Current issue: 56(2)
Result of a survey of soils supporting forest plantations in Wisconsin in the United States indicated a close correlation between the levels of fertility of non-phreatic, coarse-textured soils and the growth of red pine (Pinus resinosa Roezl) stands aged from 15 to 32. This relationship, however was not observed in plantations established on deep-gley soils, underlain at a depth of 3–9 fl by ground water.
The survey encountered 20 red pine plantations on soils underlain by a deep ground water table accessible to tree roots thorough their contact with gley horizon or with extended capillary fringe. The average growth of the stands was 80 cubic feet/acre (5.6 m3/ha) at the age of 22 years. Thus, mensuration analysis suggested that the soils are the choice grounds for forestry enterprise. However, the analysis of soil samples showed that in many instances the soils are extremely low in mineral colloids, organic matter and nutrients. Many of the sites would be regarded as critically deficient in nitrogen, phosphorus and potassium.
The following hypothesis are suggested to explain this discrepancy:
a) The moisture content of coarse-textured non-phreatic soils remain near the wilting point during a large apart of the growing season with subsequent reduction of transpiration and uptake of nutrients. If a capillary fringe provides a supply of water for the root system, trees may derive an adequate supply of salts and exchangeable ions from comparatively infertile substrata.
b) The suitably located ground water provides adequate aeration of the surface soil layers which is not impeded by capillary fringe, increasing activity of mycorrhiza, and a mycotrophic uptake of nutrients from unweathered minerals.
c) The above effects of natural subirrigation should change the concept of soil fertility based on mere chemical analysis. The time during which the roots are engaged in active absorption appears to be of equal importance as the concentration of nutrients in available form.
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