Mineral nutrient uptake and removal by canola, Indian mustard and Linola in two contrasting environments, and implications for carbon cycle effects on soil acidification

Field studies of the oilseed crops canola, Indian mustard, and linseed/Linola were conducted over 2 seasons at 2 contrasting sites in the cropping regions of central and southern NSW to determine the uptake of mineral nutrients and quantities removed in seed. The sites were in the Junee region where production of these oilseeds is common, and at Condobolin, which is regarded as marginal for production of the crops. The 2 rates of nitrogen (N) fertiliser applied were either none or rates that growers in the Condobolin and Junee regions would apply to achieve high seed yields after a cereal crop in the rotation when soil mineral N is low. Concentrations of total N, the major cations (K, Ca, Mg, Na) and major anions (P, S, Cl), and the micronutrients Fe, Mn, Zn, and Cu were determined in shoots harvested at flowering and maturity. Nitrate and sulfate were also measured, and estimates were made of excess cation concentrations in the plant material. The ranges of concentrations of excess cations in shoots of the oilseeds at flowering were 83–206 cmolc/kg and 43–121 cmolc/kg for straw at maturity. Linola had lower concentrations of excess cations in vegetative material (83–108 and 43–82 cmolc/kg at flowering and maturity, respectively) than canola or Indian mustard. Concentrations of excess cations in seed of the crops were lower than for vegetative material, and ranged from 30 to 49 cmolc/kg. Nitrogen fertiliser had relatively little effect on concentrations of mineral nutrients or excess cations in either shoots at flowering and maturity, or in seed. However, N fertiliser increased the growth and seed yields of the crops, and thus the amounts of mineral nutrients and excess cations in shoots and seed. The results are discussed in the context of the depletion of soil nutrients due to their removal in harvested seed of the N-fertilised crops. The contribution of excess cation removal in seed to soil acidification is also discussed. It is estimated that the quantity of lime required to neutralise the acidity resulting from removal of 1 t seed is 22.4 kg for canola, 17.0 for Indian mustard, and 20.8 for linseed/Linola. Estimates of the relative contributions of seed removal and N fertiliser to soil acidification are presented.