ASSESSING SPATIAL AND TEMPORAL NUTRIENT DYNAMICS WITH A PROPOSED NUTRIET BUFFERING INDEX

Authors: MYERS, D - UNIV OF MO; Kitchen, Newell; Sudduth, Kenneth - Ken

Submitted to: North Central Extension Industry Soil Fertility Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 11/19/2003
Publication Date: 11/19/2003
Citation: MYERS, D.B., KITCHEN, N.R., SUDDUTH, K.A. ASSESSING SPATIAL AND TEMPORAL NUTRIENT DYNAMICS WITH A PROPOSED NUTRIET BUFFERING INDEX. PROCEEDINGS NORTH CENTRAL EXTENSION INDUSTRY SOIL FERTILITY CONFERENCE. 2003. P. 190-199.

Interpretive Summary: When plant nutrients are applied as fertilizers to crop fields by farmers, not all of the nutrients are used by the crop. After fertilizer application, the nutrients react with the soil so that some of them are adsorbed by the soil and no longer available for crop roots to uptake. This is called a soil's nutrient buffering. Thus, fertilizer recommendations often are adjusted to account for added nutrients that will not be available for plant growth. However, soils differ widely in how much nutrient buffering they will have. Currently fertilizer recommendations are adjusted for nutrient buffering based on state- or region-wide estimates. To improve fertilizer recommendations, methods are needed to develop field-specific understanding of soil nutrient buffering. Using yield maps (from a combine yield-monitoring system) and soil samples (taken on a grid over multiple locations in a field), we were able to develop a method for mapping the variation of soil nutrient buffering for potassium and phosphorous nutrients. We called this calculated value the buffering index (BI). We found that BI for potassium was always positive. The interpretation was that potassium fertilizer applications were insufficient to prevent soil test K from decreasing in the field. For phosphorous, much of the field was positive like potassium, with a similar interpretation. However, in some localized areas the BI was negative. This occurred at locations corresponding to historic homestead sites, field entrances, and sedimentation areas. Soil phosphorous available to plants at these locations would be sustainable with reduced fertilizer applications. For farmers, identifying field areas with variable soil nutrient buffering will allow for developing fertilizer recommendations that can be more accurate for meeting crop needs. In areas where fertilizer reductions are identified, an economic return is realized by the farmer. Reduced fertilizer applications in these areas will also benefit the public since it will potentially result in a reduction of nutrients in surface runoff that enters into rivers and streams.

Technical Abstract: Continued adoption of precision agriculture will lead to the accumulation of spatially and temporally dense soil fertility and yield data. Current soil fertility recommendation strategies use regional estimates of soil buffering properties to adjust application rates. The objective of this investigation was to present and illustrate the concept of using temporal, site-specific yield and soil-test data to calculate a nutrient buffering index (BI) that can be potentially used to improve soil fertility management. BI is a quantity-intensity relationship (delta Q/delta I) where delta Q is the net balance of a nutrient, and delta I is the change in soil test concentration. Since 1995, grid soil sampling (every 2 years) and combine yield monitoring (each year) was obtained for a 35-ha claypan soil (fine, smectitic, vertic epiaqualfs) field near Centralia, Missouri. Fertilizers were applied uniformly to the field. Yield soil test results were interpolated following generally accepted geostatistical procedures. For each year, total nutrient removal was calculated as the product of yield and a generalized nutrient concentration. Delta Q was then calculated by subtracting total crop removal from fertilizer additions. Ordinary linear least squares regression was used to estimate delta I for soil test P (1995-2003) and K (1997-2003). The value of BI is interpreted as the quantity of nutrient balance responsible for one unit of change in soil test value. For K, delta I and delta Q were both negative across the field giving a positive BI. K applications were insufficient to prevent soil test K drawdown everywhere in the field. For P, much of the field was positive because of negative delta Q and I, and the interpretation was similar to K. In some localized areas BI was negative. Since delta Q was negative over the whole field, where BI was negative it was the result of soil test P increasing. This occurred at locations corresponding to homestead sites, field entrances, and sedimentation areas. Buffered nutrient supply at these locations would be sustainable with reduced P applications. While buffered P (for most of the field) and K supply are not sustainable without increased additions, not every location in the field needs to receive the same amount to sustain P and K supply. Sites with a positive BI and a value near zero are the least buffered, and should receive lower rates of P and K fertilizer, but perhaps more frequently. Also, st these locations small amounts of removal cause large reductions in soil test values. Fertilizer additions, whether maintenance or buildup applications, could be modified by the locally derived BI to provide an application rate that more efficiently meets the crop's need, with potentially less environmental damage.