Tuesday, January 17, 2012

The Role of Micronutrients in Bangladesh Agriculture





Introduction

At least 16 elements are essentially needed by all higher plants for their proper growth and development. Among the 16 elements (CHONPKCaMgSFeMnZnCuBMoCl) carbon, hydrogen and oxygen are taken by the plants from CO2 (in atmosphere) and water. The rest 13 elements are obtained from the soils. Six of these (NPKCa Mg S) are required in large quantities and hence are known as major or macro nutrients. The other seven elements (FeMnZnCuBMoCl) are required in small quantities and classified as minor or micro nutrients (Table 1). The need for micronutrients varies with crop, soil conditions and farm management.

Table 1. Relative and average concentration of plant nutrients, forms absorbed and their
              mobility.

_________________________________________________________________________
Nutrients               Relative                 Average                    Forms                 Mobility in
                              Concentration*       concentration           absorbed*           plant with age

                                                                   (%)
________________________________________________________________________

Macronutrients

Nitrogen      (N)          1000,000              1.5                 NH4+, NO3-                   Mobile

Potassium    (K)            400,000              0.2                 K+                             Mobile

Calcium      (Ca)           200,000               0.5                 Ca2+                           Relatively immobile

Magnesium (Mg)          100,000               0.2                 Mg2+                         Mobile

Phosphorous (P)              30,000               0.2                 H2PO4- , HPO42-      Mobile

Sulfur            (S)              30,000               0.1                 SO42-, SO2              Relatively immobile

______________________________________________________________________________



________________________________________________________________________
Nutrients              Relative                 Average                    Forms                 Mobility in
                              Concentration*       concentration           absorbed*          plant with age

                                                                   (mg/kg)
________________________________________________________________________

Micronutrients


Chlorine       (Cl)        3,000                          100                          Cl-                  Mobile

Iron              (Fe)        2,000                          100                          Fe2+             Relatively immobile

Boron           (B)         2,000                            20                           BO32-         Relatively immobile

Manganese (Mn)        1,000                            50                          Mn2+            Relatively immobile

Zinc             (Zn)           300                            20                          Zn2+                   Variably mobile

Copper         (Cu)          100                              6                          Cu2+             Relatively immobile
                                                                                                                           but mobile under
                                                                                                                           sufficiency
                                                                                                                           conditions


Molybdenum (Mo)           1                         0.01                          MoO42-         Moderately mobile


Source:  Tisdale et al. (1993).  * Concentrations relative to average concentration of Mo in plant tissues.


Functions and Deficiency Symptoms of Micronutrients

For all crops, micronutrients are equally vital as the macronutrients. The only difference is the amount. Maize for instance, will contain nitrogen and zinc in the ratio of about 100 to1. But due lack of zinc can make the difference between 3-4 tons per hectare and even complete failure of the crop. The Hidden Hunger for one or more micronutrients can be very costly in the long run

Constant attention should be given to micronutrient levels and needs. A crop can suffer from
Hidden Hunger without showing a specific deficiency symptom. If the symptom is evident, the crop is already suffering a yield reduction. It would be an economic safeguard to check and avoid or at least check and remedy any yield robbing Hidden Hunger problems.
Nutrient deficient or starved plants cannot speak, but they can call for attention with a hunger sign. When the hunger sign becomes evident, a yield reduction is already underway. Corrective measures are imperative because crop growth is retarded and the plant becomes increasingly vulnerable to insects, diseases and even weather damage. Nutrient deficiencies show definite patterns. But when the plant is suffering from more than one deficiency then it becomes really difficult to identify. Functions and deficiencies of the nutrient elements should be understood as correctly as possible. Functions, deficiency and toxicity symptoms of each micronutrient are shown below:

Iron
Functions

  • The role of iron in plant metabolism was established in 1844 by Gris when he observed that iron promotes the formation of chlorophyll although it is not a constituent of chlorophyll.
  • Iron is involved in several oxidation-reductions in plants and is, therefore, essential for the synthesis of proteins and several metabolic reactions.
  • Iron helps the roots and root hairs to grow.

Deficiency symptoms

  • Iron deficiency causes chlorosis and stunted growth with sharp distinction between green veins and yellow interveinal tissues.
  • Deficiency symptoms first appear on the younger leaves at the top of the plants.
  • Severe deficiency may turn the entire plant yellow to bleached white.
  • Iron deficiency is thought to be caused by an imbalance of metals such as Mo, Cu or Mn.

