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ARS Home » Midwest Area » Morris, Minnesota » Soil Management Research » Research » Publications at this Location » Publication #71731
Title: CALCULATION OF OPTIMAL FERTILIZER RATES: A COMPARISON OF THREE RESPONSE MODELS

Author
item Olness, Alan
item EVANS, SAMUEL - UNIVERSITY OF MN RETIRED
item ALDERFER, RICHARD - ALDERFER AG CONSULTING

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/9/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Over $3 billion is spent on nitrogen (N) fertilizer each year. In some cases, excessive amounts of N are applied to crop land. In these cases, excess N often leaches to the groundwater and pollutes it in the form of nitrates. A better mathematical model is needed to separate the effects of water and of N so that benefits of adding either can be better assessed. A comparison of three models shows that a hyperbolic tangent model was the best in assessing the amounts of N to add to a crop. Results using data from a six-year long study of five N-rates, four tillages and two corn hybrids were compared. Because water and temperature are also important factors in crop yield, none of the models accurately predicted yields all the time. It appears that models should be applied for each soil, tillage, and hybrid of corn used. The quadratic model both overestimated and underestimated the best N-rate with different tillages and hybrids. Use of a hyperbolic tangent model results in a recommendation to add about 5 to 10 lb more N per acre than is currently being recommended through use of some models. The cost of this additional N ranges from about $1.50 to $3.00 per acre. With ridge tillage or chisel plow tillage, (preferred conservation tillage systems), the predicted gain in grain yield averaged about 16 bu per acre for a 95 day corn hybrid; at current prices this amounts to about $60 an acre in added income. Slightly greater additions of N resulted in more N being removed in grain than is applied as fertilizer. Use of correct models should improve profits in some management systems by >$50 per acre per year. Use of the hyperbolic tangent model will permit more accurate assessment of the value of inputs in corn production than many currently popular models.

Technical Abstract: Determination of optimal N-fertilization rates for crop production which minimize risk of environmental degradation requires accurate application of a response model. Three models, quadratic, a modified Mitscherlich and a tanh(N), which closely fit field data were applied to 48 data sets and compared for their relative accuracy. Data were collected from a Tara silt tloam soil over a 6-yr period using 2 maize hybrids and 4 tillage treatments, mold-board plow, chisel plow, ridge tillage and no-tillage. In about one-third of the cases, all models performed about equally well. Generally, the tanh(N) and modified Mitscherlich models gave better fit between N-rate and grain yield data. The economic optimal fertilizer-N rate ranged from about 140 to 170 kg ha**-1 for the modified Mitscherlich model, 136 to 184 kg ha**-1 for the tanh(N) function, and 124 to 173 kg ha**-1 using the quadratic model. Estimated grain yields at these rates ranged between 6.58 +/ 1.30 to 7.59 +/ 1.69 Mg ha**-1 for the 90-day MR hybrid and between 7.52 +/ 2.40 to 8.72 +/ 1.70 Mg ha**-1.