Determining the Need for Micronutrients

Diagnosing a micronutrient deficiency can be a difficult and time consuming process. To identify a micronutrient deficiency follow these steps :

  • Ensure that poor crop growth is not the result of a macronutrient deficiency, drought, salinity, disease or insect problem, herbicide injury or some physiological problem.

  • Find out if a micronutrient deficiency has been identified before in a particular crop or soil type in the area.

  • Examine the affected crop for specific micronutrient deficiency symptoms.

  • Take separate soil samples from both the affected and unaffected areas for complete analysis, including micronutrients.

  • Send plant tissue samples from both the affected and unaffected areas for complete analysis that includes tests for micronutrient levels.

  • If all indications point to a micronutrient deficiency, apply the micronutrient to a specific, clearly marked out affected area of land to observe results in subsequent seasons.
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.

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).

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.

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 (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 .
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