KORELASI UJI N TANAH DENGAN HASIL TANAMAN

Bahan kajian MK Pupuk dan PemupukanDiabstraksikan oleh Prof r Ir Soemarno MS

Jur Tanah FP UB Oktober 2011

“Nitrogen (N) is essential for plant growth. It ranks behind only carbon, hydrogen, and oxygen in total quantity needed and is the mineral element most demanded by plants. Because N is mobile within the plant, deficiency symptoms are expressed on older leaves. These leaves are generally uniform pale green or yellow. When N is limiting, crop growth is slow and yields are reduced. Too much available N may lower yields and lessen crop quality. If soil N supply is greater than crop demand, excessive nitrate (NO3-) may enter ground or surface water. Nitrogen in the vegetation of field crops is approximately 2-3 percent of the dry matter weight. The quantity needed is more than Mississippi soils have the capacity to provide during a growing season, thus supplemental N is usually required for economical crop production. Nitrogen behavior is complex, but must be understood so growers may manage N for maximum profitability and for minimum environmental impact”.

Uji Tanah untuk Nitrogen

Uji tanah tentang nitrogen tersedia tidak pernah sepopuler uji tanah untuk fosfat dan kalium walaupun bagi banyak tanaman dan terutama sekali varietas unggul baru ternyata nitrogen merupakan merupakan persyaratan pertama di antara unsur hara utama lainnya. Kebutuhan nitrogen biasanya direkomendasikan berdasarkan hasil percobaan lapangan dan relasi-relasi hara, tetapi jarang melalui determinasi langsung.

Alasan bagi kondisi yang agak mengherankan ini ialah karena adanya kenyataan bahwa proses mikrobiologis dalam tanah dapat mengakibatkan mineralisasi senyawa nitrogen organik tanah menjadi ammonium dan nitrat yang tersedia bagi tanaman. Beberapa laboratorium menentukan nitrogen ter-sedia dan analisis yang lebih umum adalah:

(1). determinasi N anorganik setelah inkubasi (2). determinasi dengan cara yang sama setelah proses oksidasi lemah(3). estimasi pelepasan nitrogen berdasarkan atas kandungan bahan

organik dan tekstur tanah(4). determinasi langsung NO3

- bebas.

Disamping itu, semua laboratorium uji tanah telah menggunakan informasi tentang tanaman sebelumnya terutama berkenaan dengan nitrogen karena tanaman legume akan meninggalkan banyak nitrogen dalam tanah. Dengan diketemukannya suatu elektrode yang mampu mengukur secara langsung konsentrasi nitrat maka determinasi nitrat ini menjadi semakin penting. Dua

peneluan ilmiah penting dalam hal ini adalah: (a) bahwa untuk uji nitrat tersedia dalam tanah maka pengambilan contoh tanah harus lebih dalam dari lapisan olah. Di USA dan Kanada kedalaman contoh tanah ini sekitar 60 cm. (b) dengan meningkatnya kedalaman titik pengambilan contoh tanah dan pengeringan-udara contoh tanah ternyata kandungan nitrat tanah berkorelasi nyata dengan hasil dan respons tanaman.

Dengan demikian jelas bahwa nitrat yang bersifat sangat mobil tidak bertahan di dalam lapisan olah melainkan bergerak ke arah bawah bersama dengan air perkolasi. Nitrat ini juga diambil bersama dengan air oleh akar-akar tanaman. Hal ini menyatakan bahwa kedalaman sampling yang optimum akan terpengaruhi oleh kondisi iklim.

E.S. Marx, J. Hart, and R.G. Stevens. 1996. Soil Test Interpretation Guide. 1996 Oregon State University

Plant-available nitrogen (nitrate and ammonium)Plant-available forms of nitrogen are nitrate (N-NO3-) and ammonium (N-NH4+). Soil concentrations of N-NO3- and N-NH4+ depend on biological activity, and therefore fluctuate with changes in conditions such as temperature and moisture. Nitrate is easily leached from the soil with high rainfall or excessive irrigation. Soil tests can determine N-NO3- and N-NH4+ concentrations at the time of sampling, but do not reflect future conditions. When you collect samples for nitrogen testing, keep them cold, or dry them immediately to prevent N-NO3- and N-NH4+ concentrations from changing.

