Experiment 1.

On 27 Jan. 2012, 6-inch-diameter plastic containers (ITML Horticultural Products, Middlefield, OH) were filled with each of the six previously described substrates. ‘Profusion Orange’ zinnia seeds were direct, double sown at a depth of 1 cm in the substrate-filled containers. The seedlings were grown in a polyethylene greenhouse in Raleigh, NC, and grown with 23/17 °C (day/night) air temperature settings (temperatures not monitored during growth trial). All plants were watered as needed (when the surface began to dry) depending upon weather conditions. Equal volumes of irrigation solutions (≈150 mL) were applied to all substrates at each irrigation as a result of the water-holding capacities of all substrates were within 7% of each other (data not shown), according to substrate physical property analysis performed on all substrate blends before potting, using the North Carolina State University porometer method. On 2 Feb., one of the double-sown zinnia seedlings was pinched at the substrate level to remove it and allow a single seedling to remain in each pot.

Plants were treated with constant liquid fertilizer at each watering, beginning with the first watering after thinning, with 100, 200, or 300 mg·L⁻¹ N, provided by equal N combinations of 20N–4.4P–16.6K water-soluble fertilizer (Peters Professional Peat-Lite Special; Scotts, Marysville, OH) containing 8.1% ammonical (NH₄) N and 11.9% nitrate (NO₃) N and 13N–0.9P–10.8K water-soluble fertilizer (Cal-Mag; SQM North America, Atlanta, GA) containing 0.3% NH₄-N and 12.7% NO₃-N. The fertility treatments consisted of 25, 50, or 75 g of 20N–4.4P–16.6K and 39, 77, or 115 g of 13N–0.9P–10.8K fertilizers to produce 100, 200, or 300 mg·L⁻¹ N, respectively. The water-soluble fertilizers were weighed and dissolved together in separate 100-L fertilizer tanks to achieve 100, 200, or 300 mg·L⁻¹ N. Fertilizer solutions were supplied to plants by irrigation lines constructed with drip rings. Five containers of each substrate were randomized on irrigation lines at bench level, and drip rings were placed on top of the substrate for each container, corresponding to the fertility treatment. Substrate solution was extracted 1 h after irrigation and collected weekly for 6 weeks using the pour-through method (Wright, 1986) and was analyzed for EC and pH. The experiment was a factorial design arranged in a completely randomized manner with five single-plant replications × six substrates × three N concentrations. On 23 Mar., a final growth index [GI = (height + widest width + perpendicular width) ÷ 3] of each plant was recorded at the first sign of anthesis, and total plant height was measured (from the substrate surface to the top of the bloom). Stems were severed at the substrate surface, dried at 70 °C for 1 week, and weighed.

Experiment 2.

Except where indicated, procedures were the same as described in Expt. 1. On 12 Mar. 2012, ‘Moonsong Deep Orange’ marigold seeds were sown into 288 plug trays containing Fafard Germination mix (Conrad Fafard, Anderson, SC) in a glasshouse in Raleigh, NC. For Expt. 2, the same six substrate ratios used in Expt. 1 were formulated on 11 Jan. and stored in sealed plastic bags without dolomitic limestone incorporation. Sealed plastic bags were stored under shelter and protected from the weather. On 30 Mar., dolomitic limestone was incorporated at 10 lb/yard³. A lower amount of lime was incorporated in Expt. 2 to achieve pH of 5.8, determined by initial substrate pH. On 4 Apr., 4-week-old germinated seedlings were transplanted in 6-inch plastic containers filled with each substrate. Substrate solution was collected weekly for 5 weeks and analyzed for EC and pH. On 11 May, final GI of each plant was measured and recorded. Stems were severed at the substrate surface, dried at 70 °C for 1 week, and weighed. EC data were subjected to analysis of variance by the general linear model procedures and regression analysis (version 9.4; SAS Institute, Cary, NC). Plant height, diameter, GI, dry weight, and pH means were separated by Duncan’s multiple range test (P ≤ 0.05).

Substrate solution EC.

Zinnia substrate solution EC was not influenced by percentage of aggregate (10%, 20%, or 30%); therefore, substrate solution EC data were pooled and analyzed by aggregate type (perlite or PWC) and N concentration. There was a significant linear and quadratic relationship between substrate solution EC and N rate at 14, 21, 28, 35, 42, and 49 DAP for perlite- or PWC-amended substrates (Fig. 2A and B). EC levels of substrates amended with perlite or PWC followed a similar trend from 14 to 49 DAP. Substrate solution EC was similar between 14 and 42 DAP but different at 49 DAP for each N concentration.

