From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Winter 1997, No. 7


    Computer visualization tools enable engineers and architects to make models of the complex structures they design, to assist in making decisions about how, when, and where to build. The models are simplified representations of the expected reality. In the same way, a simulation model of the soil-plant-atmosphere system can help agronomists make decisions in designing a crop nutrient management plan, to build a sound fertility program.

    Crop producers pay a great deal of attention to the weather. Indeed, complete automated weather stations for the farm are becoming popular. However, much of the weather data goes unused because of the lack of mathematical procedures that directly relate weather event A to management practice B. Decision support systems based on mathematical crop growth models need to be applied, to convert the data into information on which to base wise management choices.

    The most common form of crop growth model uses a daily time step. For each day, the model calculates the growth opportunity for the crop based on the stage of growth, the maximum and minimum temperature for the day, the amount of rain received or expected, and the available sunshine. At the same time, the movement and status of water and nutrients in the soil are calculated as influenced by temperature and rainfall interacting with soil type and crop growth. Weather also drives the nutrient uptake function, setting the crop's potential growth rate and nutrient demand.

    The nutrient most frequently considered in crop simulations is nitrogen. Nitrogen loss mechanisms are more closely related to soil water than those of any other nutrient. Each of the major loss processes...leaching, denitrification, and volatilization...depends on soil water status. In addition, the impact of nitrogen imbalances on crop quality can make it worthwhile to expend considerable effort in accurately modeling the yield, quality and environmental impacts of nitrogen management.

    Dynamic modeling of phosphorus in soil-plant systems has been applied to predict crop responses and the transformations of available phosphorus fractions in the soil. It has also been incorporated into environmental impact models such as EPIC (the Erosion/Productivity Impact Calculator). Such modeling has the power to predict how changes in cropping practices will affect productivity of current and future crops and the potential losses of phosphorus from the soil.

    Potassium uptake by crops depends on the rate of transpiration, the aeration of the soil, the opportunity for roots to explore the soil volume, and the redox status of the soil. Although crop models are less frequently applied to potassium, there is potential to improve the prediction of its availability in the soil.

    The weather up to the point of planting or side-dressing influences nutrient availability. A model can estimate what that influence was and, connected to various probable future patterns of weather, predict the outcome of crop nutrient management strategies. Although weather forcasting and climate prediction are far from certain, the sciences are improving. While the "discernible human influence" on climate change is still highly debated, the current investment in climate research may someday pay off in enhanced predictive skill.

    The tradition in soil fertility management has been to decide optimum levels of nutrients based on calibrations averaged over a number of years and locations. However, no year is average, and decisions based on average conditions are not to the maximum financial advantage of producers in most years. As the revolution in information technology continues, more and more attention must be paid to making the best use of weather data and knowledge of its impact on crop nutrient requirements.

--- TWB ---

For more information, contact Dr. Tom W. Bruulsema, Director, Eastern Canada and Northeast U.S. PPI, 18 Maplewood Drive, Guelph, Ontario N1G 1L8, Canada. Phone (519) 821-5519. E-mail:
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