Homegrown: Know your fertilizer

I try to stay away from commercial fertilizers, but it's important to know how they work. Linn County Extension Master Gardener, Lisa Slattery, explains what the N, P and Ks mean on the labels and much more:

We all know compost is a great additive to the garden but what about fertilizers are they a good thing too?  Commercial fertilizers work very well and are effective and economical for supplying some of the mineral elements needed by plants, but they should be used wisely.  Adding too much fertilizer to your home gardens can be as bad as not adding any or not adding enough.

Every fertilizer on the market (including organics) by law must state the analysis of the product on the label.  The analysis of fertilizer is the percentage of nitrogen or N, percentage of phosphorus or P and percentage of potassium or K.  For example, a 6-10-4 fertilizer contains 6 percent nitrogen, 10 percent phosphoric acid or phosphorus, and 4 percent potash or potassium.  These numbers vary by product.  Commonly seen ones are 10-10-10 or 12-12-12 or 5-10-10.  Iowa State University Extension mentions as a rule, those with the higher numbers are more economical to use since the cost per unit of actual nutrient is lower.  Almost all commercial fertilizers (including organics) also include several minor trace elements that aid in plant growth as well including iron, boron, manganese, nickel and zinc.  Even though some of these trace elements are found they are rarely lacking in our excellent Iowa soils.

So now you know the percentages, but what do these minerals mean?  Nitrogen is used for foliage growth, phosphorus will help plants bloom and root and potassium aids plants in their ability to take in water and aids in fruit and vegetable development.  That's why you see different numbers of percentages on different types of fertilizers.  For example bloom booster fertilizers have a higher number as do rooting fertilizers that are specifically helpful with plant transplants.  A good overall all-purpose fertilizer will still be 10-10-10 or 12-12-12 or similar.

Not only do fertilizers vary by amount of minerals but they are available as granular, slow-release granular and water soluble products.  Both granular and water-soluble are great choices but with the water soluble you want to be careful not to mix it incorrectly and don't apply it when temperatures are over 80 degrees or when humidity levels are very low since this can damage plant foliage.

OK, so how do you safely use fertilizers?  Always thoroughly read the manufacturers label and follow product directions exactly.  As for favored fertilizer application, ISU recommends side dressing granular as the most efficient method of application.  Side dressing means to spread the fertilizer evenly along the row of plants (or individual plant) in a band.  Rake it in below the soil surface or irrigate with an inch or more of water.

Some plants like more fertilizer than others.  For example during the growing season garden crops like cucumbers, squash, broccoli, and cabbage like to be fertilized by side dressed applications every four to six weeks - with about 2 tablespoons of a complete fertilizer, but again follow manufacturers directions.  Tomato yields might be increased by side dressing, but don't apply fertilizer until the first cluster of fruit has set.  If you over-fertilize you'll get lots of beautiful leaves and foliage at the expense of fruit set.  If rainfall has been below normal make sure you are adequately watering plants after fertilization.

So if you are new to using fertilizers in your home garden start with a small bag of an all purpose fertilizer, read labels and follow directions.  Fertilizer used correctly will enhance your garden's blooms, fruits and vegetable yields.