Growth analysis of soil-grown spinach plants at different N-regimes

Spinach plants were grown in pots under controlled conditions in three different soils (a loamy sand, a silt loam at low mineral-N level and a silt loam at the double mineral-N level). The nitrogen uptake pattern varied considerably between the three soil types and was used to validate an equation between the relative growth rate and nitrogen content. This equation is based on the growth response of spinach plants grown hydroponically at equal environmental conditions either at optimum nitrogen supply (complete nutrient solution) or with a relative nitrate addition rate of 0.30 day^–1, 0.225 day^–1 or 0.15 day^–1 effecting an exponential increase in nitrogen uptake. Growth in potted soil was slightly overestimated. Part of this bias was explained by the lower shoot weight ratio observed for the soil grown plants. This was demonstrated by the improvement in growth predictions when using net assimilation rate rather than relative growth rate as the driving variable in the model.