Georgia Gardener Newsletter Design Tip: February 19, 2009

Using Fertilizers

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A common type of fertilizer

As we get closer to spring, Walter and I are besieged with questions about what to fertilize and when. On top of that we often get calls and emails about plants that are stressed for a variety of reasons and the first thing most people want to reach for is a bag of fertilizer. STOP!

Americans are addicted to fertilizers, most of which are synthetically-made fossil fuel based products. According to the EPA, homeowners use as much as three times the amount of fertilizer (per acre) as farmers and up to eight times the amount of pesticides. Homeowner use of garden chemicals is a significant contributor to non-point source pollution throughout the country.

Frequent or over-use of fast-acting fertilizers isn't good for the plants and the soil, either. Stimulated by a high dose of fertilizer, plants can put out green growth too quickly which may be susceptible to insects, disease, drought and frosts. When using fertilizers, less is best and organic (non fossil fuel) is often the preferred slow way to go.

Fertilizer Labels

All fertilizers whether they are synthetic or organic have three numbers separated by dashes somewhere on the label. The common synthetic fertilizer pictured above is 10-10-10. These numbers are referred to as the NPK content of the fertilizer. A bag of 10-10-10 fertilizer contains 10% nitrogen (N), 10% phosphorus (P) and 10% potassium (K) which are considered the macronutrients for most plants to help produce healthy foliage, roots, flowers, fruits and general health. The remaining 70% of the fertilizer's content can be any number of micronutrients and/or inert ingredients. Milorganite which is produced from treated sewage has an NPK of 5-2-0.

You may think that using a fertilizer with higher NPK numbers will mean more bang for the buck. However, as the numbers get higher, the fertilizer can get "hotter" a term we use to mean it can cause chemical burning to various parts of the plant. Many plants can't absorb that much nutrition quickly so the remainder of the fertilizer is left to run off into nearby waterways.

To be honest, most landscape plants shouldn't require supplemental fertilizer if they have been planted in good soil that is covered with an organic (not plastic or crumb rubber) mulch. As the organic mulch decomposes, it slowly releases nutrients back into the soil that are easily absorbed by the plants. There are a few exceptions. Plants such as annuals, vegetables, fruit/nut bearing plants, plants that have been bred to have extremely long bloom seasons or fast foliage growth and plants in containers may need some supplemental fertilizer to maintain vigor. If planted in good soil, the amount of fertilizer used can be decreased.

I'm a believer in organic fertilizers - those derived from natural sources such as bone meal, blood meal, composted plant material, cottonseed meal, fish emulsion, greensand, Milorganite, etc. Their NPK contents may be lower, but they tend to not "overdose" plants with too much nutrition too quickly. Synthetic fertilizers that have been developed to be slow release (and are usually labeled as such) are better than the "flash-in-the-pan" quick release fertilizers that have a high NPK.

Holly-tone fertilizer (4-6-4) Milorganite from treated sewage (5-2-0)

When To Fertilizer...And When Not To

As a general rule, you only want to fertilize when the soil has warmed up and plants are actively growing and ready to use the nutrients supplied to them. For most plants, that would be spring through early fall. Some plants like winter annuals and fescue can be fertilized during warmer periods in the winter. Trees, shrubs, perennials and warm season annuals should only be fertilized during the growing season. For warm season turf grasses (bermuda, zoysia, centipede and St. Augustine) fertilizing should not begin until the grasses have almost completely greened up in the spring.

Having a detailed soil analysis performed every 2-3 years is also a great way to make sure that your soil has the proper nutrients in the proper concentrations and the soil pH is correct for the plants you're growing. Many plants have difficulty in obtaining nutrients if the soil is too acidic or alkaline even if the nutrients are present.

Be careful about fertilizing too late in the fall with high nitrogen fertilizers or plants might be stimulated into a flush of new green growth which can be damaged during hard frosts. Use of some synthetic fast-release fertilizers during drought can also have negative effects. Potent fertilizers can stimulate plants into flushing out with new growth which may cause more water stress on the plant during dry conditions and some synthetic fertilizers can cause a build up of certain salts in the soil which can actually pull water back from plant roots. And finally, unless a plant is clearly suffering from a nutrient-deficient problem, do NOT apply fertilizer to stressed plants until the problem has been identified. Fertilizer can't compensate for compacted soil, poor drainage, inadequate mulch, improper light, insects and certain diseases.

Remember, less is best and always read the fertilizer label carefully.

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