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Fluid Journal : Fluid Journal 1996-1998
Ammonia volatized - % 100 75 50 25 0 012510 ATS (volume ratio) - % Figure 5. Effect of ATS in reducing ammonia volatilization. surface, it dries and a series of chemical and biochemical reactions begins. These reactions are varied and complex and are dependent on environmental factors such as temperature, moisture conditions, level of organic matter, buffering capacity, and pH. It generally has been observed that greater N losses occur on calcareous soils. Liming may also increase the level of ammonia volatilization by shifting the NH4+ concentration to higher levels of NO3, which can be subsequently lost as a gas. Investigators have noticed that if all other conditions are similar, volatilization is greater in coarser-textured soils. This is attributed to the higher CEC capacity of finer-textured soils. With a high CEC, more NH4+ is held on the ex- change complex and less NO3 - is present is the soil solution. There seems to be varying effects of organic matter on volatilization but the results appear to be linked with CEC reactions. Usually volatilization is greater where sod or residues are present. This is probably caused by conversion of urea to NH3 by hydrolysis on the surfaces of growing leaves or residue particles. Since CEC of the residues is limited, much less NH4+ can be retained than on soil surfaces. Proper placement. Surface banding, subsurface injection or soil incorporation of manures or urea-based fertilizers essentially eliminates ammonia volatil- ization. Precipitation. Where mechanical incorporation is not available or prac- ticed (conservation tillage), precipitation acts as a natural incorporator. Table 1 provides a guideline for the amount of loss under different moisture conditions. A light rain is sufficient to incorporate UAN and minimize N loss, but the longer the dry spell after application, the greater the chance for volatilization losses. NBPT. The most active compound identified as a urease inhibitor is N-(n- butyl) thiophosphoric triamide, generally known as NBPT (Agrotain®). Urease is an enzyme that catalyzes the conver- sion of urea into its basic components. It is commonly thought to be present under almost all soil conditions and in large amounts. Without active urease, most soil microbes would be unable to break down urea because of the high amount of energy required to begin the conversion process. A comprehensive study of 78 trials performed by university scientists showed NBPT to be effective at rates ranging 0.55 to 1.1 pounds active ingredient/A. Note in the summation of trials (Figure 2) how an average of 74 lbs/A of additional unamended urea-N was required to equal the yield advan- tage the NBPT additive gave by inhibit- ing losses of N applied at 92 lbs/A. Such N losses did not occur at all locations every year. Conditions giving rise to large yield losses are as unpre- dictable as the weather. When favor- able, where N losses are minimized, use of NBPT would be a financial waste, as would an extra 74 lbs/A not be taken up by the crop. Losses from the root zone could find their way to groundwater. It is also important to understand that, individually, additional N required would be greater than the 74-lb/A average difference in the model used above, since the average includes sites where N losses were low or negligible. On 6 of 21 sites, for example, the yield differential between high and intermedi- ate N levels was 5 bu/A or less. DCD. Recent research conducted on potatoes by the USDA-ARS, Kimberly, Idaho, showed impressive yield gains from the use of the inhibitor dicyandiamide (DCD), an agent be- lieved to inactivate an enzyme neces- sary for nitrification. An application of 28-0-0-5S + DCD outyielded an applica- tion of 32-0-0 by 61 cwt/A (Figure 3). N-Serve® is most commonly used to inactivate the enzyme necessary for nitrification and should be applied preplant. Research on N-Serve® is extensive. Some beneficial effects are shown in Figure 4. Ammonium thiosulfate (12-0-0-26) may also act as a urease inhibitor. Research conducted with different rates of ATS showed a 38 percent reduction in volatilization when ATS was mixed in UAN at a 10 percent rate (volume basis) and nearly a 25 percent reduction at the 2 percent rate (Figure 5). Dr. Lohry is agronomist for Nutra-Flo Company in Sioux City, Iowa. !
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