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Hannah Mathers, The Ohio State University, Horticulture and Crop Science |
The ability to absorb nutrients is also an important component of plant adaptation. Plants also have an ideal temperature range for optimum absorption of mineral nutrients (Pisek etal., 1973).
Calcium (Ca) has been found to increase heat tolerance in plants (Bakanova, 1970). However, improper overwintering and supra-optimal root-zone temperatures can cause Ca deficiencies. Only young root tips in which the cell walls are still unsuberized can absorb calcium. Calcium is required for cell division and elongation. Injury to young roots can result in a "Catch 22" in nursery culture. Young roots are required for Ca uptake; however, Ca is necessary for young root formation. Once young roots are injured, it is hard to correct the problem.
Young plants require higher levels of available nutrients relative to older plants. Young plants have the highest nutrient requirements per unit of root system, and nutrient deficiencies are more common with young plants.
Mineralization (e.g., conversion of organic Nitrogen [N] to inorganic N) of composted container substrates is also affected by high temperatures (Kraus etal., 2000). Both nitrate (N03-) and ammonium (NH4+) are inorganic forms of N that can be taken up and metabolized by plants. Nitrate is often a preferential source for plants but much depends on the plant species and other environmental factors. A number of reports indicate that the uptake of both N-forms is temperature dependent, with rates of uptake being depressed by lower temperatures (Clarkson and Warner, 1979).
In general, composts are considered valuable amendments for container substrates. However, most nutrients in compost are not readily soluble and are only released as the organic material breaks down. Most of the N mineralization studies have been conducted at soil temperatures found in field production. Mineralization of organic N is microbially mediated, and the rate of nutrient availability is regulated by environmental conditions such as temperature, moisture, and pH (Haynes, 1986).
Since temperatures of container substrates can reach 103F and above, mineralization rates under these conditions will be quite different from field conditions. Kraus etal. (2000) found that the mineralization rates of certain composted materials in the first weeks of production were too rapid to support adequate N to maximize growth over the container production season. In recent studies, we have found that compost-containing media improves plant survival at high temperatures vs. traditional bark-based media (Johansen and Mathers, 2002).
Fiber pots, used in the nursery industry for years, especially for seasonal, bare-root crops, have recently had copper-based fungicides added to them for increased pot longevity. Fiber pots have many advantages, one being cooling effects for root systems. Overheating in black plastic containers occurs because of the large influx of energy from the sun combined with insufficient loss of the incoming heat. Due to high temperatures, the membrane integrity of the root is lost, and the roots are injured or killed (Larcher, 1995). Conventional plastic containers act as "black heat-sinks" whereas fiber pots do not (Ruter, 1999). Fiber pots also allow for evaporative cooling through their sides. With fiber containers, some of the absorbed heat is dissipated enough that supra-optimal root-zone temperatures are avoided (Beattie etal., 1999). Fiber pots, due to their porous-walls, also increased air exchange throughout the depth of the container, which improves root development by decreasing the potential for waterlogging (Ruter, 1999).