Artefacts of the pot environment on soil nutrient availability: implications for the interpretation of ecological studies

Pot experiments are frequently used to examine plant, soil, and microbial interactions that cannot be studied in situ. Although impacts of the pot environment on seedling growth have long been recognized, they are rarely addressed directly in current ecological studies. Here we examine how commonly used soil conditioners, often necessary to maintain adequate drainage in pots, can influence nutrient availability and seedling growth. Balsawood (Ochroma pyramidalis) seedlings were grown in central Panama in soil combined in a 50:50 mix with one of the following soil conditioners: marine sand, pure sand, perlite, vermiculite, or rice husks. Soil nutrient availability, foliar N and P, and seedling growth were compared after 1 month. Rice husks dramatically reduced soil nitrate, seedling growth, and foliar N. Other conditioners had smaller effects on growth, although vermiculite may be a significant source of base cations in infertile soils. Marine sand had a strong neutralizing effect on acidic soil due to the presence of calcareous shell fragments. Effects of soil conditioners need to be added to other known artifacts of the pot environment influencing nutrient availability, including alterations to N mineralization rates relative to bulk soil, and pot-size effects on N:P availability. These artifacts can strongly influence plant performance and therefore the interpretation of ecological studies.