Abstract:

The aim of this research was to lay a scientific basis for alfalfa （Medicago sativa） soil testing and fertilizer recommendation in the Loess Plateau region of China. The research methodology included a literature survey of alfalfa fertilization experiments conducted on the Loess Plateau， extracting data on soil nitrogen content and forage yield of N fertilizer treatments and where no N fertilizer was added. An algorithm for scattered experimental data integration and a new formula based on nutrient balance were employed to develop a sufficiency index （SI） of soil N with a scale of 1 to 6 and the recommended N fertilizer application rates （RNFAR） for alfalfa in the study region. It was found that for the planting year of an alfalfa crop （Y₁） on the Loess plateau， the values for soil alkali-hydrolyzable nitrogen for SI levels 1 to 6 were， respectively， ≥73， 55-73， 39-55， 26-39， 13-26 and <13 mg·kg^-1. Values for soil total nitrogen for SI levels 1 to 5 for Y₁ were， respectively， ≥1.5， 1.1-1.5， 0.7-1.1， 0.4-0.7 and <0.4 g·kg^-1. Values for soil organic matter for SI levels 1 to 6 for Y₁ were， respectively， ≥19， 13-19， 9-13， 5-9， 2-5 and <2 g·kg^-1. For second and subsequent seasons， values for soil alkali-hydrolyzable nitrogen for levels 1 to 5 were， respectively， ≥79， 51-79， 30-51， 13-30 and <13 mg·kg^-1. For the whole crop life （WCL） of alfalfa， values for soil total nitrogen for SI levels 1 to 4 were， respectively， ≥1.9， 1.2-1.9， 0.5-1.2 and <0.5 g·kg^-1. Values of soil organic matter for SI levels 1 to 5 for the WCL were， respectively ≥24， 13-24， 6-13， 2-6 and <2 g·kg^-1. For a target yield of 9.0 t DM·ha^-1 and an N fertilizer use efficiency of 40%， the RNFAR for SI levels 1 to 6 were， respectively， 0， 68， 135， 203， 270 and 338 kg N·ha^-1， while for a target yield of 22.5 t DM·ha^-1 and an N fertilizer use efficiency of 40%， the corresponding values were， respectively， 0， 169， 338， 506， 675 and 844 kg N·ha^-1. This research has provisionally developed a system for calculating recommended fertilization rates for alfalfa crops on the Loess Plateau， based on soil nitrogen abundance and deficiency indicators and target crop yield and thereby provides a scientific framework for soil testing and nitrogen application to alfalfa crops in the region.

Key words: alfalfa, soil testing and fertilizer recommendation, organic matter, total nitrogen, alkaline hydrolysable nitrogen, sufficiency index, fertilization index, fertilizer application rate