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Substrate solution EC of both substrates fertilized with 100 mg·L⁻¹ N resulted in a steady decline from 1.71 to 0.72 mS·cm⁻¹ for perlite-amended substrates and 1.02 to 0.66 mS·cm⁻¹ for PWC-amended substrates from 14 to 49 DAP (Fig. 2A and B). Whipker et al. (2001) indicated that the recommended pour-through extract EC range for zinnia was 1.0 to 2.6 mS·cm⁻¹. Therefore, plants fertilized with 100 mg·L⁻¹ N achieved the recommended EC levels between 14 and 28 DAP for both substrates and continuously declined beyond 28 DAP.

Substrate solution EC of plants supplied with 200 mg·L⁻¹ N held fairly constant (Fig. 2A and B). In perlite-amended substrates, solution ECs were consistent and within recommended EC levels from 14 to 42 DAP, but increased to 1.42 mS·cm⁻¹ at 42 DAP, which was above the recommended range for zinnia. Unlike substrates amended with perlite, substrate solution EC of PWC-amended substrates fertilized with 200 mg·L⁻¹ N remained relatively consistent between recommended EC levels from 14 to 49 DAP. Kang and van Iersel (2009) found a similar trend when ‘Gnome White’ petunia were irrigated with a fertilizer solution EC of 1.5 mS·cm⁻¹.

Substrate solution EC of zinnia plants grown in substrates amended with perlite or PWC and fertilized with 300 mg·L⁻¹ N resulted in a steady increase from 2.10 to 3.93 mS·cm⁻¹ and 2.00 to 3.95 mS·cm⁻¹ for perlite- and PWC-amended substrates, respectively, from 14 to 49 DAP (Fig. 2A and B). Kang and van Iersel (2009) also reported constant liquid fertilizer concentrations above 2.5 mS·cm⁻¹ supplied to petunias to increase substrate solution EC, as a result of additional fertilizer applied to the substrate than taken up by the plant. Plants grown in both substrates amended with perlite or PWC and fertilized with 300 mg·L⁻¹ N maintained recommended EC levels from 14 to 21 DAP. Substrate solution EC of both substrates continuously increased above recommended EC levels for zinnia beyond 28 DAP. An increase in substrate solution EC with increasing fertilizer concentration throughout the experiment is consistent with previous findings (James and van Iersel, 2001; Kang and van Iersel, 2001, 2009; Nemali and van Iersel, 2004).

Data pooled by N concentration and sampling time for marigold plants grown in perlite- and PWC-amended substrates resulted in a significant linear and quadratic relationship between 11, 18, 25, 32, and 39 DAT (Fig. 3). From 11 to 39 DAT, substrate solution ECs of marigold plants were similar and followed a similar trend of zinnia plants. Marigold plants grown in perlite- or PWC-amended substrates and fertilized with 100 mg·L⁻¹ N resulted in a steady decline from 0.99 to 0.49 mS·cm⁻¹. Substrate solution EC of plants fertilized with 200 mg·L⁻¹ N, held fairly constant over the duration of 5 weeks, while plants fertilized with an additional 100 mg·L⁻¹ N resulted in an increase from 2.77 to 4.16 mS·cm⁻¹ from 11 to 39 DAT. According to Whipker et al. (2001), substrate solution EC of marigold plants were in recommended pour-through extract EC range of 1.0 to 2.6 mS·cm⁻¹ when fertilized with 200 mg·L⁻¹ N. Substrate solution EC levels of perlite- or PWC-amended substrates were not within recommended EC range for marigold plants when fertilized with 100 or 300 mg·L⁻¹ N.

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Substrate solution pH.

Zinnia substrate solution pH was the same for aggregate type and percent of aggregate; therefore, substrate solution pH data were pooled and analyzed by N concentration. Results indicate as N concentration increased, substrate solution pH decreased. The mean substrate solution pH of zinnia plants grown in both perlite- and PWC-amended substrates fertilized at 100, 200, and 300 mg·L⁻¹ N were (mean ± se) 6.2 ± 0.37, 5.7 ± 0.30, and 5.5 ± 0.32, respectively (data not shown). Overall, though, substrate solution pH for zinnia plants fertilized with 100 mg·L⁻¹ N was within the acceptable range of pH 6.2 to 6.5 for zinnia growth (Nau, 2011). For marigold, the substrate solution pH decreased with decreasing aggregate amendment. This trend is supported by Jackson et al. (2009b) who observed pH to be highest in 100% PTS substrate compared with a PL substrate, and pH generally decreased as the proportion of peat increased. Overall, substrates amended with 30% perlite or PWC and fertilized at all fertility concentrations had the highest substrate solution pH, which decreases with increasing peat. Similarly, Wright et al. (2008) found that substrate pH followed the normal response of decreasing with increasing fertilizer rate for PL and PTS. Overall, substrates amended with 20% perlite or PWC and fertilized with 100 mg·L⁻¹ N were in acceptable pH range for marigold growth (pH 6.2 to 6.5). However, plants grown in substrates amended with 10% and 30% perlite or PWC and fertilized with 100 mg·L⁻¹ N, achieved a pH 6.1 and 6.7 (higher pH values than is recommended for marigold growth), respectively. Also, in marigold plants grown in 10%, 20%, or 30% perlite- or PWC-amended substrates and fertilized with 200 and 300 mg·L⁻¹ N, substrate solution pH values were high and not within the recommended marigold substrate pH range.

