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Need for fertilizers in wood production


A prepared at the request of FAO

TEMPERATURE, moisture, and fertility are three essential factors of the environment and habitat determining the growth performance of forest trees. There are other factors, too, but these three are major ones. Foresters, traditionally, have looked upon all these factors as being subject, at best, only to rather weak and mainly indirect control. The nature and magnitude of their influence, to be sure, have long been recognized, as controllable to some degree through silvicultural manipulations capable of altering stand density, structure, or composition. However, at any given latitude or altitude, the productivity of forest land has been accepted by the forester largely as a gift of the local environment, that is, climate, physiography, soil, etc. In large measure, these factors determine how far from the optimum for growth of any species in question the temperature, moisture, and nutrient levels are likely to be. At any rate, the productivity of forest land has been assumed to remain essentially stable, or susceptible to only minor change, during a single rotation.

Excluding swamps and bogs where water control may prove practicable, or the rare and rather limited areas where irrigation may prove justifiable, the temperature and moisture regimes in a forest environment must still be accepted largely in the degree and amount that Nature provides. Not so with fertility. Nutrient levels can be altered even drastically from existing levels, and raised presumably closer to optimum by adding chemicals to the land in sufficient quantity and appropriate kind. The practice, obviously, involves certain risks; it requires capital, wisdom and know-how, and a venturesome spirit. These are the principal ingredients required for control of the fertility vector of land productivity, and thus for exercising substantial control of wood production, and perhaps of wood quality, in greater measure than was thought possible in the past.

T. EWALD MAKI is Carl Alwin Schenck Professor of Forest Management, School of Forestry, North Carolina State University, Raleigh, North Carolina, United States of America. Note that this paper does not attempt to deal with use of fertilizers in forest nurseries, seed orchard development, game food production, and related practices for which methods and schedules are sufficiently known for the attainment of reasonably satisfactory results,.

Nevertheless, time and uncertainty are still proving to be very powerful forces in arriving at production decisions, and of the limited resources usually available merely for carrying out the traditional forest management activities, more often than not, suggestions to allocate any portion of meager operating or other available funds into forest manuring ventures are likely to encounter some resistance among land owners and managers alike. Have not they been told that forest trees, particularly coniferous species, are supposed to have capacity to attain commercial maturity on very meager nutrient budgets? Why tamper now with such a neat arrangement? It is not surprising, then, that in most forest regions unanimity concerning either the need or the feasibility of fertilizing forest land may be hard to find.

This lack of consensus, deepseated and widespread, is understandable, but regrettable too. It hinders development of a casual attitude toward forest fertilization from which too often tremendous and dramatic responses are expected; it retards adoption of the practice as simply another tool available to the forest manager to use as he does prescribed burning, herbicide application, site preparation, etc., on the occasions and in the situations that in his judgment are ripe and right.

This paper presents some general views about forest fertilization, its probable need in future wood production, and about the stages in forest land management and stand development where its employment may result in acceptable gains. It seems relevant first to consider some continuing developments in forestry and land use that would appear to substantially augment and strengthen the case for fertilizer use on forest land.

Major factors and considerations relating to forest fertilization

There are several major factors or considerations that would appear to bear quite heavily on the question of need for fertilizers in future wood production. Without attempting to indicate any order of priority or urgency, at least six factors or developments will be considered here, emphasizing why or how they might influence the need for fertilizers.


First, one might consider the instability or impermanence of land use in many parts of the world, since it relates to the question of land quality available for forest production. High quality land for wood growing purposes is not abundant now, and seems destined to suffer continual attrition, especially where new ground from forest is opened up for field crops, orchards, meadow, or pasture. With minor exceptions, where men have lived longest, particularly in the temperate and subtropic regions, lands are in the worst condition. The shifting pattern of land exploitation falls basically into much the same mold everywhere. Forests recede from centers of habitation and wood consumption; soils suffer abuse, becoming depleted in fertility, unfavorable in physical characteristics, or both. To re-establish forest growth on such depleted sites will usually require strenuous effort and special measures. In the majority of instances, where macroclimate favors forest growth, it is possible to establish commercial wood production on the decimated areas. However, to achieve satisfactory results in the shortest time requires almost certainly the use of fertilizers at time of regeneration, at some later stage, or both.


