Relationships between the critical concentrations of nitrogen, phosphorus and potassium in 17 different vegetable crops and duration of growth

Fifty‐three field experiments were carried out to define the way in which the critical concentrations of nutrients in vegetable crops decline with increasing age so as to provide better criteria for diagnosing deficiencies and to provide inputs to models for fertiliser response. Each of 17 vegetable crops were grown with levels of nitrogen (N), phosphorus (P) and potassium (K) fertiliser that resulted in near maximum yields and little luxury consumption of nutrients. Between five and 20 individual plants were harvested at intervals (up to 13) during the growing period and analysed separately. A model was derived and used to summarise the data as curves of % organic‐N, P and K, and sum of cations (milli‐equivalents) in the dry matter with time. Mean plant dry weight‐time relations were also obtained. Unexplained variation about the best fitting curves was substantial and was not appreciably reduced by taking account of mean plant weight as well as time. Much of this unexplained variation was attributed to differences between plants within a harvest and some of it to harvest to harvest deviations from the model. However, little was associated with year to year variability. The net result is that 95% confidence limits were about ±25%. The significance of these findings to detecting nutrient deficiencies and providing inputs to models are discussed.