Zinnia plants fertilized at all N treatments did not exhibit any visual nutrient deficiencies or toxicities. Marigold plants grown only in the substrate amended with 10% perlite and fertilized with 100 and 300 mg·L⁻¹ N exhibited lower leaf purpling at 15 DAT. The discoloration of marigold leaves is likely caused by the high substrate pH and its possible effect on phosphorus availability or due to excessive irrigation. However at 39 DAT, plants did not exhibit any nutritional deficiencies or toxicities.

Difference of substrate pH for both species may be caused by the rates of 200-mesh (60% screening) dolomitic lime amended to the substrates. Materials used to formulate substrates to grow zinnia had an initial pH of about 4.2 before liming. Dolomitic lime was incorporated at 12 lb/yard³, 3 d before use to allow for lime equilibration, resulting in pH ≈6.2; which was within the recommended pH range for zinnia plants. Substrate formulated for marigold had an initial pH ≈4.1 before liming. Based on previous studies, a lower rate of dolomitic lime 10 lb/yard³ was incorporated in the formulated substrates to achieve a target substrate pH ≈5.8.

The reactive fraction or the fraction of dolomitic limestone that initially reacts to increase pH to a stable level, 5 to 10 DAP was achieved for zinnia and marigold plants. However, the residual or the unreacted limestone affects the substrate’s long-term buffering capacity or the ability to minimize fluctuations in pH (Argo and Fisher, 2002) may have been reduced when dolomitic lime of 10 lb/yard³ was incorporated in substrates to grow marigold plants. Argo and Fisher (2002) demonstrated that the amount of residual lime contained in a substrate can influence the overall pH reaction in the substrate and override the effects of fertilizers. In this study, the normal response of substrate pH decreasing with increasing N concentration for substrates amended with perlite or PWC was observed. It has been reported by Wright et al. (2008) that 100% PTS normally has a proper pH range for plant growth with no addition of dolomitic limestone incorporated into PTS compared with PB and PL. Therefore, preplant/pre-pot initial testing and lime reactivity is recommended for any substrate with wood components (of any percent) in them.

Frequent monitoring of substrate solution EC and pH is recommended (Cavins et al., 2000) as well as matching a suitable fertility program to the crop. Despite similar EC and pH data in these trials, there is other evidence and discussion in recent literature to suggest that different wood components can alter substrate chemical properties in different ways. Most notably, increased ratios of some wood components have been shown in other works to increase substrate pH and decrease EC during crop production (Jackson and Wright, 2009).

Plant height.

Zinnia height was not influenced by aggregate type, percentage, or N concentration with a mean overall height of 14.3 cm for all treatments. Marigold height was not influenced by aggregate type or the percentage of perlite- or PWC-amended substrates; therefore, plant height data were pooled and analyzed by N concentration with significant differences between N concentrations. In perlite- and PWC-amended substrates, plant height was greatest when fertilized with 100 mg·L⁻¹ N and was 4% and 7% shorter when fertilized with 200 and 300 mg·L⁻¹ N, respectively, as represented visually in Fig. 4. Shorter plant height could be contributed to high substrate solution EC, not recommended for marigold culture when fertilized with 300 mg·L⁻¹ N. Also, the substrate pH was not within the recommended pH range when plants were fertilized with 200 and 300 mg·L⁻¹ N, which may contribute to shorter plant height.

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Diameter.

Zinnia and marigold plant diameter was not influenced by aggregate or percentage of perlite- or PWC-amended substrates; therefore, plant diameter data were pooled and analyzed by N concentration. For zinnia, plant diameter was smaller as N concentration increased while marigold plant diameter increased as N concentration increased (Table 1). Zinnia plant diameter was 3% and 8% smaller when fertilized with 200 and 300 mg·L⁻¹ N, respectively, compared with plants fertilized with 100 mg·L⁻¹ N. In both perlite- and PWC-amended substrates, marigold plant diameter increased by 9% and 8% when fertilized with 200 and 300 mg·L⁻¹ N, respectively, compared with plants fertilized with 100 mg·L⁻¹ N. Visually, zinnia and marigold plant diameters were similar.

