By V. T. AALTONEN, Professor, Forest Research Institute, Helsinki, Finland
The key subjects for forest soil research are phenomena which, in one way or another, affect the fertility of the soil: what soil properties determine the fertility, how permanent this fertility is by nature, and what the possibilities are for improving the productivity of poor soils. The following is a brief report on what has so far been done in this field in Finland, where forest soil research has now been conducted for a quarter of a century.
In order to make the general trend and the objectives of this research work more comprehensible, it is best to explain at the outset some of the natural conditions and the characteristics of forestry and forest research in Finland.
First, many phenomena that are or have been foremost in forest soil research elsewhere are not found in Finland. For example, the competition between forest and heath, which is a problem in Denmark and northwest Germany, does not occur in Finland; neither does competition between forest and steppe, or between forest and prairie. Saline soils (like those in Hungary, for example), which are difficult to afforest, do not exist here, and soil erosion is unknown. Moreover, the forest soils in Finland are rich in mineral nutrients and have not been exhausted by such practices as the Central European use of forest litter, for example. Nor does the humus layer, as a rule, form a serious obstacle to natural forest regeneration, as in parts of Central Europe. Soil improvements of any kind on felling areas are rare in Finland.
When one considers that, in addition, the practical evaluation of forest soil quality in Finland is based exclusively on vegetation (the forest site types of Cajander), it is not surprising that both in practice and in forest research much less attention has been devoted to the soil itself than in many other countries. Thus, forest soil science has been a subject of higher forestry education in Finland for only four years. The author of this report does not know' therefore, why soil research was given so important a position at the Forest Research Institute that in 1918, when the Institute began its work, a third department for soil investigation was founded in addition to the departments of silviculture and forest survey, each department headed by-a professor. (The department for soil investigations started its work in 1923.)
The general trend of forest soil research, as well as other forest research in Finland, has been fundamentally influenced by the forest type theory devised by Cajander. In his far-reaching program for forest soil research in Finland, Cajander ascribed the greatest importance to studies of the extent to which forest types are representative of natural site classes. Science cannot be content, he remarks, with merely stating a certain correlation between forest type and habitat, but must analyze more closely the biological value of the habitat. A reliable foundation for measures that aim at improved forest soil productivity can be laid only by examining the factors that determine site productivity. Nor did Cajander think it impossible that, by doing so, one might arrive at a purely climatic and pedological evaluation of site quality.
Chemical soil properties. The early forest soil investigations in Finland were concentrated principally upon plant nutrients and soil-acidity. Chemical soil analyses (using cool, weak hydrochloric acid as solvent) showed that improved productivity of the forest type was correlated with distinctly increased lime and total nitrogen content. A correlation with potassium was not quite so clear, and the phosphoric acid content showed a decrease with improved habitat. Research on nitrogenous compounds in particular, has shown that the ammoniac nitrogen content increases with: improved forest type; nitrate nitrogen, on the other hand, is found mainly only in grass-herb forest soils. The total content of bacteria and microscopic fungi, too, has been demonstrated to increase with improved forest type, and the presence of so-called physiological bacteria groups (nitrifying, cellulose de nitrogen-fixing, and other bacteria) varies in a certain proportion to the forest type. Furthermore, it has been demonstrated that the acidity of the humus layer decreases with improved forest type.
With regard to the results already mentioned, two principal facts should be noted: (1) In these investigations neither tree species nor physical soil characteristics have been taken into consideration, but only the forest type based on tile lesser vegetation. (2) The results characterizing different forest types are average values derived from a huge amount of material; with regard to individual soil characteristics, one and the same forest type may show considerable variations.
Textural properties. The influence of grain size and superficial deposit on soil productivity has been the subject of numerous investigations in different countries. In general, the correlation between soil properties in this respect and soil productivity has been proved to be much closer than in the case of chemical soil properties.
According to the Cajander theory, a forest type is not determined by soil texture (sand, loam, etc.). The same kind of soil may support widely varying forest types, and one and the same forest type may appear on very different soils. The forest type represents the integrated result of the biological total value of the site - the joint effect of all primary climatic and soil factors on the vegetation. Although this is generally the case, the statistical material obtained with regard to productive soils in Finland in connection with the second national forest survey (covering about 8,000 sample plots) shows a fairly clear correlation between soil texture and forest type.