Ammonium-nitrogen does not accumulate in the soil, as soil temperature and moisture conditions suitable for plant growth also are ideal for conversion of N-NH4+ to N-NO3- . Ammonium-nitrogen concentrations of 2 – 10 ppm are typical. Soil N-NH4+ levels above 10 ppm may occur in cold or extremely wet soils, or if the soil contains fertilizer residue.Soil nitrate-nitrogen measurements are most useful as a post-harvest “report card” to evaluate N management. Nitrate remaining in the soil after harvest can leach during winter rains, contaminating surface and groundwater. If residual nitrate levels are consistently high, reduce fertilizer N inputs in future growing seasons.

In arid regions, soil nitrate (N-NO3-) is evaluated by measuring N-NO3- to the expected rooting depth of the crop to be grown. If test results are reported in ppm, convert to lb/acre . Then subtract the soil nitrate from the crop requirement to determine a fertilizer rate. Failure to account for N-NO3- in the soil can lead to over-application of nitrogen fertilizers. Also, irrigation water should be analyzed for N-NO3- content, and fertilizer rates reduced accordingly. Proper irrigation increases N use efficiency and reduces nitrate leaching.

Ketersediaan N Tanah

The quantity of N in soils is intimately associated with organic matter levels. Legumes such as soybeans and alfalfa convert atmospheric N2 to plant available forms via a symbiotic biological process involving Rhizobium bacteria and the plant roots. This fixed N may either return to the soil to ultimately become part of soil organic matter and serve as a N source to subsequent crops, or be removed in harvested plant materials. Small quantities of soil N are provided by residue from plants that do not fix atmospheric N. Organic matter in Mississippi soils typically ranges from 0.5 to 2.0 percent by weight of the upper six inches. Typically organic matter is approximately 5 percent N, so total N in the topsoil ranges from 500 lb/acre to 2000 lb/acre. However, only a very small portion of the total N is available to plants within a growing season. Organic matter is replenished by returning crop residues to the soil or introducing other organic sources such as manures or animal bedding. Almost all N in commercially available fertilizers is derived by combining atmospheric N2 with H2 to form ammonia (NH3), which may be used as fertilizer (anhydrous ammonia), or as a starting point in the manufacture of other nitrogen fertilizers. Anhydrous ammonia is an efficient source of fertilizer N, but because it is under high pressure, it requires specialized handling and stringent safety precautions. Because of theses requirements for anhydrous ammonia, other N products have increased in popularity. Animal manures are important sources of N in the environment. The quantity of N in the manure depends on the animal species, their age and diet, and bedding materials. Manure begins contributing to plant nutrition and soil organic matter when added to agricultural soils, but not all the N within manure is immediately available to plants. Nitrogen is in organic and inorganic forms in soils. Over 90 percent of soil N is associated with soil organic matter. Nitrogen is in compounds identifiable as part of the original organic material such as proteins, amino acids, or amino sugars, or in very complex unidentified substances in advanced stages of decomposition. These uncharacterized substances resist further microbial degradation and account for the very slow availability of soil N (http://msucares.com/crops/soils/nitrogen.html).

Plants may use either ammonium (NH4+), or nitrate (NO3-) which behave quite differently in soils. Positively charged NH4+ is attracted to negatively charged sites on soil particles as are other cations. It is available to plants, but the electrostatic attraction protects it from leaching. Conversely, negatively charged NO3- does not react with the predominately negatively charged soil particles, so it remains in the soil solution and moves with soil water. Therefore NO3- may leach out of the root zone when rainfall is excessive, or accumulate at the soil surface when conditions are dry.

Siklus Nitrogen

Untuk melukiskan proses siklus nitrogen secara sederhana, N bergerak dari tanah memasuki tumbuhan, dan kembali lagi dari tumbuhan ke tanah, seringkali dengan intermediasi binatang atau manusia. Pada kenyataannya, situasinya jauh lebih kompleks, karena senyawa-senyawa N mengalami berbagai transformasi dalam tanah (mineralization, immobilization, fixation, nitrification dan denitrification). Senyawa-senyawa ini mengalami proses pertukaran antara tanah dan atmosfir (melalui proses-proses volatilization, denitrification, biological N fixation, atmospheric deposition) dan antara tanah dengan hydrosphere (melalui proses-proses pencucian, erosion/runoff, drainage, irrigation).

