Site-Specific Management Zones for Efficient Nitrogen Management

Variable N application based on yield potential increases N use efficiency.

Traditionally, application of nitrogen (N) fertilizer for corn production has been at uniform rates across an entire field. Determination of the amount of N to be applied has been a function of expected yield and N credits based on composite soil sampling. While this technique has proven effective for overall grain production, it does not take into account the amount of soil variability (because composite soil sampling requires too few samples and provides an average number for the entire field), or environmental concerns with nitrate leaching into ground water. The traditional technique of uniform N application may result in under and over application of N in various parts of the field. This has two implications: (i) under application of N leads to crops inability to maximize its growth potential (ii) over application of N in certain areas of the field leads to potential nitrate leaching below the root zone.

With the development of global positioning systems, and geographic information systems, it is now possible to vary the rate of N applied to certain areas of a field. Also, a field can now be separated into areas or zones that have similar characteristics, and these areas can be managed variably. We call these areas with similar characteristics "site specific management zones" (SSMZ). Currently, these SSMZ are delineated using aerial imagery, topography, and the farmer's experience of the field. These SSMZ are then differentiated into categories of high, medium, and low yield potential. For the high, medium, and low productivity zones, the amount of N applied is varied such that the plants receive optimum amounts of N needed to maximize growth in each zone, while minimizing potential nitrate leaching.

An ongoing study on two sites near Greeley, CO (Site I), and Wiggins, CO (Site II) is testing the management zone concept compared to traditional N application methods. Site I is under furrow irrigation, and Site II is under sprinkler irrigation (Figure 4). The N management treatments and their corresponding N application rates for the two sites are presented in Table 4. Corn grain yields within each N application treatment across each zone are examined to determine which N management treatment performed best for yield and apparent nitrogen fertilizer use efficiency.

Yield results at the furrow irrigated study site I, showed that the SSMZ - HNHZ and the grid sample based N application produced the highest grain yield with an average of 215 and 204 bu/ac, respectively (Table 5). Both treatments, (SSMZ-HNHZ and grid sampling) account for spatial variability in soils. However, economically speaking delineating management zones on a field is much less expensive than performing grid sampling. Besides, management zones are stable and can be used for managing N for a number of years. Also, the Apparent Nitrogen Use Efficiency (ANUE, Table 5) was the highest among all treatments for SSMZ - HNHZ treatment.

Yield results at the sprinkler-irrigated study site II are somewhat similar. The SSMZ - HNHZ treatment produced the highest grain yield and ANUE (Table 5). While Site II did not show the separation in yields among treatments that we had hoped for, it does again demonstrate that by using SSMZ we can achieve yields as good as traditional methods with far less input.

To date, this study has shown that by using SSMZ we can achieve yields as good as, or better than, any other current method of N application management. The SSMZ - HNHZ method of N management results in high yields, and low potential for environmental degradation, both of which are important for the future of irrigated corn production.

Raj Khosla and Tim Shaver
Assistant Professor and Extension Specialist, and Research Associate
Precision Ag Program

Table 10: Nitrogen management treatment and application rates at two study sites in Northeastern Colorado.

Site	N management1 Treatments	Zone2	Rate -- lb N ac-1 --
Study Site I (Furrow irrigated)	Uniform	High Medium Low	164 164 164
	Grid Sampling	High Medium Low	171 - 230 210 - 246 203 - 235
	SSMZ-LNHZ	High Medium Low	167 177 187
	SSMZ-HNHZ	High Medium Low	164 142 122
Study Site II (Sprinkler irrigated)	Recommended-50	High Medium Low	113 113 113
	Recommended+50	High Medium Low	213 213 213
	Grid Sampling	High Medium Low	160 - 185 160 - 185 160 - 185
	SSMZ-LNHZ	High Medium Low	133 163 193
	SSMZ-HNHZ	High Medium Low	182 159 126
1 The N application treatments were based on uniform application, grid sampling, control (zero N), recommended uniform application plus or minus 50 lb N ac-1, and site specific management zones (SSMZ). The HNHZ received high and low N rates for the high and low productive zones, respectively. The LNHZ received low and high N applications to the high and low productive zones, respectively. 2 Level of soil productivity.

Table 11: Nitrogen treatment, corresponding mean N application rates, grain yield,
and apparent nitrogen use efficiency for the two study sites.