VARIATION OF EARLY GROWTH AND NUTRIENT CONTENT OF NO-TILL CORN AND SOYBEAN IN RELATION TO SOIL-PHOSPHORUS AND POTASSIUM SUPPLIES

Knowledge of relationships between variation in early plant growth and soil nutrient supply is needed for effective site-specific management of no-till fields. This study assessed relationships between soil tes t phosphorus (STP) and potassium (STK) with early plant growth and P o r K content of young corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] plants in eight no-till fields. Composite soil (0-15 cm depth) and plant (V5-V6 growth stages) samples were collected from 400-m(2) a reas at the center of 0.14-ha cells of a 16-cell square grid and from 2-m(2) areas spaced 3 m along each of two 150-m intersecting transects . Correlation, regression, multivariate factor analyses were used to s tudy the relationships between the variables. Variability was higher f or samples collected from the transects. Plant dry weight (DW), P upta ke (PU), and K uptake (KU) usually were correlated with STP and STK bu t the correlations varied markedly among fields. Relationships between soil and plant variables could not always be explained by known nutri ent sufficiency levels for grain production. Plant P concentration (PC ) was not always correlated with STP and sometimes it increased linear ly with STP, but other times increased curvilinearly until a maximum w as reached. Plant K concentration (KC) usually was correlated with STK , however, and increased linearly with increasing STK even in fields w ith above-optimum STK. The results suggest greater susceptibility of e arly growth to STP than to STK and greater plant capacity to accumulat e K compared with P over a wide range of soil nutrient supplies. Varia tion in STK likely is a major direct cause of variation in KC over a w ide range of conditions but variation in STP is not likely a major dir ect cause of variation in PC when high STP predominates.