Soil Fertility Lab: Liming, fertilization, and soil testing

In lecture we discussed the concept of Liebig’s Law of the Minimum and its application to assessment of plant nutrient deficiencies. The practical illustration of that concept was provided by graphing a measure of plant growth on the Y axis against plant tissue nutrient concentration on the X axis. The plant is the integrator of the environment; tissue nutrient concentrations reflect the capacity of that environment to supply essential elements to the plant. The same concept can be applied using soil nutrient concentrations instead of foliar nutrient concentrations. This lab deals with practical applications of Liebig's law through application of fertilizers to supply those nutrients that may be limiting tree growth.

Ultimately, the relationship between tree growth and nutrient (foliar or soil) concentration can be used to determine if application of nutrient elements would increase growth. In forestry applications, this relationship may also provide some indication of the effects of harvesting on the ecosystem, or, may indicate the fertility status of a proposed planting site. In the latter case, the forester can decide which species would be more appropriate to plant with regards to nutrient requirements. Deficiency can be recognized by comparing lab results with the graph of plant growth vs. nutrient concentration.

Soil testing refers to the general procedure used to assess soil fertility. Soil samples selected from the rooting zone of the area in question are analyzed in the laboratory for a variety of exchangeable nutrients, texture, and organic matter content. In and of themselves, the soil chemical data have no intrinsic value. The nutrient concentrations measured from laboratory analysis of soil samples do not provide a direct measure of plant available nutrients; we do not know exactly what the plant root 'sees' as available. However, correlation of soil test results with tree or crop productivity provides a tool that is useful to predict likelihood of response to fertilization. Once this relationship is established, nutrient concentration data (foliar or soil) can be used to assess likelihood of fertilizer response. Without this relationship, nutrient concentration data are of no use. It is important to point out that this empirical relationship requires considerable time and effort to develop.

In comparison to trees, nutrient uptake for annual crops is more easily related to exchangeable soil nutrients. Yield data for crops are obtained each year, in contrast to only once in 30, 60, or 120 years for trees. Although weather influences crop yields, the effect is limited to the year during which crops are grown. The extended rotation for trees effectively increases the variability contributed by climate. Tree roots occupy the soil rooting volume effectively and for a long period of time. They are able to extract nutrients in forms that are not readily available to field crops, and they are able to extract these nutrients from great depths in the profile. Hence, it is difficult to simulate nutrient absorption by tree roots. Nevertheless, there has been tremendous success in developing tools to predict the likelihood of response to forest fertilization on the basis of plant and soil nutrient analyses. Considering (a) the complexity of forest systems in both space and time relative to agricultural systems, and (b) the length of the rotation for trees, the level of success that has been attained in predicting probability of growth response from forest fertilization is remarkable.

Intensive management has reduced rotation age tremendously. One of the industrial forest corporations in the southeast manages loblolly pine pulpwood on an 11 year rotation. During that time period, fertilizer is applied three times and pests are controlled annually. Culture of willow plantations for bioenergy here in the northeast is based on a three year cycle. Cuttings are planted in the spring and the plants are coppiced the following fall to encourage vigorous growth the following spring. Two growing seasons later, the plantations are harvested. The material is chipped and delivered to electricity generating plants where it replaces some of the coal fuel.

The two examples of intensive management described above require a large capital investment for site preparation and inputs of nutrients, approaching agriculture in the level of management. Dr. Earl Stone showed that the development of forest soils information is a direct result of the increase in intensity of management. As the level of investment in forest practices increases, the risks of failure become greater. It would be foolish to invest $400 per acre to establish a plantation of fast growing trees on a site where the soil properties would not support a reasonable level of growth. It would be even worse it the plantation failed due to poor soil conditions. When those levels of invesment are involved, adequate understanding of the soil system is an absolute necessity to insure a positive rate of return.

After becoming familiar with Soil Survey Reports, you may wonder why the seemingly vast array of soil chemistry data ((referred to as soil characterization data) included in most reports cannot be used to identify nutrient deficiencies. The wide variation in exchangeable nutrients within map units combined with the wide array of soil map units included in a typical soil survey makes it impossible to use the data for this purpose. Consider how the information for soil map units is collected i the field. Representative soil profiles of a soil series, termed modal profiles, are described and sampled in detail within soil survey reports. After developing a soil map for the hillside at Heiberg Forest and comparing your results to those presented in the soil map, you can appreciate the degree of natural variation that occurs in soil morphology alone. The variability in soil chemical data is an order of magnitude greater than that for the morphological data. This means that the relationship between tree growth and nutrient concentrations (soil and plant) must be developed on a site specific basis.

It is clear that knowledge of the nutrient requirements of the various tree species enable us to make wise forest management decisions. This information is generated through sound research. Much less is known about fertility status and management of the soils of developing countries. Unfortunately, this is where the most rapidly growing segments of the world population exist. If world food and fiber supply are to significantly increase, effort must be made to develop that knowledge. The technology of temperate region soils and crop production cannot be directly applied to the developing countries because of the differences in soil physical and chemical properties.

Two continents already fully utilize much of the available arable land - Asia and Europe. However, large acreages of potentially arable land exist on the African continent in the Congo Basin and in South America in the Amazon Basin. One of the more common examples of the misapplication of western agricultural technology is the clearing of lateritic soils in the tropics. Lateritic soils, one of many soil types in the tropics, require special management practices for their preservation. These soils cannot be laid bare by intensive cultivation following removal of forest cover because they dry, harden, and erode, reducing their potential productivity. The system of shifting agriculture practiced by native peoples over the centuries more or less preserved those soils. After clearing, the soils were primitively farmed for several years until readily available nutrients were depleted and yields declined. Only a small area was cleared at any one time. After nutrient reserves were depleted, the land was abandoned and another patch of forest was cleared. The forest reestablished in the abandoned field and nutrients were replenished. This type of inefficient farming cannot support the large present day population.

