ROOT MASS AS A DETERMINANT OF CORN HYBRID RESPONSE TO STARTER FERTILIZER

Many studies have shown variable response to starter or row applied fe rtilizer on corn (Zea mays L.) hybrids. Field experiments with approxi mately 25 hybrids over three seasons showed that some hybrids responde d to row applied fertilizer with large growth and yield increases whil e others did not respond. The objectives of this study were to compare root and shoot growth of responsive and non-responsive corn hybrids t o starter fertilizer and their uptake of nitrogen (N) and phosphorus ( P). Two glasshouse experiments were conducted with hybrids from the fi eld studies found to be most and least responsive to row applied ferti lizers. In the first experiment, P was applied at levels of 0, 10, 20, or 30 mg kg(-1) and was mixed with either 25 or 100% of the total soi l volume (1.5 L). Nitrogen was mixed with the total soil volume in all treatments at a rate of 100 mg pot(-1). Phosphorus rates in the secon d experiment were 0, 30, or 60 mg kg(-1) and the N rate was 200 mg pot (-1). Fertilizer N and P were mixed with total soil volume or banded 5 cm below the soil surface and 5 cm away from seed. The soil was obtai ned from the A horizon of Norfolk loamy fine sand (fine loamy, siliceo us, thermic, Typic Kandiudult). Time from seeding to harvest was 33 da ys for the first experiment and 41 days for the second. Root weight of the non-responsive hybrid was 31% higher in the first experiment and 48% higher in the second than of the responsive hybrid. Each hybrid re sponded similarly to starter P (62 to 78% increase in top growth). The responsive hybrid produced a significant (P 0.05) increase in top (92 %) and root (76%) weight due to starter N, but the non-responsive hybr id did not respond to N placement. The lack of response to starter N-f ertilizer was attributed to greater root growth in the non-responsive hybrid because its top and root weight with broadcast N were about the same as those of the responsive hybrid with banded (starter) N. Since the hybrids differed only in response to starter N, a convenient meth od to classify corn hybrids with respect to starter fertilizer respons e is to measure top growth at six weeks after planting with banded ver sus broadcast N applied at planting.