Sphagnum peat mushroom casing soils: composition, function and microbiology.

In the mushroom growing process, Agaricus starter culture (spawn) is worked into the mushroom substrate (compost following pasteurization) and allowed to grow throughout the substrate for 14 days. After colonization of Agaricus mycelia in the substrate is complete, a 1.5-inch layer called casing, consisting of peat soil amended with calcium carbonate (to neutralize pH) and water (about 80 percent moisture), is applied on top of the compost bed. This layer is called the casing soil (Figure 1). The casing soil enhances the retention of irrigation water on the growing beds, and promotes mushroom fruit body formation.

[FIGURE 1 OMITTED]

Background and Composition of the Mushroom Casing Soil

The major component of casing soil used by mushroom growers in North America is sphagnum peat. Sphagnum peat is primarily decomposed sphagnum moss. Sphagnum moss typically grows in an aquatic bog environment. The bog water has a pH near 4.0, low levels of oxygen and few mineral nutrients. Each year, dead sphagnum moss accumulates in the bog to form peat. Due to conditions in the bog, sphagnum moss peat decomposes slowly. Over thousands of years, it can form layers up to 4-20 feet thick (PHI 2005).

Canada is the world's largest producer of horticultural peat. Currently, of Canada's 270,000,000 acres of peat wetlands, 40,000 acres are under harvest (PHI 2005). Peat bogs are composed of approximately 92 percent water. In preparation for peat harvest, the surface of the peat bog is cleaned by removing surface foliage, large root systems and debris. Drainage ditches are dug around and through the bog to drain the peat. The peat bog is harrowed to a depth of six inches, air-dried and the top two inches are removed with vacuum harvesters (CSPMA 2005).

Peat, with its high humus content and weed-free nature, is an ideal amendment for raised horticultural beds or small gardens. Soil amendment with peat improves the nutrient-and water-holding capacity of sandy soils, and improves the drainage and aeration capacity of clay soils. It is for these reasons peat is widely used by the greenhouse, and fruit and vegetable industries as a soil amendment.

Functions of the Mushroom Casing Soil

Peat is ideal for preparing mushroom casing soils because of the following:

1. Retention of irrigation water

2. Initiation of the sporophores Retention of irrigation water

Retention of irrigation water

Water is involved in several important functions in the growth of Agaricus bisporus (Lange) Imbach cells. Water makes up about 90-94 percent of the fruiting body mass. The growth of Agaricus mushrooms depends critically upon the ability of the cells to translocate water (Beyer et al. 2000). Water uptake by the mushroom has been suggested to be a limiting factor in yields (Kalberer 1991). Also, yield, size and quality of harvested mushrooms are correlated to the amount of moisture in the substrate or casing (Schroeder & Schisler 1981; Kalberer 1985; Kalberer 1987). Hence, a critical function of the mushroom growing medium (compost and casing) is to act as a continuous reservoir of water for the rapidly growing mushrooms. The high water retention capacity of peat based casing soils makes it an ideal growth substrate for mushroom growing.

Initiation of the sporophore

An important process during mushroom growing is the pinning of mushrooms. Pinning is the initiation of the transition of Agaricus bisporus from the vegetative stage to the reproductive stage. Addition of casing soil onto the surface of Agaricus colonized compost stimulates pinning. One key factor to this transition, resulting in fruiting body (basidiome) initiation is the casing microflora (Eger 1972). The casing soil supports an active, aerobic bacterial flora (Hayes & Nair 1976), and the fluorescent Pseudomonad spp. represents up to 50 percent of the bacterial population in the casing layer (Samson 1986). Pseudomonas putida (Trevisan) Migula has been identified as an important species involved in basidiome initiation (Hayes etal. 1969; Rainey et al. 1990). The mechanism by which P. putida stimulates fruiting is not well understood, but the bacterium is thought to remove "self-inhibitory substances" produced by the vegetative mushroom mycelium (Eger 1972; Wood 1976).

