Better Growing
By DR. FRANCIS R. GOUIN
Professor Emeritus
University of Maryland
College Park

6.01.2010

Any one who is experienced in growing azaleas, rhododendrons, mountain laurel, leucothoe, andromeda, blueberries, and so forth, are well aware that these plants need to be grown in acid soils. When one speaks to most growers, it appears that few are knowledgeable as to why these species require an acid soil in which to grow. One of the most common reasons given is that these species have a high iron requirement and that iron is most available in acid soils.
In mineral soils iron is most available at a pH between 4.0 and 5.5. However in organic soils iron is almost equally available between a pH of 4.5 and 8.5. This is the reason why I always recommend amending soils with liberal amounts of compost to raise the organic matter concentration which allows many different species to be grown in close proximity to each other, including those species that have high iron, zinc, manganese and copper requirements.
What most gardeners do not realize is that soil pH has a greater effect on the type of nitrogen that is available than any other soil factor. Ericaceous plants, which include azaleas, rhododendrons, and blueberries, can only utilize ammonium nitrogen (NH4). Nitrate nitrogen can actually be toxic to these species if present in large quantities.
In several small experiments that I conducted in commercial nurseries and at the University of Maryland, I found that fertilizing azaleas during the growing season with ammonium nitrate, calcium nitrate or nitrate of soda results in a change in foliage color from a bright glossy green to a dull green within a week after application. The application of nitrate nitrogen also caused the plants to stop growing and set buds in late July.
When organic matter decomposes, the bodies of the microorganisms, which are mostly proteins and amino acids, become the organic source of nitrogen. This form is primarily ammonium.
Ammonium in an acid soil remains as ammonium because the soil is low in oxides and rich in hydrogen ions. The conversion of ammonium to nitrate by Nitrosomonas bacteria occurs as the oxide levels in soils increase and is very slow. If you understand the definition of pH, (the reciprocal of the hydrogen iron concentration) you can understand why ammonium remains as ammonium longer in acid soils than in less acid soils. Acid soils have a much higher concentration of hydrogen ions than less acid soils.
As the pH of the soil increases due to the introduction of oxides, such as those found in limestone, there is a lower concentration of hydrogen ions present in the soil. The bacteria nitrosomonas converts ammonium to nitrite NO2, which is a form of nitrogen that is not utilized by plants. As oxide levels in the soil increase nitrobacter convert nitrite to nitrate NO3, which can also be utilized by plants except ericaceous species. Bacteria responsible for changing the forms are nitrogen are referred to as autotrophic bacteria. Thus trying to grow ericaceous species in only slightly acid mineral soils (pH between 5.5 and 7.0) is difficult because ammonium is rapidly converted to nitrate. However, this is less of a problem if the plants are being grown in soils rich in organic matter, because as the organic matter decomposes ammonium is always the first source of nitrogen that becomes available. Also, in high organic soils there is an abundant supply of humid and fulvic acids present.
Based upon this knowledge, one can quickly realize that there are many benefits from growing plants in high organic soils and the benefits from adding compost Compost becomes compost when most of the readily digestible carbon in the bulking waste material are no longer recognizable. The carbon compounds of cellulose and hemicellulose have been digested and released into the atmosphere as carbon dioxide. As the microorganisms begin to die from the lack of carbon (starvation), their bodies decompose into ammonium. Thus a soils with 8 percent organic matter is capable of generating approximately 80 pounds of ammonium nitrogen per year, which is an abundant source of nitrogen to grow ericaceous species, while a soil that has less than 3 percent organic matter can only generate between 20 to 25 pounds of ammonium nitrogen per year per acre. Since iron and most trace elements are readily available at a wide range of soil pHs, due to the presence of high levels of humic and fulvic acids, availability of trace elements in high organic soils is not a problem
Acid soils with less than 3 percent organic matter are most likely to be deficient in calcium. However, soils rich in organic matter are likely to contain adequate levels of calcium to support plant growth. Thus one can appreciate the benefits of amending soils with compost because in addition to providing an abundant supply of ammonium nitrogen, the compost is also rich in plant residues that contain calcium.
I recently pulled from my shelves “The Standard Cyclopedia of Horticulture” published by Liberty Hyde Bailey in 1900. In the section on preparing a soil for gardening, he strongly recommends “the incorporation of liberal amounts of organic matter” and emphasizes the use of compost. I feel certain that Dr. Bailey did not fully understand the complexities in high organic soils with regards to releasing ammonium nitrogen or the availability of trace elements, but he certainly had a great appreciation for the importance of organic matter in the growth of plants. Enough said.