The soil nitrogen cycle (Adapted from Hofman and Van Cleemput, 2004) (http://www.fertilizer.org/ifa/HomePage/SUSTAINABILITY/Climate-change/Nitrogen-cycle.html)

Nitrogen conversions depend on soil moisture conditions, soil acidity, temperature, and microbial activity. Because the climate is warm and humid, microbial transformations occur throughout the year. This constant breakdown results in lower organic matter levels in soils than in cooler, drier climates.

Siklus nitrogen Tanah-atmosfir (Sumber: Nitrogen Fertilization of Corn . Agronomy Facts 12. http://cropsoil.psu.edu/extension/facts/agronomy-facts-

12)

Fiksasi Nitrogen Atmospheric nitrogen must be processed, or "fixed" (see page on nitrogen fixation), to be used by plants. Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make their own organic compounds. Most biological nitrogen fixation occurs by the activity of Mo-nitrogenase, found in a wide variety of bacteria and some Archaea. Mo-nitrogenase is a complex two component enzyme that has multiple metal-containing prosthetic groups. [5] Some nitrogen fixing bacteria, such as Rhizobium, live in the root nodules of legumes (such as peas or beans). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates. Nutrient-poor soils can be planted with legumes to enrich them with nitrogen. A few other plants can form such symbioses. Today, about 30% of the total fixed nitrogen is manufactured in ammonia chemical plants.

Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase.

The reaction for BNF is:

N2 + 8 H+ + 8 e− → 2 NH3 + H2

The process is coupled to the hydrolysis of 16 equivalents of ATP and is accompanied by the co-formation of one molecule of H2. In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway.Enzymes responsible for nitrogenase action are very susceptible to destruction by oxygen. (In fact, many bacteria cease production of the enzyme in the presence of oxygen). Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with a protein such as Leghemoglobin.Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as clovers, soybeans, alfalfa, lupines, peanuts, and rooibos. They contain symbiotic bacteria called Rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil[1][5] The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional and organic farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of clover or buckwheat (non-legume family Polygonaceae), which are often referred to as "green manure."

Konversi N2

The conversion of nitrogen (N2) from the atmosphere into a form readily available to plants and hence to animals and humans is an important step in the nitrogen cycle, which distributes the supply of this essential nutrient. There are four ways to convert N2 (atmospheric nitrogen gas) into more chemically reactive forms:

1. Biological fixation: some symbiotic bacteria (most often associated with leguminous plants) and some free-living bacteria are able to fix nitrogen as organic nitrogen. An example of mutualistic nitrogen fixing bacteria are the Rhizobium bacteria, which live in legume root nodules. These species are diazotrophs. An example of the free-living bacteria is Azotobacter.

2. Industrial N-fixation: Under great pressure, at a temperature of 600 C, and with the use of an iron catalyst, hydrogen (usually derived from natural gas or petroleum) and atmospheric nitrogen can be combined to form ammonia (NH3) in the Haber-Bosch process which is used to make fertilizer and explosives.

3. Combustion of fossil fuels: automobile engines and thermal power plants, which release various nitrogen oxides (NOx).

4. Other processes: In addition, the formation of NO from N2 and O2 due to photons and especially lightning, can fix nitrogen.

Asimilasi Nitrogen

Plants take nitrogen from the soil, by absorption through their roots in the form of either nitrate ions or ammonium ions. All nitrogen obtained by animals can be traced back to the eating of plants at some stage of the food chain. Plants can absorb nitrate or ammonium ions from the soil via their root hairs. If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll. In plants that have a mutualistic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules. Animals, fungi, and other heterotrophic organisms obtain nitrogen by ingestion of amino acids, nucleotides and other small organic molecules.

Model asimilasi NitrogenThe ODE model describes the processes by which ammonia is taken up (N-assimilation) and nitrogen is distributed to biosynthetic pathways (N-utilization). The concentration of species within the dashed boxes (GLU, GLN, ASP, intracellular NH3, and unmodified GS and PII) are calculated as variables by the model, whereas those outside the boxes ( -KG, OA, and extracellular NH3) must be provided as inputs. N-assimilation consists of five metabolic and regulatory reactions: (1) glutamate dehydrogenase, GDH; (2) glutamine synthetase, GS; (3) glutamate synthase, GOGAT; (4) adenylyltransferase/adenylyl-removing enzyme, AT/AR; (5) uridylyltransferase/uridylyl-removing enzyme, UT/UR. Ammonia enters the cell by passive diffusion across the membrane. N-utilization consists of two different classes of reactions, which consume amino acids: those in which a nitrogen group is transferred to an acceptor

molecule, recycling the carbon skeleton (N-donation) and those that consume the entire metabolite (protein and biosynthesis). In the model, the rate of GLN, GLU, and ASP consumption for biosynthesis and protein production was taken to be proportional to growth rate.

