Nitrogen Decision After a Wet Spring
A 40-hectare field of feed corn follows a soybean crop and received 60 cubic metres per hectare of liquid dairy manure in October. The grower planned 150 kg nitrogen per hectare as sidedress but could not enter the field for three weeks because of 140 mm of rain. Tissue tests now show pale lower leaves and a SPAD reading averaging 38 against a well-fertilised reference strip at 47. The soil is a sandy loam with low organic matter. The field is at the six-leaf stage, and the grower has 90 kg nitrogen per hectare of urea-ammonium nitrate on hand. A neighbor argues that the manure and soybean credit make additional nitrogen unnecessary.
1. Judge whether the neighbor's reasoning holds, using the evidence in the case.
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The neighbor's reasoning is partly right in principle but wrong here. Soybean and manure do supply nitrogen, but the October application on a sandy loam with low organic matter is vulnerable to leaching and denitrification, and 140 mm of rain in three weeks is more than enough to move nitrate below the root zone. The reference strip comparison is the decisive evidence: a 9-point SPAD gap at six-leaf stage indicates the crop is nitrogen-limited now, and the pale lower leaves are consistent with that. The credits should be counted, but they cannot be assumed to still be present. The correct judgement is that a rescue application is justified, sized to close the observed gap rather than to the original plan.
2. Recommend an application rate and method, and explain the risk of simply applying all 90 kg per hectare at once.
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A reasonable approach is a split application of 45 to 60 kg nitrogen per hectare now, applied as a dribble or coulter-injected band to reduce volatilisation and keep the material in the root zone, then reassess with another SPAD reading in seven to ten days. Applying all 90 kg at once is risky for three reasons: it commits the whole budget before the response is known, it raises leaching exposure on a sandy loam if more rain follows, and it can push vegetative growth at the expense of grain fill. The reference strip provides the calibration for deciding whether the second increment is needed, so the split converts a guess into a measurement-driven decision.