The results are illustrated in Figures 1 and 2; the soils are divided into three textural classes according to the degree of coarseness: A = coarse-grained soils (sand, gravelly soil, etc.); B = medium-coarse soils (ordinary morainic soils); C = fine-textured soils (clay soils, etc.). The soil-forming areas (1, 2, etc.) according to which the research material is divided will be discussed later. In areas 3 and 4, the soil is 12 to 18 percent A-soil, 72 to 77 percent B-soil, and 10 to 11 percent C-soil.
With regard to the distribution of forest types according to different soils, it has been noted that with a change from coarse to fine soils under uniform climatic conditions or on the same kind of soil from northern Finland to southern Finland, the number of forest types decreases, i.e., a few types become dominant. Of the three leading species, the Scots pine (Pinus sylvestris) is dominant on coarser soils, the Norway spruce (Picea abies) and the birch on finer soils.
Within the same forest type, moreover, the growth of the trees may vary considerably according to the texture and grain size of the soil. In forests of Myrtillus and Vaccinium types, for example, the yield on very stony morainic soils is only 50 percent of that on a normal forest type. Certain special investigations have shown furthermore that the productivity of morainic soils intimately depends on the content of fine earth in the surface soil, if this content is calculated as a proportion of the soil as a whole (including also blocks, stones, etc.). In glaciofluvial deposits, on the other hand, the deeper soil layers, too, must be taken into consideration.
Soil-forming processes. The investigations already described have been concerned with the chemical composition and the texture of the soil in forests of different forest types. Other research studies that should be mentioned are those on soil-forming processes.
Among the main soil-forming processes of the globe, the development of Podzol soil is dominant in Finland, where the climate is humid and the soil poor in lime. Due to climatic variations that occur even in Finland, the conditions for soil formation are not uniform all over the country. Leaching, moreover, is greatly dependent on soil texture and moisture conditions.
Only a few results of investigations on soil-forming will be mentioned here, and chiefly those that merit consideration from a practical point of view.
First, if the thickness of the A-horizon (of mineral soil) is taken as a measure of leaching strength, Finland may be divided into soil-forming areas as indicated in Figure 3. Of these, areas 1, 2, 3, and 4 represent geologically old soil, and 2a and 3a represent young soils. Among the former the soil is most weakly leached on area 4, somewhat more strongly on area 3, and most strongly on areas 1 and 2. Area 3a is most weakly leached. The variations - besides those due to the age of the soil - are explained by humidity variations.
Fig. 2. Distribution of soils among forest types of soil-forming area d. Lh = grass-herb: forests, PyT = Pyrola type. (For other symbols., see Fig. 1.):
In the sample plot investigations conducted in connection with the second national forest survey, the thickness (on productive soils) of the humus layer and the A-horizon was also measured. For the former, the classes < 2 cm., 2-4 cm., and > 4 cm. were used; for the A-horizon:-(= lacking or very weak) < 3 cm. and 3+ > cm. The results for different forest types from areas having produced the most abundant observation material are indicated in Figures 4 and 5. Productive soils in which the humus layer is considerably thicker than 4 cm. or the A-horizon considerably over 4-5 cm. are fairly scarce.
Fig. 3. Soil-forming areas in Finland
Why the soil profile varies with forest type in the way previously described has not yet been satisfactorily demonstrated, but most likely it is a question of the influence of soil texture (and moisture conditions).
In professional literature on soil science, tree species are generally regarded as an important, if not decisive, soil-forming factor. As appears from Figures and 7, however, soil development (the thickness of humus layer and A-horizon) in Finland is-fairly independent of tree species.
A clear correlation, on the other hand, exist between soil texture and soil profile. As is evident from Figures 8 and 9, the humus layer grows thicker and the A-horizon thinner when the soil becomes more fine-grained.
It should be mentioned that these results refer to heath forests, the humus of which is mor, but that the so-called biological activity of the humus increases in the direction CT ® OMT. Generally, when considering the relationship between tree species and humus layer, one should note that in Finland the forest soil is nearly everywhere covered with a more or less abundant ground vegetation and that the litter from this ground vegetation plays a very important role in the formation of the humus layer. Since the nature of the ground vegetation is closely dependent on the nature of the mineral soil and on moisture conditions, the primary characteristics of the mineral soil generally have a decisive influence on the humus-forming.
It may furthermore be mentioned in this connection that certain studies in Finland on the influence of the Siberian larch (Larix sibirica) compared with other tree species on the chemical, physical, and biological characteristics of the soil have not manifested distinct differences between various species. The larch seems to have a degrading rather than an improving effect on the soil.