A second major consideration 'concerns the notable emphasis and effort currently directed toward, and expended on, forest genetics. Among other improvements, one expected outcome of this activity is the development of potentially more rapid-growing strains of timber trees through selection and breeding. Abundant experience with agricultural crops has demonstrated convincingly that the improved strains will give high yields of fruit or fodder, grain or grist, oil or fiber, only when grown on sufficiently fertile soil to capture more fully the genetic potential achieved through selection and breeding. The outcome from forest genetics is certain to be the same. How else could it be? It is certainly unrealistic to assume that rapid-growing strains of forest trees with thrifty crowns and thick bark are going to thrive on a meager mineral and nitrogen budget, despite any gains that may have been achieved also in photosynthetic efficiency. Thus, where large expenditures have gone toward developing improved strains of trees, in all probability it will be necessary to grow them only on good sites in order to reap more fully the rewards from their proved genetic potential. If sites of high native fertility are scarce or lacking, it will be necessary to develop them out of ordinary stuff, and this sort of site building, plus subsequent maintenance, is sure to involve a well-planned schedule of fertilizing.


A third major consideration involves the principle of resource substitution. There doubtless are numerous situations in which fertilizers could be used to replace land. In this type of situation or substitution, inputs of fertilizer could conceivably be added up to about the point where the cost of the inputs would equal the cost reductions resulting from use of less land. At least superficially considered, this form of resource swapping is beginning to look increasingly attractive because land prices are undergoing inflation and ad valorem taxes seem to be headed nowhere but up, while fertilizer materials are tending to remain at relatively stable price levels. Substitution need not be restricted to land only. Within the supply-radius of manufacturing plants dependent on wood there undoubtedly are forest tracts within which fertilizers could be applied to stimulate greater production of timber. Gradually savings would accrue because of shorter hauls, and these savings could then be plowed back into the manuring program for the tracts in question. Obviously, to develop prescriptions for effective employment of resource substitution will require critical study; at this stage it is not possible to cite specific illustrations of how well it would work in this context, but the idea has merit, and the principle is basically sound.


A fourth major consideration centers on the growing realization that extensive areas of peaty swamp and bog land comprise a practically forgotten domain and a seriously neglected province of possibly great potential for forest production. Very often peat lands are found to be too deficient in one or more of the major elements to support acceptable growth of commercial tree species. Nevertheless, it is believed they are destined for an exciting future because of their abundant moisture supply (usually excessive) and their large reserves of nitrogen (mainly in non mobile form). In many situations of this sort it is possible to manipulate the water supply toward more favorable or even optimum levels, and through concomitant improvement in temperature and aeration to mobilize sufficient nitrogen for satisfactory tree growth. With unlimited moisture, the necessary minerals and even additional nitrogen can be added to trigger the growth of suitable species to commercial dimensions. On the basis of present knowledge the peat and muck lands appear to -provide the sites where growth response from single applications of fertilizer can be sustained for the longest period, thus greatly increasing the likelihood that fertilizing coupled with proper water control will find wide application on peaty sites.


A fifth element to consider in this rationale is the mounting evidence that infertility of forest soils is widespread, though as yet a clear idea is lacking of how much it may be limiting wood production. What is known definitely now is that, in many forest stands where by accident or design the right, elements have been applied, the growth response has been so great as to indicate clearly that the available nutrients were not nearly at optimum levels or that serious deficiencies in one or more elements did exist. In fact, in numerous instances a single element or compound produces a surprisingly large improvement in growth. Moreover rather modest dosages often seem to satisfy the requirements for adequate growth stimulation, making aerial application more feasible. Under these circumstances, neglect or failure to employ fertilizers for boosting the production of wood begins to appear somewhat indefensible.