Toxicity symptoms

  • Iron toxicity discolors the interveinal areas of the leaves.
  • Iron toxicity produces necrosis and reduces the uptake of manganese.


Manganese
Functions

  • Manganese functions primarily as a part of the enzyme system in plants.
  • Manganese activates several important metabolic reactions.
  • Manganese plays a direct role in photosynthesis by adding chlorophyll synthesis.
  • Manganese accelerates germination and maturity.
  • Manganese increases the availability of P and Ca.

Deficiency symptoms

  • Manganese deficiency appears first in younger leaves with yellowing between the veins.
  • Sometimes a series of brownish-black specks appears.
  • In small grains, grayish areas appear near the base of the younger leaves.
  • Manganese deficiencies may appear from an imbalance with other nutrients such as Ca, Mg and Fe.

Toxicity symptoms

  • Manganese toxicity turns the leaves completely white.

Copper

Functions

·         The essentiality of copper for the growth of higher plants was first recognized in 1931.
·         Copper is necessary to form chlorophyll in plants.
·         Copper catalyses plant processes.
·         Copper is known to act as an electron carrier in enzymes which bring about oxidation-reduction and regulates respiratory activity in plants.
·         Copper is necessary in promoting plant processes though it is not usually a part of the product(s) formed by those reactions.

Deficiency symptoms

  • Common symptoms of Cu deficiencies include dieback in citrus and blasting of onions and truck crops.
  • Many crops show Cu hunger with leaves that lose turgor and develop a bluish-green shade before becoming chlorotic and curling.
  • Most often the plants fail to flower.

Toxicity symptoms

  • Excessive application of Cu depresses the growth of plants.
  • Excessive application of Cu may also cause Fe deficiency symptoms to appear.

Zinc
Functions

  • Zinc aids in producing plant growth substances and enzymes.
  • Zinc is essential for promoting certain metabolic reactions.
  • Zinc is necessary for producing chlorophyll and forming carbohydrate.
  • Zinc increases the resistance of crops to diseases
  • Zinc regulates the nutrition of phosphorus.
  • Zinc increases the yields of leguminous crops.


Deficiency symptoms

  • Zinc is not translocated within the plant, so symptoms first appear on the younger leaves
      and then on other plant parts.
  • The old leaves turn brownish and young leaves become whitish.
  • The leaves become smaller and curling appears on the margin.
  • Zinc deficiency in maize is called white bud because the young bud turns white or light yellow in early growth.
  • Zinc deficiency causes sterility in rice.
  • Stalks of fruits become smaller; as a result, the plant bears the fruits in cluster and the fruit yield decreases drastically.
  • Other deficiency symptoms include bronzing of rice, rosette of pecans and little leaf of fruit trees.

Toxicity symptoms

  • Zinc is toxic to plants when applied in large quantities
  • Excessive Zn decreases the protein synthesis.



Boron
Functions

  • Boron is essential for germination of pollen grains and growth of pollen tubes.
  • Boron is essential for cell wall formation.
  • Boron forms sugar/borate complexes associated with sugar translocation.
  • Boron is important in protein formation.
  • Boron helps in producing good quality seeds

Deficiency symptoms

  • Boron deficiency generally stunts plant growth- the growing point and younger leaves are are first affected. This indicates B is translocated in the plant.
  • Thus it causes death of growing point and the stem turns blackish.
  • Boron deficiency decreases the growth of the roots and root hairs.
  • Cell wall is broken due to B deficiency.
  • The whole plants become brittle.
  • The fruits of the papaya become irregularly shaped.
  • Hallow spaces occur in cauliflower stem.
  • Scab and leaf rolling appear in potato.
  • Grains of hybrid rice cannot develop properly; as a result, yield is reduced.
  • Midrib of beans rotten and look like blackish in colour.

Toxicity symptoms

  • Boron toxicity damages the growing point and young leaves.
  • The yield of crops is drastically reduced.


Molybdenum
Functions
  • Molybdenum is required to form the enzyme, nitrate reductase.
  • The enzyme reduces nitrates to ammonium in the plant.
  • This is vital for helping legumes form nodules which are essential for symbiotic N fixation.
  • It is also needed to convert inorganic P to organic forms in the plant.
  • Molybdenum helps in the translocation of iron.

