Submitted to: North Central Extension Industry Soil Fertility Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 1, 2010
Publication Date: November 17, 2010
Citation: Kitchen, N.R., Sudduth, K.A., Drummond, S.T., Sheridan, A.H. 2010. What do you do when your N-rich reference fails?. In: Proceddings of North Central Extension Industry Soil Fertility Conference, November 17-18, 2010, Des Moines, Iowa. p. 52-59. Technical Abstract: In recent years, canopy reflectance sensing has been investigated for in-season assessment of crop N health and fertilization. Typically, the procedure followed compares the crop in an area known to be non-limiting in N (the N-rich area) to the crop in a target area, which may be inadequately fertilized. Measurements from the two areas are used to calculate a relative reflectance to represent the potential need for additional N fertilizer. Establishing N rich areas or strips is often inconvenient for farmers, since this coincides with other demanding spring operations. The objective of this study was to answer the question of what do you do when the N-rich reference fails. Two studies were conducted. In the first, a total of 16 field-scale experiments were conducted over four growing seasons (2004-2007) in three major soil areas of Missouri. Multiple blocks of randomized N rate response plots traversed the length of each field. Each block consisted of 8 N treatments from 0 to 235 kg N/ha on 34 kg N/ha increments, top-dressed between vegetative growth stages V7 and V11. Adjacent to the response blocks, N-rich (235 kg N/ha) reference strips were applied at or just after planting. Crop canopy reflectance sensor measurements in the format of inverse simple ratio values (Vis/NIR) were obtained from the N response blocks and adjacent treatment strips at the time of top-dress N application. Viewed in frequency distribution diagrams, canopy sensor ISR values for target corn were almost always higher than those for N-rich corn, had a greater range of values, and were more positively skewed. A model was developed that successfully predicted 75% of the variation in average N-rich reference sensor measurements. In the second study the effect of hybrid on canopy sensor readings was explored. Canopy sensing (Crop Circle) readings were taken through most growth stages from V3 to V10 on 11 (2008) and 8 (2009) hybrids. Variability within and between hybrids was most noted for corn less than 60 cm in height. Results showed that soil type and soil surface wetness impacted canopy readings more than hybrid. The results could prove useful in determining reasonable ranges for N-rich reflectance values in variable rate N applications.