A method to quantify N fertilizer requirement

Abstract

A methodology to quantify N fertilizer requirement for a barley crop is proposed. The method consists of establishing a relationship between barley N demand (DEM) and an index of soil N supply (NS). NS values were obtained adding the inorganic N, i.e, (N-NO^- ₃ + N-NH⁺ ₄, (Ni), measured just before planting in the soil to an index of soil N mineralization (Nor):

NS = Ni + Nor

Soil organic matter (SOM) was selected as an index of Nor:

Nor = 25.9 + 17.9 (SOM) R²; = 0.89; p < 0.01

and Ni was measured in a soil extract obtained with KCl (1N).

Ns values were related to both grain yield (Y) and N accumulated in the above ground biomass of barley (AB) grown under greenhouse conditions. The following relationships were obtained:

Y = 0.81 + 0.99 (NS) - 0.0002 (NS)² R² = 0.77; p < 0.01

AB = -0.006 + 0.002 (NS) - 0.000003 (NS)² R² = 0.84; p < 0.01

A graphic relationship was established between DEM and NS, in order to extrapolate the greenhouse results to field conditions. Using this relationship, AB was converted from g N pot^-1 to kg N ha^-1, and NS from μg g^-1 of N in the soil to kg ha^-1.