Finally, it seems pertinent to consider the assumption, long-prevailing among many foresters and others too, that forest soil management principally consists of maximizing the effects of forest detritus through the medium of silviculture. The litter-fall through annual recurrence, subsequent breakdown, liberation of minerals, and the associated energy transformations are supposed adequately to subserve the nutritional requirements of timber stands in perpetuity. This assumption has strong overtones of logic, and also elements of fiction. It may be recalled that about a century ago studies were initiated to determine whether fears of soil impoverishment through repeated litter removal (streunutzung) had any basis in fact, and the conclusions then indicated that the fears were founded and the dangers were real. Comprehensive investigations in recent years have generally confirmed the results of the earlier study, suggesting that repeated removal of timber crops, though involving only the usable portion of the bole and the bark enveloping it, could eventually reduce the nutrient supply in the soil to a significant degree. The depletion might reach critically low levels particularly in soils of relatively low native fertility; in them the consequent debilitating effects of lowered stand thrift could doubtless be averted by appropriate manuring. If the intent and need were to sustain a reasonable level of wood production in perpetuity on such sites, then manuring would surely be warranted.


In summary, six major considerations or elements of a rationale have been discussed in the preceding paragraphs to emphasize that, as exploitation of forest resources gives way to intensive management of forest lands, the need for serious consideration of forest fertilization grows more urgent and the justification much stronger. All six factors or elements have already, or soon will, become "facts of life" in modern forest land management. One is thus led to the conclusion that, if forestry remains a significant use of land and wood continues to be an important raw material, then the years ahead will see the application of fertilizers on a very substantial scale for the express purpose of increasing the production of wood.

Having thus prejudged the case for fertilizer use in the wood production context, it appears mandatory to examine some results of past study and experience to show how feasible the practice appears to be at different stages of stand development and land management, and also, if possible, to determine the basic soundness of our expectations and prognoses. In other words: "Now that we have buried the corpse, let's have a look at it."

Some experiences and results with forest land fertilization


Published results of investigations on fertilizer use in stand establishment and stand growth stimulation alone number hundreds of citations, most of them based on some sort of field trials. The individuals who appear to have responded to some overpowering urge to spread fertilizer salts in the woods are legion. Unlike investigations of tree nutrition in the laboratory, greenhouse, nursery, or seed orchard where fairly well-tested procedures often are followed, and unlike existing standardized methodologies for mensuration, silviculture, progeny-testing and related field installations, there are as yet no comparable ground rules or operational procedures to guide field experimentation in forest fertilization. Individuality and imagination, particularly the latter, are extremely valuable assets in research, but in past forest fertilization studies there appears not to have been enough of these of the right kind. Future research in this area would profit from adoption of a few basic ground rules and requirements, thus ensuring a sounder basis for comparison and assessment of reported results, and rendering information retrieval easier and more meaningful.


The probable conservation of nutrients through recycling of annual litter fall, the usually extensive mass of soil available as rhizosphere, the many years tree crops require as a rule to reach usable size, and similar arguments have helped to create a comfortable feeling that worry about nutrient drain is needless. The lullaby goes something like: "Forester, sleep well tonight; weathering, even of primary minerals, is surely rapid enough to keep apace of removal suffered in the harvest of bole and bark." It is not the intent here to undertake any detailed analysis of this somewhat controversial subject. However, Table I provides a basis for making judgments about the magnitude of values represented in the drain of some elements.