Sumber: Metabolomics-driven quantitative analysis of ammonia assimilation in E. coli. Jie Yuan, Christopher D Doucette, William U Fowler, Xiao-Jiang Feng, Matthew Piazza, Herschel A Rabitz, N.S. Wingreen and J.D.Rabinowitz. 2009. Molecular Systems Biology 5 Article number: 302.

AmmonifikasiWhen a plant or animal dies, or an animal expels waste, the initial form of nitrogen is organic. Bacteria, or fungi in some cases, convert the organic nitrogen within the remains back into ammonium (NH4

+), a process called ammonification or mineralization. Enzymes Involved:

GS: Gln Synthetase (Cytosolic & PLastid) GOGAT: Glu 2-oxoglutarate aminotransferase (Ferredoxin & NADH

dependent) GDH: Glu Dehydrogenase:

o Peranannya dalam asimilasi ammonium relative minor.o Penting dalam katabolisme asam amino.

Nitrifikasi

Nitrification is a process of nitrogen compound oxidation (effectively, loss of electrons from the nitrogen atom to the oxygen atoms):

1. NH3 + 1.5 O2 + Nitrosomonas → NO2- + H2O + H+

2. NO2- + 0.5 O2 + Nitrobacter → NO3

-

3. NH3 + O2 → NO2− + 3H+ + 2e−

4. NO2− + H2O → NO3

− + 2H+ + 2e−

The conversion of ammonium to nitrate is performed primarily by soil-living bacteria and other nitrifying bacteria. In the primary stage of nitrification, the oxidation of ammonium (NH4

+) is performed by bacteria such as the Nitrosomonas species, which converts ammonia to nitrites (NO2

-). Other bacterial species, such as the Nitrobacter, are responsible for the oxidation of the nitrites into nitrates (NO3

-).[3] It is important for the nitrites to be converted to nitrates because accumulated nitrites are toxic to plant life.Due to their very high solubility, nitrates can enter groundwater. Elevated nitrate in groundwater is a concern for drinking water use because nitrate can interfere with blood-oxygen levels in infants and cause methemoglobinemia or blue-baby syndrome.[7] Where groundwater recharges stream flow, nitrate-enriched groundwater can contribute to eutrophication, a process that leads to high algal, especially blue-green algal populations and the death of aquatic life due to the algae's excessive demand for oxygen. While not directly toxic to fish life, like ammonia, nitrate can have indirect effects on fish if it contributes to this eutrophication. Nitrogen has contributed to severe eutrophication problems in some water bodies. Since 2006, the application of nitrogen fertilizer has been increasingly controlled in Britain and the United States. This is occurring along the same lines as control of phosphorus fertilizer, restriction of which is normally considered essential to the recovery of eutrophied waterbodies.

DenitrifikasiDenitrification is a microbially facilitated process of nitrate reduction that may ultimately produce molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products.Denitrification is the reduction of nitrates back into the largely inert nitrogen gas (N2), completing the nitrogen cycle. This process is performed by bacterial species such as Pseudomonas and Clostridium in anaerobic conditions. They use the nitrate as an electron acceptor in the place of oxygen during respiration. These facultatively anaerobic bacteria can also live in aerobic conditions.Denitrification takes place under special conditions in both terrestrial and marine ecosystems.[14] In general, it occurs where oxygen, a more energetically favourable electron acceptor, is depleted, and bacteria respire nitrate as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere denitrification only takes place in anaerobic environments where oxygen consumption exceeds the oxygen supply and where sufficient quantities of nitrate are present. These environments may include certain soils[15] and groundwater,[16]

wetlands, oil reservoirs, poorly ventilated corners of the ocean, and in seafloor sediments.

Denitrification generally proceeds through some combination of the following intermediate forms:

NO3− → NO2

− → NO + N2O → N2 (g)

The complete denitrification process can be expressed as a redox reaction:

2 NO3− + 10 e− + 12 H+ → N2 + 6 H2O

This reaction shows a fractionation in isotope composition. Lighter isotopes of nitrogen are preferred in the reaction, leaving the heavier nitrogen isotopes in the residual matter. The process can cause delta-values of up to −40, where delta is a representation of the difference in isotopic composition. This can be used to identify denitrification processes in nature.

