Applied Fertility Management for Irrigated Soybean Production

Expected Outcome: It is expected that P, K, and N fertilizer application will have a significant positive effect on soybean yield, as demonstrated by Lamond and Wesley (2001) and Gordon (2008). Yield response to S is currently unknown. However, given the recent increase in crop S deficiency across the United States attributed to more stringent air quality regulations and reduced atmospheric S deposition, a positive yield response to S fertilization is possible. If significant yield response to N and S is evident in the small plot portion of the study, steps will be taken to include a these nutrients in the subsequent year’s field scale component. Yield response to P and K fertilizer on the producer’s field will be impacted by the spatial distribution of soil properties that are have been shown to govern P and K availability. If a significant positive yield response to P and K fertilizer is shown on the producer’s field, local adaptation to intensive soybean fertility management will improve, possibly increasing irrigated soybean productivity in NCK.


Justification

From 2000-2009, irrigated soybean yields in North Central Kansas (NCK) have averaged ~53 bu/A (USDA-NASS, 2011). Although yields have demonstrated a slight upward trend, producers in the area are largely unsatisfied, especially given the yield and profit potential of irrigated corn in the Republican River Valley. Research conducted at the Kansas State University North Central Kansas Experiment Field in Scandia, KS has demonstrated that proper fertility management involving P and K has the potential to significantly improve irrigated soybean yields in the area (Gordon, 2008). Similar to many soybean producers across the United States Corn Belt, growers in NCK have been slow to adopt intensive fertility management programs for soybean production. The purpose of this research is to continue the work done by Gordon (2008), and expand on it by including a field scale, farmer-cooperative component to increase awareness of irrigated soybean yield potential with proper fertility management.

Recent research has indicated that crop response to P fertilizer at sub-field scales is not always explained by soil test P (Wittry and Mallarino, 2004; Bermudez and Mallarino, 2007; Scharf et al., 2011). Research conducted by Scharf et al. (2011) has also indicated that soil test K levels may not significantly predict yield response to K fertilizer in some fields. Extensive soil testing on the producer’s field will aid in understanding what particular soil chemical and physical properties are governing crop response to P and K fertility. This will further the knowledge of producers who may have applied P and K fertilizer based on soil test results in the past with limited success.


Objectives

1. Develop P, K, N, and S fertility management techniques to maximize irrigated soybean production.
2. Understand soybean yield response to P and K fertilizer at the field scale.

Methodology

Research will be conducted at the North Central Kansas Irrigation Experiment Field, (Scandia) and in cooperation with a producer in the Republican River Valley of North Central Kansas. Research will be a combination of small fertilizer response plots conducted at the Irrigation Experiment Field and a field scale study on a producer’s field. Treatments in the small plot study will be similar to that of Gordon (2008). Results from the study of note indicate that addition of P and K fertilizer increased irrigated soybean yields from 18-34 bu/A, while row spacing, plant population, and preplant N fertilizer had little to no effect. Thus, treatments in the small plot study will focus on specific rates of these three nutrients, with a possible addition of an S fertilizer component. Specific treatments will depend on soil test results from the experimental site. Preliminary treatments for small plots include:

1. 30 lb/a P₂O₅ and 80 lb /a K₂O,
2. 30 lb/a P₂O₅ and 120 lb /a K₂O,
3. 80 lb/a P₂O₅ and 80 lb /a K₂O,
4. 80 lb/a P₂O₅ and 120 K₂O,
5. 80 lb/a P₂O₅, 120 lb/a K₂O, 25 lb/a S,
6. 80 lb/a P₂O₅, 120 lb/a K₂O, 25 lb/a S, and 30 lb/a N,
7. untreated check plot.

All treatments will be applied preplant and incorporated, with the exception of the N treatment. N will be side-dressed at R3 growth stage, as recommended by Lamond and Wesley (2001). The side-dress N treatment will be included to determine if application timing could be a factor in the lack of response shown by Gordon (2008). Field scale plots will consist of three treatment combinations of P and K fertilizer:

1. 30 lb/a P₂O₅ fertilizer only,
2. 80 lb /a K₂O fertilizer only,
3. a combination of P and K (30 and 80 lb/a, respectively).

These treatments will be replicated twice in strip plots across the field landscape. An untreated check strip will be included in each replicate. Tissue testing will be done on all plots at the Experiment Field site, and in site-specific areas of the producer’s field to be determined by soil analysis. Prior to harvest, plant height, number of nodes, and pod counts will be measured. Plots will be machine harvested for yield.