Dr. Grow: Understanding fertilizers can help your garden grow

Last week I was at the bank, a friend of mine said, "I've got a question to ask you about fertilizer. How come there are so many kinds on the market? Can you use the outdoor fertilizer on indoor plants? Can I use acid-loving fertilizer on plants other than on azaleas? What is the difference between regular fertilizers and so-called organic forms?"

As you can see this one question led to a lot more questions. I normally find that if one person asks a question, the same question is on the minds of a lot of other people. Before I answer the questions, I need to give you a short course on fertilizers.

Today's we'll be concerned only with fertilization of landscape plants in native soils. Other considerations come into play when you are concerned about fertilization of plants in containers, plants in disturbed soils or annual plants.

First the pH of the soil must be adjusted to the needs of the plants you are growing. A whole textbook can be written on pH, so I don't want to get more involved in the subject except that pH determines the availability of stored soil nutrients to plants.

There are 16 elements or nutrients that are required for plant growth. Carbon, hydrogen, and oxygen are assimilated in plants from water and air through photosynthesis. The rest of the nutrients, obtained from soil, are broken down into two groups, the macronutrients and micronutrients.

Micronutrients are elements that are required in very small quantities for plant growth. These elements, zinc, boron, iron, manganese, molybdenum, coppery and chlorine are usually found in adequate amounts in most clay and loamy soils in the region. They may be in short supply in sandy soils. An occasional application of these nutrients, once in every two or three years, is usually enough to keep these elements at an acceptable level for plant growth.

Calcium, magnesium, and sulfur are secondary macronutrients. Again they are usually found in adequate amounts in native soils. As with micronutrients, an occasional application may be needed.

Nitrogen, phosphorous and potassium are the primary micronutrients. The three numbers you find on every fertilizer bag refer to the percentages of N, P, and K -- nitrogen, phosphorous and potassium. Both phosphorous and potassium are usually found in adequate amounts in native soils. Nitrogen, on the other hand, is a nutrient that must be added to soils each year for plants to grow at optimum rates.

I have suggested that several nutrients are usually found in adequate amounts in native soils. A soil test done on your soil every three to five years will tell you for sure if the major nutrients are at adequate levels in your particular soil. For a small fee, the local University Extension Office will run these soil tests for you.

There are some other things that you need to know. Plants tend to grow at a rate that is in equilibrium with the nutrient levels in the environment in which they are growing. If you increase the nutrient levels in the soils, then they will tend to grow at a faster rate, to a point.

Plants do not distinguish between nutrients originating from inorganic and organic sources. In fact, most nutrients, whatever the sources are absorbed as inorganic forms. Microorganisms convert organic nutrient sources into inorganic forms in soils. Organic fertilizers provide benefits for soils and plants other than just being a source of nutrients. For example they may enhance soil tilth.

Plant response to nutrients in soils may depend on the ratios of certain nutrients. For example, if too much nitrogen in relation to phosphorous and potassium is applied to flowering plants, they may grow very rapidly but not set many flowers.

Nitrogen applied in the form of nitrates will raise soil pH. Some forms of nitrogen, such as ammonium or urea forms, will result in decrease in soil pH. These fertilizers are called acid-loving.

Nitrogen and a few other nutrients can be formulated to quickly release to the environment. Others, releasing over time, are called slow-release fertilizers. In fact, manufacturers can determine the length of time over which they want a fertilizer to release. Some release in six to eight weeks, some release over an entire growing season, and some release over a three-year period.

Now to the questions.

"How come there are so many different kinds of fertilizers on the market?"

There are several reasons for this. Some plants respond better to some macronutrient ratios compared to others. For example, if you are fertilizing fruit trees, you generally want nitrogen concentrations to be lower or nearly the same as that of phosphorous. Some plants are very sensitive to nutrient deficiencies. If pecans are grown in soils with low levels of zinc, they suffer from a disease called rosette.

"Can you use outdoor fertilizer on indoor plants?"

Many fertilizer formulated for indoor plants often contain micronutrients, because these elements are usually deficient in potting soils or potting mixes. On the other hand, fertilizers formulated for landscape plants may or may not contain micronutrients. These outdoor fertilizers can be used on potted plants for short periods of time. If they are used for long periods of time, without the additional application of micronutrients, deficiencies may result and abnormal growth patterns may develop in the indoor plants.

"Can I use acid-loving fertilizer on plants other than on azaleas?"

Acid-loving fertilizer used over a long period of time will reduce the pH of the soil to which it is applied. If you are growing azaleas, you want the soil pH to be very low. On the other hand, if you are growing barberries, you want the soil pH to be neutral. Use of an acid-loving fertilizer for a short period of time will not cause a problem for the barberries. Continued use of acid-loving fertilizer will cause a pH change in the soil, and consequently the barberries will suffer because they will no longer be growing in the right soil conditions.

"What is the difference between regular fertilizers and so-called organic forms?"

First, as mentioned above, organic fertilizers from sources such as manures will not only add nutrients to the soil, but they also enhance soil tilth. Organic fertilizers usually have lower percentages of nitrogen, phosphorous, and potassium in them compared to inorganic sources. The plant doesn't seem to care whether it gets its nutrients from organic or inorganic sources. In both cases the nutrients must be converted by microorganisms to the right form for the plant to take it into its roots.

I hope this short course in soils and soil nutrition has helped you understand fertilizers. If not, send me your questions via the Southeast Missourian and I'll try to clear up any confusion that you may have. If you want to talk to someone in person, contact your University Extension Center or stop by your local garden center and talk to someone who knows fertilizers.