Have you ever found yourself on the fertilizer aisle at the garden center overwhelmed with all the different options? My sister and my sister-in-law both did earlier this year. (And I really hope I helped them out when they texted me with questions.)

But I had almost forgotten about those incidents until I mentioned last week in my garden update that my cherry tomatoes were doing much better this year, thanks largely to the additional fertilizer I gave them.

So I thought I’d share a quick and brief explanation of just what those fertilizer labels mean.

Just like humans, plants require certain nutrients for normal, healthy growth. They get some easily through the atmosphere (think hydrogen, carbon, and oxygen), but others they need to get through their roots. If there aren’t adequate amounts of essential nutrients available in the soil, you’ll have to supplement with fertilizer. Some nutrients are only needed in trace amounts, but others, particularly nitrogen, phosphorous, and potassium are needed in higher quantities. It’s these three that are generally applied through fertilizer, though others might be included, because these three are the most likely to limit plant growth – nitrogen is essential for green growth and good vigor, phosphorous is needed for good flowering and fruiting, and potassium helps with overall structural quality that protects against environmental stresses and can improve fruit/flower quality.

As a general rule for gardening in Utah soils, nitrogen is easily depleted and is always needed but many times phosphorous and potassium levels are adequate, or even high. Having your soil tested will let you know what the situation is in your garden, and whether or not you’ll need to add additional fertilizer.

And that’s where it gets tricky. But I’m here to tell you some basics about understanding that label so you can get the best results. Many different nutrients may be included in the fertilizer, but regardless of what major or minor nutrients are in the fertilizer, or what else is on the label, you’ll always find three numbers. These three numbers are the most important to pay attention to, but until you know what they mean they could also be the most confusing since they are all different

A typical fertilizer label. The three numbers at the top (12-4-8) indicate the relative amounts of nitrogen, phosphorous, and potassium.

A typical fertilizer label. The three numbers at the top (12-4-8) indicate the relative amounts of nitrogen, phosphorous, and potassium. 

For example, one container might say 10-10-10 while another says 13-2-0 or maybe even 0-0-60. What may seem like a cryptic code is actually telling you the percent of the three major nutrients (nitrogen, phosphorous, and potassium) that are included in the fertilizer. And, luckily for us all, they are always in the same order: N-P-K*

*nitrogen (in the form of ammonia, nitrate,or urea), phosphorus (in the form of phosphate), and potassium (in the form of potash)

So a 10-10-10 code would tell you that the fertilizer has 10% nitrogen, 10% available phosphate, and 10% available potash. Easy peasy.

Really, the most critical time for fertilization would be at the time of or just prior to planting. Being the good gardener that you are, you would have had a soil test done that would tell you your N, P, and K levels so you would know whether or not you would need that 10-10-10, or if you just need to supplement with nitrogen and get something like a 12-2-1.

But, like I mentioned in my bucket garden update, my plants have really benefited from additional applications of fertilizer throughout the growing season since the plants go through the limited amount of nutrients available in a container much quicker than they would if planted in the soil. The same goes for my houseplants – they get regular feedings in the summer time while undergoing active growth, but if you take a look at the labels for fertilizers targeted at houseplants you’ll notice the ratios are much smaller than what you might find for putting in your garden.

So add a soil-test to your list of garden chores for this fall and you can start next spring off on the right foot!