N-P-K Compost Fertility

By Felicia Fredd
​Enchanted Garden Productions

This is a heads up to all the people out there growing, or thinking about it, who are unaware that compost is NOT actually loaded with nutrients. I've had this conversation with many people trying to grow plants both in-ground, or in pots, using "tons of good compost", or 'premium' compost potting mixes with dismal results. The initial reaction to the breaking news that their plants are probably starving is a kind of flustered disbelief. A friend of mine once shrieked “So what’s the $%! point!”

Soil, compost, and plant structural and nutrient interactions are really complex subjects (agricultural chemistry territory). I know just enough to get by myself, but hopefully, I can explain the basics well enough to provide someone else with a decent starting point.

Soil vs. Compost
The 'point' of compost (decomposed organic material) is to improve the texture, water holding capacity, microbial activity, oxygen flow, cation exchange (complicated, but important with regard to nutrient exchange dynamics), etc. of a soil for the benefit of many of kinds of plants - especially the fast & high production plants we tend to cultivate.

Soil refers to dirt. It differs from compost in that it is composed of decomposed rock which provides structure (stability) and mineral content. Soils do include compost (organic material) to varying degrees, but the reverse is not so much true.

Generally speaking, well prepared compost IS great, but it does not actually contain many available nutrients. By the time compost is ‘finished’ the microorganisms breaking down that organic material have themselves extracted (converted) most of the stored nitrogen and carbon energy. So, it is, by definition, a low nutrient product. If a commercially available compost or potting soil advertises a reasonably high nutrient content, those nutrients were most likely added back in the form of fertilizer that will again deplete fairly rapidly depending on the demands of the plants cultivated.

My Approach to Fertilizing
To grow, most plants need the primary macronutrients: nitrogen (N), phosphorous (P), potassium (K), and other micronutrients to varying degrees. People who know a few things may get fancy with it by increasing and decreasing nutrient ratios through the growing season as needed for better leaf, flower, or fruit production.

But, before you do anything at all, you need to know what you're starting with - whether that's a home compost-soil mixture, or 100% potting soil. You can find out by using inexpensive soil test kits that measure macronutrient levels. They’re sort of weird at first because the color matching system has you comparing opaque color against color suspended in liquid, but once you’ve done it a few times, you gain confidence in interpreting the readings.

You really do need to know this because you need to match what a fertilizer does have (in many combinations of N-P-K ratios) to what your growing medium doesn't have. If you add high nitrogen fertilizer to a medium that's really only deficient in potassium, at baseline, you're potentially creating a problematic ‘unbalanced’ situation.

Since this obviously takes a minute to figure out, I recommend purchasing individual bags of blood meal (nitrogen source), bone meal (phosphorous source), and greensand (potassium source) - no blends. If you have these things separately, you can use them separately to adjust for one or two specific deficiencies. Greensand also covers bases with regard to micronutrients. These products are organic, but not vegan; however, there are alternative primary nutrient sources such as soybean meal, or urea based nitrogen, etc. There are many ways of doing things, but for me blood meal, bone meal, and greensand is easiest and least costly. If I am container gardening (not growing in-ground), I also typically make a point of adding basic indiscriminate dirt to my compost and potting ‘soil’ blends for minerals and stability. I have no advice on ratios or particulars regarding soil types; it’s just an instinct.

And Then There’s pH…
It's also very important to know whether the plants you’re trying to grow are compatible with your soil pH, or acidity level. A mismatch can cause big problems. The same basic soil test kit for macronutrients typically includes a test for pH level. Decreasing pH is typically accomplished with elemental sulfur, and to increase pH, limestone is commonly added.

For a deeper dive into this, and many other garden science topics, I’ve found both Linda Chalker-Scott at Washington State University, and Robert Pavlis at “Garden Fundamentals” to be great online podcast resources.