Fertilizer use tips

• Use slow-release fertilizers (organic or chemical) to reduce groundwater contamination through the loss of excess nitrogen.

• Avoid applying nitrogen late in the growing season (both chemical and organic sources) when most plants cannot use it efficiently and winter rains will just leach it from the soil.

• Plant a cover crop if you must apply composted manure in the fall. The cover crop will use the fertilizer and decrease environmental risks of runoff and leaching.

• Make several small applications during the growing season instead of a large amount all at once when using quick-release chemical fertilizers.

• Instead of scattering granular fertilizers widely, place them in a band alongside of row crops or in the same area as the seed (but slightly above or below to prevent burning) when you plant to concentrate nutrients near the root zones, where it is needed.

• Consider that certain animal manures tend to have higher concentrations of nitrogen than others. For instance, chicken manure has a higher nitrogen level than steer manure.

• Do not use manure as the only source of nitrogen in a garden because manures usually are high in phosphorus relative to nitrogen and excess phosphorus can move into surface water, harming water quality and aquatic organisms.

Plants need many different nutrients to grow normally. They get some of these nutrients from the air and water; most they get from the soil. Three nutrients that plants get from the soil — nitrogen, phosphorus and potassium — are needed in relatively large quantities. As a result, these nutrients (often called the primary nutrients) are at the heart of most plant fertilizers.

Fertilizers by the numbers

Every commercial fertilizer is labeled with three numbers that indicate the percentage of total fertilizer weight represented by the three primary nutrients. The percentage of nitrogen (N) always is the first number, followed by phosphorus (P) and then potassium (K). (The K is for kalium, the Latin word for potassium.)

The three numbers together are called the nitrogen-phosphorus-potassium (or N-P-K) ratio.

For example, if the numbers on a bag of fertilizer are 5-10-10, then by weight, 5 percent is nitrogen, 10 percent is phosphorus and 10 percent is potassium. This fertilizer is lower in nitrogen than phosphorus and potassium.

But what is the remaining 75 percent of the bag by weight? It is “inert ingredients” which make the fertilizer easier to distribute. “Inert ingredients” vary by manufacturer and usually are sand or other materials that have no nutritional value for plants.

Chemical versus organic

What is the difference between “chemical” fertilizers (sometimes called “processed”) and “organic” fertilizers? Chemical fertilizers are manufactured or extracted from natural materials that often undergo extensive processing. The nitrogen in chemical fertilizers is derived from petroleum and natural gas.

Phosphorus is derived from rock and undergoes treatment with sulfuric acid. Most potassium is mined from underground deposits of soluble minerals.

Chemical fertilizers are highly concentrated. When applied to plants, they give the plant a big shot of pure nutrient; they do not provide organic matter.

Organic fertilizers are natural materials that undergo little to no processing. Organic sources of nitrogen include animal manures, blood meal, fish meal and alfalfa meal. Phosphorus can be found in bone meal and rock phosphate while potassium can be found in greensand and kelp meal. Organic fertilizer producers may use a combination of these materials to give a balanced fertilizer product.

Organic fertilizers break down slowly, giving plants a smaller dose of nutrients over a longer period of time. They also “feed” the soil because they contain a lot of organic matter which is broken down by beneficial soil organisms, increasing their number and diversity.

Smart use

Adding fertilizer when it is not needed can damage plants, contaminate groundwater and is a waste of money. Do not fertilize your plants unless you have a specific reason to do so. Do a soil test before adding nitrogen or any other nutrient.

Time fertilizer use with the plant’s life cycle when it needs particular nutrients. Apply nitrogen just prior to and during periods of rapid growth, when plants need it most.

During the flowering phase, focus more on phosphorus, using a fertilizer that has phosphorus in equal or greater amounts compared to the nitrogen such as 5-10-5.

Whether using organic or chemical fertilizers, do not over-irrigate. Plants take in nutrients that are in solution (dissolved in water) but too much water following an application of nitrogen fertilizer can distribute nitrates below the root zone of the plants (a process called leaching) and pollute groundwater.

Because chemical fertilizers do not contain organic matter, using chemical fertilizers as the only source of plant nutrients can be detrimental to the health of the soil and the beneficial organisms that live in the soil.

When using chemical fertilizers, also apply compost to “feed” the soil organisms.

Careful use of fertilizers is good for your plants, the environment and your pocketbook.

Pearl of Wisdom

Because manures can carry microorganisms harmful to humans, they should be well-composted before use in the vegetable garden so as not to spread infectious diseases.

Furthermore, if you are applying well-composted manure in spring before planting, allow at least 60 days between the application of the composted manure and harvesting of the vegetables.

Always wash vegetables thoroughly before eating.

For more

For more gardening information, attend the Brown Bag Series at the Clallam County Courthouse. These free educational events are held at noon on the second and fourth Thursdays of each month.

On June 9, veteran Master Gardener Bob Cain will talk about growing Brassicas including broccoli, kale, cauliflower and Brussels sprouts.

Judy English is a WSU-certified Clallam County Master Gardener.