Deficiency symptoms

  • Molybdenum deficiency symptoms show up as general yellowing and stunting of plant.
  • Molybdenum deficiency can cause N deficiency in legume crops such groundnut and soybean because soil bacteria on legumes must have Mo to help fix N from the air.
  • Molybdenum deficiency causes whiptail symptom.
  • Curling appears on the leaf margin.
  • Molybdenum deficiency causes the flowers to shade and as a result the yield is reduced

Toxicity symptoms

  • Excessive Mo is toxic especially to grazing animals. Cattle eating forage with excessive Mo content may develop severe diarrhea.
  • Molybdenum toxicity stunts the plant growth.
  • The leaves of the plants look rusty and gradually rotten.

Chlorine
Functions

  • Chlorine is known to be essential for plant growth.
  • Little is understood about the role of chlorine.
  • Chlorine is thought to be essential for the photosynthetic process.
  • Chlorine has been reported to interfere with P uptake and seems to enhance maturity of small grains on some soils.

Deficiency symptoms

  • The plant looks diseased and rottening starts in many places.
  • The growth of roots and root hairs becomes stunted.
  • Maturity of the crops is delayed.

Toxicity symptoms

  • Excess Cl can reduce the quality of certain crops, especially tobacco.
  • It produces thick, brittle tobacco leaves with rolled-up margins.
  • It also lowers burning quality.




Cobalt

Functions

  • Cobalt has not been proven essential for higher plant growth.
  • Nodulating bacteria need it for fixing atmospheric N in legumes.


Soil Conditions Responsible for Micronutrient Deficiencies and Susceptible Crops

The soil type and conditions where micronutrient deficiency may occur and the crops which
are most susceptible to micronutrients are described below:

Micronutrient      Soil conditions where deficiency                    Most susceptible crops
                                    may occur
     
Iron                Occurs in alkaline soils, in calcareous           Beans, soybeans, maize, rice
                       soils when cold and wet, in soils where         lemons, lime and tree fruits           
                       phosphate has been excessively applied

Manganese     Occurs in sands, peats and mucks, alkaline    Soybean, small grains, tree fruits,
                     and particularly calcareous over-limed          cotton, leafy vegetables and
                     soils. Also in areas low in organic matter.      beans

Copper         Occurs in sandy soils, peats and mucks,           Small grains, maize, wheat,
                     over-limed soils and in high concentrations     vegetables, tree fruits, carrots
                            of iron and manganese.                                     and onions


Zinc           Occurs in calcareous soils after leaching           Beans, soybeans, citrus, tree fruits,
                  and erosion, in acid-leached soils, in coarse     maize, potatoes, wheat, cotton, rice
                  sands and soils where phosphate has been       and sweet corn.
                  excessively applied. Also in low organic
                  or over-limed soils.


Boron          Occurs in acid-leached soils, coarse             Citrus, cotton, mustard, wheat, maize,                          textured sandy soils, peats and mucks,        cabbage, cauliflower, potatoes,
                    drought conditions, over-limed soils,           tomato, tree crops and tobacco
                    alkaline or low organic matter soils.

Molybdenum 
                        Occurs in acid soils and highly                 All legumes, citrus, tobacco and
                         weathered acid-leached soils.                  cauliflower
                        And in soils with low phosphate levels.


                   
                       
Micronutrient Status and Research in Bangladesh

Although Bangladesh is a small country it has wide variety and complexity of soils at short distances due to a diverse nature of physiography, parent materials, lands, hydrology and drainage conditions. As a result, continuous changes are taking place in the soil fertility status due to organic matter depletion, nutrient deficiency/toxicity, drainage impedance/water logging followed by degradation of soil physical and chemical properties and soil salinity/acidity.

The fertility status of Bangladesh soils is extremely variable (Table 2). Most of the soils are depleted and are in urgent need of replenishment with organic manure and fertilizers if projected crop production target is to be obtained. At present the farmers use only about 132 kg nutrients (104 kg N + 12 P2O5 + 13 kg K2O+3 kg S plus Zn, B and others/ha/year while the estimated removal is around 200 kg/ha/year. Thus a continuous stress is being put on the country’s soils. As a result the productivity of our soils which is a fundamental resource for the farmers and ecosystem is low and decline in crop yields is reported in many soils.                               















Table 2. Nutrient status of some important soils series of Bangladesh (Islam 2006).














