Submitted to: North Central Extension Industry Soil Fertility Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/28/2011
Publication Date: 11/16/2011
Citation: Allphin, E., Kitchen, N.R. 2011. Assessing nitrogen management and claypan soil variability effects on switchgrass using reflectance sensing. 2011 North Central Extension-Industry Soil Fertility Conference, November 16-17, 2011. 27:158-166. Interpretive Summary: For claypan soils of the US Midwest, the topsoil is often described as the depth of soil over the top the subsoil claypan horizon. This topsoil depth can be highly variable within fields and is a major factor causing grain yield variability. Switchgrass is a prairie grass that is a potential bioenergy crop to be grown on these soils since it is projected to have more stable yield even with variable topsoil, than grain crops. Switchgrass also has an advantage of providing improved soil conservation outcomes than grain crops. In order to help farmers expand switchgrass into more acres, tools are needed to help explain switchgrass growth and make management decisions. The purpose of this study was to explore, on claypan soils, the use of canopy reflectance sensors to assess stand and nitrogen health of switchgrass. We found that reflectance sensing early in the growing season was able to detect differences in switchgrass management. Reflectance sensing values were different between switchgrass varieties, and between nitrogen rate treatments. Also, reflectance sensing was also able to identify areas where switchgrass stand was less than desired. Switchgrass harvested in the fall was related to the early-season reflectance values, but primarily when stand was sparse or nitrogen rates inadequate. Soil topsoil depth had little impact on reflectance measurements. We concluded reflectance sensing may be a useful tool for producers to use in assessing switchgrass health and altering management for improved production.
Technical Abstract: The topsoil depth or depth to the claypan (DTC) can be highly variable across the landscape for some Midwest soils. This makes managing crops on these soils difficult because their productivity can be highly variable. Due to the high variability on theses soils, switchgrass (Panicum virgatum L.) has been suggested as a potential crop for these soils, since production is estimated to be more stable even with variable topsoil, than grain crops. The objective of this study was to investigate the relationship of active-light reflectance sensing on switchgrass stand and growth as impacted by management and DTC variation. The research was conducted on a site in Columbia, MO where 32 25- by 20-m blocks were constructed with varying topsoil depths (ranging from 0 cm to 90cm DTC). Switchgrass was planted in the spring of 2009 with eight different management treatments initiated at that time. Canopy reflectance sensing measurements were obtained at multiple times during early season growth for 2010 and 2011 using a Crop Circle ACS210 (Holland Scientific, Lincoln NE). The sensor was held between 60 and 90 cm above the canopy and two 7-m long passes along each plot were recorded, giving 90-120 readings per plot recorded at a 10 hertz rate. Reflectance was expressed using the ratio of the visible wavelength to the near-infrared (NIR) wavelength, sometimes called the inverse simple ratio (ISR). Plant height measurements were taken at the same time as reflectance sensing. Reflectance sensing was able to detect differences in management well during early summer growth. Reflectance for switchgrass variety Cave-n-Rock was significantly less than the Kanalow variety. Sub-optimal stand and inadequate N fertilization were found to be important factors affecting reflectance measurements. Under these conditions, reflectance sensing and biomass yield were related (RMSE=912 kg/ha; r^2=0.53). Soil topsoil depth had little impact on reflectance measurements. We conclude reflectance sensing may be a useful tool in assessing switchgrass for management decisions.