Crop Monitoring Technologies to Assess Nitrogen Status

  • Fox R
  • Walthall C
  • Schepers J
 et al. 
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Visual methods to assess crop N status have probably been used for millennia, al-though early farmers were not aware that the observed symptoms of defi ciency were due to a lack of N per se. Visual observation is still commonly used by growers to quickly assess the adequacy of N supply for crop growth, as visual symptoms of N defi ciency are characteristic for many crops. Tissue tests of specifi c plant parts for ei-ther total N concentration or total nitrate concentration became the preferred meth-od to determine crop N adequacy during the early to mid-1900s. A relatively inex-pensive, handheld instrument was developed in the early 1980s (SPAD meter) (Fox and Piekielek, 1998) to estimate the chlorophyll concentration of plant leaves using leaf spectral transmitt ance. This allowed researchers to develop a means of quickly inferring crop N status from chlorophyll concentration throughout the growing sea-son (e.g., Fox et al., 1994; Piekielek and Fox, 1992; Turner and Jund, 1991). Research has also been conducted since the 1970s to develop remote sens-ing methods to detect and estimate crop stress including N status. Relationships between leaf spectral refl ectance and N status have received the most att ention. Refl ectance changes of full canopies due to N defi ciency have been under investiga-tion more recently with handheld, tractor-mounted, airborne, and satellite sensors, and active systems using laser illumination to excite and measure fl uorescence. Tucker (1984) wrote a general review of crop monitoring to assess crop N sta-tus. The visual symptoms of N defi ciency, and critical N concentrations required for maximum yields of crops have been included in several books on plant nu-trient defi ciency symptoms (Bergmann, 1992; Chapman, 1966; Grundon, 1987; Sprague, 1964). Schröder et al. (2000) have recently published a review comparing soil and plant tests for diagnosing the N status of crops. Visual Methods Farmers have known the value of manure and legumes for increasing yield since Greco-Roman times and therefore most likely knew the plant symptoms of

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  • Richard H Fox

  • Charles L Walthall

  • J. S. Schepers

  • W. R. Raun

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