     Soil series
pH
OM (%)
meq/100g



μg-1







Ca
Mg
K
NH4-N
P
S
B
Zn
Cu
Fe
Mn
Tejgaon
5.1
1.5
0.7
0.45
0.17
8
4
10
0.5
2.0
3
272
27
Chhiata
5.3
0.6
2.0
0.57
0.12
2
5
4
0.5
2.0
3
291
23
Kalma
5.1
1.7
1.5
0.28
0.16
5
3
10
0.4
2.0
8
301
15
Khilgaon
5.0
1.6
2.0
1.57
0.21
12
14
12
0.3
5.0
6
200
10
Savar
5.6
1.5
3.0
1.62
0.11
13
18
23
0.9
3.9
10
100
20
Sonatola
5.7
1.3
1.6
0.83
0.12
5
15
8
0.1
1.0
4
42
9
Ghatail
6.2
1.1
2.0
0.77
0.10
12
2
Tr
0.4
1.0
5
150
7
Shilmandi
5.4
0.9
1.2
0.67
0.11
10
5
8
0.3
2.0
10
204
            25
Bonarpara
6.5
1.0
1.6
0.68
1.11
13
5
8
0.3
2.0
6
203
9
Kaunia
6.4
1.0
4.3
0.85
0.47
9
7
10
0.1
2.0
9
94
5
Gangachara
6.5
1.2
1.5
0.51
0.15
7
10
12
0.5
1.0
6
38
10
Polashbari
6.0
0.9
3.5
0.69
0.15
6
11
4
0.1
1.0
4
42
9
Gopalpur
7.7
1.4
23.0
4.00
0.22
2
10
4
0.3
1.5
3
25
5
Sara
7.9
0.9
18.7
2.50
0.15
16
16
4
0.3
1.0
8
24
14
Ghior
6.8
1.2
8.0
2.50
0.34
5
8
10
0.2
1.0
3
140
13
Darsona
8.0
1.4
20.0
4.00
0.33
7
8
9
0.3
1.0
5
65
7
Domar
6.0
1.0
1.5
0.5
0.20
4
10
5
10
0.1
4
50
Tr
Barkal
5.5
1.4
1.6
0.9
0.43
14
12
10
0.1
1.4
3
180
16
Mirpur
7.2
0.8
4.0
1.70
0.15
3
10
5
0.1
1.0
3
116
6
Lauta
5.2
1.6
1.6
0.70
0.18
2
9
2
0.1
1.0
3
211
4
Ekadala
5.6
1.5
1.7
0.71
0.16
8
10
11
0.1
1.0
3
211
5
Ishurdi
7.9
1.3
18.0
1.20
0.30
5
7
4
0.4
0.4
3
26
3
Amnura
5.5
0.9
3.0
1.62
0.10
5
2
6
0.3
2.0
5
30
4
Chandina
5.5
1.5
2.8
1.24
0.07
10
5
10
0.3
2.0
7
413
27
Debidwar
5.7
1.2
2.7
0.92
0.15
7
12
10
0.5
1.0
4.6
243
22
Nachole
6.3
1.3
3.0
2.0
0.27
5
12
8
0.4
1.7
5
270
10
Amjhupi
7.2
1.5
19.0
4.08
0.22
3
4
6
0.2
1.5
6
28
25
Kaptai
5.3
1.8
2.1
1.0
0.17
8
10
11
0.1
1.0
3
589
113
Jaflong
5.4
0.8
Tr
0.12
0.07
3
4
6
0.3
1.0
1
144
7
Critical Level


2.0
0.8
0.20
75
14
14
0.2
2.0
1
10
5
                                                    Tr-Trace


                                                             Zinc

The importance of zinc in crop nutrition has received considerable attention in the recent years in Bangladesh. The incidence of zinc deficiency is widespread in most calcareous and alkaline soils Table 2). The problem is more acute in wetland rice culture. The increasing incidence of zinc deficiency in wetland rice as well as on various upland crops indicates that zinc deficiency might have become more extensive than we presently know that and it is likely to increase with vertical expansion of cropping. Besides rice, zinc deficiency is now prevalent on maize, different vegetable crops, oilseeds, citrus and tea.
The critical levels of available zinc as established by different extracting procedures are shown below:

Extractant
Critical limit (mg/g)
Dithzone+NH4OAC

DTPA
EDTA+NH4Cl
0.0 N HCl
Light soils: 1.0
Heavy and calcareous soils: 2.0
                                              0.8  
                   1.5-1.8
             1.0

Boron

Boron deficiency in Bangladesh agriculture being a very recent observation has rapidly drawn the attention of soil scientists. Light textured soils of the country are deficient in available boron where significant leaching losses of borate ions might have depleted soil boron level. The available boron content of the major soils of Bangladesh varies between 0.1 and 1.9 mg g-1 soil. Most of the light textured soils of Rangpur, Dinajpur and terrace soils of Gazipur and hill soils of Srimongal contain low level of available B (0.1-0.3 mg g-1 soil). Field studies have shown that sterility problems in wheat, chickpea and mustard grown on sandy soils of Rangpur were significantly improved by the application of boron. Wheat yield after boron application was increased by than 50% and this was because of increased number of grain per spike. Sterile and partially filled seeds are produced in mustard and chickpea due to boron deficiency. In papaya deformed, uneven and smaller sized fruits are produced and the fruits also drop prematurely.

Other Micronutrients

Other micronutrients like Fe, Mn, Mo and Cl have attracted less attention in Bangladesh
agriculture. Generally they are seldom needed to be applied in crop production in most soils. However, recently Cu and Mn application in calcareous soils has appeared to be beneficial for higher yields in some field crops. Recent studies have also indicated that Mo deficiency is widespread in cabbage and legumes like groundnut in acid soils. Appreciable yield increases of these crops in presence of added molybdenum have also been recorded. Deficiency of Cl has been detected in coconut and betel nut plants. Proper potassium fertilization with muriate of potash prevents the occurrence of Cl deficiency problems in most cases. Iron is the only micronutrient present in available form abundantly in Bangladesh soils.

Micronutrient content and critical levels in crops

In order to make proper nutrient management a crop’s nutrient content (optimum and critical) should be understood. It should always be remembered that the soil and tissue samples must taken properly, handled and sent to the laboratory correctly in order for the analysis to give a correct reading. If the soil and tissue tests indicate a critical near critical levels of zinc, boron, manganese, iron or copper, an adequate application of these nutrients must be made to correct the deficiency.

Optimum Values and Critical Levels

Crops
μg g -1
Fe                Mn                     Cu                   Zn               B
Rice (leaf blade)
Optimum values
Critical levels

120               145                     6                    19                5
  25                 20                     3                     8                  2
Wheat (mid till)
Optimum values
Critical levels                               


100              50                       2                   50                    8
20                10                        1                  14                    4
Maize (before tasselling)
Optimum values
Critical levels

150              100                      10                50                  20
 30                 20                        5                20                 10

Potato (tubers half grown)
Optimum values
Critical levels

100                200                      8                 40                 30
  20                  30                      4                  15                12
Groundnut (early pegging)
Optimum values
Critical levels

150                  200                   2                  30                  35
30                      40                   1                  12                 15
  Mustard (pre flowering)
Optimum values                           
Critical levels

150                  250                  20                 35                  40
  30                    50                    8                  15                 20
Soybean ( early pre flowering)
Optimum values
Critical levels

200                   250                 10                  30                 150
  50                      60                  5                  12                   20

Chickpea (early flowering)
Optimum values                           
Critical levels

150                   120                 10                   40                  45
  30                     20                   5                   15                  18
Lentil
Optimum values
Critical levels

145                   120                 10                   40                  45
  25                     22                   4                   12                  15
Black gram
Optimum values
Critical levels

140                   110                  8                   30                  40
  30                     20                  4                  10                   15
Mungbean
Optimum values
Critical levels

140                   100                 8                    25                 30
  30                     20                 4                    15                 12


Cabbage (head)
Optimum values
Critical levels

200                  180              6                         50                 45
  40                     35              3                        20                25
Cauliflower
Optimum values
Critical levels

240                  200              8                         60                50
  50                    40              4                         30                35
Tomato (13 leaves)
Optimum values
Critical levels

200                   300              5                         100             80
  50                     60               2                          50             60
Sugarcane (rapid growth, 7 month)
Optimum values                           Critical levels