Table 1.Effect of nitrogen (N) concentration on zinnia and marigold height, diameter, growth index (GI), and shoot dry weight, grown in substrates amended with 10%, 20%, and 30% (by volume) perlite or pine wood chips fertilized with 100, 200, or 300 mg·L⁻¹ N.

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Growth index.

Zinnia and marigold GI was not influenced by aggregate type or the percent of aggregates; therefore, plant growth data were pooled and analyzed by N concentration. In general for zinnia growth, as N concentration increased, overall plant size was less. Zinnia plants fertilized with 100 mg·L⁻¹ N were the largest with a mean GI of 18.36 cm, compared with plants fertilized with 200 and 300 mg·L⁻¹ N (Table 1). Growth indices of zinnia plants fertilized with 200 mg·L⁻¹ N were similar to GI of plants at 100 or 300 mg·L⁻¹ N. Overall, marigold plants fertilized with 200 mg·L⁻¹ N were largest with a mean GI of 23.43 cm, compared with plants fertilized with 100 or 300 mg·L⁻¹ N (Table 1). However, GIs of marigold plants fertilized with 300 mg·L⁻¹ N were similar to plants at 100 and 200 mg·L⁻¹ N. Visually there were no differences in overall zinnia and marigold plant size. Larger zinnia and marigold plant size can likely be attributed to similar substrate physical properties at each percent/rate of each aggregate type (data not shown) and acceptable substrate solution pH. Lower substrate solution EC and smaller overall plant size indicate plants were receiving insufficient amounts of nutrition when fertilized with 100 mg·L⁻¹ N. As a result, high substrate solution EC from excessive fertility provided by 300 mg·L⁻¹ N led to less plant growth for both species.

Shoot dry weight.

Zinnia and marigold shoot dry weight was not influenced by aggregate type or percent of aggregate; therefore, shoot dry weight data were pooled and analyzed by N concentration. Zinnia shoot dry weight of plants increased by 7% when fertilized with 200 mg·L⁻¹ N and decreased by 5% when fertilized with 300 mg·L⁻¹ N, compared with plants fertilized with 100 mg·L⁻¹ N (Table 1). There was 11% reduction in shoot dry weight between 200 and 300 mg·L⁻¹ N; however, zinnia shoot dry weight was similar at all N concentration. Marigold shoot dry weight increased by 19% when plants were fertilized with 200 mg·L⁻¹ N and by 9% when fertilized with 300 mg·L⁻¹ N, compared with plants fertilized with 100 mg·L⁻¹ N concentration. Shoot dry weight that was 10% smaller occurred between plants when fertilized with 200 and 300 mg·L⁻¹ N, as a result of higher than recommended substrate solution EC levels. Marigold shoot dry weight was different in all N concentrations with the highest shoot dry weight occurring when marigold were fertilized with 200 mg·L⁻¹ N. Kang and van Iresel (2009) reported a similar response when petunias were fertilized with 200 mg·L⁻¹ N.

Similar plant growth from zinnia and marigold grown in substrates with 30% wood, even at lower N concentrations (100 mg·L⁻¹) provides supportive data toward better understanding the N/fertility dynamics of using raw/fresh/unaged organic components like wood in horticultural substrates. Nitrogen demands and immobilization rates are among the top concerns of academic and industry researchers, soil/substrate manufacturers, and growers relative to the perception that all fresh wood substrates or substrate components will tie-up (immobilize) much of the N in the system leading to stunted or yellow plants. It is not the opinion of these authors that this work proves that N-immobilization is absent, but instead the message here is that when using these particularly engineered blockular wood particles, their influence on the chemical properties of a peat-based substrate are negligible under the specific growing conditions outlined in this work.

Pine wood chips are engineered and processed to specific sizes and shapes to be functional as aggregates in a container substrate. Not all wood components are designed, or capable of improving/influencing the physical and chemical behavior of a substrate the same. As a result of the large particle nature of PWC, they likely have reduced surface area compared with small particles or wood components that have been processed in different ways/methods. The influence of particle size and surface area (internal and external) on microbial decomposition, nutrient immobilization, and pH buffering (among other things) is not well understood for wood substrate components, but it is believed to be highly significant. For example, wood processed into fibers (increased surface area) and amended with peat may show different N requirements (microbial populations) and plant growth response during cultivation, in addition to changing the substrate air and water relations much differently than PWC-sized particles. On the basis of the known variability of many wood components being developed, commercialized, marketed, and sold on the market today, it is highly suggested that any and all substrate wood components should not be considered the same and be tested/trialed before large-scale use.

Units

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