These data do not include the sizable quantities of nutrients represented in the root systems, the living crown, and, for the most part, the unmerchantable portion of the bole. At harvest time these quantities are left in the woods to be liberated and made available again when the respective parts decay, except, of course, the stumps of species possessing the capacity to sprout, thus keeping the nutrients in them more or less out of circulation perhaps for several rotations. But, as can be seen from the table, the drain of nutrients from the rhizosphere of the forest site appears substantial even when only the bole and its bark are harvested. Offhand, the nitrogen loss may be less serious than the removal of some other elements such as potash, since current evidence suggests that soil fungi have the capacity to fix atmospheric nitrogen, resulting actually in a buildup of N in the soil while the litter of the forest floor is being broken down. In the absence of specific data on probable rates of nutrient replenishment, there is room for numerous opinions about how serious are the consequences of the indicated drain on the productivity of forest sites. One opinion is that, if it is necessary to sustain vigorous growth of successive stands of timber, it will also be necessary to add fertilizers at appropriate intervals in the amounts and the kinds suspected or determined to be in short supply.


Species, species group, or site


Nutrient drain in bole and bark







Pounds per acre¹

Picea abies






Kvist (1964)







Rennie (1957)

Other conifers






Rennie (1957)



1 266




Rennie (1957)

Pinus silvestris






Ovington (1957, 1959)

Pinus radiata






Will (1964)

Pinus resinosa

Good site






Madgwick (1962)

Poor site






Madgwick (1962)

Pinus resinosa²






Heiberg et al. (1959)

Tanbark oak






von Schroeder (1890)

¹ 1 pound per acre = 1.12 kilograms per hectare, - ² Midpoints of the ranges reported by the authors.


Over the world much land has been ruined for agricultural use through ignorance or neglect of soil management and through lack of wise husbandry of the land. Under such circumstances tree planting has frequently appealed as the best hope and the logical restorative measure. Too often in the past foresters have undertaken these tasks of land restoration, including critical needs for quick initial soil stabilization, in the mulish faith that trees alone, stuck in the naked earth at some economically prejudged spacing, would stabilize the soil, improve the site, and at the same time grow to usable size without benefit of mulching, subsoiling, fertilizing or other ameliorative measures. The "joker" is that frequently abused and abandoned land has deteriorated so much in physical condition and fertility that it does not have the strength to produce a merchantable crop of trees without the aid of special measures.

If the bedrock is not too close to the surface, a single treatment, consisting of (a) deep subsoiling on the contour and (b) introduction simultaneously of fertilizers and organic matter into the subsoil slit, will have remarkable restorative effects on site productivity. In this procedure trees may be planted immediately, not in the subsoil slit, but 3 to 4 inches (7 to 10 centimeters) below it, i.e., on the downhill side. For some drastic sites, particularly where rapid achievement of soil stabilization is paramount, it is better to apply a more general restorative treatment consisting, perhaps, of overall cultivation, fertilization, liming where needed, mulching or sowing of a nurse crop for one or more years before undertaking to plant the trees judged suitable for the site in question. On basically infertile soils, as for example in southeast Germany where sites have been further impoverished by the old practice of streunutzung, the existing stand of decrepit timber is first removed, then the site is limed and sown to some legume (lupine, broom, black locust or even alder), followed by planting to pine.

Lime applications tend to be rather heroic in quantity, but apparently the legumes can use it to advantage and, where the legume growth succeeds, the effect on pines generally is very marked. In general, for renovating harsh sites, except when using unusually responsive species such as Robinia pseudoacacia, it is better to apply fertilizers (and lime where needed) to the soilstabilizing and the soiling crops rather than directly to the tree seedlings at planting time. Finally, with respect to harsh-site restoration problems, critical study has been meager, but valid experience has Dot. On the basis largely of experience, it is possible to assert that if it is necessary to grow timber on abused. and impoverished soil, some extra expenditures are justified, including what is necessary for appropriate fertilizing to ensure the vigorous development (if the established stand of trees.


