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How do I figure out the ppm of my fertilizer mix?

Understanding the concentration of nutrients in your fertilizer mix is essential for optimal plant growth. This article will teach you how to calculate the parts per million (ppm) of your fertilizer or fertilizer mix, and how to create a custom NPK mix to meet your specific needs.

Understanding Fertilizer Labels​

On the side of a fertilizer bag, you will find three numbers representing the percentage content of Nitrogen (N), Phosphorus (P), and Potassium (K) in the mix. These are the primary macronutrients required for plant growth. For every gram of fertilizer in one liter of water, it contributes 10 ppm of the given nutrient per percentage point. For example, a 20-20-20 fertilizer mix will provide 200 ppm of each nutrient for each gram in a liter of water.

Calculating PPM of Fertilizer Mix​

To determine the ppm of your fertilizer mix, you can use the following formula:

grams of fert per liter = A / B

Where:

  • A = your desired ppm
  • B = 10 ppm x the % of nutrient in mix
Alternatively, you can calculate ppm as follows:

your ppm = C x B

Where:

  • B = 10 ppm x the % of nutrient in mix
  • C = grams of fert per liter

Example: Creating a Custom NPK Mix​

Let's say you want to create a 200 ppm N, 100 ppm P, and 200 ppm K mix using the following fertilizers:

  1. 13-0-44 (potassium nitrate)
  2. 12-62-0 (monoammonium phosphate)
  3. 33-0-0 (ammonium nitrate)
Start by calculating the required amounts of potassium nitrate and monoammonium phosphate, which are the sole sources of P and K in this case:

  • 0.5 g of potassium nitrate (200 ppm / (10 ppm x 44 K))
  • 0.2 g of monoammonium phosphate (100 ppm / (10 ppm x 62 P))
This will yield the following nutrient concentrations in one liter of water:

  • 89 ppm N
  • 124 ppm P
  • 220 ppm K
To raise the N concentration to 200 ppm, add 0.3 g of ammonium nitrate (111 ppm / (10 ppm x 33 N)).

The final mix will provide 188 ppm N, 124 ppm P, and 220 ppm K in one liter of water. For greater precision, consider mixing larger batches or using a more accurate scale.

Adjusting pH with Fertilizer​

By mixing your own fertilizer, you can adjust the nitrogen source to meet your pH requirements, rather than relying on adding acid or base. This is particularly beneficial for both hydroponic and soil-based gardening.

Conclusion​

Calculating the ppm of your fertilizer mix and creating custom NPK mixes allows you to provide your plants with the optimal nutrient concentrations for growth. By understanding the formulas and principles outlined in this article, you can make informed decisions about your fertilizer mixtures and ultimately achieve better results in your garden.
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