JoseHempSeed

N-P-K

The first step in evaluating a fertilizer will be based on the N-P-K numbers. There are two ways to interpret these numbers: first, as a concentration, and second, as a ratio. Both methods are important to understand.

As a concentration, the numbers are the percentage, by weight, of the nutrients available to your plants. For example, in a one-gram sample of Wonder-Gro, 15% (or 0.15 gram) would be available as nitrogen, 30% (0.30 gram) as phosphorus, and 15% (0.15 gram) as potassium. The 40% “missing” balance contains the remaining macro- and micro-nutrients, but it is mostly an inert filler that does not affect plant growth.

Had our fertilizer been labeled 7-15-7, it would be about half the strength of our 15-30-15. We can apply half as much of the stronger fertilizer for the same effect as the weaker 7-15-7. But we are much more likely to burn our plants with the stronger fertilizer!

We can consider the 15-30-15 nomenclature as a ratio. In this case, the ratio would be 1:2:1 (notice the slightly different notation). This tells us that there is twice as much phosphorus as either nitrogen or potassium in our fertilizer, although it tells us nothing about the absolute concentration.

Looking at N-P-K as a ratio can be very helpful. For example, cannabis prefers more nitrogen when in a seedling stage, a 5:3:3 ratio might be appropriate. So we should be looking for an N-P-K of something like 25-15-15 or 12-7-7. For vegetative growth, nitrogen becomes even more important to the leafing plant, so a ratio of 8:3:3 would be better. We would be shopping for an N-P-K of something like 16-6-6. For flowering, we want to reduce the nitrogen and potassium, but bring up the phosphorus, so a 2:5:2 ratio may be appropriate, then a 10-25-10 or similar would be on the shopping list.

You will also notice the conversion from the optimal ratio to N-P-K is not precise. All N-P-K combinations are simply not available off the shelf. If you want something like a 5:3:3 or even a 5.5: 11: 9 ratio, you’ll either have to settle for the closest you can find, or mix your own fertilizer compound.

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JoseHempSeed

The Macro Nutrients

Nitrogen

Our label tells us that 15% by weight of our fertilizer is nitrogen, then further breaks that down to say 5.8% is derived from “Ammonicial Nitrogen” and 9.2% is from “Urea”. Let’s look a little closer at these nitrogen sources.

Ammonicial nitrogen is ammonium nitrate, (NH4NO3), a common source of nitrogen as it is cheap and has a high analysis of about 34% available nitrogen. It contains two forms of nitrogen: the NO3- (nitrate) and NH4+ (ammonium) ions. Nitrate forms make nitrogen available to the roots immediately; ammonium takes longer as it must be converted to the nitrate ion before the plant can use it. Ammonium nitrate, containing both forms, makes the available nitrogen seem to “last” longer than just a single application of either the nitrate or ammonium forms of nitrogen. As the nitrate is used (or leached), the ammonium becomes available.

Urea, [CO(NH2)2]2 is a manufactured, organic chemical, also very high in available nitrogen – 46%. Urea had a bad reputation for contamination with toxic by-products when first introduced, but that problem has been virtually eliminated from the manufacturing process today.

Urea goes through a complex chemical reaction in the soil where it is converted to ammonium carbonate by enzymes, which then is converted to ammonia (NH3) and carbon dioxide in the root zone. It is this ammonia gas that combines with water, producing the ammonium ions (NH4+), then finally to nitrate ions (NO3-). This is a comparatively lengthy process and the nitrogen is not immediately available as a nutrient.

Our Wonder-Grow contains a nice mixture of nitrogen compounds: the nitrate will become available immediately, then the ammonium will kick in, then finally the urea will be available.

Another source of nitrogen you may encounter in commercial fertilizers is ammonium sulfate, [(NH4)2SO4], (21% N). Like urea, it makes the relatively slow ammonium ion (NH4+) in soil, which then must be converted before being available for uptake.

Keep in mind that your plant doesn’t care what type of nitrogen compound you fertilize with. All nitrogen (including organic) must be converted to the same ionic form before the plant can utilize them. The agronomic difference between the different sources we are concerned with is the the time to availability by the plant. Commercial farmers are concerned with loss risks, which most container and home gardeners are not concerned.

Phosphorus

The second number in the N-P-K is the phosphorus content. Phosphorus is almost always expressed as phosphate (P2O5). Phosphorus is usually obtained from phosphate rock, a mined mineral. Our fertilizer is 30% available phosphorus.

Potassium

Last of the three numbers is the percentage of potassium (K). It’s always expressed as potash (K2O). Our Wonder-Gro contains 15% available potassium.

The Rest of the Label

The balance of the list contains whatever macronutrients and micronutrients the manufacturer has decided we need. Our Wonder–Gro is pretty good stuff – it contains the macronutrients magnesium (Mg) (especially good for mj) and sulfur (S), although doesn’t contain any calcium (Ca). We may want to supplement with some form of calcium to round out the macronutrient list, although it’s probably not necessary.

Out of the eleven micronutrients (Boron, Chlorine, Copper, Iron, Manganese, Sodium, Zinc, Molybdenum, Nickel, Cobalt, and Silicon), our product contains six. That’s not too bad, however, more – or different - micronutrients may be available in other brands of fertilizer. We could certainly do worse than our Wonder-Gro. Most container gardeners rely on their potting soil for the micronutrients, and don’t supplement their regular fertilizers with micronutrients.

You’ll notice the “Chelated” iron (Fe) and manganese (Mn) on the label. “Chelated” refers to a chemical process these elements are subjected to that allows them to remain water soluble and available for uptake by the plants. All the other nutrients are specifically listed as “Water Soluble”, so they are already readily available to the plants.

It is difficult, if not impossible, to find a single fertilizer that contains the entire gamut of nutrients your plants need to sustain good growth. By knowing how to interpret the label, you can make relative decisions on fertilizer qualities and supplement your favorite brand as you like.