Dirt. I could say that dirt is the most important part of gardening but if you know anything about hydroponics you would know that the dirt is just a handy substrate for holding nutrients and water. It’s what’s in the dirt that makes all the difference to a plant.

The nutrients required for healthy plants reads like a shopping list for chemistry lab: carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulfur, calcium, magnesium, iron, molybdenum, boron, copper, manganese, zinc, chlorine and possibly a few in such small amounts that we haven’t noticed yet.

Now that you’ve seen it you can forget that list except for the group called macronutrients—nitrogen (N), phosphorus (P) and potassium (K). The N-P-K trinity can be present in soil naturally but is also leached from the soil by plants. Keep in mind the order of the trinity as you will need it later.

If your plants exhibit vigorous growth and produce beautiful blooms and juicy fruits and vegetables. And if every Fall you turn a bit of fallen leaves and composted cow manure into your garden you’ll probably never have to worry about N-P-K. If however, you notice your plants’ leaves yellowing from the top down or their stalks and stems are brittle and weak or the root systems seem stunted then you’ll need to come back and read this.

If your plants’ leaves are turning yellow from the tips of the leaves and progressing towards the stalk and from the top down, your soil is suffering from a shortage of soluble nitrogen. You have two solutions in this instance. You can buy a product labeled as ‘blood meal’ from your garden center and apply as directed on the label. Or you can plant a legume crop (e.g., peas, beans, alfalfa or clover) and till it in near the end of its growth cycle. Legume crops will extract free nitrogen from the air and leave it behind (called nitrogen fixation) in your soil.

Stunted root systems are made healthy again by the addition of phosphorus (P), usually marketed as bone meal. (Yep, made by crushing bones.) A stunted root system is really noticeable in tomato plants where the fruit doesn’t reach full size.

Plants that develop weak or brittle stalks and stems are usually suffering from a lack of potassium (K). Potassium is usually marketed as “pot ash” but sometimes comes labeled as just potassium.

Many other products will provide all or some of the N-P-K nutrients and will be labeled (in that order) with the percentage of that nutrient available in the package. For instance, a 40lb. bag of composted cow manure might be labeled 0.5-0.2-0.5 which would indicate that one-half of one percent of the 40 pounds is available nitrogen and two-tenths of one percent is available phosphorus. A bag of urea might be labeled 46-0-0 which would indicate that 46% of its mass is available nitrogen and that it provides no phosphorus or potassium.