Soil Fatigue

Soil fatigue refers to the decline in soil fertility observed in nurseries and agricultural fields where the same type of plant has been cultivated for extended periods. This condition may result not only from depletion of soil nutrients but also from disruption of the soil’s biological balance and accumulation of toxic substances.

Causes

The main causes of soil fatigue are as follows:

• Biological Factors: Increased populations of certain fungi such as Pythium de Baryanym (root rot), Phytophthora fagi, and Fusarium species can lead to seedling diseases.
• Accumulation of Toxic Substances:
• • Toxic compounds released from plant residues (roots and leaves) into the soil play a significant role. For example, in apple nurseries, a compound called phlorizin leaches from apple roots and leaves into the soil. Phlorizin is subsequently broken down by soil microorganisms into various compounds such as phloretin, phloroglucin, p-oxyhydrocinnamic acid, and p-oxybenzoic acid.
  • The decomposition of plant residues by soil microorganisms such as Penicillium expansum can also produce phytotoxic and antimicrobial substances like patulin.
• Nutrient Imbalances: Nutrient deficiencies in the soil can increase toxin production in plants. For instance, deficiencies of nitrogen, phosphorus, sulfur, and calcium in apple rootstocks elevate the concentration of phlorizin in roots.
• Activity of Soil Organisms: In soils sterilized by steam, soil organisms rapidly reestablish themselves, whereas chemical sterilization often yields less favorable results. The number of bacteria in sterilized soils can increase significantly compared to non-sterilized soils.

Symptoms and Effects

• Reduced productivity is observed in old nursery soils.
• When the same or closely related plant species are replanted in the same location, growth retardation and developmental disorders occur. For example, apple is more susceptible to soil fatigue than pear.
• Increased incidence of seedling diseases and seedling mortality.

Factors Influencing Soil Fatigue

• Toxin Concentration and Persistence: The physiological effects and concentrations of organic compounds introduced into the soil are critical. Phlorizin and its breakdown products are not highly persistent in soil and can degrade within a few weeks. Similarly, antibiotics such as patulin are also believed to have limited persistence in soil.
• Plant Species Dependency: Resistance to soil fatigue varies depending on the plant species and cultivar. Patulin production from apple roots is higher than from peach, sour cherry, pear, and quince roots.
• Soil Properties: Factors such as soil structure, pH, and water content can influence toxin formation and accumulation.
• Nutritional Status: The quantity of nutrients in the soil can affect both the toxin content of plants and the ability of microorganisms to produce toxins. Higher levels of phlorizin have been observed in unfertilized or poorly managed nurseries. An increase in mineral nutrient levels in the soil may reduce patulin synthesis by P. expansum.

Control Methods and Recommendations

Cultural Practices:

• Deep plowing.
• Rotating the soil with different crops for several years (crop rotation).
• Green manuring.

Soil Sterilization:

• Steam Sterilization: Sterilizing soil using steam at 3–4 atmospheres of pressure and temperatures of 120–140°C in an autoclave is an effective method. Soils sterilized by this method show increased seedling numbers and improved growth, with better root system development. Steam sterilization can also be performed in practical nursery operations using specialized equipment such as “Pronto.”
• Chemical Sterilization: Chemicals such as formalin may be used, but they are reported to be less effective than steam sterilization. Carbon disulfide was initially used as a sterilizing agent. Copper-based preparations (copper acetate, copper sulfate) have also been tested.

Other Applications:

• Seed treatment with substances such as ceratan has been attempted, but no significant effect on seedling mortality has been observed.

The mechanisms for alleviating soil fatigue and enhancing plant growth have not yet been fully elucidated. One possibility is that sterilization increases the solubility of organic matter in the soil, thereby improving nitrogen cycling. Additionally, it has been determined that soil sterilization causes significant changes in the soil’s bacteriological activity.