Soil tests to predict optimum fertilizer nitrogen rate for rice C.A. Russell a,d,1 , B.W. Dunn b,d , G.D. Batten a,d , R.L. Williams b,d , J.F. Angus c,d, * a School of Agriculture, Charles Sturt University, P.O. Box 588, Wagga Wagga, NSW 2678, Australia b NSW Department of Primary Industries, Yanco Agricultural Institute, Yanco, NSW 2703, Australia c CSIRO Plant Industry, G.P.O. Box 1600, Canberra, ACT 2601, Australia d Co-operative Research Centre for Sustainable Rice Production, Yanco, NSW 2703, Australia Received 16 June 2005; received in revised form 28 October 2005; accepted 31 October 2005 Abstract Productivity and response to nitrogen (N) fertilizer were measured on 84 irrigated rice crops growing over two seasons on farms in the Riverina region of south-eastern Australia. The results were correlated with 18 tests of the N status of the top 10 cm of soil to develop a method to specify optimum N fertilizer application before sowing. In this environment, nitrogen fertilizer applied before sowing produces higher yields than the existing pre-sowing-topdressing split, provided the application rate is not so high that it leads to cold damage or lodging. Yield, biomass and N uptake varied greatly between the 84 crops, but the mean apparent N recovery of 50–60% and the N-use efficiency of 22 kg grain kg N 1 were high by commercial standards. Of the 18 soil-N tests, the one most closely correlated to crop productivity used anaerobic incubation for 21 days at 40 8C. The standard error of this test for predicting crop biomass was at least 74 kg N ha 1 , which is double the standard error of the current plant test and is unacceptably high for providing recommendations. A comparison of commercial N- application rates and the economic optimum rates showed that ricegrowers tended to overfertilize crops growing on the most fertile fields and underfertilize crops growing on the least fertile fields. Suggested strategies to increase N-use efficiency are to improve the accuracy of the soil test by considering factors such as the status of other nutrients, to inform ricegrowers of their tendency to overfertilize high-yielding crops and underfertilize low-yielding crops, and use a soil test only to discourage overfertilization. Crown Copyright # 2005 Published by Elsevier B.V. All rights reserved. Keywords: Rice; Nitrogen; Mineralization; Anaerobic incubation; Amino sugar test; Fertilization 1. Introduction The Australian rice industry started in the Murrumbid- gee valley of southern New South Wales in the 1920s and from then until the 1960s the only significant sources of nitrogen (N) were mineralization of soil organic matter and the residues of legume-based pasture of up to 8 years duration (Boerema and McDonald, 1965). The use of N fertilizer increased as the rotations narrowed, so that by 2001 the average N-fertilizer rate was 145 kg ha 1 , with a range from 0 to 400 kg ha 1 (J. Lacy, personal commu- nication). On average, 95 kg N ha 1 is incorporated into dry soil in the form of urea or anhydrous ammonia in mid- October, a few days before the field is flooded to a depth of 1–5 cm and seeded by aircraft. The remaining N fertilizer is topdressed as urea at the panicle initiation stage, which normally occurs in early January, about 80 days into the 150-day life cycle of the crop. Soil N-supply depends on previous land use, which varies considerably from field to field (Beecher et al., 1994). For example, rice crops, which are grown after many years of legume-based pasture require little or no N fertilizer to achieve the economic optimum yield. At the other extreme, continuously cropped rice require a large amount of N fertilizer to achieve optimum yield. Farmers currently base decisions about the amount of N fertilizer to apply before sowing on the productivity of previous rice crops and the sequence of previous pastures and crops. The greatest cause of year-to-year variation in yield is cold damage to the developing pollen cells when minimum www.elsevier.com/locate/fcr Field Crops Research 97 (2006) 286–301 * Corresponding author. Tel.: +61 2 62465095; fax: +61 2 62465255. E-mail address: john.angus@csiro.au (J.F. Angus). 1 Present address: University of Western Australia, Centre of Excellence in Natural Resource Management, 444 Albany Highway, Albany, WA 6330, Australia. 0378-4290/$ – see front matter. Crown Copyright # 2005 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2005.10.007