Vol. 72 No: 2

Title:
The Nitrogen Requirement of Vegetables: Comparisons of Yield Response Models and Recommendation Systems

Authors:
GILLIAN GOODLASS, C. RAHN, M.A. SHEPHERD, A.G. CHALMERS and FRANCES M. SEENEY

pp: 239-254

Abstract:
Data from 26 experiments covering eight vegetable species, were examined to determine the best model to describe the yield response of vegetables to nitrogen (N) fertilizer. Although no single model was identified as the best on all occasions, the exponential and line plus exponential gave satisfactory estimates of Nmaxima (the N application rate which gave maximum yield) on most occasions. The Nmaxima from the best fitting models were used to compare seven recommendations systems to determine the best method of predicting the N requirement of vegetables. The aims were to apply N fertilizer so as to provide not only optimum yields, but also to leave the minimum residue of unused N, as estimated from the N balance (Napplied-Nuptake at harvest). Of the methods tested, a soil mineral nitrogen system using measurements from 0–30 cm depth was closest to the N requirement for maximum yield most often. The method which left the least fertilizer residue on most occasions, whilst still achieving some acceptable yields was a different soil mineral nitrogen system (using measurements for samples from 0–90 cm depth). The method which gave the best overall combination of yields and residual N was a computer model which used local weather data to modify measurements of soil mineral nitrogen and simulate crop growth. Methods which rely on look-up tables are easiest for growers to use, but these do not cope with the variability of soil N supply in vegetable rotations as well as methods based on actual measurements. Growers need to be encouraged to adopt the latter methods as an aid to both improved yield and reduced risk of N leaching.

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