Establishment of threshold leaf colour greenness for need-based fertilizer nitrogen management in irrigated wheat (Triticum aestivum L.) using leaf colour chart

Over and untimely application of fertilizer nitrogen (N) are the major constrains in improving fertilizer N recovery efficiency in irrigated wheat ( Triticum aestivum L.). Large field to field and seasonal variability further lower fertilizer N recovery efficiency when broad based blanket recommendations are followed. Six field experiments were conducted during 2005 to 2009 at different locations to establish threshold leaf colour greenness to guide in-season need-based fertilizer N topdressings in wheat. Colour of the first top fully exposed leaf as measured by comparison with different shades of green colour on a leaf colour chart (LCC) and wheat grain yield were significantly correlated. The Cate-Nelson plot of leaf greenness expressed as LCC values against relative grain yield of 0.91 indicated that leaf greenness between LCC shade 4 and 5 may guide crop demand driven N applications in wheat. A series of experiments with progressive refinement in treatments were conducted to develop adequate need-based N management strategy using a single threshold LCC value in irrigated wheat. It was found that a dose of at least 25 kg N ha −1 should be applied at planting. At 1st irrigation (crown root initiation) stage leaf greenness cannot be quantified properly using LCC due to small leaf size and thus it did not lead to adequate fertilizer N management decision. At 2nd irrigation (maximum tillering) stage leaf colour of the first fully exposed leaf served as best indicator of inherent soil N supply as well as crop N needs and thus helped guide need-based fertilizer N top dressing for improving fertilizer N use efficiency in wheat. Fertilizer N management strategy based on application of prescriptive doses of 25 kg N ha −1 at planting and 45 kg N ha −1 at 1st irrigation and then a dose of 30 or 45 kg N ha −1 at 2nd irrigation stage depending on colour of the leaf to be ≥LCC 4 or