Abstract

The ‘Law of Optimum’ is put forward as the unifying concept in plant nutrition for realizing ‘targeted yield of crops’ through soil test-based nutrient management. This concept has been calibrated using a novel factorial field experiment technique, designed and used under the All India Coordinated Soil Test Crop Response (STCR) project. This initiative was conducted in India on a range of soils and crops over four decades and was validated through hundreds of demonstration trials in farmers’ fields.

Early results established that the relationship between wheat grain yield and the total nutrient uptake by the plant followed a linear relationship implying that, for obtaining a given yield, a definite quantity of nutrients must be absorbed by the plant. Based on crop nutrient uptake required to obtain a desired yield level (targeted yield), the ‘Law of Optimum’ calculates nitrogen (N), phosphorus (P), and potassium (K) application doses, taking into account nutrient contribution from three measurable sources: 1. soil fertility (available nutrients, based on chemical soil tests); 2. added fertilizers; and, 3. added organic manure. Over 2,000 demonstration trials in farmers’ fields conducted so far have validated the concept, realizing the yield targets within a 10% deviation. Operationally, the ‘Law of Optimum’ harmonizes the much debated approaches of ‘fertilizing the soil’ versus ‘fertilizing the crop’, ensuring a real balance is achieved among available nutrients. The principles underlying the ‘Law of Minimum’, ‘Law of Diminishing Returns’ and the ‘Law of the Maximum’ governing plant nutrition are strongly embedded in the ‘Law of Optimum’. Furthermore, this law also provides a basis for maintaining consistent soil fertility with high productivity and efficient nutrient management in ‘Precision Farming’, to achieve sustainable agriculture.