Data from fertility studies generally suggest that little added P is needed to achieve high switchgrass yields in bioenergy cropping systems (Hall et al.1982; Muir et al. 2001). This may be different in forage production settings, as in the case of a study by (Rehm 1990), who tested P amendments from 0 to 90 kg ha-1 in Nebraska over 4 years. Rehm (1990) reported a curvilinear response to P with production gains at rates up to 45 kg ha1. Others have reported increased establishment-year production with added P, although effects of P were not observed in subsequent seasons (McKenna and Wolf 1990).

Several studies have reported little to no response to P (Brejda 2000), despite low soil P status. In studies with once-per-year harvest, no response to P was observed after several (3 or 7) years of biomass removal (Muir et al. 2001). Switchgrass grown on low-P soils in Iowa did not respond to P (Hall et al. 1982) and in the southern Great Plains, switchgrass response to P was only benefited at one of two sites (Kering et al. 2012a) over three seasons. In the Kering et al. (2012a) study, P applications of 45 kg ha-1 yr-1 increased yields on a low-P (3.4 mg P kg-1) soil, but no response to any P rate (0, 15, 30, or 45 kg P ha-1) was observed at a second location with soil P concentration of 3.1 mg P kg-1.

Switchgrass’ relationship with the soil microbial community may be a common denominator in the oft-observed variable and limited responses to P and N. In the case of P uptake, switchgrass’ role as host to vesicular arbuscular mycorrhizae greatly improves the grass’ ability to extract and uptake P. Several studies have shown that these root colonizing fungi can greatly improve P acquisition in conditions of high soil acidity, high aluminum and low P (Koslowsky and Boerner 1989; Boerner 1992a, b; Clark et al. 1999; Clark 2002). Adding these fungi to sterilized, low-P soils can eliminate a response to P (Brejda 2000). Conversely, eradicating mycorrhize in low-P soils can reduce switchgrass production if fertilizer P is not added to the system (Bentivenga and Hetrick 1991).

Without returning nutrients to the system, repeated harvests will reduce soil P concentrations in switchgrass biomass production systems (Schmer et al. 2011). With modest yields (5.8 Mg ha1) of switchgrass harvested at anthesis, annual losses of 1.5 kg1 y-1 P ha were reported in production fields in the Great Plains (Schmer et al. 2011). Although greater losses would be expected with greater biomass yields, this factor must be weighed against the stage of plant development at the time of harvest, as the effects of higher yield would be offset by lower P concentrations with plant maturity and senescence (Parrish and Fike 2005; Lemus and Parrish 2009).

There is little research to suggest that switchgrass is particularly responsive to K, whether in field or greenhouse studies (Friedrich et al. 1977; Smith and Greenfield 1979; Hall et al. 1982). Typical recommendations are to maintain K at a medium level based on typical soil test ranges (Teel et al. 2003; George et al. 2008; Douglas et al. 2009). This apparent lack of response may in part be a function of K being recycled to the soil through leaf leaching when switchgrass is harvested after senescence (Parrish and Fike 2005).

As with the other nutrients, response to limestone applications can be variable. This may be less a function of pH change than of the availability (or lack of availability) of other mineral nutrients or toxins (Parrish and Fike 2005). Switchgrass strains display differences in terms of tolerance to soil acidity, with some lines being productive—as opposed to merely tolerant—at pH 4.9 (Bona and Belesky 1992). These differences also may play a role in the variable yield responses reported. Screening for such traits may prove useful if truly marginal sites such as reclaimed mine sites are to be utilized for a future bioenergy industry.