Expt. 1.

Baled switchgrass was obtained from a farm in Crawford County, PA. Switchgrass was grown and allowed to dry in the field before baling in early spring. Bales were processed in a tub grinder that included a hammermill with 1.25- and 2.5-cm screens (H-1100 Haybuster II; Duratech Ind., Jamestown, ND). Hereafter, switchgrass ground with this machine is referred to as coarse switchgrass. Particle size distribution (Table 1) was determined by passing ≈100 cm³ oven-dried coarse switchgrass (60 °C) through 19.0-, 12.5-, 6.30-, 4.0-, 2.8-, 2.0-, 1.4-, 1.0-, 0.71-, 0.50-, 0.35-, 0.25-, 0.18-, and 0.11-mm soil sieves. Particles 0.11 mm or less were collected in a pan. Sieves and pan were shaken for 3 min with a RX-29/30 Ro-Tap® test sieve shaker (278 oscillations/min, 150 taps/min; W.S. Tyler, Mentor, OH).

Table 1.

Switchgrass (Panicum virgatum) particle size distribution for coarse switchgrass processed through a hammermill equipped with 1.25- and 2.54-cm screens and fine switchgrass processed through a smaller hammermill with a single 0.47-cm screen (n = 3).

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Treatment design was a 3 × 3 factorial with three substrates and three nitrogen (N) fertilizer rates. The three substrates included coarse switchgrass amended with 0%, 30%, or 50% sphagnum peatmoss (v/v). All substrates were amended with 0.9 kg·m⁻³ Micromax (The Scotts Co., Marysville, OH) micronutrients and 1.2 kg·m⁻³ gypsum. Containers (15 cm i.d.) were filled with substrate and planted with Oso Easy™ Paprika roses (Rosa ‘ChewMayTime’) that were ≈10 cm tall and 15 cm wide at the time of planting. Plants were potted 28 Oct. 2008 and placed in a glass greenhouse supplemented with sodium vapor lights providing 13 h of lighting from 0600 hr to 1900 hr. Thermostat heat and cool points were set at 18 and 24 °C, respectively. Fertilizer treatments were initiated on 30 Oct. 2008 and is designated as the experiment starting date. Plants were fertigated with 100, 250, or 400 mg·L⁻¹ N from ammonium nitrate (NH₄NO₃) and 90 mg·L⁻¹ phosphorous (P) and 225 mg·L⁻¹ potassium (K) from dibasic potassium phosphate (K₂HPO₄). Plants were fertigated 4 d each week (Monday, Tuesday, Thursday, and Friday) and watered with tap water the remaining days. Fertigation and irrigation volume were adjusted to maintain ≈30% leaching fraction.

Substrate physical properties were determined before the experiment by packing in 347-cm³ aluminum cores (7.6 cm tall × 7.6 cm i.d.) according to methods described by Fonteno and Bilderback (1993). There were three replications for each substrate. Aluminum cores were attached to North Carolina State University Porometers™ (Horticultural Substrates Laboratory, North Carolina State University, Raleigh, NC) for determination of air space (AS). Cores were weighed, oven-dried for 4 d at 72 °C, and weighed again to determine container capacity (CC). Total porosity (TP) was calculated as the sum of AS and CC. Bulk density (D_b) was determined using oven-dried (72 °C) substrate in the same 347-cm³ cores. Substrate pH was measured using the pourthrough procedure at 2, 4, 6, and 9 weeks after treatment initiation (WAT). Foliar samples were harvested (Mills and Jones, 1996) by first rinsing with deionized water and then drying at 72 °C for 3 d. Samples were ground in a Tecator Cyclotec mill (Tecator AB, Hogenas, Sweden) through a 0.5-mm screen. Foliar N was determined with a Vario Max CN analyzer (Elementar Americas, Mt. Laurel, NJ). Other macronutrients and micronutrients were determined with a Thermo Iris Intrepid ICP-OES (Thermo Fisher Scientific, Waltham, MA). Shoot dry weight (SDW) and root dry weight (RDW) were determined at the conclusion of the study (9 WAT) by harvesting roots and shoots separately, rinsing them free of substrate, and oven drying them at 72 °C for 3 d.