Studies into the chemistry and physics of soil-forming have demonstrated in detail the chemical and physical changes of the mineral soil in dry, moist, and damp heath forests, as well as in grass-herb forests. These investigations have shed new light upon the following phenomena in particular:
(1) development of Podzol profile with growing age of soil;
(2) dependence of soil-forming on grain size of soil;
(3) mineralogical and chemical composition of the soil;
(4) leaching of calcium and other nutrients;
(5) soil-forming types.
The land-raising process, which in Finland has continued since the glacial period, offers an opportunity to trace the development of the soil profile from its beginning to over 10,000 years of age. These investigations have made possible an analysis of the special importance of time as a soil-forming factor; they have shown that in some respects the development of the Podzol profile differs from earlier concepts insofar, at least, that the B-horizon grows from the bottom up. Chemical changes may be analytically demonstrated in soils of from 200 to 300 years of age, but the changes are nevertheless so slow that in ordinary forest soils a period of 1,000 to 1,500 years is needed for the profile to reach maturity.
As mentioned previously, there is a definite correlation between soil profile and geological superficial deposit. Mechanical soil analyses have clarified this correlation in detail and have proved it to be fairly constant. Leaching is strongest in soils coarse enough to be permeated by water, but yet fine-grained enough to maintain the amount - of moisture - necessary for weathering. In clay soils, leaching is weak or non-existent; in moist sand, it is strongest. Hardpan is mainly found only in paludified coarse-grained soils; in ordinary productive soils it seldom occurs. Lodging of the finest particles to a greater depth is manifested only in clay soils.
With regard to mineralogical and chemical composition, the forest soils in Finland are rich in nutrients; poor sandy soils like those in northern Germany, on Jutland in Denmark, and elsewhere do not occur. However, calcium carbonate is found only in a few places in Finland in small amounts, and a considerable part of the silicate calcium is leached away. As a rule, the investigations have not been extended below a depth of 50 to 60 centimeters; but on the basis of certain observations it may be inferred that part of the calcium is leached away from a surface layer at least 1 meter in thickness, which likewise denotes that the soil is weathered to at least this depth. In addition, total analyses have demonstrated that, with a decrease in the size of the soil particles, the silicon content is reduced, whereas the content of calcium, aluminum, and iron is increased.
The author has divided the soil-forming types in Finland that are most important from the standpoint of forestry into two principal classes, the humus layer of the former being mor and that of the latter, mull. The main types in the former class are Podzol, Brown, and Gray soils; those of the latter class are Brown and Gray soils.
Among the subtypes of the Podzol soil the most important are iron Podzol and humus Podzol, the former dominant in ordinary productive forest soils, the latter in moist and wet soils. Morphologically these types differ from each other particularly in that the B-horizon of the iron Podzol is reddish or brown, whereas that of the humus Podzol is chocolate-brown or almost black (a result of abundant humus). Gray soil is the soil-forming type of clay. Brown soil occurs in various types of forests; mull-brown soil, however, only in grass-herb forests. The last-mentioned type may be considered as a northern variety of the Central European Brown soil (Ramann).
A question of vital importance, not only from the standpoint of forestry but with regard to land utilization in general, is the regional variation in soil fertility and its causes. Closely related to this problem is the question of the stability of soil fertility.
According to information on the occurrence of forest (and swamp) types and so-called exacting plant species, Finland is divided into the fertility areas indicated in Figure 10. The northern half of the country is considerably more barren than the southern half way but even in the southern part the fertility shows notable regional differences. The small, extremely fertile areas, or so-called grove concentrations, are not visible on the map.
Fig. 7. Thickness of A-horizon and tree species on soil - forming area 4. (For symbols, see Fig. 6.)
The major fertility variations, such as the difference between the northern and southern halves of the country, are due to climatic differences. In similar climatic conditions, fertility differences are caused mainly by textural and topographical variations, viz., geological factors. To some extent, indeed, a greater abundance of fertile soils has been demonstrated in regions where the bedrock has a higher content of basic kinds of rock than usual; however, these constituents form but a small proportion of the bedrock area. In Finland the bedrock for the most part is granitic primary rock.
The evaluation of forest soil quality is closely bound up with the question of the extent to which the quality classes are either static or dynamic. The more constant the quality classes are by nature, the slighter are the possibilities of changing soil productivity by silvicultural measures, for example.
From the standpoint of the stability of soil fertility in Finland, leaching merits particular attention. The general leaching of nutrients to a greater depth means a continuous exhaustion of the soil. Simultaneously, the soil acidity increases and the litter decomposition is delayed. These changes are so slow, however; that they need not be considered in practical quality classifications and yield calculations.
