gübre

Jimmy J. Street and Gerald Kidder2
There are 16 nutrient elements that have been proven to be essential for the growth and reproduction of plants ( Table 1 ). Thirteen of these essential elements, which may be supplied by the soil or supplemented by fertilizers, are generally divided into two groups. The macronutrients are nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg). The second group of essential elements is called micronutrients because those elements are required in small (micro) amounts by plants. They include manganese (Mn), iron (Fe), boron (B), zinc (Zn), copper (Cu), molybdenum (Mo), and chlorine (Cl). Although these elements are frequently referred to as minor or trace elements, the term "micronutrient" is preferred.

Considerable discussion can be generated over what criteria defines an essential element. In the context of this presentation, we caution that the fact that an element is essential does not mean that it needs to be added to soil as fertilizer.
Since plants obtain carbon, hydrogen, and oxygen from the air and water, there is little control over the availability of these nutrients. For most plants, the dry tissue is composed of 94 percent or more of these three elements. The other 13 elements combined represent less than 6 percent of the plant dry matter. Yet, crop production is frequently reduced and growth limited by a deficiency of one or more of these 13 elements.

The availability of nutrients to plants from the soil is strongly influenced by various mechanisms ofremoval, fixation, and release. One of the most important soil properties that affects the availability of nutrients is soil pH, a measure of the acidity or alkalinity of the soil.

Macronutrients

Nitrogen
Nitrogen is probably the nutrient that most often limits plant growth. The bulk of soil N is found within 2 feet of the surface. Soil N is present in three major forms.

1. Elemental N , found in a gaseous form in the soil atmosphere, is of direct significance to plants only as it may be involved in bacterial fixation. (e.g., symbiotic N fixation associated with a legume plant or small amounts of N fixation by free-living bacteria).
2. Organic N makes up about 5 percent of the soil organic matter (humus) by weight and about 98 percent of the total soil N. Although organic N is not available to plants, soil organisms convert a portion of it each year to inorganic forms (ammonium and nitrate) that are readily used by plants. Organic N fertilizers (e.g., manures and biosolids) are popular for lawns and gardens because of their "slow release" and long-lasting properties. The relatively low concentration of N in organic materials means several tons per acre are required to supply sufficient N for commercial field crop production. The economics of transporting these bulky materials are a major factor when considering their use as an N source.
3. Nitrogen in fertilizers for agricultural crops is largely inorganic N consisting of three types: ammonium (NH 4 + ), nitrate (NO 3 - ), and urea (CO(NH 2 ) 2 ). Although urea is an organic N fertilizer, it is rapidly converted to the ammonium form within a short time after exposure to moist, aerated soil. Therefore, under most conditions urea acts more like inorganic ammonium fertilizers than like natural organic fertilizers.

In warm, moist soils with a pH above 5.0, the majority of ammonium N is converted to nitrate N by soil organisms rather quickly (within days). Therefore, most N taken up by plants is in the NO 3 - form, although NH 4 + is taken up when present in the soil solution. Thenitrate ion (NO 3 - ) carries a negative charge which prevents its retention by the negatively- charged soil colloids. Since it is soluble and mobile, the nitrate ion is readily and easily available to plants.
Nitrate moves in the soil solution and can be leached below the plant root zone when soil moisture is excessive. The loss of nitrate by leaching is a common problem on coarse-textured, sandy soils of Florida. Leaching losses of fertilizer N are minimal when rates of application conform to recommendations consistent with the yield potential for the crop and soil in question. Nitrate N is also subject to denitrification, a process in which the nitrate ion (NO 3 - ) is reduced through several intermediate steps to a gaseous N oxide or to elemental N.

Nitrogen is a constituent of all living cells and is a necessary part of all proteins, enzymes and metabolic processes involved in the synthesis and transfer of energy. Nitrogen is a structural part of chlorophyll, the green pigment of the plant that is responsible for photosynthesis. The energy of light is combined with water and carbon dioxide through the process of photosynthesis to form simple carbohydrates essential for plant growth. Other functions of N include stimulating plants into rapid, vigorous growth, increasing seed and fruit yield and improving the quality of leaf and forage crops.

