IV. CHALLENGES TO SUCCESSFUL COMPOST USE

arboricola Hayata). Other studies (Conover and Joiner, 1966; Lumis and Johnson,

1982; Sanderson, 1980) have illustrated the negative effects on plant growth of

elevated soluble salt levels in certain types of compost products. Despite quality

control measures routinely taken by commercial composters, growers are well

advised to regularly monitor new batches of compost products as received. Excessively high soluble salt levels in compost materials can be managed by leaching,

where leaching would not pose a threat to surface or ground water resources, or by

blending the compost with substrates that have lower soluble salt levels. Indeed,

many compost based potting media recommendations contain only 20 to 30% compost, as a means of reducing damage that can be caused by high salt levels or other

phytotoxic substances that may be present in certain compost products (see Raymond

et al., 1998).

C. Compaction

Porosity is one of the more important physical parameters in container media

(Poole et al., 1981), because of the need for effective gas exchange in the root zone.

Some compost products have been reported to have satisfactory pore space at the

beginning of the plant production period but undergo compaction during the production period (Fitzpatrick and Verkade, 1991). Compost materials used as the

complete, or stand-alone, rooting substrate are more likely to settle or compact during

the production period, thereby reducing the porosity of the medium. This phenomenon is more likely to occur in fresh, immature compost products. This problem can

be treated either by allowing the compost product to age further, or by blending the

compost with materials that are not likely to undergo compaction during production.

D. Phytotoxicity

Compost products may contain phytotoxic materials that can come from a variety

of sources. The organic material from which the compost is made may contain

residues of substances that can be toxic to crops grown in the end product compost.

For example, forsythia (Forsythia intermedia Zab.) and white cedar (Thuja occidentalis L.) grown in potting mixes amended with municipal solid waste (MSW) compost suffered boron (B) toxicity due to B that had been present in the MSW feedstock

(Lumis and Johnson, 1982). This can be contrasted with the findings of Ticknor et

al. (1985), in which the authors reported very low levels of B in the biosolids compost

used and in the foliage in photinia (Photinia X fraseri Dress.), with the suggestion

that growers who use this type of compost should consider applying foliar treatments

of B to correct this deficiency. Commercial composting organizations regularly

monitor the composts they produce, both for their own quality assurance programs,

and because of governmental regulatory requirements. However, there is always the

possibility of substantial variation in the composition of the incoming organic feedstock, and small volumes of contaminated product can escape detection in random

sampling of compost materials.



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A more serious cause of phytotoxicity in compost products can come from the

composting process itself. Since commercial composting organizations derive

income from charging fees to urban waste haulers as well as from the sale of the

compost products to growers, there is an obvious and strong economic incentive to

minimize the amount of time that the organic material is composting. The technically

correct minimum amount of time that an organic substance must undergo composting

in order to have a stable end product compost is variable. It depends on several

factors, including the size of the compost pile, the pile’s aeration status, the pile’s

moisture status, the carbon to nitrogen (C:N) ratio of the material, heat levels and

range during composting, and other factors. References published prior to the widespread commercialization of composting recommend minimum composting periods

of approximately 6 months (Howard and Wad, 1931). Although it may be possible

to speed up the composting process to some degree, the economic incentives for

commercial composters to sell immature compost products are very real. Commercial plant producers who purchase compost products should be sensitive to the

seller’s incentives and should also be aware that immature compost products can

pose serious threats to the health and vigor of plants grown in them. High microbial

activity in immature composts can cause biological blockage of N from the crop.

The microbes can literally out-compete the plant for the available N, and the plant

would exhibit N deficiency symptoms. Also, the microbes that mediate the composting process secrete certain phytotoxic chemicals, such as short-chain fatty acids

like acetic acid, propionic acid, and butyric acid, during the early stages of the

composting process. Deformity or death of plant parts caused by the ephemeral

production of these chemicals at certain points early in the composting process can

be a real threat when growers attempt to use immature composts as growing media

(Jimenez and Garcia, 1989). When liners of hibiscus (Hibiscus rosa-sinensis L.)

were planted in containers with growing media amended with uncomposted biosolids, plants exhibited phytotoxicity symptoms within 5 days after planting, and the

symptoms increased in intensity as the biosolids concentration in the growing

medium increased (Figure 6.1). Plants growing in the control medium (Figure 6.2A)

did not exhibit any phytotoxicity symptoms, while plants in media amended with

uncomposted biosolids (Figure 6.2B) exhibited chlorosis, consistent with N blockage, and leaf distortion, consistent with the presence of short-chain fatty acids

(Fitzpatrick, unpublished data).

