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Predicting the Effect of Ammonium Placement on Nitrogen Uptake by Corn

Dep. de Solos, Federal Univ. of Rio Grande do Sul, Caixa Postal 776, 90.000 Porto Alegre RS Brazil
Dep. Agronomy, Purdue Univ., West Lafayette, IN 47907-7899.

^* Corresponding author.

A mechanistic model has successfully described the effect of P placement on P uptake by corn (Zea mays L.) and soybean (Glycine mat L.). Model predictions for N have not been evaluated experimentally. The objective of this paper is to compare the effect of NH⁺₄ placement on N uptake by corn found experimentally with that calculated with the mechanistic uptake model mentioned above. Observed N uptake values were obtained by growing corn in a growthchamber experiment using a sample of the 0 to 20 cm layer of Raub silt loam (fine-silty, mixed, mesic Aquic Argiudolls) soil. Ammonium as (NH⁴)²SO⁴ was applied at a rate of 270 mg N per 3 kg soil to all or part of the soil in each pot. Fractional volumes fertilized were 1.00, 0.50, 0.20, 0.10, 0.05 and 0.02. Nitrapyrin 12-chlorod-(trichloromethyl) pyridine] at 8 mg kg^–1 was added to the soil to minimize nitrification of NH⁺₄. Contribution to N uptake of each of the fertilized and nonfertilized fractions of soil were calculated with the mechanistic model from data for ion uptake kinetics, root distribution and morphology, and the soil parameters, soil solution NH⁺₄ concentration, buffer power, and effective diffusion coefficient, used in the model. Predicted values were close to observed values when N fertilizer was mixed with all the soil. Calculated values became progressively smaller than observed values as the NH⁺₄-treated fraction of the soil decreased. When increases in the size of the NH⁺₄-treated soil volume due to NH⁺₄ diffusion out of the initially treated volume and increases in the maximum N uptake rate per unit of root due to a reduction in the proportion of roots absorbing N were considered in the model, most predicted values approximated observed N uptake values.

Received for publication December 9, 1988.