Phosphorus

Like N, phosphorus (P) is an essential part of the process of photosynthesis. Plants use the energy of sunlight, and P must be present in the active portions of the plant for this energy transfer to be made and for photosynthesis to occur.

The immediate source of P for plants is that which is dissolved in the soil solution. Plants absorb P primarily as the H 2 PO 4 - and HPO 4 = ions which are predominant in most soils. The H 2 PO 4 - ion is more readily absorbed than the HPO 4 = by most plants. A soil solution containing only a few parts per million of phosphate ions is usually considered adequate for plant growth. Concentrations of phosphate ions in the soil solution may be as low as 0.001 parts per million. Phosphate ions are absorbed from the soil solution and used by plants. The soil solution is replenished fromsoil minerals, soil organic matter decomposition or applied fertilizers.

In young plants, P is most abundant in tissue at the growing point. It is readily translocated (moved about) from older tissue to younger tissue, and as plants mature, most of the element moves into the seeds and/or fruits. P is responsible for such characteristics of plant growth as utilization of starch and sugar, cell nucleus formation, cell division and multiplication, fat and albumin formation, cell organization, and transfer of heredity.

Potassium

Potassium (K) is absorbed by plants in larger amounts than any other mineral element except N and, in some cases, Ca. Potassium is supplied to plants by soil minerals, organic materials, and inorganic fertilizer. Due to the highly weathered status of Florida soils, their K supplying power is quite low in most cases. Potassium occurs in the soil solution as a monovalent cation (K + ). The cation exchange capacity (CEC) of the soil controls the retention of K + and in very sandy soils (low cation exchange capacity) under high rainfall, K is subject to leaching losses.

Potassium, unlike N and P, is not found in organic combination with plant tissues. Potassium plays an essential role in the metabolic processes of plants and is required in adequate amounts in several enzymatic reactions, particularly those involving the adenosine phosphates (ATP and ADP), which are the energy carriers in the metabolic processes of both plants and animals. Potassium also is essential in carbohydrate metabolism, a process by which energy is obtained from sugar. There is evidence that K also plays a role in photosynthesis and protein synthesis.

Calcium

Calcium (Ca) occurs in the soil solution as a divalent cation (Ca ++ ). It is supplied to plants by soil minerals, organic materials, fertilizers, and by liming materials. There is a strong preference for Ca ++ on the cation exchange sites of most soils and it is the predominant cation in most soils with a pH of 6.0 or higher.

Calcium, an essential part of plant cell wall structure, provides for normal transport and retention of other elements as well as strength in the plant. It is also thought to counteract the effect of alkali salts and organic acids within a plant. Calcium is absorbed as the cation Ca ++ and exists in a delicate balance with Mg and K in the plant. Too much of any one of these elements may cause insufficiencies of the other two.

Magnesium

Soil minerals, organic material, fertilizers, and dolomitic limestone are sources of magnesium (Mg) for plants. Magnesium occurs as a divalent cation (Mg ++ ) and is held on the exchange sites like calcium (Ca ++ ) and potassium (K + ).

Magnesium is part of the chlorophyll in all green plants and essential for photosynthesis. It also helps activate many plant enzymes needed for growth. Magnesium, a relatively mobile element in the plant, is absorbed as the cation Mg ++ and can be readily translocated from older to younger plant parts in the event of a deficiency.

Sulfur

In most soils, sulfur (S) is present primarily in the organic fraction which becomes available upon decomposition of organic matter and crop residues. The available sulfate (SO 4 = ) ion remains in soil solution much like the nitrate (NO 3 - ) ion until it is taken up by the plant. In this form it is subject to leaching as well as microbial immobilization. In water-logged soils, it may be reduced to elemental S or other unavailable forms. Sulfur may be supplied to the soil from the atmosphere in rainwater. It is also added in some fertilizers as an impurity, especially the lower grade fertilizers. The use of gypsum (CaSO 4 ) also increases soil S levels.