80                  150                   10                     40                     8  
20                    40                    4                      15                     5   
Tobacco (45 DAT)
Optimum values
Critical levels

100                 160                   40                     60                   35
  40                   50                    15                     25                  20
Tea (mature leaves)
Optimum values
Critical levels

200                  100                  20                     40                   25
 50                     30                   8                       15                  15
Mango (mature leaves)
Optimum values
Critical levels

150                  200                  15                      100                80
  40                    50                    6                        40                45
Banana (midrib of third youngest leaf)
Optimum values
Critical levels

150                  1500                15                       30                 70
  40                    50                   8                        18                 30

Correction of Micronutrient Deficiencies

Iron

Iron deficiency can be corrected by soil application or foliar spray at the rate of 15-20 kg Fe/ha. Applying soil soluble such as Fe sulphate to the soils is not very efficient because Fe converts rapidly to unavailable forms.
When these materials are applied as foliar spays, they are much more effective. Injections of dry Fe salts directly into trunks and limbs can control Fe chlorosis in fruit trees. Most Fe fertilizer sources are best applied as foliar spray. This method uses lower rates than soil application.

The most commonly used Fe fertilizers are Fe sulphate (19-23%Fe), Fe ammonium sulphate (14.0%Fe) and Fe chelates (5-14%Fe).

Manganese

Manganese deficiency can be corrected in several ways:

  1. If the deficiency is due to over liming, the pH of the soil should be maintained below 6.5. This can be done by reducing lime rates or using materials containing sulphur. They reduce pH and convert Mn into plant available form. For field crops, it is more economical to add Mn than to try lower soil pH.

  1. Soluble Mn salts such as such Mn sulphates may be applied with starter fertilizers in band. High starter P fertilizer helps mobilize Mn into the plant. A field deficiency symptom can be corrected by foliar application with 10-15 kg MnSO4/ ha.


  1. On some soils, an extremely acid pH may cause Mn toxicity to crops. Liming will eliminate the problem.

Mn fertilizers which are in common use include Mn sulphate (26-28%Mn), Mn oxides (41-68%Mn), Mn chelate (12%Mn) and Mn chloride (17%Mn)

Copper

Copper deficiency may be corrected either by soil application or by foliar spray at the rate of 4-6 kg Cu/ha. When applying copper fertilizer, it should be remembered that, like most other micronutrients, large quantities of Cu can be toxic to plants.

The most widely used copper fertilizers include copper sulphate (22.5%Cu), copper ammonium phosphate (30.0%Cu) and copper chelates containing variable amounts.

Zinc

The best way to correct Zn deficiency is to apply Zn fertilizers broadcast or with row fertilizers. Such applications often last several years. Foliar application has been used successfully as a temporary or emergency measure. Seed treatment has also been successful.

There are a number of zinc compounds which can be used to correct deficiencies in soils as well as in plants as foliar spray. The zinc fertilizers commonly used are zinc sulphate (monohydrate 36%Zn, heptahydrate 20-22%Zn), zinc oxide (78%Zn), zinc phosphate (51%Zn) and zinc chelates (10-14%Zn).

Boron

Boron deficiency may be corrected by soil application in the field or foliar sprays on standing crops at the rate of 1-2 kg B/ha.. Crops vary widely in their needs for and tolerance to boron. Yet, the line between deficient and toxic amounts is narrow than for any of the other essential micronutrients. So boron should be applied very carefully especially in a rotation involving crops with different sensitivities to B.

There is a good number of boron fertilizers which include boric acid (17.0%B), solubor (20.0%B), sodium tetra borate (fertilizer borate 46 and 65 containing 14.0 and 20.0%B, respectively), sodium penta borate (18.0%B). All these materials are water soluble and suitable for both soil application and foliar sprays. 

Molybdenum

Molybdenum deficiency in acid soils can be corrected either by soil application or through foliar spay at the rate of 0.5-1.0 kg Mo/ha using the commonly used Mo fertilizers such as sodium molybdate (39-41%Mo) and molybdic acid 47.5%Mo). These materials may be mixed with NPKS fertilizers, applied as foliar sprays or used as seed treatment. Seed treatment is probably is the most way of correcting Mo deficiency.

Mo becomes more available as pH goes up, the opposite of other micronutrients. Therefore, liming of acid soils increases the availability of Mo.