Oksidasi ammonium secara Anaerobik In this biological process, nitrite and ammonium are converted directly into elemental nitrogen (N2) gas. This process makes up a major proportion of elemental nitrogen conversion in the oceans.

ImobilisasiImmobilization is the process of incorporating inorganic into organic form by microbes or plants. Because it is largely dependent on microbes, the availability of carbon and other nutrients determine the rate of immobilization. When residues with high carbon:N ratios are being decomposed, all readily available N within the soil system may be tied up by the microbes and therefore unavailable for plant uptake. This effect eventually fades because, without external N, the microbial population dies off and decomposes, releasing N which is available to plants. The risk of immobilization is avoided by mixing plant residues into the soil well before the next cropping cycle.

Volatilisasi NVolatilization is the loss of ammonia gas (NH3) to the atmosphere from anhydrous ammonia, urea, or N solution fertilizer sources. Losses to volatilization are minimalized through management of fertilizer applications. Anhydrous ammonia applicators should have good sealing of the soil after fertilization. The first step in urea N conversion to NH4+ is the production of NH3. Escape losses from urea are more likely with warm temperatures (over 50º F) and on high pH soils (greater than 7). Ideally, urea should be incorporated into the soil or applied prior to a rainfall. Urea volatilization loss is increased when applied to vegetative cover, or plant residue, so care should be exercised using urea as a topdressing N source. Urea should never be placed in contact with seed because of toxic effects of NH3 on seedlings. The same precautions apply to N solutions as they contain urea.

PEMUPUKAN N dan PERILAKU TANAH

Initially, urea breakdown increases ammonia concentration, which makes soil immediately surrounding urea particles more alkaline. This zone can be somewhat toxic to seedlings until the ammonia is neutralized, although this process takes place rather quickly. It's usually a few days before nitrogen from the urea becomes available to the plants. Despite its initial effects, urea ultimately makes soil more acidic. Urea causes soil acidification when it is broken down to form ammonium and the subsequent conversion of ammonium to nitrate, which is the predominant form of nitrogen taken up by plant roots.(The Effects of Urea Fertilizer | eHow.com http://www.ehow.com/facts_7544774_effects-urea-fertilizer.html#ixzz1k6HrkuVK)

Pemupukan nitrogen (Urea atau Vermikompos) meningkatkan kandungan bahan organic dalam tanah, dan ketersediaan hara dalam tanah.

Respons BOT dalam tanah yang dipupuk dengan Urea atau Vermikompos dengan dosis hingga 80 kg N/ha (Sumber: Tran Hoang Chat, Ngo Tien Dung, Dinh Van Binh

and T. R. Preston. 2005).

Respons N-total dalam tanah yang dipupuk dengan Urea atau Vermikompos dengan dosis hingga 80 kg N/ha (Sumber: Tran Hoang Chat, Ngo Tien Dung, Dinh Van Binh and T. R. Preston.

2005).

Respons K-tanah dalam tanah yang dipupuk dengan Urea atau Vermikompos dengan dosis hingga 80 kg N/ha (Sumber: Tran Hoang Chat, Ngo Tien Dung, Dinh Van Binh and T. R. Preston.

2005).

Respons P-tanah dalam tanah yang dipupuk dengan Urea atau Vermikompos dengan dosis hingga 80 kg N/ha (Sumber: Tran Hoang Chat, Ngo Tien Dung, Dinh Van Binh and T. R. Preston.

2005).

Pemupukan nitrogen meningkatkan kadar nitrat daun tomat:

Sumber: Paulo Cezar Rezende Fontes and Cláudio Pagotto Ronchi (2002).

HUBUNGAN HARA – TANAH – TANAMAN

The “fertilize the soil” approach continues to apply fertilizer in the area indicated as “apply maintenance fertilizer”. Critical level is the soil test value where a nutrient is no longer applied. The extra fertilizer builds the soil test level for future crops. At this soil level, yield increases from added fertilizer may not occur or may be small. So why should a farmer ever fertilize beyond the economic maximum yield, or why would anyone recommend fertilizer beyond this point? The reason is that we cannot make a fertilizer recommendation with absolute certainty. Crop use of applied fertilizers is never 100 percent. For example, nitrogen fertilizer may be lost by leaching or denitrification, and phosphorus may be fixed or its availability greatly reduced by some soils. To reduce risk, many agronomists support the maintenance philosophy over the deficiency correction approach. They would rather a farmer not sacrifice potential yield and profit by applying beyond the soil critical level than to be somewhat below the critical level. Calculations show that profit is only marginally reduced when fertilizer is applied at slightly above the economic optimum yield point. It is usually more profitable for a farmer to slightly over-fertilize than to have yields on the low side of the economic optimum.

