Garden Patch: Organic fertilizers can be harmful too

Organic fertilizers can be harmful too

Part 2 of 2

The Texas AgriLife Extension recommends that homeowners use products that cause little or no harm to the environment, which might lead to the conclusion that only organic products should be used.

However, this is not necessarily true.

Organic substances can be harmful to people and wildlife. Nature produces powerful toxic substances as well as those with low toxicity. Always check a product’s label to determine if you may safely use it in your landscape. Choose an alternate if, for example, the item could harm aquatic species in the lake. Or use another product if the one you are considering is harmful to your pets. Match your selection to your site.

What are the chief differences between chemical and organic (natural) fertilizers? Although plants respond to the nutrients contained in both organic and chemical fertilizers, the nutrient source does affect your garden – primarily the soil. The word “organic” when used to describe fertilizers simply means that the product’s nutrients are derived solely from the remains or by-product of a once-living organism. Organic matter is an important component of healthy soil. Therefore, organic fertilizers generally improve the soil.

Chemical fertilizers may include a sufficient amount of several nutrients that a plant needs to grow, while organic fertilizers typically contain higher amounts of one element or a combination of several in very low levels. Organic fertilizers may be free, such as compost from your yard and kitchen wastes. Chemical fertilizers must be purchased. Most organic fertilizers are slow release; they have less chance of burning plants than rapid-release chemicals. On the other hand, some organic materials, such as manure, can also be harmful when applied raw rather than composted.

Most, but not all, organic products state the fertilizer ratio on the packages. In general, organic fertilizers release nutrients over a long period. Most rely on active soil organisms to break them down and make their nutrients available to plants. One drawback is that these fertilizers do not contribute substantial nutrients when the key organisms are not as active. This occurs when the soil is cold. Thus, nutrients may not be released in sufficient quantities at a time when the plants are in need of them.


Compared to synthetic fertilizer formulations, organic fertilizers contain fairly low concentrations of nutrients, but they perform other important functions that synthetics do not. These functions include increasing soil organic content, improving the physical structure and texture of the soil and increasing bacterial and fungal activity, particularly mycorrhiza fungus, which makes other nutrients more available to plants. Although you will not provide nutrients quickly, the primary benefit to using organics is that you will slowly improve the soil structure.

Cottonseed meal, bone meal, blood meal, hoof and horn meal and animal manures are examples of organic fertilizers. Even though fresh manures have the highest amount of nutrients, most gardeners use composted manure forms, which have less salts, to reduce the chance of burning tender plant roots. Other natural fertilizers include fish emulsion, compost tea and humic acid, as well as alfalfa, kelp and soybean products.

Sewer sludge is a recycled product of municipal sewage treatment plants. It comes in two forms – activated and composted. Activated sludge has higher nutrient concentrations than composted and is usually sold in dry, granular form as a general purpose, long lasting, non-burning fertilizer. Composted sludge has come under scrutiny because it often contains heavy metals that build up in the soil. It is more often sold as a soil amendment, not as fertilizer, and should be used with caution. Urea is a synthetic organic fertilizer, which is an organic substance manufactured from inorganic materials.

Chemical fertilizers are mass-produced. They are capable of delivering a fast release of soluble nitrogen in chilly weather, which is a plus for cool-season crops and spring flowers. However, to avoid burning plants, chemical fertilizers should be applied to soil that has been pre-moistened. The soil should be thoroughly watered again after fertilizing. This caution is not necessary with most organic fertilizers.

Both chemical and natural fertilizers come in liquid or granular forms. Liquids get nutrients to roots immediately, but because they last only a couple of weeks in the soil, they must be applied more often. Granular fertilizers may be broadcast or gently worked into the soil, but not directly applied to planting holes. Granular products are typically slow release and therefore cause less harm to the environment. Also, because they release nitrogen slowly and steadily, most of it is used by the plant, and very little is wasted.

To be environmentally safe, treat all organic and synthetic products with caution and use them as directed. And be aware that homemade “natural” concoctions can produce unexpected or negative results. When using any product for the first time, apply it on a very small portion of a plant. Observe the plant. If the desired results are achieved, you may consider using it in the landscape.

Caution: It may seem convenient to buy a fertilizer that is combined with herbicide (weed & feed), but buyer beware. Dangers include increased weed growth and severely damaged or dead plants.

Resources: AgriLife Extension, Easy Gardens for North Texas

For horticulture answers, call the Texas AgriLife Extension, Hood County at 817-579-3280 and ask to speak to a Master Gardener or go to