The response of hardwood species to soil nitrogen is known from the work of H. L. Mitchell and R. F. Chandler in New York. They used leaf analysis to characterize available nitrogen in second-growth stands of the Northeast. "Nitrogen tolerant" species, such as red oak, aspen, and red maple, grew relatively well with low supplies, although growth improved up to a point as more nitrogen became available. "Nitrogen demanding" species, like white ash, tulip poplar, and basswood, grew poorly at the lower levels, but growth rate increased continuously up to the maximum nitrogen levels that they found. Thus nitrogen supplies strongly affect the competitive abilities of different species. Accordingly, factors that control the availability of soil nitrogen tend to influence forest composition as well as growth.

THE QUANTITIES OF NUTRIENTS taken up by forest crops are relatively large.

A compilation of European data by p. J. Rennie in the journal, Plant and Soil, 1956, indicates that the aboveground woody parts of 100-year-old coniferous stands contain approximately 800 pounds an acre of calcium, 400 pounds of potassium, and 65 pounds of phosphorus. These values are averages from nine stands of larch, spruce, and fir. They are exclusive of foliage but include the amounts in intermediate thinnings.

About two-thirds of the calcium and one-half of the phosphorus in these amounts would be removed from the forest in stemwood and bark. Nutrients contained in the roots, foliage, and recently fallen organic matter add to the totals absorbed from the soil. Quantities taken up by hardwood species are even greater, particularly of calcium, but those by Scotch pine are much less.

The quantity of an element absorbed does not necessarily indicate the actual requirement for vigorous growth. Nevertheless, as Mr. Rennie emphasized, soils of very low inherent productivity like some weathered, highly silicious sands cannot supply the nutrients needed for sustained forest production. In other nutrient-poor soils, slow availability of nutrients and restrictions on root development limit the uptake and growth. Artificial fertilization may be the only feasible means of increasing productivity on such sites.

The texture and mineralogy of most forest soils in the United States suggest that normal release of mineral nutrients can meet requirements such as we cited, so that removal in the usual forest products is not likely to cause depletion. But attention must be directed to the condition of soils already depleted or naturally infertile and to means of maintaining fertility reserves if they are critical.

ROOT DEVELOPMENT, anchorage, and aeration, beyond what we have already said, have a profound bearing on the growth of forests as they relate to soil.

The ultimate influences of soil are not fully expressed by early growth rate. Rooting habit and development sometimes differ greatly on sites identical in growth rate and so affect the characteristics of a stand in later years.

Trees with root systems limited by compact horizons or gley layers are susceptible to windthrow damage. This possibility is increased by certain soil textures or saturated conditions that loosen the holding effectiveness of roots. Bedrock close to the surface acts similarly, unless large fissures allow a firm anchorage. Roots resting on coarse-grained rock are sometimes worn away as trees sway in violent winds, leaving large openings for entrance of decay.

Occasional periods of very poor subsoil aeration, perhaps occurring at long intervals, can prune off the deeper root system and thus affect growth.

NATURAL SOIL BODY features that can be mapped or recognized readily are especially useful in managing large forest properties. Hence there now is great interest in correlating site index, or other elements of productivity, with them. This is a somewhat different approach from the one that relates a single or few habitat factors to forest growth.

Soil properties texture, depth, drainage, mineralogy, slope, and presence of certain horizons occur together. By their combinations they characterize the various soil classification units that soil scientists recognize. Natural units such as soil series and types are based on distinctive origin and morphology. The soil units therefore include the individual factors already mentioned as associated with growth, as well as additional properties and combinations that cannot be treated separately in correlation studies. Moreover, these soil units are recurring features of the landscape and can be mapped in a practicable manner.

Many earlier attempts to relate soil type to forest type and growth rate were not fully successful, for several reasons. Tree growth does not necessarily respond to the changes in properties that separate adjacent soil units, and all species do not respond alike.

The deeply penetrating root systems sometimes encounter conditions not reflected in the surface soil. Many valuable relationships appear, nevertheless, when soil units are accurately conceived and delineated on maps.

The basis for these relationships may be the classification al units themselves or broader divisions formed by combining units of similar capabilities, as in regard to moisture retention and nutrient supply. These broader divisions may be subdivided as necessary into phases according to different depths or slope directions. Several examples demonstrate that grouped soil types or other units, skillfully used, can indicate site quality. Aside from being useful to the manager of land now in timber, soil surveys with this kind of interpretative information are useful in forecasting the growth potential of open land before purchase or planting. They can be applied also to young stands or to cutover areas where direct measurements of site index cannot be made.

Moreover, the results of many soil-site correlation studies based on specific factors can be interpreted into soil survey mapping units. For instance, site index of loblolly and shortleaf pines in the southeastern Piedmont region has been correlated with subsoil plasticity and depth of friable soil above, other factors being held essentially constant. Plasticity, however, was found to be characteristic of soil type. Accordingly, with this information, depth phases of soil types can serve as indexes of productivity.

The site type gives the forester a clue as to how well the species present in disturbed stands are adapted to the site and what trends he can expect.

Further, each mapping unit can be rated, qualitatively or quantitatively, for attributes important in forest management. Susceptibility to windthrow, trafficability, erosion hazard, and likelihood of soil damage by wet weather logging, are closely related to soil physical characteristics. Other attributes are ease of regeneration, encroachment of undesirable species, and soil-influenced diseases or insects.