In our last article (June 2014 issue), we discussed our research findings evaluating the activity and efficacy of plant growth regulator drench applications to a substrate amended with 20 percent pine wood chip (PWC) aggregates. To continue with our theme in highlighting the use of PWC aggregates (Figure 1) as an alternative to replace perlite in greenhouse substrates (Figure 2), this article will focus on evaluating the fertility requirements.
Proper Fertilization Can Minimize Nutrient Loss
While communicating with growers around the United States, it is all too common to hear them express a level of uncertainty about the use of wood-based substrates or components as an alternative for peat and pine bark. In the past, the perception and reluctance to use wood-based materials in growing mixes has derived from stories concerning the use of sawdust and resulting nitrogen (N)-deficient crops.
Since the initial concept to use saw dust as a substrate or substrate component, numerous universities have developed techniques to process wood, thus yielding wood components that have been intensively studied and reported as successful. However, consistent with previous reports about sawdust and crop health, researchers observed that plant growth in substrates composed of wood, or large portions of wood, had a tendency to become N–deficient as a result of high rates of N immobilization.
Such reports evaluated and compared the plant growth of multiple floriculture and nursery plants grown in 100 percent wood-based substrates to traditional peat or pine-bark-based substrates. Results of these studies indicated similar or minor differences in growth if plants were irrigated and fertilized more often in the 100 percent wood substrates, compared to peat or pine-bark-based substrates.
Generally, reduced plant growth in wood-based substrates is only a concern when fertility levels (primarily N) are below optimal recommended levels for plant growth and development. For example, work by Wright et al., (2008) found that ‘Baton Rouge’ chrysanthemums grown in a 100 percent wood-based substrate required an additional 100 ppm N, compared to plants grown in a peat-lite substrate composed of 45 percent peat, 15 percent perlite, 15 percent vermiculite and 25 percent bark (by volume).
Therefore, to counteract microbial N immobilization from a wood-based substrate or a substrate containing wood, growers must use a fertilization program that will supply additional nutrients needed by plants and minimize nutrient waste and loss from leaching. However, to our knowledge there is no information available regarding fertility recommendations for optimal plant growth in peat-based substrates amended with PWC.
Perlite Versus PWC Aggregates In Peat-Based Substrates
At North Carolina State University, we have developed a wood processing technique that produces bilocular and non-fibrous PWC aggregates as a perlite replacement. We investigated peat-based substrates formulated with 10 percent, 20 percent or 30 percent perlite or PWC aggregates. However, based on conversations with growers to determine the most common substrate formulation, we will present and discuss our findings of the substrates amended with either 20 percent perlite or PWC aggregates (Figure 3).
We prepared our peat-based substrates by amending either 20 percent perlite or PWC aggregates (by volume) and adjusted substrate pH to 5.8. Two plant species were trialed: ‘Profusion Orange’ zinnia and ‘Moonsong Deep Orange’ African marigolds (Figure 4, A and B). For zinnia, seeds were double sown directly into the substrates and after six days, seedlings were thinned to one seedling per pot, whereas the African marigold plugs were transplanted into the prepared substrates.
At each irrigation event, drip rings (Figure 5) were used to fertilize each plant with either a constant liquid feed rate of 100, 200 or 300 ppm N, provided by equal N combinations of Peters Professional 20N-10P-20K Peat-Lite Special containing 8.1 percent ammonium (NH4-N) and 11.9 percent nitrate (NO3-N) and Ultrasol Cal-Mag 13N-0.9P-10.8K Water Soluble Seedling Plus containing 0.3 percent NH4-N and 12.7 percent NO3-N. Using the Pour-Thru method, we extracted and measured substrate solution pH and electrical conductivity (EC) with a handheld Hanna pH meter on a weekly basis for six weeks. For each species at the first sign of anthesis, a final growth index (GI) [(height + widest width + perpendicular width) ÷ 3] and plant dry mass were determined.
Fertility Recommendations For PWC-Amended Substrates
Results of our study indicate substrate solution EC levels and pH values as recommended by Whipker, et al., (2001) can be achieved for optimal plant growth in PWC-amended substrates. At 200 ppm N, zinnia substrate solution EC was similar for both perlite- and PWC-amended substrates. For zinnia plants grown in perlite-amended substrates, recommended EC levels were maintained from 14 to 42 days after planting (DAP), while PWC-amended substrates maintained recommended EC levels from 14 to 49 DAP. A steady decline of EC levels from 14 to 49 DAP was observed when plants were fertilized with the insufficient N rate of 100 ppm N, compared to a steady increase in EC levels when plants received 300 ppm N.
Substrate solution EC of marigold plants was not influenced by aggregate type but rather by increasing N rate and followed a similar trend to the zinnias. For both species, substrate solution pH was influenced by increasing N rate and followed the normal response of decreasing pH with increasing N rate.
The recommended substrate solution pH 6.2 to 6.5 for both zinnia and marigold plants, was achieved in both substrates, when fertilized at 100 ppm N. The substrate solution pH of both species did not reach recommended pH values when fertilized with 200 or 300 ppm N. Maximum plant growth for zinnia (Figure 6) and marigold (Figure 7) was obtained when plants received 200 and 300 ppm N, respectively, and overall maximum shoot dry weight of both species, was determined when plants were fertilized with 200 ppm N.
Based on the results of this study and visual quality of plants grown in PWC-amended substrates, it is recommended that plants be fertilized with a water soluble fertilizer between 100 to 200 ppm N, without any additional N needed for optimal plant growth. The absence of significant nitrogen tie-up (immobilization) in substrates containing 30 percent PWC (or less) is most likely the result of the large particle size of the PWC aggregates. The PWC particles are larger and have less surface area than wood substrate components that have been previously developed/processed and evaluated in the past by researchers in the US. The uniqueness of the particle size of PWC is not fibrous like previous pine tree substrate materials, hence the lack of degradation and nitrogen depletion observed during greenhouse production.
When substituting PWC for perlite in greenhouse substrates, no adjustments to cultural practices or production of zinnia and marigold plants are needed. However, frequent monitoring of substrate solution EC and pH is recommended, as well as matching a suitable fertility program to the crop. The fertility recommendation provided here can maintain proper substrate solution EC levels and pH values, provide maximum plant growth and reduce excess N applied to zinnia and marigold plants grown in substrates amended with 20 percent PWC (by volume).
Whipker, B.E., J.M. Dole, I.J. Cavin, J.L. Gibson, W.C. Fonteno, P.V. Nelson, D.S. Pitchey and D.A. Bailey. 2001. Plant root zone management. N.C. Community Flower Grower’s Association, Raleigh, N.C.
Wright, R.D., B.E. Jackson, J.F. Browder, and J.G. Latimer. 2008. Growth of chrysanthemum in a pine tree substance requires additional fertilizer. HortTechnology 18:111-115.