Browsing by Author "Westfall, Dwayne G., advisor"

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Open Access
Ground based active remote sensors for precision nitrogen management in irrigated maize production
(Colorado State University. Libraries, 2009) Shaver, Timothy Michael, author; Westfall, Dwayne G., advisor; Khosla, Rajiv, advisor
Precision agriculture can increase farm input efficiency by accurately quantifying variability within a field. Remotely sensed normalized difference vegetation index (NDVI) has been shown to quantify maize (Zea mays) N variability. Ground-based active remote sensors that can determine NDVI are commercially available and have been shown to accurately distinguish N variability in maize. There are several active sensors available but no studies directly comparing active sensors have been reported. Therefore, a study was conducted to evaluate active sensor performance and develop an in-season maize N recommendation algorithm for use in Colorado using NDVI. Previous studies have demonstrated an association of active sensor NDVI with maize N content and height. However, the NDVI from a GreenSeeker™ green NDVI prototype active sensor had not yet been tested when our study began. Therefore, the green sensor was evaluated to determine if differences in plant growth across MZ could be determined by the active sensor. Results show that the prototype active sensor did not record NDVI values that were associated with MZ. The NDVI from two different sensors (Crop Circle™ amber NDVI and GreenSeeker™ red NDVI) were then examined under greenhouse and field conditions. Results show that NDVI from the amber and red sensors equally distinguished applied N differences in maize. Each active sensor's NDVI values had high R2 values with applied N rate and plant N concentration. Results also show that each sensor's NDVI readings had high R2 values with applied N rate and yield at the V12 and V14 maize growth stages. An N recommendation algorithm was then created for use at the V12 maize growth stage for both the amber and red sensors using NDVI. These algorithms yielded N recommendations that were not significantly different across sensor type suggesting that the amber and red NDVI sensors performed equally. Also, each N recommendation algorithm yielded unbiased N recommendations suggesting that each was a valid estimator of required N at maize growth stage V12. Overall results show that the amber and red sensors equally determine N variability in irrigated maize and could be very important tools for managing in-season application of N fertilizer.
Open Access
Improvement of soil test P calibration and fertilizer P management in crop rotations in Moroccan dryland agriculture
(Colorado State University. Libraries, 1997) Amrani, Mohamed, author; Westfall, Dwayne G., advisor; Peterson, Gary A., committee member; Barbarick, Kenneth A., committee member; Moore, Frank Devitt, III, committee member
Although the main limitation of crop production in the arid and semiarid regions of Morocco is lack of sufficient rainfall, phosphorus (P) nutrient deficiencies also are major obstacle to maximum crop production. Fertilizer management is an important step in sustainability of crop production where both economic and environmental concerns are important. The objectives of this study were (i) to improve fertilizer P recommendations by the inclusion of P sorption capacities of individual soils in the P requirement model and (ii) to determine the effect of direct, cumulative, and residual P on wheat (Triticum aestivum) and chickpea (Cicer arietinum L.) yields under field conditions in two cropping systems: continuous wheat and chickpea-wheat. Phosphorus buffering indices were determined from sorption isotherms developed using 19 soils from the Abda, Chaouia, and Ben Sliman zones of Morocco. The greenhouse study consisted of growing wheat (cv. Merchouch) on 13 soils with four Prates (0, 3.4, 6.7, and 13.4 mg P kg-1). Maximum buffering capacity (MBC) of soils was incorporated into the Mitscherlich model to determine P fertilizer requirement. Soils showed a contrasting ability to adsorb P. Maximum P adsorption (Xm) varied from 146 to 808 mg P kg-1 soil. The tentative calculations of P requirement, assuming the soil test P levels in all 13 soils was 3 mg P kg-1, showed that the amount of fertilizer needed for 90% of maximum yields varied from 1 to 15 mg P kg-1 soil. However, the fertilizer P recommendation by the usual method was 12 mg P kg-1 for all soils with a soil test P of 3 mg kg-1. These results suggest that the inclusion of buffer indices in determining P requirement can increase the accuracy of P recommendations. In a greenhouse study, two other crops were grown after wheat. Com (Zea mays L., cv Kamla) was harvested after 60 days and wheat was grown to maturity. The treatments were four Prates applications (0, 3.4, 6.7, and 13.4 mg P kg-1) using 13 soils on the first and third crop (wheat). This greenhouse study showed that a significant response of com to residual P occurred in soils with initial NaHC03-P test levels less than 6 mg P kg-1. The response was inconsistent between 6 and 10 mg P kg-1, and no response occurred above a soil test P level of 10 mg P kg-1. In general, soils with more than 14 mg kg-1 NaHC03-P level provided adequate P for maximum yield for three succeeding crops under greenhouse conditions. Field experiments were conducted in 1994-96 at three locations: Khmis Zemamra, Sidi El Aydi, and Khmis Sidi Rhhal. Phosphorus was applied at 0, 8.9, 17.8, and 26.7 kg P/ha on both wheat and chickpea the first year. The second year, plots were split into with and without P fertilizer treatments. Phosphorus rates of 8.9, 17 .8, and 53.4 kg P ha-1 were required to increase and maintain soil test P level to a sufficiency level for three succeeding crops at Khmis Zemamra, Sidi El Aydi, and Khmis Sidi Rhhal, respectively. The effect of cropping system was not consistent. The residual P effect did not produce maximum wheat yield. Based on the range of P rates used in this study, a single application of P will not supply adequate nutrition for the following crop. If we assume that a chickpea grain yield of 2 Mg ha-1 is a satisfactory yield in a CP-W rotation where wheat is the principal crop, P requirement for chickpea can be met by residual P. Using current wheat and fertilizer prices, the combinations of 17.8-17.8, 26.7-0, and 8.9-17.8 kg P ha-1 would be the recommended P application rates for continuous wheat, W-CP, and CP-W rotations, respectively. Based upon my results I recommend that farmers consider both soil adsorption capacity and rotation (previous P applications and cropping system) to better manage P and optimize profit from fertilizer use.