It is a practice among some mushroom growers to vary the proportion of sphagnum peat and black peat. While sphagnum peat is the major component in casing soil, some commercial mushroom growers add up to 40 percent black peat in their casing layer. It is generally reported by growers that adding black peat to the casing layer regulates pinning over the period of the mushroom crop, thus avoiding peak production days and spreading the harvesting period over the duration of the flush. This cultural practice also helps the growers to harvest and sell mushrooms with symmetrical shape, equal size, consistent maturity, and equal solids content.

Microbiology of Casing Soil

The casing layer on which the mushroom fruiting bodies develop is a significant reservoir for the microflora of fresh mushrooms (Doores et al. 1986). Doores et al. found that aerobic bacterial populations from casing material ranged between 8.2 and 8.5 log CFU/g. Samson 1986 demonstrated that fluorescent pseudomonads can represent up to 50 percent of the total bacteria in casing samples, whereas Doores et al indicated that they represented 2 percent of the total casing bacteria only. Miller et al. 1995 demonstrated that the populations of casing bacteria changed over the Agaricus growth cycle. The casing soil harbored 8.7 to 9.7 log aerobic bacteria per gram of casing soil. The proportion of fluorescent pseudomonads in casing was shown to fluctuate between 14 to 41 percent of the total bacteria present, increases coinciding with the onset of fruiting. Studies in our laboratory have also demonstrated that the casing layer on which the mushroom fruit body develops is high in microbial populations. Total aerobic bacterial populations range from 8.0 to 8.5 log CFU/gm of casing material. The major bacterial genera present in casing soil were the Pantoea genus (8.2 log CFU/gm) and the Pseudomonas genus (7.7 log/gm casing soil).

Casing soil also contains a significant population of yeasts, molds, and actinomycetes. Sphagnum peat, the major component of casing soil, is known to contain Trkhoderma and Streptomyces (Tahvonen 1993). Studies in our laboratory have shown that casing soil in the production environment harbor approximately 5.2 log CFU of molds and 6.7 log CFU of native yeast per gram. Penicillium is the predominant genera of mold present in casing soil. Species level identification based on macro-and micro-morphological features determined that the following species of Penicillium were the predominant ones: P. decumbens, P. chrysogenum, P. glabrum, P. citreonigrum, and P. digitatum. Aspergillus niger was occasionally isolated from the casing soil.

Waksman and Purvis 1932 conducted a study to characterize microbial populations of an undrained peat bog in Florida (Table 1). The study was conducted by obtaining samples from different depths of the peat bog. The upper layer of the peat bog was abundant in aerobic bacterial populations, actinomycetes, and fungi. The depth of the bog significantly influenced microbial populations. With increasing depth, populations of aerobic bacteria, actinomycetes, and fungi decreased and the number of anaerobic bacteria increased. Table 1: Microbiological population of an undroined peat bog in Florida, adapted from Waksman and Purvis 1932.

Numbers In thousands/gram of moist peat Depth of Aerobic Actinomycetes Fungi Anaerobic Peat, cm Bacteria Bacteria 2-20 890 370.0 20.0 120 23 960 290.0 10.0 180 45 410 100.0 7.0 180 75 18 13.0 0.3 16 120 30 0.3 0.0 75 165 235 3.3 0.0 380 * Moisture content of peat varied from 80.1 to 87.4 percent

While soil type significantly influences indigenous microflora, peat soils are generally abundant in actinomycetes (Table 2) (Rao& subrahmanyan 1929). Also, the preparation of casing soil involves the neutralization of peat PH by the addition of calcium carbonate. In his book, The Actinomycetes: Asummary of current knowledge (Waksman 1967), Dr. Waksman indicates that the draining of soil and subsequent addition of calcium carbonate to soil favors the development of actinomycetes. Table 2: Numbers of actinomycetes in Indian Soils, adapted from Rao and Subrahmanyan 1929. Soil Type Crop Raised Actinomycetes, thousands

per gram of soil Black Cotton 3,340 Alkaline Wheat 2,540 Peaty Paddy 2,340 Alluvial Paddy 1,000 Raddish Laterite Tea 250 Kalar Soil Fruit 680 Red Sandy Loam Coconut 40

Disease and Foodborne Pathogen Suppressive Characteristics of Sphagnum Peat and Sphagnum Peat Based Mushroom Casing Soil