Sulfur is taken up by plants primarily in the form of sulfate (S0 4 = ) ions and reduced and assembled into organic compounds. It is a constituent of the amino acids cystine, cysteine, and methionine and, hence, proteins that contain these amino acids. It is found in vitamins, enzymes and coenzymes.

Sulfur is also present in glycosides which give the characteristic odors and flavors in mustard, onion, and garlic plants. It is required for nodulation and Nfixation of legumes. As the sulfate ion, it may be responsible for activating some enzymes.

Micronutrients

Of the 16 elements known to be essential for plant growth, seven are required in such small quantities that they are referred to as "micronutrients". These are Fe, Mn, Zn, Cu, B, Mo, and Cl.

Micronutrient deficiencies are most apt to limit crop growth under the following conditions: (1) highly leached acid sandy soil, (2) muck soils, (3) soils high in pH or lime content, and (4) soils that have been intensively cropped and heavily fertilized with macronutrients. Four of the micronutrients occur predominantly as cations in the soil solution. They are iron (Fe +++ ), copper (Cu ++ ), manganese (Mn ++ ), and zinc (Zn ++ ). Two occur predominantly as anions. These are molybdenum (MoO 4 = ) and chlorine (Cl - ). Boron occurs as the neutral species H 3 BO 3 .

Manganese (Mn) is mainly absorbed by plants in the ionic form Mn ++ . Manganese may substitute for Mg by activating certain phosphate-transferring enzymes, which in turn affect many metabolic processes. High Mn concentration may induce Fe deficiency in plants.
Manganese availability is closely related to the degree of soil acidity. Deficient plants are usually found on slightly acid or alkaline soils. Liming Florida soils to pH above 6.5 frequently causes Mn deficiency.
Iron (Fe) is a constituent of many organic compounds in plants. It is essential for the synthesis of chlorophyll. Iron deficiency is often induced by alkaline soil pH and can be induced by high levels of Mn. High Fe can also cause Mn deficiency.
Copper (Cu) is essential for growth and activates many enzymes. A deficiency interferes with protein synthesis and causes a buildup of soluble N compounds. Excess quantities of Cu may also induce Fe deficiency.
Zinc (Zn) is essential for plant growth because it controls the synthesis of indoleacetic acid, which dramatically regulates plant growth. Zinc is also active in many enzymatic reactions.
Boron (B) primarily regulates the metabolism of carbohydrates in plants. The need varies greatly with different crops. Rates required for responsive crops may cause serious damage to B-sensitive crops. Boron deficiency may occur on both alkaline and acid soils but is more prevalent on the calcareous, alkaline soils.
Molybdenum (Mo) functions largely in the enzyme systems of N fixation and nitrate reduction. Plants which can neither fix N nor incorporate nitrate into their metabolic system because of inadequate Mo become N deficient. Molybdenum is required in minute amounts.
Chlorine (Cl) is needed in relatively large quantities in plant nutrition. However, the abundance of Cl from many sources in the environment means that Cl deficiencies in plants are rare. Excess and toxicity of Cl are more frequently occurring problems than are deficiencies.

References

The following references by the American Society of Agronomy and the Soil Science Society of America are recommended as comprehensive:

• Nitrogen in Crop Production (1984)
• Potassium in Agriculture (1985)
• The Role of Phosphorus in Agriculture (1980)
• Micronutrients in Agriculture (2nd ed., 1991)

Tables

Footnotes

1. This document is Fact Sheet SL-8, one of a series of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published: January 1989. Revised: June 1997. Please visit the FAIRS Website at http://hammock.ifas.ufl.edu.
2. Jimmy J. Street, former associate professor; Gerald Kidder, professor, Soil and Water Science Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611-0290.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.

Copyright Information

This document is copyrighted by the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS) for the people of the State of Florida. UF/IFAS retains all rights under all conventions, but permits free reproduction by all agents and offices of the Cooperative Extension Service and the people of the State of Florida. Permission is granted to others to use these materials in part or in full for educational purposes, provided that full credit is given to the UF/IFAS, citing the publication, its source, and date of publication.