Exponential nutrient loading: a new fertilization technique to improve seedling performance on competitive sites

Reforestation efforts in Ontario have become increasingly more reliant on containerized planting stock since these seedlings are less costly to produce and plant than bare-root stock. Container seedlings, however, tend to be more susceptible to competing vegetation when planted on weed-prone sites often requiring release by chemical herbicides. We have developed cultural techniques to improve the competitiveness of containerized seedlings by promoting initial outplanting performance thus reducing the need for early vegetation control. The approach is based on two new preconditioning practices, exponential fertilization and nutrient loading, which induce a steady-state build up of nutrient reserves in seedlings for outplanting. Exponential nutrient loading integrates these practices with high dose fertilization inducing luxury consumption. Steady-state culture corresponds better with the competitive outplanting environment, since stable internal nutrient accumulation in the greenhouse phase conforms with steady-state nutrient uptake of natural exponentially growing vegetation in the field, and exponential nutrient delivery to container-restricted root systems also better simulates nutrient flux reached by expanding roots in a field soil with constant nutrient availability. Combined with nutrient loading, the higher nutrient reserves and improved nutrient balance in seedlings contribute to enhanced stress resistance and increased growth performance. This paper reviews the theory and practice of exponential nutrient loading during seedling culture, presents results of growth and nutritional responses of seedlings planted on competitive sites, and suggests appropriate criteria for quality testing of nutrient loaded stock.