Here’s what to look for when you buy organic fertilizer.
- First, check the N-P-K number listings on the bag. These numbers refer to N (nitrogen), P (phosphorus), and K (potassium) in that order: the NPK ratio. A rule of thumb for determining if fertilizer is organic or chemical, is that if an NPK ratio adds up to more than 15, or if one of the numbers is more than 8, it is probably chemical, not organic. These lower numbers don’t mean that there is a failing in organic fertilizers; remember, that is the advantage of organic materials: their nutrients are not immediately available, but rather are released slowly, over time, at a rate the plants can use without waste.
- Second, scan the list of ingredients for words like ammonium, muriate, urea, nitrate, phosphoric, or superphosphate; if these words or their variants are part of the ingredients, don’t buy. The words phosphate and sulfate themselves are not necessarily indicators of processed or
synthesized materials, but if combined with any of the key words above, they are.
- Other ingredients to watch out for are cottonseed meal and leather
tankage, not because they aren’t natural (non-synthetic) products, but because they are frequently contaminated with harmful residues, thus making them suspect to use in an organic vegetable garden. The same points apply to liquid fertilizers.
- When using commercial organic fertilizers, follow the instructions and the
recommended application rates listed on the package.
- Don’t double up because
the listed N-P-K is lower than what you might be used to using.
- And be careful to keep track of your soil’s organic matter level; these purchased fertilizers, unless they are made from composted manures (many are), do not
add organic matter to the soil-and organic matter is at the heart of organic gardening.