Agroforestry programs have been initiated in several areas an attempt to solve some of these problems. Attempts are being made to develop a system of compatible food crops with a protective overstory of trees. The overstory protects the soil from raindrop splash, crusting and drying of the soil surface. By careful management of underplanted crop species and density, competition between the nurse trees and the crop is controlled. Expansion of these systems could increase food production in the some areas. Fertilization, and soil erosion control are important.

It causes some embarrassment for to us to know that only 15 percent of the world fertilizer production is used in the areas where over half the world population lives. One of the prime consumers of fertilizer is the golf course industry. It is evident that the technologically advanced countries are the ones which can make the greatest changes in management inputs (i.e. fertilizers, genetic engineering, pesticide applications, etc.). We can probably increase yields still further even in the United States and increase the acreage under cultivation. However, expansion will be on progressively poorer soils since the best lands are already under cultivation or have been lost to other non-farm uses. The primary soil problem - erosion, must still be controlled.


There at two ways to alter nutrient status of soils: supply nutrients in the form of fertilizer, or change alter soil reaction adding acidic or basic substances. Ground limestone is the most common material used to decrease soil acidity (raise soil pH). When applied to the soil, CaCO3 reacts with water. A Ca ion and an OH ion react to form an HCO3 ion. H ions from the soil solution then combine with the OH ion to form water. The Ca ion may be adsorbed onto colloid surfaces, or may be absorbed by plant roots or microorganisms. The H ions displaced from the exchange surfaces, or the reserve acidity, pass into active acidity (solution H ions) replacing those neutralized by the OH ions in the formation of water. In acid soils, the H ion concentration in solution is related to the hydrolysis of Al+++ or hydroxy-Al, or hydroxy-Fe+++ ions. Their hydrolysis in turn is influenced by the amount of clay and organic matter in the soil. The continued removal of H ions from the soil solution will ultimately result in the precipitation of the Al+++ and Fe+++ ions and their replacement on the adsorption sites with Ca and/or Mg and other 'base' cations.

The primary effect of liming on soils is the reduction in the activity or solubility of Al and Mn. Both of these ions in anything other than very low concentrations are toxic to most plants. Other indirect effects include alteration of phosphorus availability, with the maximum occurring near a neutral pH. Micronutrient availability is also very pH sensitive. Many micronutrients may be toxic at low pH. Liming reduces their solubility. The increase in pH from liming also favors nitrification processes and nitrogen fixation. In addition, liming also has beneficial effects on soil structure from flocculation.

Most forest tree species do not have a high lime requirement, but liming may raise the pH so that significant amounts of additional nutrients may become available. This may be critical on intensively managed soils such as short-rotation pulpwood plantations. Liming also increases the effectiveness of applied fertilizers, In some cases, the liming may simply correct a deficiency of Ca and Mg. A good example of this is in the fertilization work carried out on the High Peaks of the Adirondacks where an attempt has been made to increase the resistance of the native vegetation to wear and tear from hiker impact. A dolomitic limestone was applied since it contains both calcium and magnesium carbonates.


The fertilizer ratio and the fertilizer guarantee were discussed in lecture. Potash and phosphoric acid were more acceptable and understandable to farmers. Precise analytical methods in use today were not common 50 - 60 years ago. Today, we use elemental concentrations as the basis for fertilizer application calculations.

N tends to be the most limiting element to most forest systems. After N, P tends to be limiting. There are notable cases of K deficiency on outwash plains of New York. Although the sands are plentiful in terms of total K, it is bound in unavailable forms in crystalline structure of minerals and is not converted to an available form (K+ ion) sufficiently rapidly to meet the needs of the trees on a daily basis. Complete fertilizers (N, P, K) are often applied to soils in order to maintain a proper balance of nitrogen, phosphorus, and potassium.

Present interest in forest fertilization is at an all-time high. The trend towards more intensive forest management has made forest fertilization an operational practice in the Pacific Northwest (N application) as well as in the southeastern U.S., where P is the element most commonly applied often in combination with N. Wood has increased tremendously in value so that it can compete economically with the growth of some other crops. Hybrid or superior trees may require more nutrients, increasing the need for nutrient inputs either in the form of fertilizer or through the development of nutrient enhancing nurse crops such as red alder. Genetics also controls the response to fertilization. Root distribution across and within the soil profile is also a factor. Extensive root grafting between trees may enhance the effect of fertilization. Another point to consider is the eventual use of the tree and what attribute do we desire, and does that attribute show a response to fertilization. Thus, fertilization programs would be different for Christmas tree production as compared to seed production, pulpwood, or timber production.

Fertilizers also have the characteristic of being slow- or rapid-release materials. Rapid-release fertilizers are composed of the more soluble fertilizer carriers and also may be uncoated so that the granules dissolve rapidly. The slow-release fertilizers are composed of the less soluble fertilizer materials and are usually coated with resins or plastic which control the dissolution of the nutrient compounds. A variety of granule sizes will have the same effect as coating since the smallest granules will dissolve the most rapidly. Fertilizers purchased for pastures and lawns will be of the slow-release type, or a mix of both. Fertilizers are hygroscopic and therefore must be protected from moisture and humid conditions. Plastic-lined bags prevent the absorption of moisture.

It must be remembered that fertilizers are acid-forming when applied to the soil, especially those containing nitrogen materials. In the process of nitrification of the fertilizer nitrogen, H ions are released which contribute to the hydrogen ion activity. Thus, liming may be required to offset this effect. Sometimes the fertilizer manufacturer includes some ground limestone in the fertilizer for this purpose.

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