Expt. 2.

Coarse switchgrass described in Expt. 1 was used again. In addition, a fine switchgrass material was generated by passing dried and baled switchgrass (same source material as Expt. 1) through a smaller hammermill (No. 30 with blower discharge; The C. S. Bell Co., Tiffin, OH) equipped with a single 0.48-cm screen. Particle size distribution for each material was determined as in Expt. 1 (Table 1).

Treatment design was a 2 × 2 × 3 factorial with two switchgrass grades (fine and coarse), two peat amendment rates (0% and 30%), and three N rates, the same as those used in Expt. 1. Substrates were amended and irrigated similar as those in Expt. 1. The same rose cultivar and containers were used, and greenhouse conditions with respect to light and temperature were the same. Plants were potted 30 Jan. 2009. Substrate physical properties were determined as described in Expt. 1. Substrate pH was measured using the pourthrough procedure at 2, 4, and 7 WAT. Foliar nutrition, RDW, and SDW were determined at the conclusion of the experiment, 7 WAT.

In both experiments, there were six single plant replications per treatment combination arranged in a completely randomized design. Data were subjected to analysis of variance (ANOVA) and repeated measures ANOVA where data were measured more than once over time. Linear or quadratic trends over N rate were determined with orthogonal contrasts. Least significant difference (lsd) values for each parameter were calculated with Fisher's lsd test.

Physical properties.

Physical properties of the three substrates differed with respect to AS and CC are shown in Table 2. Air space decreased from 53% to 36% with increasing peatmoss volume. Ideal AS levels are thought to be 10% to 30% (Yeager et al., 2007). By this standard, coarse switchgrass alone has extremely high AS, whereas that amended with 50% peatmoss is close to the ideal range. The ideal range for CC is thought to be 45% to 65% (Yeager et al., 2007); thus, the substrate containing only switchgrass would have less than ideal CC, whereas the other two substrates are within acceptable ranges. Although the substrate containing 70% switchgrass has significantly lower TP than the other two substrates, a difference of 2% is inconsequential. Bulk density of the substrate containing 100% switchgrass was lower than with the other two substrates, but a difference of just 0.003 g·cm⁻³ is likely inconsequential.

Table 2.

Physical properties of switchgrass (SG) (Panicum virgatum) substrates amended with 0%, 30%, or 50% peatmoss.^z

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In Expt. 2, physical property values of coarse switchgrass substrate were similar to those found in Expt. 1 when amended with 0% or 30% peatmoss. In Expt. 2, AS, CC, TP, and D_b were affected by interactions between switchgrass grade and peatmoss amendment (Table 2). Peatmoss amendment decreased AS and increased CC in coarse switchgrass but had no effect on fine switchgrass. Regardless of peatmoss amendment, fine switchgrass had less AS and greater CC than coarse switchgrass. Bulk density of fine switchgrass was greater than coarse switchgrass, likely as a result of the finer particles that compress more efficiently in a given unit volume. Compared with the aforementioned recommended ranges for physical properties, fine switchgrass with or without peatmoss had closer to ideal physical properties than coarse switchgrass.

Substrate pH.

