Relationship between inorganic soil nitrogen and maize grain yield with controlled traffic practices

Abstract

Traditional N fertilizer guidelines were based on relationships between N application rates and yield response. These relationships varied greatly according to residual inorganic N content in the soil. The objective of this study was to establish inorganic N threshold values for maize production on a specific soil that could also be extrapolated to similar conditions. During the 1993/94 season a site situated near Viljoenskroon (27"10'S, 26°55'E) on a sandy Clovelly soil form was selected for a NxP fertilizer trial. Five N application rates (0, 40, 80, 120 and 180 kg N ha-l ) and five P application rates (0, 10,20,40 and 60 kg P hal ) were factorially combined and replicated thrice according to a randomised block design. Treatments were repeated for five consecutive seasons on the same plots. Over seasons 100% relative yield was obtained at an inorganic N threshold value of 74 kg N ha-l , 90% at 56 kg N ha-l , 80% at 45 kg N ha-l , 70% at 35 kg N ha· l , 60% at 27 kg N hal and 50% at 20 kg N ha-l , measured approximately six weeks after planting in the 0-600 mm soil layer for the total soil volume. Values for soil volumes over rows were very similar. An average N requirement factor of 2.3 kg N applied per kg N measured was calculated for soil volumes over rows, as well as for total soil volumes, in the top 600 mm soil according to linear models. Thus by taking the inorganic N threshold values, the N requirement factor and the measured inorganic N level in the soil into account, an actual N recommendation can be calculated in order to obtain a specific relative yield between 50 and 100%. Farmers should for economic reasons be encouraged to manage inorganic N levels in the soil to obtain a certain percentage of the expected yield. Inorganic N threshold values can be extrapolated to similar soils varying in inorganic N content.

Maize fertilizer research over the last 25 years – an overview

“…Bloem (1999) proposed a threshold value of 7 mg NO 3 -N kg -1 for a 2.0 t ha -1 yield and 15 mg NO 3 -N kg -1 for a 6.0 t ha -1 yield respectively. A sampling time of six weeks after planting was supported for small scale farmers (Schmidt & Adriaanse, 2004), and under controlled traffic practices (Schmidt et al, 2006). For soil samples taken before planting, Van Biljon et al (2008a) proposed a lower threshold value of 70 kg NO 3 -N ha -1 and an upper threshold value of 161 kg NO 3 -N ha -1 , a biological optimum of 280 kg NO 3 -N ha -1 and an estimated N mineralization during the growing season of 40 kg NO 3 -N ha -1 .…”