There are numerous tests that can be conducted to determine whether a compost

product is sufficiently mature (Jimenez and Garcia, 1989), but most of these tests

require equipment and facilities that are not directly available to the typical nursery

crop grower. One type of test that can be conducted by most nursery growers is the

bioassay procedure. A seed flat is filled with the compost material being considered,

and a second flat filled with a control growing medium that is known to be stable.

Seeds of a species with rapid germination and rapid growth, such as radish (Raphanus

sativus L.), are sown in the flats and the germination and growth characteristics of

plants in both flats are observed for a 1 to 2 week period. If significant levels of

phytotoxic substances are present in the compost material under consideration, visual

symptoms should be apparent during this time.



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Figure 6.1



Hibiscus (Hibiscus rosa-sinensis L.) liners 5 days after planting in 25 cm diameter

nursery containers filled with a growing medium amended with uncomposted

biosolids. The row on the far left is the control medium, with no biosolids, and no

phytotoxicity symptoms are apparent on these plants. The row on the far right

contains the medium with the highest biosolids concentration, 30% of the total

growing medium, and plants in this substrate have the most severe symptoms.

The five middle rows contain biosolids concentrations of 5, 10, 15, 20, and 25%

(left to right), and phytotoxicity symptoms appear to be increasing as the biosolids

concentration increases.



V. IMPORTANT FACTORS IN A CONTAINER GROWING MEDIUM

Although there is no perfect growing medium for all ornamental crops under all

growing conditions, numerous authors have described general recommendations.

For example, Poole et al. (1981) recommend for container grown foliage crops the

following general parameters: bulk density — 0.30 g·cm–3 (dry), 0.60 to 1.20 g·cm–3

(wet); pore space — 5 to 30%; water-holding capacity — 20 to 60%; pH — 5.5 to

6.5; soluble salts — 400 to 1,000 mg·L–1; cation exchange capacity — 10 to 100

meq per 100 cm3.

Frequently, commercially made compost products have pH levels higher than

those listed above; ranges of pH 6.7 to 7.7 are common (Conover and Joiner, 1966;

Fitzpatrick, 1989; Fitzpatrick and Verkade, 1991). High pH values can result from

the chemical qualities of the feedstocks, or from materials added to the feedstocks.

For example, composts made from biosolids frequently have high pH values because

of chemical stabilizers, such as lime, added before composting. Unless milled, (see

Fitzpatrick, 1989), pore space and water-holding capacities of commercially made

compost products are usually within the acceptable ranges. Soluble salt levels, cation

exchange capacity, and bulk density may all be significantly influenced by the

composition of the parent material or by preprocessing, so growers of ornamental

crops should monitor these parameters regularly.



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Figure 6.2



Hibiscus (Hibiscus rosa-sinensis L.) liners 5 days after planting in 25 cm diameter

nursery containers filled with a growing medium amended with uncomposted

biosolids; (A) plants growing in the control medium, with no uncomposted biosolids

incorporated, do not exhibit any phytotoxicity symptoms, while (B) plants in medium

amended with uncomposted biosolids exhibit chlorosis and leaf distortion.



VI. USING COMPOST PRODUCTS BENEFICIALLY IN NURSERY CROP

PRODUCTION

There are several published general reviews illustrating compost use in nursery

crop production, including Fitzpatrick and McConnell (1998), Fitzpatrick et al.

(1998), Sanderson (1980), and Shiralipour et al. (1992). Generally, compost is used

in nursery crop production as a less expensive substitute for peat and other organic

components of the growing medium. Also, some compost products have been demonstrated to accelerate growth in some species, thereby decreasing the production

period for these crops. Some compost products also have been demonstrated to have

a suppressive effect on some plant pathogens (see chapter 12 by Hoitink et al. in

this book).

A. Field Nursery Production

Although most published work on compost utilization for ornamental crop production focuses on container plant culture, some publications have detailed the uses

of compost products as amendments in field nursery soil. In one of the earlier

references on compost use in ornamental crop production, DeGroot (1956) reported

enhanced growth in gloxinia (Gloxinia X hybrida Hort.) grown in five rates (1, 2,

3, 4, and 5 kg·m–2) of MSW compost applied to planting beds, and enhanced growth

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