In Expt. 1, repeated measures analysis indicated that pH was affected by an interaction among date, substrate type, and N rate (P = 0.0001). At 2 WAT, pH in substrates containing 100% switchgrass responded quadratically to increasing N rate, although differences among N rate treatments were minor and averaged 6.4 (Table 3). Amending substrates with 30% or 50% peatmoss reduced pH to 4.1 and 3.9, respectively, averaged across N rate. Substrates amended with 30% peatmoss also responded quadratically to increasing N rate, whereas those amended with 50% peatmoss did not. By 9 WAT, substrates containing either 0% or 30% peatmoss were again affected by N rate with pH decreasing linearly with increasing N rate. Similar to results at 2 WAT, substrates containing only 50% switchgrass were unaffected by N rate. Averaging across N rate, pH for substrates containing 0%, 30%, or 50% peatmoss were 5.9, 4.6, and 3.9, respectively, and each differed significantly from the others (contrast analyses, P ≤ 0.01 across all comparisons). Argo and Biernbaum (1996) reported that a water-soluble fertilizer containing 50% NH₄⁺-N reduced pH over time. The water-soluble fertilizer used in this experiment contained 50% NH₄⁺-N and had a similar effect on pH. The depression in pH was N rate-responsive in the two substrates containing primarily switchgrass. Previous unpublished research by the author has shown that switchgrass is poorly buffered against changes in pH from other amendments or fertilizers.

Table 3.

Effect of peatmoss amendment and nitrogen (N) rate on pH of switchgrass (Panicum virgatum) substrates over time and root and shoot dry weight of roses (Rosa ‘ChewMay Time’) growing in those substrates (n = 6).

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In Expt. 2, substrate pH was affected by switchgrass grade, peat amendment, and N rate (Table 4). At 2 and 4 WAT, pH increased and then decreased with increasing N rate in substrates amended with 30% peat, but did not respond to N rate in nonamended switchgrass. By 9 WAT, all substrates responded either linearly or quadratically to increasing N rate. Averaging across peatmoss amendments and N rates, pH was higher in fine switchgrass than coarse switchgrass (P = 0.0001). This observation is consistent with findings in other research with switchgrass by the authors (J.E. Altland, unpublished data) in that pH increases as the particle size of switchgrass is reduced. Similar to Expt. 1, peatmoss amendment reduced substrate pH.

Table 4.

Effect of coarse of fine switchgrass (Panicum virgatum) substrate amended with 0% or 30% peatmoss and fertilized with three nitrogen (N) rates on substrate pH and rose (Rosa ‘ChewMayTime’) root and shoot growth grown in those substrates (n = 6).

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Foliar nutrient content.

Peatmoss amendment rate and N rate interacted for all foliar elements analyzed (Table 5). There is no information on nutrient sufficiency ranges for landscape shrub roses; however, inferences can be made from sufficiency ranges developed for cut-flower hybrid tea roses (Mills and Jones, 1996). All nutrients with the exception of calcium and iron were within sufficiency ranges (1% to 2% and 56 to 200 mg·kg⁻¹, respectively) suggested for the nine cultivars listed by Mills and Jones (1996). Foliar N increased with increasing N rate with all three substrate types. Roses grown in substrates containing 30% peatmoss had higher foliar N than roses in substrates with 0% peatmoss and numerically greater (although not statistically greater) foliar N than substrates with 50% peatmoss. If switchgrass were immobilizing N within the substrate such that less N was available for uptake, one would expect foliar N to increase with increasing peat amendment rate. However, increasing peat volume from 30% to 50% did not increase foliar N and, in fact, caused a decrease when 250 mg·L⁻¹ N was applied. Differences in foliar N are thus likely the result of differences in available water among the three substrates tested. A similar trend was observed with foliar P and K where increasing N rate tended to cause an increase in foliar levels with substrates containing 30% peatmoss resulting in higher foliar P and K than other substrates. Calcium, boron, iron, and silicon all tended to decrease with increasing N rate and increasing peat amendment rate, suggesting higher available concentrations of these nutrients in switchgrass than peatmoss. Foliar magnesium, sulfur, manganese, copper, and zinc all varied by treatments but either showed only minor differences or showed no identifiable trend with respect to N or peatmoss rates.

Table 5.

