BOISE -- On a warm day like this, you want to get out and work in your yard and garden, but you have to remember that plants need food too, to grow well and look good, and that's where fertilizer comes in.

It can be a little confusing knowing what kind to use, how to apply it, and what the numbers on the package mean.

Our master gardener Jim Duthie helps shed a little light on fertilizers.

Why fertilize?

Like vitamin supplements for plants. We have different types of soil, and much of it is deficient in nutrients for many of the plants we want to grow. Plants basically need air and water to provide carbon, hydrogen and oxygen. Fertilizers will boost other necessary nutrients, particularly nitrogen, phosphorus and potassium (N-P-K). Other trace elements are also provided, like calcium, sulfur, magnesium and others.

What do the numbers mean?

The numbers refer to the percentage by weight of each of the three primary elements. For example, 10-10-10 means 10% Nitrogen (by weight), 10% Phosphorus (P) by weight, and 10% Potassium (K) by weight. The other 70% is made up of some trace elements, but mostly fillers that make the nutrients easier to spread and get absorbed. Nitrogen stimulate green growth; Phosphorus stimulates root growth & blooming; Potassium makes plants stronger - stronger stems, better able to withstand weather extremes, disease and pests. A 10-10-10, would be a general purpose fertilizer. Some plants, like trees and grass, need more Nitrogen, so you would want to go with a lawn fertilizer such as 20-3-3. Fruits, veggies & flowers need more Phosphorus and Potassium to stimulate production, so you might go with a 9-12-12.

How much?

Always follow the directions. More doesn't mean better with fertilizer. The plants can only take up so much at a time, and you can chemically burn grass and damage other plants and trees with too much Nitrogen, for example, called "too hot". Also, the excess fertilizer will just leach out, or run off, with the water and end up in the ground water or rivers.

Chemical vs. Organic?

Plants really can't tell the difference - they just need the nutrients. The main difference is in the nutrient availability and the long-term effects on soil, plants and environment.

Organic - means the product is minimally processed, and the nutrients remain bound up in their natural forms, rather than being extracted and refined. Not the same connotation as "organic" food. Usually made from plant or animal waste or powdered minerals - manure,

Compost, bone meal, cottonseed meal. Might be processed in a factory or a farm.

Chemical - refined to extract nutrients and bind in specific ratios with other chemical fillers.
May be from petroleum products, rocks, may be naturally occurring but are refined to pure state.


Organic - Improve structure of soil and increase capacity to hold water and nutrients. Slow release fertilizers, so difficult to over fertilize and possibly harm plants. Little to no risk of toxic buildups of chemicals and salts. Renewable, biodegradable, sustainable & environmentally friendly. Can be expensive, but there are cheap alternatives and you can make your own.

Chemical - Nutrients are available to plant immediately, so rapid response. Highly analyzed, so exact ratio of nutrients is known. Easy to understand ratios and chemical sources. Are relatively inexpensive.


Organic - Requires microorganisms to break down and release nutrients. Need warmth and moisture, so limited seasonally. Take longer to break down, so slower, gradual response. Nutrient ratios are somewhat vague or unknown.

Chemical - Mostly made from nonrenewable sources, including fossil fuels. Grow plants but do little to sustain and build the soil. Don't replace trace elements in soil that are gradually depleted by repeated crop plantings, causing long-term damage to the soil. Danger of over-fertilization which can kill plants and upset ecosystem. Tend to leach out, requiring repeated applications. That in turn can lead to toxic buildup of chemicals and salts in the soil, which can make their way into fruits and vegetables. Long-term use can change the soil pH, upset beneficial microbial ecosystems, increase pests and even contribute to release of greenhouse gases.

How to apply?

Liquids or dissolved crystals - sprayer or watering can. Pellets or granules -spreader or shaker. Spikes or pills- push into soil or hammer into ground.