Having good soil is key to successful gardening.

While we spend considerable time selecting plants, digging the perfect hole and applying fertilizer and lime, how many of you actually take the time to pull a soil sample?

Without a soil sample, you are playing a guessing game when it comes to deciding how much fertilizer your soil needs.

Let’s use the analogy of filling up your car’s gas tank.

If you need a gallon of gas in your tank to get where you are going, a gallon is all you need to put in the tank. If you put less than a gallon you’ll find yourself on the side of the road.

You can get to your destination on one gallon just as easy as if you filled your tank with 10 gallons.

Try to pump 100 gallons in your car and you will probably be cleaning up a spill.

There are 18 nutrients that plants require for proper growth but in our soils only four of the tanks normally need refilling.

You know how a gas tank holds gas. Soil holds nutrients a little different. Have you ever seen static electricity hold balloons to the ceiling or hair to cling wrap? On a molecular level, static electricity also holds nutrients to soil particles.

The simplest nutrient to understand is potassium. Plants need potassium. Contrary to certain marketing hyperbole, they don’t care where they get it. Plants can get potassium from potassium nitrate, potassium sulfate, potassium chloride, wood ashes or a few hundred other sources.

Once a plant has enough potassium it won’t need any more. Adding more potassium is like adding extra gas when you already have enough.

Clay soils hold potassium fairly well, so an occasional soil test will help you figure out how much potassium you have. On the NCDA soil test, any number over 50 will get you where you are going. Sandy soil and container media doesn’t hold potassium as well as clay soils. You should either soil test more frequently or figure out how much to add each year.

Let’s look at phosphorus next. Phosphorus may come from super phosphate, diammonium phosphate, phosphoric acid, bone meal or a few hundred other sources.

Clay soils do a super job of holding phosphorus and levels can build up to several times what the plants need.

Once the soil level is built up you are just adding more gas than the tank needs.

I know gardeners that have gone several years at a time without adding phosphorus.

Most horticulturists don’t consider excess soil phosphorus to be a problem but it can suppress beneficial fungi.

Things change when we look at nitrogen. Nitrogen is highly mobile in the soil so there is little if any buildup. A soil test doesn’t check for nitrogen because it might be gone by the time you get the results. In most situations you can get a response when you add additional nitrogen.

Nitrogen has a variety of sources including ammonium nitrate, urea, and sodium nitrate.

Since plants need nitrogen, phosphorus and potassium it shouldn’t be a surprise that we find all three nutrients in anything that was once alive assuming it hasn’t been processed too much. For example, soybean meal, cottonseed meal, compost and manure each contain some nitrogen, some phosphorus and some potassium.

We can get nitrogen, phosphorus and potassium in the same fertilizer bag by mixing different sources together. A bag of 10-10-10 fertilizer has all three major nutrients.

Nitrogen, phosphorus and potassium are represented in that order by the numbers on a fertilizer bag. I don’t know why they are in that order. It is alphabetical which makes it easier to remember. It may be the order chemists and horticulturists figured out plants needed them. They are not listed in order of need by weight. Plants actually use more pounds of potassium than they do phosphorus.

With that background, let’s imagine you are sitting at a full service gas station (and yes, some do still exist!).

What would you tell the attendant when he asks how much gas you need?

I bet you looked at the fuel gauge before coming up with an answer.

A soil test is your guage to determining how much nitrogen, phosphorus and potassium your soil has in its tank.

Now is a great time to pull a soil sample of your lawn, flower bed or vegetable garden.

Visit your local Cooperative Extension Center for a soil sample kit.

The testing is offered free of charge by NCDA to all residents of North Carolina.

Don’t let your soil’s tank run empty.

Heather Odom is an Agriculture Agent with the North Carolina Cooperative Extension service in Northampton County.

She can be reached at 252-534-2711 or heather_odom@ncsu.edu.