The perils of pot experiments

Experiments with plants in pots can be beset by several often-unrecognised artefacts. The soil or other medium at the bottom of a freshly watered and drained pot is inevitably saturated with water; if the pot is short the whole of the medium may have such little air-filled porosity that it becomes hypoxic. This is an especial problem with field soils used in pots, for these typically do not contain many pores large enough to be drained at the small water suctions that prevail. Such suctions are zero at the bottom of a freshly drained pot and rise by 1 kPa for every 100-mm increment in height, leading to suctions at the top of a typical pot that are much less than the 10-30 kPa that occurs in soils in the field that have drained to 'field capacity'. Commercial potting mixes overcome this problem by using coarse materials in the mix, which create many large pores (>1 mm diameter) that drain at small suctions. Fine potting mixes used for growing Arabidopsis in small pots are likely to lead to hypoxia throughout the pot. Pots in glasshouses may experience temperatures that are substantially higher or lower than the air temperature. Sun striking the sides of dark pots can raise the temperature by several degrees. In heated glasshouses cold tap water added to warm pots can greatly affect the growth of plants in those pots. Simple precautions can at least partly ameliorate these difficulties.