Do the same thing for each of the fertilizers you have available.
When you’re finished your spreadsheet should look like this, if you’re using the same fertilizers I’ve listed here.
Fertilizer – %’s Bag Weight Cost/bag “To Purchase” Cost N(125) P (68) K (125) SmBtch kg #
46-0-0 40 kg $35.20 272 kg $ 240 125 0 0 39 86
0-45-0 ” 25.20 151 kg 95 0 68 0 22 48
0-0-60 ” 39.20 208 kg 204 0 0 125 30 66
TOTALS 611 kg $ 539 125 68 125 91kg 200#
I’m sorry it is such a mess. The narrow columns don’t lend themselves to these things. I’ll save this article as a Word file in the Files section of the MittleiderMethodGardening@yahoogroups.com for anyone who cares to see it in a better format.
Using the materials shown above you end up with an NPK mix of approximately 20-11-20 at a cost of $.88 per kg, or $35.20 for a 40 kg bag. This is much less – for a much better NPK ratio – than the $58.50 you would pay for 13-15-13 (this is figured in cost to Samoa from Australia).
Including smaller batch amounts allows you to mix the size batch that you need without losing accuracy. I chose the amounts I did to give you an amount of NPK in pounds that you can easily use with the pre-mixed Micro-Nutrient Mix available from the Foundation’s website at www.growfood.com in the Materials section. 200# of NPK would require 5 packages (10 packets) of MicroMix and 30# of Epsom Salt to yield 235# of Mittleider Magic Weekly Feed Mix.
If the fertilizers available are different than the ones I’ve listed above you will need to solve for the amounts a little differently. Start with the compound that is the only choice for a particular nutrient (DAP – or 18-46-0 for instance). Figure out how much P you will need to buy, then figure out how much nitrogen you received “free” in the deal and put that in the N column. When solving for nitrogen you need to buy less of the next compound because of what you already have in the DAP.
You can do this even when you have all three represented, such as 13-23-13.
