Site Specific Nutrient Management

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Nutrient Omission Plot Technique Version 1

Experimental setup

Fertilizer N, P, and K must be applied at sufficiently high rates to eliminate limitations to crop growth. The addition of fertilizer N, P, and K is consequently adjusted based on the yield anticipated with full fertilization. More fertilizer N, P, and K are applied at higher anticipated yields (Table 1).

The rates of added N, P, and K in omission plots are completely unrelated to fertilizer practices of farmers, recommendations of extension, and the SSNM recommendation. They do not reflect a rate to be recommended for use by farmers.

Table 1. Fertilizer addition to omission plots based on an estimate of the yield attainable with full fertilization of N, P, and K.

Plot	Fertilizer addition (kg ha⁻¹)
	Attainable yield: <6 t ha⁻¹			Attainable yield: 6 to 7 t ha⁻¹			Attainable yield: >7 t ha⁻¹
	N	P₂O₅	K₂O	N	P₂O₅	K₂O	N	P₂O₅	K₂O
NPK	90	50	70	130	50	70	180	75	100
–N	--	50	70	--	50	70	--	75	100
–P	90	--	70	130	--	70	180	--	100
–K	90	50	--	130	50	--	180	75	--

Nitrogen is typically applied as urea in equal doses at the following instances: Within 14 days after transplanting (DAT) or 21 days after sowing (DAS), at active tillering, and at panicle initiation. Phosphorus is applied basally as a fertilizer containing only P, such as single superphosphate (SSP) or triple superphosphate (TSP). Half of potassium is applied basally and half at panicle initiation as muriate of potash (MOP, 60% K₂O). For rice-growing areas known to be deficient in zinc, broadcast zinc sulfate at about 5 kg Zn ha⁻¹ to all four plots.

The four plots in the nutrient omission plot technique comprise one experimental unit. At least five experimental units or replications of the omission plots are typically distributed within a rice-growing area representing a relatively homogenous soil type or land management history. The grain yields obtained with the nutrient omission plot technique can then be used for developing location-specific N, P, and K management for the rice-growing area.

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