WHY CHANGING FROM DISTRICT YIELD POTENTIAL TO BLOCK YIELD POTENTIAL AS THE BASIS FOR THE NITROGEN GUIDELINES IS NOT AS SIMPLE AS IT SOUNDS

摘要

The nitrogen (N) guidelines within the SIX EASY STEPS program are based on a combination of: 1. District yield potential (DYP) calculated from the 'highest average annual district yield' determined from mill data over a 20-year period multiplied by a factor of 1.2; 2. A target N-utilisation index value (TNUIV) of 1.4 kg N/tonne of cane up to a cane yield of 100 t/ha and 1 kg N/ tonne of cane thereafter to determine the baseline N application rate; 3. A N mineralisation index (based on soil organic carbon (%)) to take soil type into consideration. This step provides a discounted TNUIV ranging from 1.33 (for ratoon cane grown on soils with less than 0.4% organic C) to 0.66 (for plant cane grown on soils with more than 2.4% organic C). Others have suggested that block yield potential (BYP) should be considered as an alternative to DYP for determining N application rates. Their suggested BYPs have included: 1. A 10-year average block yield plus two standard deviations. N rate was calculated by multiplying this BYP by 1.4 kg N/t cane. 2. An average farm yield potential (FYP) applied to each block on the farm. The N rate was determined by multiplying FYP by 1.4 kg N/t cane up to 100 tc/ha and 1.0 kg N/t cane thereafter. In both cases N mineralisation based on soil OC% was not included in determining N fertiliser rate. These possibilities were tested using commercial yield data from higher-yielding blocks of cane located on Lugger and Tully soils in the Tully district. Both approaches led to needlessly higher N rates than those suggested by the SIX EASY STEPS and the N management strategies currently used by the particular growers. This is a perverse outcome for high-yielding blocks given the need for increased N-use efficiency within a sustainable sugarcane production system. The SIX EASY STEPS N guidelines therefore currently remain the most appropriate basis for industry best management practice. Any movement towards amended N rates needs scientific evaluation using long-term field trial and commercial data.