Gardening Tip: What Kind of Fertilizer Should I Use? (Part 2)

N-P-K Values on a Fertilizer LabelIn my last post, I shared a little bit about what fertilizer is.  Here, I’ll de-mystify those little numbers on the package!

Nitrogen (N), phosphorus (P), and potassium (K) are the three primary nutrients in fertilizer.  Manufacturers use N-P-K values to show the percentage of each of those nutrients in their products.  For example, 21-7-14 indicates that you’re getting a product that contains 21% nitrogen, 7% phosphorus, and 14% potassium.  In a 100 lb. bag, that would mean 21 lbs. of nitrogen, 7 lbs. of phosphorus, and 14 lbs. of potassium.

Getting down into the dirt, nitrogen helps produce that lush green foliage that we love, while phosphorus helps with root growth and fruiting/flowering.  And potassium helps with overall plant health and disease resistance.

Now, that’s pretty cool, right?  But you’re probably wondering what the other 58% is.  Truth is, with organic fertilizers, the rest is some combination of secondary nutrients like magnesium, sulfur, and calcium along with some trace nutrients.  Fillers and other ingredients that help with application and absorption make up the remainder of most inorganic fertilizers.

A 21-7-14 fertilizer (which is what I use on my bermudagrass) is what’s known as a complete fertilizer because it has all three primary nutrients.  An incomplete fertilizer lacks at least one of the primary nutrients and, therefore, would have a zero value…for example, 1-11-0 is an incomplete fertilizer because it does not contain potassium.

Now comes the fun part:  organic and inorganic fertilizers.

Organic fertilizers, as you can probably guess, are made from the by-products of living things. They can be either naturally made or manmade, but they almost always contain carbon.  Compost is a naturally occurring complete fertilizer made from yard and kitchen waste.  If you use compost, you will probably not have to use any other fertilizers.

Other organic fertilizers include blood meal, bone meal, alfalfa, coffee grounds (yes!), fish emulsion, liquid seaweed, guano, and some others.  While they can be expensive and slower to absorb, these fertilizers often require fewer applications, contain beneficial trace nutrients, unleash bacteria that the soil loves, and improve the soil.

Inorganic or synthetic fertilizers are cheaper, absorb more quickly, and contain more super-charged nutrient levels.  But it’s also easier to go overboard with these guys, resulting in deliriously happy, but disease-susceptible plants and fertilizer burn.

So…which is the best one to use?  Hint:  Our native and desert-adapted xeriscape plants in our Phoenix desert gardens generally make their own nitrogen.  Look for part 3, how to choose fertilizers, coming next!


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