Ornamental Plants -- Annual Reports and Research Reviews 1998

Special Circular 165-99

Growth of Ornamental Plants in Compacted Soils and Media

Michael Knee, Ruth A. Brake, Nicole D. Cavender, and Laura C. Thomas

Summary

This study investigated the ability of a number of Ohio native grasses and flowering plants to grow in compacted media. In one experiment, three native species grew better in moderately compacted media than five species of traditional bedding plants. Ability to grow in compacted media seem-ed to be related to the ability of roots to ex-pand under pressure and not to growth at low-oxygen levels. Digital photography and image analysis showed that 11 of 13 species of prairie grasses and forbs (herbaceous flowering perennials) grew larger in compacted topsoil or greenhouse medium than in uncompacted media. Size differences persisted in the field for the first growing season. Type of medium did not influence size at transplanting, but in the field, seedlings from greenhouse medium grew larger than those from topsoil. Seedlings of these species were larger after growth in untilled plots than in tilled areas.

Introduction

In urban situations ornamental plants are often planted in soils that are compacted or have poor structure (1). The silt-loam soils characteristic of much of Ohio are particularly subject to compaction and loss of struc-ture after stripping of native vegetation, construction activity, and everyday human traffic. Plant roots growing in compacted soils experience restricted aeration and physical resistance; these conditions are generally expected to reduce plant growth (2). Native, unselected plant species are sometimes promoted for their ability to tolerate stresses better than exotic species or horticultural varieties. Indeed, one catalog lists a number of prairie grasses and forbs as "clay-busters," raising the expectation that they can tolerate soil compaction (8).

Adaptations to growth in compacted soils could include the ability for roots to grow at low oxygen concentrations or to force apart soil particles (6). This study tested for the ability of the radicle of germinating seeds to elongate at low oxygen concentration or in the presence of high atmospheric pressure. High pressure simulates the resistance of the surrounding media to root cell expansion. Roots could adapt to low oxygen or physical restriction in other ways. For example, roots can develop aerenchyma, open channels through the tissue that allow oxygen to diffuse from the above-ground parts of the plant (5). Adaptations to physical resistance include the secretion of polysaccharides by the root tip that lubricate passage through the soil, or the ability to follow tortuous paths between soil particles (1, 4).

This study originally set out to test the hypothesis that native plants could tolerate soil compaction better than horticultural bedding plants and to determine whether low oxygen or physical resistance was the important factor. Observing that some prairie species actually seemed to benefit from mild compaction in greenhouse conditions, the study went on to determine whether this was a general response for other species. This study was also interested in determining whether field performance of plants was influenced by production of seedlings in compacted media or by production in soil as opposed to artificial medium. An additional aspect of this research has been to determine whether digital photography can be used for reliable, nondestructive measurement of plant growth.

Materials and Methods

For the first experiment comparing prairie and bedding plants, seedlings were grown in 24-cell flats under a range of soil-compaction levels in field soil (Crosby Silt Loam) or greenhouse medium (Metro Mix 360, W. R. Grace, Cambridge, Mass.). Cells were loosely filled (uncompacted) or packed with a pestle to three compaction levels, the highest being the maximum that could be pressed into the cell with hand pressure.

Macropore space was estimated from the change in weight of water-saturated media that were allowed to drain to field capacity. Plastic tubes were embedded in selected cells for analysis of the soil atmosphere; the lower end of the tube was open and the upper was sealed with a rubber septum at the soil surface. A hypodermic syringe was used to take gas samples through the rubber seal; these were injected into a gas chromatograph set up to analyze carbon dioxide, oxygen, and ethylene.

The bedding plants were grown in the greenhouse for six weeks and the prairie plants for 12 weeks. At the end of this time, shoots were harvested and dry weights were measured.

For the second experiment, 13 species of prairie plants were grown in 48-cell flats in greenhouse medium (Metro Mix 360) or a commercial topsoil (Saginaw Series from Michigan marketed by Southland Soil Products, Greensboro, N.C.). Cells were loosely filled or compacted with an extra 20% of greenhouse medium or 40% of topsoil.

After 12 weeks growth in the greenhouse, seedlings were transplanted to a site in The Ohio State University's Chadwick Arboretum (Columbus) where subsoil from the excavation of a lake had been used to create a mound. A square area on the mound was divided into four plots each 2.25 m². Two of the plots were cultivated by turning with a fork to a depth of 25 cm, and two were left uncultivated. Each plot was further subdivided into four areas, each assigned to one of the production treatments (two media and two compaction levels in factorial combination).

Seedlings from appropriate production treatments were set out in each area according to a planting design that was intended to be visually attractive as well as statistically valid. Seedlings were planted with a minimum of soil disturbance, so that in the untilled plot, the roots were in immediate contact with compacted soil.

Plants were photographed with a Kodak DC40 digital camera at the time of transplanting and twice during the first growing season. To provide scale a 30-cm rule was included in the seedling photographs, and a meter rule was included in the plot photographs. Photographs were edited in Adobe Photoshop 3.0 to remove nonplant image, and areas of individual plants were estimated using Sigmascan 1.2. In order to normalize the distribution of error in statistical analysis, plant area data was log-transformed. Back-transformed means are shown in Tables 4 and 5.

Results and Discussion

The greenhouse medium had a much lower bulk density at all compaction levels than the field soil used in the first experiment (Table 1). However, macropore space was similar in the two media at corresponding compaction levels. The oxygen concentration detected in media decreased to a minimum of approximately 12% (Table 2) as compaction increased. This seems to have been a function of soil respiration since oxygen concentration was not affected by plant species or even the absence of a plant from a cell. Carbon dioxide concentration increased with compaction, but to a lesser extent than the oxygen decreased. Ethylene concentration tended to decrease with compaction but concentrations were usually less than 0.1 µl L^-1 (Table 2).

