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Fluid Journal : Fluid Journal 1993-1995
1 Fluid Journal Summer 1994 Summary: Nitrogen management is an increasing concern for producers. Potential loss of N by leaching and volatilization decreases income and increases potential for ground-water contamination. Nitrogen fertilizer recommendations are based on the pre- sidedress soil nitrate test (PSNT). Representative soil samples must be taken and sent to a lab for analysis. The time lapse is critical if additional N must be applied. Fields that contain bands of N are difficult to sample because of variation created by the band. In search of an alternative, we commenced studies in 1991 at two sites in Iowa. Results to date suggest that measuring chlorophyll content in corn at the V6 stage may be the way to solve the variability problem created by banded N. Chlorophyll content in corn leaves at the V6 growth stage appears to relate to soil N and grain yield. However, other factors that affect chlorophyll content, such as weather and hybrid, are making it difficult to develop a standard critical level. Anew method for assessing soil N in Iowa is the Pre-sidedress Soil N Test (PSNT). A conservative amount of N is applied preplant, followed by sidedressing in late spring. Problem is PSNT considers only soil NO3-N. Ammonium-nitrogen is not measured. Because injected N does not nitrify as rapidly as broadcast N, the PSTN assessment could underestimate the amount of N in the soil by failing to measure ammonium nitrogen. Injection also makes it difficult to obtain a representative soil sample. There is no field uniformity with injected N, resulting in either extremely high or low concentrations when soil sampling. by Chad Berghoefer and Dr. Randy Killorn Chlorophyll Meter Promising As Tool For Nitrogen Management But variances in weather and hybrids make it difficult to develop a standard critical level. A possible alternative to the PSNT method of assessing soil N, already being researched, is use of a portable chlorophyll meter. Studies include its use to spot N deficiencies in crops and, when calibrated, to estimate the amount of N fertilizer required for optimum yields, when readings are below critical value. Thus, it may be possible to walk through a field taking random chlorophyll readings and predict if additional fertilizer is required. If so, the quantity could be determined within minutes of walking out of the field. Other research has reported that leaf N concentration in corn is related to the amount of chlorophyll in the corn leaf. Similarly, a good correlation between chlorophyll meter readings and the chlorophyll concentration in corn leaves has been found. Focus of this discussion will be on whether or not chlorophyll content of corn leaves can be used as a measure of the N fertilizer needs of a crop growing in fields in which N fertilizer was injected before planting. Experiments were conducted at two sites in Iowa from 1991 to 1993. Data discussed here are from 1993 only. Three types of application were used: broadcast, spring inject, and sidedress inject. Ames site Chlorophyll content of V6 corn leaves increased with N rates. SPAD (Minolta meter) readings of V6 corn leaves increased with N rates. SPAD readings were not affected by method of fertilizer application. SPAD readings ranged from about 43 to 48. Maximum readings were reached at about 125 lbs/ AofN. Ammonium-nitrogen concentrations taken at sidedress were not affected by N rate or method of application. Most of the N fertilizer had nitrified, giving low levels of ammonium-nitrogen at the time of sampling. Nitrate-nitrogen increased with N rate. Spring broadcast applications resulted in higher concentrations of nitrate-nitrogen than injected applications. Nitrate-nitrogen content of the check treatment was below the critical level of 21 ppm, indicating the need for additional N to optimize yield. Grain yields increased with increased N rates. Figure 1 displays an almost linear relationship. Yields were also affected by method of application. Heavy precipitation leached most of the spring-applied N below the top foot of soil. The order of yields, ranging from highest to lowest, was: sidedress injected, spring injected, and broadcast, which supports the assumption that nitrification rates are lower in injected than in broadcast treatments. The relationship between SPAD readings on V6 corn leaves and grain
Fluid Journal 1996-1998