Foliar nutrient content of rose (Rosa ‘ChewMayTime’) grown in switchgrass (Panicum virgatum) and peatmoss substrates fertilized with one of three nitrogen (N) rates.^z

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In Expt. 2, foliar N content in roses showed a significant response to peat amendment (Table 5) and N rate; all other main effects and interactions were not significant. Averaging across switchgrass grades and N rates, roses grown with no peat had 2.37% foliar N, whereas roses grown in substrate amended with 30% peat had 2.72% foliar N. Foliar N and P increased with increasing N rate. Across substrates, foliar K was greatest with roses fertilized with 250 mg·L⁻¹ N. Similar to Expt. 1, calcium and iron were at or below reported sufficiency ranges for rose, whereas all other foliar nutrients were within sufficiency ranges.

Plant growth.

Shoot growth was significantly affected by amendment with peatmoss, but not by N rate or its interaction with peatmoss (Table 3). Substrates with 0%, 30%, and 50% peatmoss resulted in roses with 4.9, 5.6, and 4.7 g dried shoot mass, respectively. Root growth was marginally affected by substrate type (P = 0.1048) with root growth decreasing from 1.8 to 1.4 g as peatmoss level increased from 0% to 50% of substrate volume. Nitrogen rate affected RDW with RDW decreasing from 1.9 to 1.5 g as N rate increased from 100 to 400 mg·L⁻¹.

In Expt. 2, root and shoot growth were affected by switchgrass grade, peat amendment, and N rate (Table 4). Root growth decreased linearly with increasing N rate. Root growth was greatest in substrates with coarse switchgrass and no peatmoss. In this experiment and others conducted by the senior author, root growth of roses appears to increase in response to decreasing water-holding capacity. Shoot growth increased linearly or quadratically with increasing N rate among all switchgrass grade and peatmoss combinations. Averaging across N rates, no substrate combination provided superior growth among those tested.

In summary, switchgrass can be processed and modified such that it is suitable for production of short-term woody crops such as landscape shrub roses. Physical properties of switchgrass substrates will depend on the degree to which it is ground and amended with peatmoss and other components. Container capacity of switchgrass seems primarily governed by its particle size distribution. In these experiments, switchgrass processed through a 0.47-cm screen provided ideal physical properties, whereas switchgrass processed through a larger screen required amendment with 30% or 50% peatmoss to achieve the proper ratio of AS to CC. Processing with smaller screens will require more energy and higher production costs and thus will be an important consideration in development of switchgrass substrates.

Substrate pH of 100% switchgrass has been between 6.5 and 7.5 in this and other experiments by the senior author (J.E. Altland, unpublished data) with pH increasing as the proportion of fine particles increases. Although the various species of plants grown in nurseries respond differently to substrate pH, the aforementioned range of switchgrass pH is higher than what is typically considered ideal. Furthermore, switchgrass seems poorly buffered against change from irrigation water or fertilizer amendment. Future research will evaluate amendment with composts with the intent of buffering against pH change.

Foliar nutrient content data suggest switchgrass provides a suitable medium for delivering plant nutrients through typical nursery fertilizer methods. Foliar calcium levels were low in some treatments. Because substrate pH is typically high, amendment with lime will not likely be needed. Lime amendments supply a large portion of calcium fertility; thus, some alternative calcium source (perhaps gypsum) will be explored. Foliar iron levels were similarly low, but this can also be remedied by amendment with iron sulfate.

Overall plant growth was vigorous in all treatments regardless of substrate type. Roses have been reported to grow over a wide range of pH (McCall, 1980) and are considered better suited to well-drained substrates (J. Lee, personal communication). Thus, it is not surprising that roses in this study grew well in substrates with pH ranging from 3.76 to 6.86 or in substrates with suboptimal CC. Future experiments will evaluate amendments to lower and stabilize pH, moderate physical properties, supplement additional calcium and iron, and document substrate conduciveness to root pathogens. A wider range of plant materials with more specific requirements for pH and substrate water content will also be used.