Nitrogen (N) fertilizer recommendations made without adequate knowledge of the N supply capability of a soil can lead to inefficient use of N. Proper crediting of N from manure and legumes as well as mineralization of N from organic matter is difficult. Remote sensing techniques that use the crop to indicate its N status show considerable promise for improving N management. Objectives of this paper were twofold: 1) to compare the N Reflectance Index (NRI) calculated from ground-based radiometer measurements acquired over irrigated corn (Zea mays L.) at a nadir view (0Њ) and an oblique view (75Њ) with measured plant N and 2) to evaluate the NRI obtained from both view angles for correcting in-season N deficiencies in a commercial corn field. The NRI calculated from canopy reflectance was not representative of plant N at the sixth leaf growth stage (V6) for either view angle because of the soil background influence on canopy reflectance. However, the oblique view NRI was a good predictor of plant N at V9 and V12 as was www.dekker.comThis article is a U.S. government work, and is not subject to copyright in the United States.the nadir view NRI at V12. The nadir view NRI was not as sensitive as the oblique view NRI at the V9 growth stage because soil was still visible through the canopy. Consequently, the nadir view NRI provides a conservative estimate of plant N prior to complete canopy cover. Use of the nadir view NRI to detect in-season corn N deficiencies for the 1999 growing season reduced N application during the growing season by 39.2 kg N ha Ϫ1 without reducing grain yield. If the oblique view NRI would have been used to assess the plant N status, the first fertigation would not have been recommended which would have saved additional N.
The yield in any given field or management zone is a product of interaction between many soil properties and production inputs. Therefore, multi-year yield maps may give better insight into determining potential management zones. This research was conducted to develop a methodology to delineate yield response zones by using two-state frequency analysis conducted on yield maps for 3 years on two commercial corn fields near Wiggins, Colorado. A zone was identified by the number of years that yield was equal and greater than the average yield in a given year. Classes producing statistically similar yield were combined resulting in three potential yield zones. Results indicated that the variability of yield over time and space could successfully be assessed at the same time without the drawbacks of averaging data from different years. Frequency analysis of multi-year yield data could be an effective way to establish yield response zones. Seventeen percent of the field #1 consistently produced lower yield than the mean while 43% of the field produced yield over the mean. Corresponding values for field #2 were 6% and 42%. The remainder of the fields produced fluctuating yields between years. These spatially and temporally sound yield response maps could be used to identify the yield-limiting factors in zones where yield is either low or fluctuating. Yield response maps could also be helpful to delineate potential management zones with the help of resource zones such as electrical conductivity and soil maps, along with the directed soil sampling results.
Soil productivity is affected by soil physical properties that play a crucial role in planning drainage systems. Improper planning of drainage systems can create high water table problems and, in turn, an unsuitable environment for plant growth. Therefore, drainage systems should be well planned and monitored regularly. It is very labor-intensive and time-consuming to determine the spatial and temporal changes in drainage parameters such as groundwater (GW) depth, elevation, hydraulic gradient and salinity by conventional methods over large areas. Geographic information systems (GIS) can be used to assess the spatial and temporal changes efficiently and rapidly. This study was conducted to determine drainage problem areas and to suggest the most suitable drainage systems for those areas by evaluating spatial and temporal changes in GW depth, elevation and salinity, with regard to drainage criteria used by the State Hydraulic Works (DSI) responsible for the development of land and water resources in Turkey. A pilot area of 8494 ha in the Lower Seyhan Plain, Adana, Turkey, was selected. The elevations and coordinates of 85 drainage observation wells were obtained from the existing maps with a scale of 1/5000 and 1/25 000, respectively. The GW depth and GW salinity data, the latter only available for July, were obtained from an archived long-term monthly data set of the 6th Regional Directorate of the DSI. Running ArcView 3.0a, the salinity, as well as the seasonal maps of the GW depth and elevation, were developed to assess the spatial and temporal changes. Results indicated that about 99.8% of the study area was suffering from various levels of drainage problems. Main, secondary and tertiary drainage canals in the region seemed to be malfunctioning due mostly to siltation and weed problems. GW fluctuation did not seem to be of great importance, indicating a need for a subsurface drainage system. It was concluded that some areas are likely to be prone to potential GW salinization risk during the peak irrigation season if in the future, irrigation water is sparingly used and irrigation efficiency is improved. The GW hydraulic gradient did not significantly change temporally, but it did change spatially.
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