A THEORETICAL EXPLANATION OF THE PIPER-STEENBJERG EFFECT

The relation between plant yield and plant nutrient concentration is s ometimes found to be negative, a phenomenon called the Piper-Steenbjer g (PS) effect. A model was used to examine the underlying causes of th e PS effect, and the conditions under which it is most likely to occur . The model uses the nutrient productivity concept for plant growth an d a nutrient uptake equation in which root growth rate and external nu trient concentration determine the uptake rate. The study suggests tha t the PS effect occurs when the fast growth of plants grown in an init ially higher nutrient medium eventually leads to a more rapid depletio n of external nutrients than the slow growth of plants grown in an ini tially lower nutrient medium. The fast growth of plants combined with a rapid decrease of nutrient uptake leads to a fall in plant nutrient concentration. When these large plants with very low nutrient concentr ations are compared with the smaller, slow-growing plants, a PS effect may be Found depending on the time at which the plants are harvested, and on the range of initial values of the external nutrient content, When it occurs, the effect is greatest when the depletion volume per u nit new root (V-d) is lowest, and when the mobility of nutrients in th e medium is highest (alpha=1). The results are sufficiently general to apply to a variety of nutrients, plant species and growth media.