Because there were large differences in the growth of plant species, dry weights were normalized relative to the average for a species:

normalized weight = actual dry weight 4 average dry weight

After this manipulation, it is possible to compare effects of medium and compaction level without being distracted by species effects (Figure 1). Higher levels of compaction tended to reduce the dry weight of all species in both media; the first compaction level tended to decrease the dry weight of bedding plants and to increase the dry weight of the prairie species, Asclepias, Echinacea, and Schizachyrium (Figure 1). Some species (Tagetes, Zinnia, and Echinacea) had higher dry weight in the greenhouse medium than in field soil, whereas Impatiens was higher in field soil than in greenhouse medium. Other plants showed similar growth in both media.

Figure 1. Dry weights (relative to average for species) of shoots of plants grown at different compaction levels in greenhouse medium (GM) or field soil (FS). Anm Antirrrhinum majus, Art Arabidopsis thaliana, Ast Asclepias tuberosa, Ecp Echinacea purpurea, Gye Gypsophila elegans, Imb Impatiens balsamina, Scs Schizachyrium scoparium, Tap Tagetes patula, Zie Zinnia elegans.

Germinating seeds of the same species were grown for three days in different oxygen concentrations. The relationship between radicle growth and oxygen concentration was examined to determine the concentration at which elongation occurred at half of the maximum rate (C_0.5). This concentration varied according to species from 0.74% for Schizachyrium scoparium to 4.98% for Zinnia elegans (Table 3). There was no obvious relationship between growth in compacted soil and C_0.5. Germinating seeds were also exposed to high atmospheric pressure. Radicle elongation was more sharply reduced by pressure for the bedding plants than for the prairie species. The contrast was par-ticularly clear at 1.14 MPa (11.4 bar) where radicle length of the bedding plants was reduced by 55% or more, but growth of the prairie species was reduced by 20% or less (Table 3). Root response to pressure did not account for all of the responses to soil compaction in the greenhouse experiment. However, there was a correlation between response of whole plants to the first compaction level and the response of radicle elongation to 1.14 MPa (Figure 2).

In the second experiment, a wider range of prairie species was grown in two kinds of media and at two compaction levels. At the time of transplanting, the size of seedlings in topsoil and greenhouse medium was similar, but seedlings in compacted media were larger than those in uncompacted media (Table 4). The larger size of plants from compacted than from uncompacted media was maintained in the field, but the proportional difference tended to decrease (Tables 4 and 5). The size of plants from seedlings raised in greenhouse mixture increased faster than those from topsoil, and plants in untilled soil grew larger than those in tilled soil (Table 5).

Although severe compaction adversely affects plant growth, soil can also be too loose. There appears to be an optimum level of compaction at which root-soil contact is maximized, but root growth is not restricted (6). The rate of plant growth at any time tends to be proportional to plant size, so relative differences in size are expected to persist, as they did in the second experiment. In research with lettuce transplants, seedlings grew larger in compacted media than in uncompacted, but the difference did not persist in the field (7). It is sometimes argued that artificial media may be ideal for plant growth in the greenhouse, but that plant roots may have difficulty growing into mineral soil when planted out in the field. From this research it appears that there is no advantage over greenhouse medium in the use of topsoil to produce seedlings of the plant species tested. From the comparison with bedding plants, it appears that prairie species grow well at levels of soil compaction that inhibit other plants. This may be related to their ability to grow under conditions of high atmospheric pressure and their apparent success in untilled soil in the second experiment.

In the second season the plants have grown so that it has become more difficult to separate and estimate their areas in digital photographs. Further experiments involving destructive sampling and estimation of dry weight are underway to test some of these preliminary findings. More research is planned to investigate the environmental adaptations and landscape uses of prairie grasses and forbs.

Literature Cited

1. Bennie, A. T. P. 1996. Growth and mechanical impedance. pp. 453-470. In: Waisel, Y., A. Eshel, and U. Kafkaki. Eds. Plant Roots: The Hidden Half. Marcel Dekker, New York.

2. Boone, F. R. and B. W. Veen. 1994. Mechanisms of crop responses to soil compaction. pp. 237-264. In: Soane, B. D. and C. van Ouwerkerk. Eds. Soil Compaction in Crop Production. Elsevier, Amsterdam.

3. Craul, P. J. 1992. Urban Soil in Landscape Design. Wiley, New York.

4. Ijima, M. and Y. Kono. 1992. Development of the Golgi apparatus in the root cap cells of maize (Zea mays L.) as affected by compacted soil. Ann. Bot. 70:207-212.

5. Konings, H. and H. Lambers. 1991. Respiratory metabolism, oxygen transport, and the induction of aerenchyma in roots. pp. 247-265. In: Jackson, M. B., D. D. Davies, and H. Lambers. Eds. Plant Life Under Oxygen Deprivation: Ecology, Physiology, and Biochemistry. SPB Academic Publishing, The Hague.

6. Lindstrom, M. J. and W. B. Vorhees. 1994. Responses of temperate crops in North America to soil compaction. pp. 265-286. In: Soane, B. D. and C. van Ouwerkerk. Eds. Soil Compaction in Crop Production. Elsevier, Amsterdam.

7. Nicola, S. and D. J. Cantliffe. 1996. Increasing cell size and reducing medium compression enhance lettuce transplant quality and field production. HortScience. 31:184-189.

8. Prairie Nursery, Inc. 1998. Catalog and Growing Guide. Westfield, Wisc.