Hubungan antara ketersediaan hara dalam tanah dengan hasil relative tanaman.

Yield response as influenced by soil test level and soil test recommendation approach. (Hergert, 1997). Sumber: The Scientific Basis for Making Fertilizer Recommendations.

http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447047&topicorder=4&maxto=6&minto=1.

Pemupukan nitrogen hingga dosis optimumnya biasanya akan diikuti oleh peningkatan hasil biomasa dan hasil biji jagung.

Sumber: http://nitrogenes.cropsci.illinois.edu/

Menggunakan Pupuk N secara efisien:Fertility management of phosphorus and potassium should always be based

on a soil testing program. The frequent interchanges of N between forms limit the value of N soil testing as a predictive tool in warm, humid Mississippi. Supplemental N fertilization is usually necessary for economic production of nonlegume crops, but heavy N applications cannot substitute for poor management.

Waktu aplikasi pupuk N dalam hubungannya dengan fase pertumbuhan tanaman sangat menentukan efisiensi hara pupuk. Pada tanaman jagung waktu aplikasi pupuk nitrogen yang tepat diabstraksikan pada gambar berikut.

Ancaman kehilangan hara N dari aplikasi pupuk sangat besar, sehingga sekitar 50 - 90 % dari total hara N yang dibutuhkan oleh tanaman jagung diaplikasikan dalam bentuk pupuk secara sidedress ketika tanaman jagung sudah tumbuh tingginya mencapai 10 - 20 inches. (Sumber: Nitrogen Fertilization of Corn . Agronomy Facts 12. http://cropsoil.psu.edu/extension/facts/agronomy-facts-12).

HUBUNGAN PEMUPUKAN N DENGAN HASIL TANAMAN

Nitrogen diperlukan oleh tanaman untuk sintesis khlorofilnya, kecukupan nitrogen dapat menjamin khlorofil berkembang dengan baik. Hasil-hasil penelitian membuktikan bahwa status khlorofil tanaman sangat dipengaruhi oleh pemupukan N, demikian juga kandungan N-organik dalam tubuh tanaman.

Relationship between relative chlorophyll meter values and the N-rate deviation from the economic optimum N rate. Sumber: http://www.ipm.iastate.edu/ipm/icm/2007/5-

14/measuren.html

Sumber: Paulo Cezar Rezende Fontes and Cláudio Pagotto Ronchi (2002).

Sumber: Paulo Cezar Rezende Fontes and Cláudio Pagotto Ronchi (2002).

Kurva respon hasil biomasa tanaman terhadap pemupukan nitrogen dirumuskan dalam bentuk kuadratik (gambar di bawah).

Tran Hoang Chat, Ngo Tien Dung, Dinh Van Binh and Preston (2005):The field trial was conducted at the Goat and Rabbit Research Centre, Sontay, Hatay, Vietnam from April to December 2004. The aim was to evaluate the response of water spinach to fertilization with increasing levels of nitrogen (0, 10, 20, 30, 40, 50, 60 kg N/ha over 28 days) in the form of earthworm compost or urea. The biomass

yield response to fertilizer N was positive and curvilinear and was greater for the earthworm compost at the higher levels of application of N. Increasing application of fertilizer N provoked linear responses in DM content, which decreased, and in crude protein content, which increased. Soil fertility was improved by the worm compost, but not by urea, as measured by the organic matter, phosphorus and potassium contents of the soil at the end of the trial.

Respons Hasil Biomasa tanaman water spinach yang dipupuk Urea atau Vermikompos dengan dosis hingga 80 kg N/ha (Sumber: Tran Hoang Chat, Ngo Tien Dung, Dinh Van Binh

and T. R. Preston. 2005).

Pemupukan nitrogen dapat memperbaiki ketersediaan N tanah bagi tanaman tomat, dan mempengaruhi hasil biomasa tanaman dan hasil buah tomat, kurva responnya berbentuk non-linear.