Application of fertilizers to seedlings at the very moment they are being set in the ground is believed to provide a clear economic advantage by eliminating the need of a separate visit to the planting site later. Yet the weight of opinion and recorded evidence are preponderantly against fertilizing as an integral part of the planting operation (Wakeley, 1954; Lunt, 1946; Zehetmayr, 1960; Tamm, 1965; Viro¹). The objections in part have a biological basis. The critical need of the seedling at time of planting is for moisture not salts. During this period, when the seedling is struggling to recover from transplanting shock, fertilizing may do little more than stimulate vigorous growth of adjacent grass and weeds which reduce the moisture supply and may quickly overtop the little tree, particularly if it is a slow-starting species. In part, the objections are raised because the economic aspects appear so questionable. The expenditures for fertilizing, though perhaps modest-appearing at the start, accumulate carrying charges compounded over many years, often decades. Even under the best circumstances the observed responses, particularly with most coniferous species, are frequently not large. What does, say for example, a 20 percent increase in height of treated seedlings over controls at the end of one or two years after planting really amount to at some specified condition of maturity years hence, even if now it is very highly significant statistically? As yet there are no convincing answers to the types of arguments that have been advanced against this practice.

¹ Unpublished manuscripts of P.J. Viro, Finnish Forest Research Institute Helsinki. Citations are merely a small sample of many similar reports.

Despite the objections, both biologic and economic, raised against the practice of fertilizing seedlings at planting time, there still are some reasons why exploration of this practice deserves to be continued. Dipping seedling roots in clay slurries has yielded marked protective benefits, and when fortified, the clay may produce worthwhile starter effects (Slocum and Maki, 1956; Tabor and Davey, 1964). Deep placement of certain fertilizers has given beneficial growth stimulation to pine seedlings without invigorating weed and grass competition (Hughes and Jackson, 1962). Use of slow-acting fertilizers has resulted in good growth response of Douglas fir without danger of salt burn (Austin and Strand, 1960), and development of mechanical devices shows promise for automating application of fertilizers at time of planting (Staroska et al., 1962; Anon, 1963). It is clear, also, that essentially all the unsatisfactory experiences and results in this context have arisen from treatments of coniferous planting stock. It may be worth noting here that some tremendously impressive results have been obtained from application of fertilizer to Robinia pseudoacacia at the time of planting. This species appears to have an almost -unlimited capacity to respond to a variety of fertilizer salts when supplied in sufficient dosages. It seems reasonable to assume that there are several other species in this world with a like capacity to respond and a like tolerance of salts as has black locust; if such species were also found to produce usable wood, they would provide a valuable addition for programs requiring fertilizer application to be integrated with the planting operation. Applying fertilizers at any stage of plantation establishment after planting, especially when the seedlings are ready to compete with associated vegetation, or if placement is such that it benefits the seedlings and not adjacent weeds, grass, and brush, has frequently been shown to improve the vigor of the seedlings. In brief, from a biologic standpoint, fertilizing at time of plantation establishment, when properly done, can be definitely beneficial. Whether it proves to be economically warranted is still a matter of conjecture. Currently there is speculation, not wholly baseless, that initially observed small differences will expand gradually until they represent 5- to 10-foot height differences by the time the stand reaches marketable size; if so, then there is, indeed, hope of economic justification for this practice.


The management of peat soils for forest production is not simple, for it -usually involves effecting suitable control of the water economy as well as improving the nutrient status (Heikurainen, 1964). Some of the older and biologically interesting fertilization experiments in peat swamps are the wood-ash treated plots in Norra Hällmyren, Sweden (Tamm. & Carbonnier, 1961) and those at Kaakkosuo, Finland (Huikari, 1953). While perhaps of little practical significance because of the large quantities of ashes applied, these experiments do indicate the large biologic potential of the peat soils for tree growth and tell something about the long-lasting effects of this type of treatment. In the regeneration of swamps with nitrogen-rich peats, the application of phosphorus and potassium at relatively low rates to plantings of Pinus silvestris one year after planting has resulted in very striking benefits for height growth (Huikari, 1961; Paarlahti, 1965). For some types of peat, fertilizers appear essential for triggering tree growth to commercial dimensions, In a stand of Betula pubescens with an understory of Picea abies on drained peat soil, potassium at 100 kilograms per hectare completely corrected a chlorotic condition of the birch and more than doubled volume growth (Tamm, 1960). Even where water control through appropriate ditching alone may substantially stimulate tree growth, manuring can produce marked additive effects in wood production. On many peat soils the biologic justification for manuring seems already well beyond question; the ultimate economic justification in many instances is still conjectural


