F.G. Fernández, D. Schaefer, and K. Greer

 

Row-crop agriculture in the Mississippi River Watershed is under intense pressure to reduce
nitrogen (N) and phosphorus (P) loads that can eventually exacerbate the hypoxia zone in the
Gulf of Mexico. To reduce nutrient and sediments losses, more precise application and
placement of nutrients, (especially N and P) along with less tillage, are being proposed. No-till
has been a difficult challenge for corn (Zea mays L.) growers in Illinois because of early spring
wet and cool soil conditions that delay planting and/or seedling development. In addition, the
lack of incorporation of surface-applied P and K in no-till systems creates stratification of these
nutrients with higher concentrations in the surface compared to the subsurface. Nutrient
stratification under no-till can cause runoff to have higher P concentrations, which could create a
greater concern for surface water quality. In recent years, strip-till, which is another conservation
tillage method, has emerged as an alternative. Strip-tillage is done in the fall by disturbing the
soil 7-8 inches deep and creating a residue-free band 4-6 inches wide and 1-2 inches high. The
cultivated strip provides the benefits of conventional tillage by creating a good seedbed zone
with warmer and dryer soil conditions in early spring; while the undisturbed soil between strips
provides the benefits of soil and water conservation of no-till systems (Morrison, 2002). Better
seedbed conditions with strip-till have improved crop growth and yield (Morrison, 2002; Randall
and Vetsch, 2008; Farmaha et al., 2011). Additionally, in strip-till soil P levels in the soil surface
could be effectively reduced as P and K fertilizers are placed deep in the soil during the tillage
operation (Farmaha et al., 2011).

The introduction of government programs to encourage soil and water conservation along with
the potential to lower costs of operation and all the potential benefits just described is helping
strip-till gain popularity in Illinois (Frazee, 2006). One of the most commonly perceived
challenges by farmers in relation to adoption of conservation tillage systems is fertilizer
management. Traditionally, surface broadcasted P and K fertilizers were incorporated through
tillage, resulting in an even distribution of these nutrients in the plow-layer. Conservation tillage
systems are characterized by minimal soil disturbance. Limited mobility of surface broadcast P
and K fertilizers in combination with reduced mixing of the soil often creates strong vertical
stratification with highest P and K concentrations in the surface 2 inches of the soil (Buah et al.,
2000; Crozier et al., 1999; Hargrove, 1985; Holanda et al., 1998; Howard et al., 1999). As band
applications of P and K are becoming more common, not only vertical stratification, but also a
horizontal pattern of high and low fertility across the field is being observed in fields. This is
because crops use only a portion of the applied P and K in a band and the residual fertilizer
creates a zone of concentrated nutrients that persist for a long time (Miner and Kamprath, 1971).
This horizontal pattern of high and low fertility across the field is most likely to occur in
controlled-traffic and strip-till systems where RTK satellite navigation technology makes it
possible to plant and band fertilizers always in the same location.

Application of P and K represents a large investment for Illinois farmers, and improving
management practices that optimize returns on fertilizer investment is at the forefront of a
farmer’s agenda. Optimization of P and K fertilizer use can be accomplished by correctly
predicting availability of these nutrients for the crop to be grown through soil testing. The
vertical stratification and horizontal patterns described for these nutrients present a challenge to
accurately predict, through soil sampling and testing, the P and K status of the soil and the need
for additional fertilization. Current soil sampling guidelines were developed for fields receiving
intensive tillage that evenly incorporated fertilizers throughout the plow-layer. Using these
standard soil sampling guidelines might not provide accurate information to aid P and K
management under conservation tillage systems with fertilizer band applications. In fact, many
Illinois producers that for several years have been doing P and K banding in their conservation
tillage fields are facing the challenge of not knowing what the real P and K status of their fields
are. Currently in Illinois, as well as in the Midwest, we do not have adequate guidelines on how
to collect soil samples from these fields. The few studies that have focused on the interaction of
fertilizer placement and strip-till have done so for starter fertilizers (Bermudez and Mallarino,
2004; Vetsch and Randall, 2002). There are no studies that have attempted to determine the
impact of subsurface fertilizer band application on soil surface P levels and the potential benefit
to lower environmental impact. Finally, the need to determine an appropriate soil sampling
scheme for conservation tillage systems has been recognized, but there is still only limited
information available, especially in relation to strip-till (Bermudez and Mallarino, 2007;
Mallarino, 1996; Mallarino and Borges, 2006; Tyler and Howard, 1991). In a report from the
North-Central Region 13 (NCR-13) committee, the topic of soil sampling fields under
conservation tillage systems was addressed, but the final conclusion of the report was that there
is no universal agreement on the best way to collect soil samples from such fields (Rehm, et al.,
2002).

Since it is relatively simple to combine deep placement of P and K with the strip-till operation,
these nutrients are normally applied in the row at 6-8 inches below the soil surface. Deep
banding of P and K has been shown to enhance fertilizer use efficiency, nutrient availability, and
yield (Hairston et al., 1990; Bordoli and Mallarino, 1998; Ebelhar and Varsa, 2000; Borges and
Mallarino, 2000), while others have reported no or small benefit to deep-banding compared with
broadcast applications (Hudak et al., 1989; Yin and Vyn, 2002a, 2002b; Borges and Mallarino,
2003; Rehm and Lamb, 2004; Farmaha et al., 2011). Despite the increasingly greater interest and
adoption of strip-till systems for corn and soybean [Glycine max (L.) Merr.] production in
Illinois and the Midwest, relatively little is known about P and K nutrient management for this
cropping system. The lack of knowledge on how to collect soil samples in strip-till systems to
accurately predict P and K fertilization for corn and soybean, and the need to effectively supply
those nutrients in the cropping system to improve the profitability of farming operations and
sustainability of the environment, constitutes the basis of this study.