Sumber: Paulo Cezar Rezende Fontes and Cláudio Pagotto Ronchi (2002).

Pemupukan nitrogen meningkatkan hasil biji jagung, kurva responnya non-linear, sehingga dapat ditentukan dosis optimum pupuk N

Pengaruh pemupukan N terhadap hasil biji jagung. (Sumber: Nitrogen Fertilization of Corn . Agronomy Facts 12. http://cropsoil.psu.edu/extension/facts/agronomy-facts-12).

Rekomendasi Pemupukan N tanaman jagung1. If possible, plan corn production as part of a rotation with legumes. Take full

advantage of the residual N by reducing fertilizer N.2. Keep crop yield records for each of your fields.3. Soil test every three years or when changing crops in a field.4. Provide yield goal and previous crop information on soil test questionnaire for best

N recommendation.5. Account for available manure N for each field.6. Don’t waste fertilizer dollars by applying excess N.7. Avoid N volatilization and loss, either by choice of N source or through

incorporation.8. Avoid denitrification and leaching losses of N by delaying application of the bulk of

the N to be applied.9. If UAN is your N source, apply it as a dribbled band rather than spraying it.10. Use the PSNT on fields with a history of organic N additions.11. Keep track of soil surface pH in no-till corn production.

(Sumber: Douglas B. Beegle and Philip T. Durst. 2003. Penn State’s College of Agricultural Sciences. Web: agsci.psu.edu)

BAHAN BACAAN

Aerts, Rien and F. Berendse. 1988. "The Effect of Increased Nutrient Availability on Vegetation Dynamics in Wet Heathlands". Vegetatio 76 (1/2): 63–69.

Chapin, S.F. III, Matson, P.A., Mooney H.A. 2002. Principles of Terrestrial Ecosystem Ecology. Springer, New York 2002 ISBN 0387954430, p.345

CLAASSEN, M. E. T. and G.E.WILCOX. 1974. Effect of nitrogen forms on growth and composition of tomato and peas tissue. Journal of the American Society for Horticulture Science, Alexandria, v. 99, p. 171-174, 1974.

FERNANDES, M. S. and R.O.P.ROSSIELO. 1995. Mineral nitrogen in plant physiology and plant nutrition. Critical Reviews in Plant Sciences, Boca Raton, v. 14, n. 2, p. 111-148.

Moir, J.W.B. (editor) 2011. Nitrogen Cycling in Bacteria: Molecular Analysis. Caister Academic Press. ISBN 978-1-904455-86-8.

Paulo Cezar Rezende Fontes and Cláudio Pagotto Ronchi. 2002. Critical values of nitrogen indices in tomato plants grown in soil and nutrient solution determined by different statistical procedures. Pesq. agropec. bras. vol.37 no.10 Brasília Oct. 2002

SMEAL, D. and H. ZHANG. 1994. Chlorophyll meter evaluation for nitrogen management in corn. Communications in Soil Science and Plant Analysis, New York, v. 25, p. 1495-1503.

Smil, V. 2000. Cycles of Life. ScientificAmerican Library, New York., 2000) Smith, B., R. L. Richards, and W. E. Newton. 2004. Catalysts for nitrogen fixation :

nitrogenases, relevant chemical models and commercial processes. Kluwer Academic Publishers, Dordrecht ; Boston.

Steven B. Carroll; Steven D. Salt. 2004. Ecology for gardeners. Timber Press. p. 93. ISBN 9780881926118. http://books.google.com/books?id=aM4W9e5nmsoC&pg=PA93.

Tran Hoang Chat, Ngo Tien Dung, Dinh Van Binh and T. R. Preston. 2005. Effect on yield and composition of water spinach (Ipomoea aquatica), and on soil fertility, of fertilization with worm compost or urea. Workshop-seminar, Making Better Use of Local Feed Resources 23-25 May, 2005, MEKARN-CTU.

Vance, C. P. 2001. Symbiotic Nitrogen Fixation and Phosphorus Acquisition. Plant Nutrition in a World of Declining Renewable Resources. Plant Physiol. 127:390-397.

Vitousek, P.M.; J.Aber; R.W.Howarth; G.E.Likens; P.A.Matson; D.W.Schindler; W.H.Schlesinger; G.D.Tilman. 1997. "Human Alteration of the Global Nitrogen Cycle: Causes and Consequences". Issues in Ecology 1: 1–17.