Not, more than 25 years ago, there was little, if any, discussion that some day manuring established stands for the primary purpose of stimulating volume increment would be an acceptable silvicultural practice. Today it is being practiced in some countries on a commercial scale in the firm belief that it pays. One company in Sweden, for example, fertilized some 37,000 acres (15,000 hectares) in 1965, and plans to fertilize nearly 100,000 acres (40,000 hectares) in 1966 (von Schoultz, 1966). Current manuring practice in northern Europe on upland sites is confined mainly to well-stocked stands occupying sites of average quality and approaching a size suitable for a substantial revenue thinning or a final harvest cut. Manuring is done usually not more than 10 years ahead of the thinning or harvesting. Nitrogen alone appears to suffice for producing annual volume increases of 20 to 30 cubic feet per acre (1.4 to 2.1 cubic meters per hectare) above that of check areas of the same site quality. For example, two applications of urea, five years apart, each at rates of about 65 to 100 pounds of nitrogen per acre (75 to 115 kilograms per hectare) have produced the extra growth for the 10-year period (Tamm, 1962, 1966; Viro, 1962, 1966; Paarlahti, 1964). High stumpage prices make manuring more attractive, especially in northern Europe (Viro, 1966; von Schoultz, 1966), and the relatively light dosages required make aerial application more feasible. Obviously, the short period prior to a major thinning or final harvest keeps carrying charges low. In the United States, despite lower stumpage rates, interest in manuring established stands is definitely rising. As one example, Curtis (1964) reported a cost of only U.S. $7.25 per acre ($18.25 per hectare) for aerial application of phosphate fertilizer at 200 pounds per acre (230 kilograms per hectare) to a 630-acre (260-hectare) plantation of slash pine; lie estimated this venture as having the possibility of returning 12 percent on the investment.

With prudent selection of sites for treatment, manuring invariably stimulates basal area increment, but the effect of this growth increase is not yet fully understood; very likely it depends on the species, climate, physiography, or related factors. Posey (1964) found nitrogen significantly stimulated loblolly pine volume growth in a Piedmont upland site in North Carolina, but the wood formed after manuring was lower in specific gravity, with thinner cell walls and shorter tracheids. Neither Tamm (1962) in Sweden nor Jensen et al. (1964) in Finland have observed any adverse effects on wood properties traceable to the sudden increase in growth, and the latter have not found sulfate pulp to be adversely affected by the wood from such trees.

Fertilizing entails risks, which though not necessarily universal, are no less real. It can greatly increase the incidence of fusiform rust infection in some pine species; it makes seedlings and saplings more susceptible to browse damage by rabbits, deer, moose, etc.; it makes cones and seed more attractive to squirrels and related predators; it may adversely affect the quality of water flowing from fertilized watersheds or treated swamps. These are merely examples of the nature and breadth of the spectrum of liabilities that the practice of manuring may involve.

Despite the likelihood that the practitioner will encounter a variety of perplexing problems and disappointing results as manuring comes into wider use, the current outlook and prognosis are that the practice will continue to expand rapidly unless stumpage prices drop or fertilizer prices rise, or both. Application already seems well ahead of solid research knowledge that is essential for guiding the -practice toward more precise biologic and economic optima in the variety of situations encountered in operating on a practical scale. Be that as it may, today it is fulfilling a need, genuine or chimerical, and its momentum has brought it rapidly to a place among silvicultural and forest management operations where it can no longer be dismissed as someone's wild dream.


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