Kesuburan tanah Lanjutan Sub Topik: Boron (B) dan Molibdenum (Mo) Oleh: Dr. Ir. Hamidah Hanum, MP Sekolah Pascasarjana USU

Function and mobility of B  Boron has a primary role in cell wall biosynthesis and structure and plasma membrane integrity.  It is required for carbohydrate metabolism, sugar transport, lignification, nucleotide synthesis, respiration, and pollen viability.  B is not an enzyme constituent and does not affect enzyme activities.  It is relatively immobile in rice plants. Because B is not retranslocated to new growth, deficiency symptoms usually appear first on young leaves. B deficiency symptoms and effects on growth White, rolled leaf tips of young leaves.  B deficiency results in reduced plant height and the tips of emerging leaves are white and rolled (as in Ca deficiency,).  Plant height may also be reduced. Severe deficiency results in the death of the growing point, but new tillers continue to be produced.  Rice plants may fail to produce panicles if they are affected by B deficiency at the panicle formation stage. B

1. Cu-Mineral Cu menyusun 10 ppm kerak bumi, Mineral primer B: tourmalin, boronsilicat B dpt bersubstitusi dgn Al 3+ dan atau Si 4+ B –tanah 2. B -Larutan tnh H 3 BO 3 0 mrpkn btk dominan di larutan tnh yg diserap tanaman pd pH 5-9. pH > 9.2 H 2 BO 3 - terjadi hidrolisis menjadi H 4 BO 4 yg segera dipindah ke larutantnh utk diserap tanaman mlli massflow dandifusi 4. B-kompleks organik Sumber utama B tersedia dalam tanah adalah bentuk komplek B-bahab organik 3. B-teradsorbsi Adsorbsi dan desorbsi dpt membuffer B- larutan dari kehilangan –leaching. Adsorbsi B terdapat pd tnh alkaline, tnh dgn kadar B yg tinggi

Faktor-faktor yg mempengaruhi ketersediaan B 1.pH. Konsentrasi B menurun dgn naiknya pH (6,3-6,5), Pengapuran menurunkan ketersediaan B krn diadsorpsi Al(OH) 3, max, adsorpsi pd pH Tekstur. Kandungan B larutan tnh pd tnh bertekstur pasir selalu lebih rendah pd tnh berpasir, BO rendah 3. Interaksi dgn hara lain. Jk Ca- tersedia tinggi, mk mbatas ketersediaan B. Utk mengetahui status B tan digunakan ratio Ca/B. Defisien B jk ratio Ca/B > 1200: 1 4. Kelemababan tnh. Defisien B terjadi pada iklim kering dan kelembaban tanh rendah 5. Bahan organik. Ketersediaan B pd top soil > subsoil 6. Faktor tanaman. Keragaman genetik menentukan B- uptake

B deficiency can be caused by one or more of the following factors: Small amount of available B in soil B adsorption on organic matter, clay minerals, and sequioxides Reduction in B mobility because of drought Excessive liming Causes of B deficiency B deficiency Occurrence of B deficiency B deficiency is not very common in rice, but can occur in the following soils: Highly weathered, acid red soils and sandy rice soils in China Acid soils derived from igneous rocks. Soils formed from marine sediments contain more B than those formed from igneous rocks. High organic matter status soils in Japan $

Preventive strategies for B management Treatment of B deficiency management Water management: Avoid excessive leaching (percolation). B is very mobile in flooded rice soils. Fertilizer management: On B- deficient soils, apply slow-acting B sources (e.g., colemanite, at intervals of 2–3 yr. B fertilizers have a longer residual effect in silty and clayey soils (apply 2–3 kg B ha-1) than in sandy soils (apply 3–5 kg B ha-1). In rice-wheat systems, B applied to wheat can alleviate B deficiency in the subsequent rice crop. Do not apply excessive amounts of B to avoid B toxicity B deficiency should be treated as follows: Apply B in soluble forms (borax) for rapid treatment of B deficiency (0.5–3 kg B ha-1), broadcast and incorporated before planting, topdressed, or as foliar spray during vegetative rice growth. Borax and fertilizer borates should not be mixed with ammonium fertilizers because this will cause NH3 volatilization.

When soil pH is <6, B is present mostly as undissociated boric acid–B(OH)3–and uptake depends on mass flow. Above pH 6, B(OH)3 is increasingly dissociated and hydrated to B(OH)4- and plant uptake becomes actively regulated. An increase in soil pH results in a greater amount of B adsorbed to organic matter, sequioxides, and clay minerals. Therefore, flooding acid soils decreases B availability, whereas flooding alkaline soils increases B availability. The pH of wetland soils decreases upon drying so that B is desorbed and can be leached out. B toxicity may also be related to B concentration in the irrigation water and the amount used. B toxicity may become more severe in the dry season when B concentration in deep-well irrigation water is larger and there is little rainwater to dilute the large B concentration in irrigation water and/or leach B from the soil and rice plants. Effect of submergence on B toxicity Preventive strategies for B toxicity management Varieties: Plant B-toxicity tolerant varieties (e.g., IR42, IR46, IR48, IR54, IR ). B-toxicity tolerant varieties can yield up to 2 t ha-1 more than susceptible varieties. Water management: Use surface water with a low B content for irrigation. Groundwater must be monitored regularly if used for irrigation. If the B concentration is too great, dilute the water with water from a different source containing a small amount of B. Soil management: Plow when the soil is dry so that B accumulates in the topsoil. Leach with water containing a small amount of B. Treatment of B toxicity Leach with low-B irrigation water if percolation is sufficient and a suitable water source is available