Mo can affect Cu metabolism. For example, animals grazing pastures low in Mo may develop Cu toxicity, if Cu soil levels are high enough. But animals eating forage high in Mo may develop Cu deficiency leading to a disease called molybdenosis. It can be corrected by feeding Cu sulphate orally, injecting Cu containing medicines or applying Cu sulphate to the soil.


Chlorine

Fortunately, chloride is seldom limiting in soils. Fertilizing soils with muriate of potash enriches the soils with chloride. Chlorides are not retained in the soil to any extent, so are subject to considerable leaching. They leach more rapidly through sandy soils.

Chelated Micronutrients

Chelate is a derivative from the Greek word chele meaning claw. A chelate is an organic compound which combines the positive charged cation (zinc, manganese, iron or copper) with an organic, negative charged chelating agent. The organic molecule surrounds the positive charged metal and protects the new chelated form of cation from chemical tie up in the soil. The most effective synthetic chelating agents are EDTA for Zn, Fe, Mn and Cu; HEDTA for Zn and Fe.


Approved Micronutrient Fertilizers

The Government of Bangladesh has standardized so far more than 85 fertilizers and fertilizer materials for their import, production, marketing and their use within the country. The Government has also fixed their specifications for monitoring and proper use within the country.
Although fertilizer business is with the private sector, the Ministry of Agriculture is responsible for quality control of the imported fertilizers as well as domestically produced fertilizers and fertilizer materials by the Fertilizer Management Law published and revised from time to time.

Soils and Chemistry Laboratories of five organizations such as BARI, BINA, DU, SRDI and BSTI have been entrusted for analyzing the fertilizers and fertilizer materials following the Fertilizer Analytical Manual Published by BARC. The major responsibility for quality control of fertilizers and fertilizer materials as well as their proper use at the field levels lies with the Department of Agriculture (DAE). The DAE officials and scientists working with fertilizers and fertilizer materials in the field and laboratories should be familiar with their specifications so that proper use of fertilizers is ensured and no abuse can take place. The specifications of the micronutrient fertilizers so far approved by the Government are described below:

  1. Zinc Sulphate (monohydrate, granular)

i)  Total zinc content percent by weight, minimum                                        36.00
ii) Water soluble zinc content as percentage of total zinc, minimum              95.00
Iii) Sulphur content (as SO4-S) percent by weight, minimum                        17.60
iv) Moisture content (free water) percent by weight, maximum                      2.00
v) Particle size: 90 percent of the material shall pass through 4-2 mm IS sieve
vi) Physical condition: Granular, free flowing 


  1. Zinc Sulphate (heptahydrate)

 i) Total zinc content percent by weight, minimum                                        21.00
ii) Water soluble zinc content as percentage of total zinc, minimum              96.00
Iii) Sulphur content (as SO4-S) percent by weight, minimum                        10.50
iv) Moisture content (free water) percent by weight, maximum                     
                                                 a) Crystalline                                                              3.00
                                                 b) Granular                                                                   2.00
           v) Particle size: For granular product, 90 percent shall pass through 3-1 mm IS sieve
vi) Physical condition: Granular, free flowing/crystalline




  1. Chelated Zinc

i)  Total zinc content percent by weight, minimum                                        21.00
ii)  Moisture content (free water) percent by weight, maximum                      7.00
iii) Physical condition: Pale yellow amorphous powder powder/crystalline
iv) pH: not less than                                                                                          5.00



  1. Boric Acid                 

i) Total boron content as percentage, minimum                                              17.00
ii) Lead content percent by weight, maximum                                                 0.0015
iii) Arsenic content percent by weight, maximum                                            0.0008
iv) Solubility in grams per 100 ml of hot water                                              39.10
v) Solubility in cold water: sparingly soluble
vi) Physical condition: White crystalline

  1. Solubor
                                                                 
i) Total boron content as percentage, minimum                                              20.00
ii) Lead content percent by weight, maximum                                                 0.0005
iii) Arsenic content percent by weight, maximum                                            0.0004
iv) Physical condition: White amorphous powder
   

  1. Sodium Molybdate

         i) Total molybdenum content as percentage, minimum                                     39.00
         ii) Physical condition: white crystalline powder
         iii) pH: minimum                                                                                                  8.700
 



Nutrient Availability from Zinc and Boron Fertilizers


Zinc fertilizers

Zinc sulphate- both monohydrate and heptahydrate as well as chelated zinc are generally used to correct zinc deficiencies in sensitive crops. Zinc sulphate is highly water soluble and chelated zinc are highly water soluble. Zinc sulphate is generally applied to the soils, while chelated zinc is sprayed to foliage. Zinc availability in soils is largely controlled by soil pH. The higher the soil pH, the lower is the availability.

