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News / Life / Clark County Life

Gardening with Allen: A primer on fertilizers and soil

By Allen Wilson, Columbian freelance writer
Published: May 4, 2024, 6:07am

I checked my leftover fertilizers and I have six different kinds of fertilizer. Do I really need that many? Could you give us some basic information about plant fertilizers?

I don’t think anyone needs more than about three kinds of fertilizer. So lets start with the basics. There are three major or macro fertilizer or nutrient elements used in large quantities by plants: nitrogen, phosphorus and potassium. N for nitrogen, P for phosphorus and K for potassium. K is used for potassium because it is the first letter for the Latin name for potassium. They are always listed in that order N-P-K.

There are three other major or macro nutrient elements that are generally readily available in most soils or available in compounds with N, P and K. These are calcium, magnesium and sulfur. These three have a major effect on soil pH and are sometimes applied to make the soil more alkaline (calcium and magnesium) or more acid (sulfur).

There are eight minor or micro nutrient elements that are used in small quantities by plants. There are iron, zinc, manganese, molybdenum, copper, nickel, boron and chlorine. Iron is the most likely to be deficient in soils and is frequently included in many fertilizers.

Nitrogen is important for leaf growth, so lawn fertilizers are high in nitrogen. They have a typical N-P-K balance of 20-5-10 percent. In order to reduce the phosphorus in underground water supplies the state of Washington has eliminated phosphorus in lawn fertilizer, resulting in a typical ratio of 20-0-10. General purpose fertilizers usually have an N-P-K ratio of 1-1-1, such as 16-16-16. Flower and vegetable fertilizer ratios are typically 5-10-5 or 6-10-4.

When I am fertilizing my lawn in the spring, I usually scatter lawn fertilizer on trees and shrubs because they are producing new leaves also. Sometimes I use general purpose fertilizer on trees and shrubs, especially when planting new ones. I use general purpose fertilizer on flowers and vegetables so I do not buy flower or vegetable fertilizer.

The only other fertilizer I purchase is Osmocote, which is a timed-release fertilizer. It has a 14-14-14 balance. Fertilizer granules are encapsulated in a plastic coating with microscopic pores. Each time water is applied to the soil, a little water flows through the pores, dissolves some fertilizer and flows out again. This gives gradual fertilization for three or four months. I use Osmocote on all my container plants because normal fertilizer is quickly dissolved and leeched from the soil due to frequent watering. I use it for newly planted plants to give them a good, steady start.

Columbian freelance writer