Bonnie Orr

Bonnie Orr, WSU Master Gardener

The array is almost overwhelming — bright colors, assorted numbers, abbreviations, name brands, pictures of lush flowers and huge vegetables —how do you sort out which fertilizer you want to purchase for your garden?

First, make a plan. What do you need fertilizer for? The first question — and one that many people refuse to ask themselves — is why do they need fertilizer? Have you ever had a soil fertility test done on your soil? How do you know what amendments you need to add?

Often, people bring their soil test results to the Diagnosis Clinic for the master gardeners to advise and interpret. And many times the soil has an excess of either N, P or K — nitrogen, phosphorous or potassium. These are the most common macro nutrients in a bag of either lawn or garden fertilizer.

Our native soils in Eastern Washington contain sufficient phosphorous. Excess amounts of phosphorous or nitrogen can cause as many growing problems as low levels of fertility, in addition to contaminating the groundwater and rivers. You are wasting money buying and spreading product that the soil does not need. It would be a good idea to get a soil test done before this year’s gardening season starts.

The next question: Do I use organic or conventional fertilizer? The bottom line is that the plant does not know the difference. Organic nitrogen or synthetic nitrogen is going to provide the same benefits to the plant. Nitrogen is nitrogen — the same with other supplements. A gardener makes the choice based on how he or she wants to deal with the soil and with the fertilizer residues in the soil.

Using fertilizers seems to be problematic. The adage “more is better” is truthfully detrimental to your plants and to your soil. Read the directions on the package. These directions are not merely suggestions, but are based on the company’s research before marketing their product.

Many people mistakenly believe that compost is fertilizer. Compost has very low fertility — usually less than 3%. Adding more compost to the garden is counter-productive because compost is a soil amendment. Too much compost prevents enough soil contact for the roots to gather nutrients and perform such magic functions as cation exchange! Compost’s function is to help the soil retain water, create more loft in heavy soils and provide insulation from high or low heat. Worms and other organisms break down the compost during the growing season.

The research varies on the volume of compost to add to the soil. I use 10% each year. The percentage generally varies from 5% to 15%. Fifteen percent, the high end, is actually not very much. You can prove to yourself how much to dig into the soil. Try this experiment: Take a gallon of soil, about 8 measuring cups. Add ¾ of a cup of your homemade, screened compost and stir it thoroughly. It really does not make much of a visual difference in the soil does it?

Organic supplements such as rabbit, llama, chicken or cow manure must be totally rotted down before they are added to the garden for two reasons. The first reason is the amount of salts in the manure that can accumulate in the soil. The second reason is that some manures are very high in nitrogen and will negatively affect the growth of your garden. The two most common problems are huge, lush plants and no blooms or fruit production and blossom end rot in tomatoes and peppers.

Make your 2020 garden a happy, healthy experience.

A WSU Master Gardeners of Chelan County column appears weekly in The Wenatchee World. Bonnie Orr is one of four columnists featured. To learn more, visit or call 667-6540.

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