CLA appears to be a protein-chitin powder. Ammonium (NH4+) is released during microbial breakdown of chitin, and is also produced during CLA decomposition in soil. When CLA was tested as a nematicide in Chinese cabbage, high concentrations (0.4% w/w) were toxic to the crop but a concentration of 0.2% increased plant weight. Because Chinese cabbage is sensitive to high levels of ammonium, a study was conducted with this crop to determine the effect of CLA as a slow-release nitrogen fertilizer. Ureaformaldehyde (UF), a slow-release compound subject to microbial mineralization, was included in the study for comparison.
Chinese cabbage seedlings were grown in pots of sandy loam, in temperature-controlled tanks at 15, 20, or 35oC (Normal field root temperatures). Seedlings were planted after 0.2% CLA or 0.1% UF was mixed into the soil (available N is 10% in CLA and 20% in UF). A third treatment was irrigated weekly with 4-2-8 liquid fertilizer. Soil ammonium and nitrate (NO3-) levels, leaf number, top weight, and plant N level were measured.
After 14 days after planting (DAP), soil ammonium levels were highest at the higher temperatures for CLA and UF treatments. Ammonium levels were higher in UF-treated than in CLA-treated soils at all temperatures except 15oC. Soil nitrate levels were higher at higher temperatures in all three treatments. These results indicate higher rates of mineralization for CLA and UF as temperature is increased. By 30 DAP, ammonium was found only in the UF-treated soil at 15oC, and soil nitrate levels were low in all treatments.
Plant weights at 14 DAP were low for UF- or mineral-treated plants grown at 35oC, but early growth did not differ among the other treatments. By 30 DAP, CLA and UF treatments had increased plant weight over mineral treatments at all temperatures, except for lower weights with UF at 35oC and no difference between CLA mineral treatments at 15oC. This fertilizing effect of CLA and UF was reflected in high plant N levels for these treatments.
As root temperature increased to 30oC, plant weight increased as well, especially with UF and CLA. UF is broken down by microbes at a faster rate as temperature increases, and CLA is probably mineralized by a similar process. Thus, more nitrogen from UF and CLA is available to plants at higher temperatures. However, although soil nitrogen levels were higher at 35 than at 30oC, plant weights were lower at 35oC. This was most significant in UF-fed plants: roots deteriorated and 50% of the plants wilted at 35oC. When roots metabolize ammonium, sugars are depleted and oxygen is used. Excess ammonium released by UF at high temperatures may have been toxic to the plants. By contrast, CLA-fed plants had better-developed roots at 35oC than did UF-fed plants, and weights of CLA-fed plants were not reduced as much by the high temperature. In addition, UF-fed plants had highest nitrogen levels at the lower temperatures, but nitrogen levels in CLA- and mineral-treated plants were generally independent of soil temperature.
These results suggest that CLA may be able to control release of ammonium. Other compounds such as UF may have value as slow-release nitrogen fertilizers, but they release much more ammonium into the soil at higher temperatures. This can make them toxic to ammonium-sensitive crops like Chinese cabbage. CLA holds promise as a slow-release nitrogen fertilizer, especially where high soil temperatures limit the use of other fertilizers in ammonium-sensitive crops.
JOURNAL OF HORTICULTURAL SCIENCE 63(4): 621-627.