Fertilizer Nitrogen Management in Irrigated Transplanted Rice Using Dynamic Threshold Greenness of Leaves

Abstract

Application of fertilizer N doses to rice (Oryza sativa L.) whenever leaf greenness as measured by chlorophyll meter (SPAD) and LCC falls below a fixed threshold is being extensively used for site-specific management of fertilizer N. As fixed threshold greenness varies with regions, rice cultivars and seasons, a dynamic threshold greenness concept in the form of SPAD-based sufficiency index has been introduced. For rice, the dynamic threshold greenness is defined as 90 % of the SPAD meter reading of leaves in an N_rich or over fertilized strip. In the present investigation, four field experiments with six cultivars were conducted at two locations to test fertilizer N management following two variants of the LCC-based dynamic threshold greenness concept—the leaf greenness one or 0.5 unit less than the LCC reading of the N_rich strip (LCC-N_rich), vis-à-vis SPAD meter-based sufficiency index approach and blanket recommendation of 120 kg N ha⁻¹ applied in three equal split doses. The fertilizer N management following threshold greenness of one unit less than the LCC-N_rich resulted in total N application of only 60 kg ha⁻¹ and thus sub-optimum rice grain yields. The grain yield of rice, N uptake and fertilizer N use efficiency measured with N management following threshold leaf greenness of 0.5 units less than LCC-N_rich were at par with the SPAD meter-based 90 % sufficiency index approach and were better than the blanket fertilizer recommendation in the region. The LCC-based dynamic threshold greenness strategy holds promise to efficiently manage fertilizer N rice as it can effectively take care of variations in the rice cultivars, locations and seasons.