Among the chelated zinc available in the market, Librel zinc imported and marketed by National Agricare Import & Export Ltd is a quality product. Application cost is also cheaper as compared to other zinc fertilizers. This chelated zinc is produced by Ciba Specialty Chemical PLC in Switzerland. This product is widely being used under the trade name Librel Zinc in about 120 countries of the world to correct Zn deficiency in most agricultural, horticultural and ornamental crops. Librel Zn is compatible with all other crop care chemicals.
Depending on the severity of deficiency Librel zinc may be applied at the rate of 0.5 – 1.00 kg/ha. Details of application methods for different crops of Bangladesh are available with National Agricare Import & Export Ltd.


Boron fertilizers

In Bangladesh only two grades namely boric acid and solubor have been standardized for use as boron fertilizers. Solubor is highly water soluble, while boric acid is soluble in hot water. Boron is one of the seven micronutrients essential to all plant growth and its deficiency was first identified in mid eighties on mustard and papaya plants. Boron deficiency has been recognized in a wide range of crops. The deficiency can be remedied by the correct application of either boric acid or solubor through soil application or foliar spray.

Boron deficiency is clearly visible in defined ways in many crops. By the time visible symptoms are seen, yields will already have affected. The best way to establish need is either through soil testing or through tissue analysis. Plants take up boron in borate ion BO32‑. When boric acid or solubor is applied to the soils they dissociate and BO32‑ would be available for plant uptake.

Boron is usually recommended at the rate of 1-4 kg/ha for soil application in B deficient soils for boron susceptible crops such as sunflower, cotton, mustard rape, radish, carrot and papaya. The sufficiency and toxicity level of boron in crop plants is very narrow. That’s one should be very careful in fertilizing the crops. Half of soil application rate may be recommended when correcting B deficiency through foliar spray.

In Bangladesh market it is very difficult to get good quality boron fertilizers. In fact, the most adulteration taking place in fertilizer business is with marketing of boric acid. Unapproved and substandard boron fertilizers are marketed freely in the country. One boron product named as Libel Boron 20 is being imported and marketed by National Agricare Import & Export Ltd. This is a quality product and can be widely used at the rate of 2 -5 kg/ha to correct deficiency in most susceptible arable, horticultural, fruit and ornamental crops. This is also an economic product. Per hectare application cost is lower than other products. Detailed brochure on this product is available with National Agricare Import & Export Ltd.


Conclusion

Micronutrients supplement forms an integral part of balanced fertilization which is the key to successful crop production. Bangladesh is now facing challenges in food production because of shrinking of natural resources and increasing teeming millions. Every year at least two million people are added to the total present population of 150 million. Crop production in Bangladesh is frequently constrained by natural calamities such as devastating floods, cyclonic storms and surcharge, tornados, drought; and depleting soil fertility. Most of the Bangladesh soils are deficient in nitrogen, phosphorus, potassium and sulphur. Almost all acidic soils are deficient in magnesium. According to present estimate, more than two thirds of the total soil resources are deficient either in one or more micronutrients. As mentioned earlier, micronutrients are absolutely necessary for successful crop production. Yield reduction is already happening in many areas of Bangladesh. It has been estimated that up to 30 percent yield reduction is possible for the sensitive and susceptible crops. The whole situation has been further complicated by irregular and inadequate supply of fertilizers (both macro and micronutrients). Fertilizer distribution system in the country is not at all satisfactory. Every year hue and cry among the farmers is being heard at the beginning of each crop season.

Moreover, the quality of the majority of imported and locally produced fertilizers especially the micronutrients does not confirm to the standards specified by the Government. Therefore, it is high time that all out efforts must be taken to monitor quality of fertilizers as well as to ensure their supply at the door step of the farmers so that projected crop production target can be achieved. At the same time, the fertility of the soils should be maintained at high level. 








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