INTRODUCTION
Highbush blueberry roots, like those of most ericaceous plants, host a fungal symbiont which increases the plant’s ability to forage for nutrients, most notably nitrogen. The fungi benefit from this relationship because it utilizes the host plant’s photosynthates as its carbohydrate source. The fungal symbiont, usually an ascomycete for ericaceous plants, infects the root and forms mycorrhizae (which literally means “fungus roots”). While many studies have been conducted on the effects of organic matter (both pre-plant and as a mulch) and nitrogen form and level on blueberries, and some studies have been conducted on the effect or level of mycorrhizal infection on blueberries, none of these studies have reported the relationship between nutrition and planting practices with the level of mycorrhizal infection.
Organic matter and nitrogen studies. Soil amendments for preplant incorporation improve blueberry establishment and production on mineral soils. Perlmutter and Darrow (1942) found that blueberry seedlings grew best in a media that was 2/3 “forest litter” and 1/3 soil. More recently, Odneal and Kaps (1990) found that aged or fresh pine bark was as good as sphagnum peat for preplant incorporation, with no differences in plant height, spread, number of canes or plant yield or size. Rotted sawdust in the planting hole has also proven to be a good material for preplant incorporation (Eck, 1988).
Mulches are considered essential for blueberry production on mineral soils. Numerous studies have been undertaken to evaluate the effect of mulches. On rabbiteye blueberries grown on a fine sandy loam, mulching increased plant height, and shoot and root weights (Patten et al., 1988). Many researchers have examined different types of mulches for their effectiveness. Savage (1942) found that 8 cm (3 in) of sawdust mulch (at least 1 year old) was superior to rye straw or oak leaf mulches, with better plant survival and growth. All mulches, however, were notably better than clean cultivation. Skiruin and Otterbacher (1984, 1986, 1987) and Meador et al. (1984) have found that many mulching materials work well. Sawdust and chopped corn stalks were particularly effective. Leaf mulch (of oak, locust, beech, maple, sycamore and some pine needles) also performed as well as sawdust and chopped corn stalks, but had to be replenished yearly rather than once every two years. All mulches were aged 1–2 years. Wood chips and straw produced the lowest yields and shortest plants. Pine bark and well composted manure or stable bedding have been reported to work well (Eck, 1988), however, the latter materials may increase soil pH. Korcak (1988) cites the reasons for improved performance using organic mulches as improved water-holding capacity, reduced temperature fluctuations, weed control and improved soil tilth. Mulches also maintain a more constant media pH, and in some cases contribute to acidification of the soil.
A study by Spiers (1986) found that on rabbiteye blueberries, mulch was the most important component, followed by incorporated peatmoss and irrigation. Mulching resulted in a uniform root distribution from the plant crown outward, with most roots in the upper 15 cm (6 in) of soil. Incorporated peatmoss tended to concentrate the root system near the crown area (most at the 30–45 cm [12 to 18 in] depth). After three years, all of the plants that had received none of the treatments (no mulch, peat moss or irrigation) had died.
Recommended levels of nitrogen fertilizer application for mineral soils are between 45 and 91 kg of actual nitrogen/ha (40 and 82 lbs of actual nitrogen/A) (urea or ammonium sulfate) depending upon plant age (Goulart et al., 1994). There has been considerable debate regarding the nitrogen form utilized by highbush blueberry plants (Townsend, 1967; Hammett and Ballinger, 1972; Rosen et al., 1990). Most studies conclude that there is preferential utilization of ammonium nitrogen by Vaccinium, making it an unusual species, since most plants prefer to take up nitrate nitrogen (Peterson et al., 1988; Townsend, 1967; Merhaut and Darnell, unpublished). Studies on Calluna vulgaris and Vaccinium macrocarpon resulted in increased total N, increased N concentration in the shoot, and increased shoot growth on mycorrhizal versus non-mycorrhizal plants (Read and Stribley, 1973; Stribley and Read, 1974). Mycorrhizal plants also contained more total nitrogen than non-mycorrhizal plants, suggesting that the mycorrhizal plants were better able to utilize non-ammonium sources of nitrogen than non-mycorrhizal plants (Stribley and Read, 1974). Other research demonstrated that ericoid mycorrhizae can assimilate amino acids, peptides and proteins from the soil, and transfer N from these sources to host plants, providing an additional source of N which plants otherwis...