Variable Source N Fertilizer Applications to Optimize Crop N Use Efficiency

Introduction:
Use of nitrogen (N) fertilizer in corn has long been essential for improving yields and increasing economic returns. The efficient utilization of N fertilizer is becoming increasingly more important because of rising fertilizer costs and the possible negative impact of environmental N loss. Soil N losses, such as leaching and atmospheric loss, and poor plant N use utilization, can lower crop yields and be harmful to the environment.

The loss of applied N fertilizer may vary across a field due to several factors including differences in soil physical properties that affect soil water. In addition, enhanced efficiency N fertilizers, such as polymer-coated urea (PCU), a slow-release form of N fertilizer, may improve N utilization by delaying N release under conditions which may promote N loss or by providing N later in the growing season when plant N requirements are higher. Results of previous field research at the Greenley Paired Watershed Study show that gaseous N losses from applied N fertilizer primarily occurred within the first 40 days after N fertilizer application suggesting that delayed N release may be an advantage in improving N use efficiency. In addition, that study showed that the N losses varied by landscape position.

This research investigated the concept of variable source N fertilizer application. For this concept, slow-release N fertilizer is applied to field areas with a higher probability of N loss due to wet conditions and conventional N fertilizer is applied to low-risk field areas. This N fertilization strategy may insure sufficient N fertilizer availability throughout the field, reduce risk of N loss, and make use of slow-release or treated N fertilizer more economical. The objectives of this are:

1. To determine methods to delineate and map areas in fields which are more vulnerable to N loss due to wet conditions.
2. To examine the use of a variable source strategy to optimize crop N fertilizer use efficiency.
3. To calculate the cost-effectiveness of using this variable source strategy compared to uniform applications of conventional or other N fertilizer sources.

Methods:
A two-year field trial was initiated in 2005 at the MU Greenley Experiment Station. The site was mapped for elevation and apparent electrical conductivity (EC_a) using a EM-38 sensor (Fig. 1). Measurement of soil EC_a gives an indication of relative depth to the claypan subsoil layer. Soil gravimetric water content was determined on 6 in depth soil samples collected in a 10 by 25 ft grid over the Greenley site on March 31^st in order to compare the effects of spatial distribution of elevation and EC_a on the distribution of soil water content.

Nitrogen fertilizer treatments included a control and 4 N source treatments at 150 lb N/acre: 1) urea; 2) polymer-coated urea (PCU) (ESN, Agrium, Inc.); 3) a 50% urea/50% polymer-coated urea mixture; and 4) anhydrous ammonia. N fertilizers were injected (#4) or broadcast-applied and incorporated (#1,2, and 3) in 10 ft by 1500 ft strips across three landscape positions representing shallow, deep and low-lying areas. Corn silage and grain yields were determined at each site. Site harvest locations are shown in Fig. 1. Total aboveground biomass tissue samples at harvest and periodically during the growing season were taken and are currently being analyzed for total N content in order to determine fertilizer N use efficiency.

The rate of soil N₂O gas loss or efflux was also measured periodically over the growing season for each N fertilizer treatment and landscape position. Soil N₂O gas was collected using small sealed chambers fitted with rubber septa inserted into PVC collars in the soil. The head space gas was collected from the chambers in the different treatments and analyzed by gas chromatography (GC).

Results:
Rainfall during the 2005 cropping year was relatively low during the growing season with a long period of drought after the middle of June (Fig. 2). This lack of rainfall may have reduced possible crop N response.

Measurement of the spatial distribution of soil water content in the top 6 in depth was undertaken prior to planting to evaluate whether measurements of elevation and ECa would assist in predicting spatial differences in soil water content that might affect the fate of applied N fertilizer. Initial evaluation of the distribution of surface soil water content (Fig. 3), suggests that elevation may be a better predictor of spatial patterns of surface soil water content. However, this analysis did not take into account the possible effect of differences in soil water availability deeper in the soil profile on the fate of applied N fertilizer. In claypan soils, the amount of available water in the soil profile is probably affected by the depth to the claypan layer which is a property related to soil EC_a.

Visual symptoms of plant N deficiency were observed both in control plots and in low-lying areas, possibly due to water collecting in those areas from spring and early summer rainfall. However, lack of sufficient water for crop growth after the middle of June also affected corn growth response to added N fertilizer. Grain yields increased 19.9 to 46.2 bu/acre with added N fertilizer at the summit (shallow) and low-lying landscape positions (Table 1). The PCU fertilizer (ESN) and anhydrous ammonia had grain yields that were 24.4 and 23.5 bu/acre greater than urea in the low-lying area and had the highest yields of all the N sources in the low-lying area (Table 1).

Conclusions:

• Both polymer-coated urea and anhydrous ammonia had higher grain yields compared to urea in the low-lying area at Greenley. However, there were no differences among these fertilizer sources at the other landscape positions, which suggests that response to N fertilizer source may vary across fields depending on landscape position.
• This first year of research indicates that the concept of variable source application may have some validity for N fertilizer management. However, further research is required under wetter climatic conditions when risk of N loss is higher.

Acknowledgements:
The authors would like to acknowledge the Missouri Fertilizer and Aglime Advisory Board, Agrium, Inc. and MFA, Inc. for supporting this research.

Table 1. Effects of N fertilizer source and landscape position on corn grain yields at the Greenley site.

^*NS = not significant

Figure 1. Map showing the spatial distribution of elevation and EC_a at the Greenley site. Circles on the map show the location of the sampling collars for soil N₂O gas loss and the approximate location for the grain and silage harvests.

Figure 2. Daily and cumulative precipitation (in mm) for the Greenley site.

Figure 3. Spatial distribution of soil gravimetric water content at the Greenley site on March 31^st.