Fertilizer

Back to Home

Factors to Consider in Choosing a Fertilizer

Nutrient content

The Three Main Nutrients (N, P, and K)

Nitrogen (N) is the main nutrient for new, green growth. Plants that are almost all leaf (such as lawn grasses) need plenty of nitrogen, so the first number is especially high in fertilizers for lawns because grass must continuously renew itself after mowing. The higher the number, the more nitrogen the fertilizer provides.

Phosphorus (P) promotes root development which helps strengthen plants. It also increases blooms on flowers. Lots of phosphorous is great for bulbs, perennials, and newly planted trees and shrubs. They depend on strong roots, so fertilizers meant for these plants often have high middle numbers.

Potassium (K) improves the overall health of plants. It helps them withstand very hot or cold weather and defend against diseases. Most soils already have some potassium, so the third number in the fertilizer analysis is usually smaller than the other two. Fertilizers for some tropical plants, especially palms, contain extra potassium because these plants have a special need for it. Fertilizers meant for fall, such as Winterizer, also contain extra potassium to help prepare plants for cold weather.

Other Important Nutrients                                                                                        

Calcium (Ca) improves general plant vigor and promotes growth of young roots and shoots.

Magnesium (Mg) helps regulate uptake of other plant foods and aids in seed formation. It is also important in the dark green color of plants and to the ability of a plant to manufacture food from sunlight.

Sulfur (S) helps maintain a dark green color while encouraging more vigorous plant growth.

Minor, or trace, elements are used in small amounts but still essential to plant health. They help ensure dark green color, vigorous development, and healthy growth. Iron (Fe) is the minor element most often lacking from poor soils. It helps plants maintain a dark green color.

Look carefully at a label to shop wisely for fertilizers. The numbers on the bag tell only part of the story. The most important ingredient, nitrogen, comes in many different forms. That is what distinguishes commodity fertilizers and some slow-release fertilizers from the latest technology such as timed-release nitrogen.

Soil Texture

Soil with good texture can hold adequate water, oxygen, and nutrients for plants. Soil with poor texture won’t.

You can improve the texture of any soil by mixing in organic matter such as purchased planting mixes, soil conditioner, peat moss, composted manure, or compost. To add organic matter to a bed or planting hole, mix it into the soil with a turning fork or tiller as you cultivate. You will want to use a layer at least 2 inches deep to mix into the soil; more if the ground is very hard clay or extremely sandy.

Most soils are either a porous sand or a heavy clay. In some regions, very fine silt is also common. Clay and silt pack down hard and do not drain well. Sand particles are much larger than particles of clay or silt, leaving more space for water to run through, which is why sandy soils dry out so fast. Clay soils stay waterlogged after a heavy rain.                                                                                                    

The ideal garden soil is loamy. Which means that most of us end up having to improve at least part of the soil in our yard, especially in new neighborhoods where most of the topsoil was scraped away before building. Organic matter in the soil holds moisture yet improves drainage (figure that one out). It also increases oxygen, which is needed by roots and helps the soil store nutrients until plants are ready to use them.

Tips for Fertilizing Sandy Soil                                               

     Because of its structure, sand does not hold on to nutrients very well. Many plant nutrients dissolve in water, so they tend to leach out of sandy soil fast. For this reason, it is important to use a timed-release fertilizer that won't wash through the soil before plants can make use of it. Gardeners having sandy soils, such as are common Florida and along the coasts, often have to fertilize more than gardeners in areas such as the Midwest, which have naturally rich, loamy soils.

Sandy soil dries out quickly, so it's important to use mulch to slow down evaporation from the soil's surface.

Tips for Fertilizing Clay Soil

Clay soil has many tiny spaces that hold on to water and nutrients. Because water that holds dissolved nutrients moves through clay soil slowly, it's important to mix fertilizer into the soil well.

If you have a problem site that stays wet and packed down all the time, consider making raised beds. Besides improving the soil's drainage and texture, people are less likely to walk in clearly marked beds. Footsteps squeeze the air out of clay soil, which makes it even more hard and dense.

Soil pH

The subject of pH (the measure of soil acidity) often intimidates new gardeners, but it shouldn't. Soil acidity is easy to test, and problems are usually easy to correct.

Soil pH affects the ability of the soil to release the nutrients in fertilizer. If the pH level is too high or two low, nutrients can get "locked up" in the soil chemistry and become unavailable to plants.

Soil pH is measured with the numbers 1 to 10. A pH below 7 is acid. A pH above 7 is alkaline. Most plants thrive in a pH of 6 to 6.8. Check soil pH by buying a soil test kit or pH meter. You can also find out how to send soil samples to your state soil testing lab by contacting your County Extension Service.

• If needed, adjust the pH of your soil by applying lime to acidic soil or by applying sulfur or gypsum to alkaline soil. See the rates in the box to the right.

• Most plants grow well in soil with a pH between 6.0 and 6.8, but some do better if the soil pH is a little higher (cabbage family vegetables, buffalograss) or slightly lower (azaleas, centipede grass). The ideal pH of many popular garden plants is listed in the Lawn and Garden sections of this site.

• Soil pH changes slowly, but it does change. Check your soil every year or two, or more often if your plants do not grow well despite proper fertilization.

Forms of Fertilizer

Superior Granular Fertilizers These types of fertilizers are in granules and are the most common bagged products. They are applied with a spreader, or worked into the ground They are a good value because they last longer than solubles Example: Lawn and garden fertilizers that feed for 2, 3, 6, or 9 months.
	Messy Water Soluble Fertilizer These types of fertilizers are designed to dissolve in water. They can cause salts to build up in the soil and burn plants They are expensive compared to granular fertilizers They last only 1 to 2 weeks before needing to be reapplied They can be messy to mix and apply They can serve well as a supplement to granular fertilizers, especially with plants growing in containers outdoors Example: Blue, powdery types that you dissolve in water.
Limited Natural Organic Fertilizers These types are processed organic materials such as manure, dried blood, or bone meal. They have a low level of nutrients compared to most granular fertilizers They are popular with organic gardeners who object to inorganic granular or water soluble forms They are relatively expensive when compared to granular fertilizers Example: Manures, Bone Meal, and Blood Meal are the most common sources for gardeners who prefer to fertilize this way.