In a country like Finland, where most of the forest soils have been exposed to several fires in the course of time, the effect of fire on productivity represents an additional problem of forest soil science. Though no special investigations on this problem have been conducted, the general impression seems to be that, at least in dry heath forests, fires have had an exhausting effect on the soil. Cajander, for example, mentions that a Vaccinium type, and even a Myrtillus type, may for some time support a ground vegetation which does not noticeably deviate from that of the Calluna type, and that in certain cases a Vaccinium type may permanently change into a Calluna type. In spite of these and certain other vegetation changes, fires probably seldom cause a permanent deterioration of the general soil quality. The forest regeneration capacity of the soil is in most cases improved by fires, but in this respect, too, the effect is of short duration. It appears that on extremely dry sands, fire may cause a more lasting deterioration of the regeneration capacity.
An effect in many respects resembling that of forest fires was brought about by a primitive form of cultivation, formerly quite common in Finland, the so-called "burning over." Since, in this case, the soil was to some extent cultivated too, and one or several crops were harvested, the changes in the soil were greater and more lasting than those caused by forest fires. As yet, at least, there is no evidence that cultivation by "burning over" has caused any practically noteworthy and durable changes in the fertility conditions of Finnish forest soils.
All told, the forest-soil quality in Finland may be considered as fairly constant and practically static. This does not exclude the possibility of changes of the site quality caused by climatic factors.
It should be noted that the investigations referred: to in this report have not applied to the paludification of forest soils. Some idea of the importance of this process may be inferred from the fact that swamps from about one-third of the whole land area in Finland and that the major part of the present-day swamps was once firm or dry forest land. However, the paludification of forest soils, too, is a relatively slow process and nowadays it is probably even slower than before.
The most important further research is that pertaining to the chemical composition of needles and leaves, litter production, and so-called root competition. These investigations are unfinished, as yet, but it has been demonstrated that the composition of the ash and N-content of needles and leaves is dependent on the forest type as well as on the tree species. The picture of the useful nutrients of the soils thus obtained corresponds in its main features to that obtained by the chemical soil analyses previously mentioned. The nature and significance of root competition have been elucidated in studies on natural forest regeneration and the growth and yield of naturally normal stands, and also by greenhouse experiments and various root investigations.
Fig. 9. Thickness of A-horizon and soil texture. (Symbols as in Fig. 8.)
The main results to date of forest soil research in Finland, as well as their practical significance, are briefly the following:
In climatically, topographically, and otherwise similar conditions, a more or less clear correlation exists between forest types and those soil properties that are most important from the standpoint of soil fertility. This correlation, in turn, proves that the forest types represent natural site classes. It does not alone, however, decide the question of the adaptation of forest types as a means for practical evaluation of forest sites.
As to their mineralogical composition, the forest soils in Finland are comparatively rich in nutrients. Although leaching has occurred, and is continuing, the changes due to this process and to other causes are slow. The leaching has been weakest in clay soils and most intense in moist sands.
For the present, at least, the possibilities of improving the general fertility of forest soils are not great. Nor do silvicultural measures, such as thinnings, regeneration cuttings, etc., have any considerable effect in this respect. The natural regeneration of the forest, on the other hand, may be considerably advanced by suitable treatment of the soil surface (harrowing, burning, preparation by chemicals, and other devices).
As to the future tasks for forest soil research, certain unfinished lines of investigation have already been mentioned. Among these, those concerning the competition among trees for living space in the soil are specially suited for laying a theoretical basis for applied silviculture, i.e., according to habitat. Of other individual problems, those related to leaching and improvement of the soil regeneration capacity especially demand additional investigations. In order to clarify the former problem, lysimeter experiments should be arranged and, with the aid of river water analyses, the amount of nutrients that are leached away into the sea should be estimated.
Above all, the extent to which forest soil productivity is dependent on soil characteristics should be further investigated, particularly microbiological soil properties and processes which, as yet, have been comparatively little studied in Finland. An important question is the classification of site quality on glaciofluvial deposits, especially in the dry heath forests in northern Finland. The objective of all investigations belonging to this group is to develop, through forest types, a method of evaluating site quality on the basis of soil characteristics.
The objectives of forest soil research vary in different countries according to differences in natural and economic conditions. There are, however, problems that are common to all countries noted for forestry, and scientists from different countries may benefit from co-operation on the solution of these. Among the subjects previously mentioned for forest soil research in Finland, the following seem in particular to belong to this category:
(1) evaluation of site quality through soil investigations;
(2) stability of soil fertility;
(3) competition among the trees of a stand for living space in the soil.
