Complementary measures of soil texture, organic nitrogen accessibility and rhizosphere dynamics improve nitrogen supply prediction for short-cycle leafy vegetables

Accurate estimation of soil nitrogen (N) supply to plants is important for optimising fertiliser application, particularly for fast-growing, short-cycle crops with high N demand. Fertiliser recommendations for salad crops rely primarily on pre-sowing soil mineral N (SMN), which may not capture soil N supply via mineralisation during the cropping period. This study evaluated whether the plant N uptake by babyleaf spinach could be predicted by combining measures of soil N availability with soil properties. Six measures of soil N availability: KCl-extractable SMN, Total soil N, Anaerobically mineralizable N, Plant Root Simulator (PRS®)-N, Hydrolysable N and N contained in the free and lightly-occluded light fraction of SOM (FLOLF-N) were compared with plant N uptake across 15 soils. KCl-extractable SMN was a poor predictor of plant N uptake (R ² _adj  = − 0.075), while no individual N availability measure explained more than 28% of its variation. We found that sandy soils with relatively low organic matter could supply substantial quantities of plant-available N. Bayesian multiple regression identified a model including soil texture, PRS® and FLOLF-N that explained 77% of the variation in plant N uptake. These results suggest that predicting N supply for short-cycle crops may benefit from integrating rhizosphere inorganic N availability and N contained within physically accessible SOM fractions with the soil physical properties that control nutrient accessibility and transport.