1. Mo-Mineral Mo menyusun 2 ppm kerak bumi, Mineral yg mengontrol MoO 4 2- adalah PbMoO 4 dan CaMoO 4 Mo–tanah 2. Mo -Larutan tnh : MoO 4 2-, HMoO 4 - dan H 2 MoO 4 0 Konsentrasi MoO 4 2-, HMoO 4 - meningkat dgn naiknya pH

Faktor-faktor yg mempengaruhi ketersediaan Mo 1.pH. Ketersediaan MoO 4 meningkat dengan naiknya pH tnh. Aplikasi pupuk yg membtk asam (ZA) pd pd tnhbertekstur kasar menurunkanketersediaan Mo 2. Adsorpsi pd Fe/Al oksida. Mo sgt kuat diadsorpsi Fe/Al oksida shg tdk tersedia bagi tanaman 3. Interaksi dgn hara lain. P meningkatkan serapan Mo. Kadar SO 4 2- yg tinggi menekan serapan Mo. Cu dan Mn juga menurunkan serapan Mo. Mg mendorong serapan Mo. NO 3 - mendorong serapan Mo ttp NH 4 + menurunkan serapan Mo. 4. Efek iklim. Defisiensi Mo lebih sering terjadi pada kondisi tanah kering 5. Faktor tanaman. Keragaman genetik (kemampuan mengekstrak Mo tnh) menentukan Mo-uptake

Functions Molybdenum is essential for many plant functions. Some of them are It functions in converting nitrates (NO3 ) into amino acids within the plant. It is essential to the symbiotic nitrogen fixing bacteria in legumes. It is essential to the conversion of inorganic P into organic forms in the plant. Molybdenum is considered to be quite mobile as it moves readily in both the xylem and phloem conductive tissue of the plant. Still its highest concentration is in mature leaves because it binds readily with sulfur-containing amino- groups, sugars, and polyhydroxides which are usually in greater concentration in these leaves. It is found in the enzymes nitrate-reductase and nitrogenase which are essential for nitrate reduction and symbiotic Nfixation in plants. Adequate Molybdenum minimizes the presence of nitrites and nitrates in plant tissues.

Factors Affecting Availability Leaching Soil Conditions: Available soil Mo is an anion, and is therefore leachable. Soil pH: Molybdenum is the only micronutrient that has increased availability as the pH increases. At a soil pH above 6.5, unnecessary Mo applications can result in Mo toxicity to the crop, to animals eating the crop, or it may induce deficiencies of an element listed below. At pH's below 6.0, availability is rapidly diminished because Mo is easily "fixed" in the soil by free Fe(OH)3, AI(OH) 3 and Fe2O3. Soil Saturation: It is believed by some researchers that in saturated soils, Mo availability is increased somewhat. Mo:S Balance: Some work has shown that sulfate applications can cause a reduction in Mo uptake by plants. Mo:P Balance: Applications of P have increased the Mo content of plants in some research. It is thought that P reduces the adsorption of Mo compounds in the soil. NH4:NO3 Balance: Plants can often grow well in low Mo soils when fertilized with NH4 fertilizers, as opposed to NO3 fertilizers

Deficiency Symptoms Chlorosis of leaf margins, or more general chlorosis in some cases whiptail of leaves distorted curding of cauliflower fired margin and deformation of leaves due to excess NO3 destruction of embryonic tissue. Poor grain or fruit set, due to less viable pollen Foliar deficiency symptoms are somewhat rare and positive responses may occur where there are no visible symptoms. The most common visible symptom is a pale yellowing resembling nitrogen deficiency. Other symptoms include whiptail of leaves and distorted curding in cauliflower and destruction of embryonic tissue in some legumes.

Toxicity Symptoms Marginal leaf scorch and abscission as found in typical salt damage. Yellowing or browning of leaves and depressed tillering. Excess levels in plants are more of a concern to animal life, especially ruminants. Over-consumption of plant tissue high in Molybdenum can lead to a condition called Molybdenosis. Because of the intensity of interactions, toxic symptoms will normally manifest themselves as deficiencies of other nutrients.(Usually Cu). The application of Sulfur can decrease Molybdenum uptake and minimize the incidence of toxicity. Typical Application Rates (of sodium molybdate, not elemental Mo) MethodRate Broadcast6 to 12 oz./Acre Foliar1 to 3 oz./Acre Seed Treatment? to 1 oz./Acre

Using Molybdenum in a Fertility Program Most analytic laboratories presently, do not offer soil analysis for Molybdenum on a routine basis. The more capable labs may offer it as a special request. Most soil analytic techniques lack well calibrated interpretive methodology (the correlation between extractable Mo and crop response is the weakest of all the essential nutrients). Plant analyses are a better choice. But, because of limited research the "sufficiency range" in plants is quite broad and not very well defined. Generally speaking values of over 1.0 ppm are considered adequate. However, the crops previously listed will often respond to Molybdenum applications where the soil pH is below 6.2. As stated earlier, be very careful with Molybdenum applications at pH's above 6.5, as yield reductions can occur.