UNIT V FERTILISERS Fertilizers or manure are substances

UNIT V FERTILISERS

• Fertilizers or manure are substances that are added to soil to increase its fertility and for the healthy growth of plants. They are naturally occurring inorganic or organic substances or artificially made substances.

Need for fertilizer: • Plants need the following substances for their growth: water, nitrogen compounds, potassium compounds, C, H, O 2, Ca, Mg, S, Zn, Boron, Cu, Mn, Mo, Cl, Fe etc. C, H, and O 2 are derived from air and water and are called as natural nutrients. • N, P, and potassium are consumed in large amounts and so they are called as primary nutrients. Ca, Mg and S are secondary nutrients. Rest of the elements is required in minute amounts so they are called as micro nutrients.

• Soil becomes exhausted, drained by successive crops and so loses the vital minerals present in it. So fertilizers are added to i) replenish the loss due to plant growth ii) increases the fertility of soils iii) maintain the PH of the soil in 7 -8, to facilitate optimum growth and health iv) supply them additional tonic and good food, so that plants grow healthier and gives better yield.

• In the ancient period people used natural manures like cow dung, horse dung, urines and wastes from the stables, wood, ash etc. , for replenishing the soil. Organic bacteria present in these materials produce humus and humic acid which are absorbed by the soil.

Essential requirements for a fertilizer: • Every compound with N, P, Ca etc cannot be used as fertilizer, the important requirements for a fertilizers are

• Element present in the compound must be easily available to plant • soluble in water • stable, so that made available to plant for a long time • Should not be very costly • Should maintain PH of soil • Should no be a poison to plant • Should not be hygroscopic

Important functioning of fertilizers is • the movement of water in the soil- lime opens the pores of the soil and enable a free circulation of water • action of CO 2 on the material in the soil- CO 2 soluble in water and forms carbonic acid, which easily dissolves carbonates of Ca, Mg and oxides of iron, Mn which are insoluble in water.

Soil improvers: • Substances that are added to improve the soil texture is called soil improvers.

Soil amendments: • Substances that are added to correct acidity or high concentration of certain salts are called soil amendments, eg : lime, gypsum.

Fertility of the soil Virgin soil • The soil one in which plants has not grown for a long time. So the soil will be fertile.

Exhausted soil • These are soils in which crops are grown one after another and so they might have lost their nutrients. Fertility of the soil can be affected by a) amount of fixed nitrogen b) N, P, K and minerals c) PH value of soil.

PH value: • The PH value of the soil must be between 7 and 8, ie neutral or slightly alkaline. If the PH is greater than 10 or less than 3 then the soil is sterile soil. • During cultivation Ca, Mg etc are removed, so PH decreases and so acidity increases. If this is the case then add base like lime. If PH is less than 3 then no cultivation is possible.

Classifications of fertilizers Based on the mode of operation on the soil fertilizers can be classified as follows Direct fertilizers These are fertilizers which are directly absorbed by the plants. eg. Nitrates, super phosphates and ammonium compounds.

In direct fertilizers • These are fertilizers which do not act as fertilizers, but help fertilization indirectly in keeping the soils PH suitable for plant growth. eg. Lime, barn yard Complete fertilizers • These are fertilizers which provide all the essential elements like N, P, K etc. eg: Guano

Incomplete fertilizers • These are fertilizers which contain only one or two needed elements. Eg: ammonium phosphate, KNO 3

Straight and mixed • Most crop need more than one nutrient and they are prepared by mixing appropriate amounts of ammonium salts, super phosphates, and potassium salts etc. • Straight fertilizers are used in special situations, like giving plants additional supply of nitrogen. Eg : potash , phosphates.

• Various materials are used in mixed fertilizer. Eg : ammonium solution and super phosphates + KCl. Ammoniate solution contain NH 3 along with ammonium nitrate or urea or both. This supplies supplemented nitrogen. • The following proportion can be mixed and used – 2% N, 8% P 2 O 5, 2% K 2 O etc and this differ from crops and season.

Other classifications I : Based on number of nutrient elements i) single ii) compound or multiple. Single: only one single nutrient Compound: Two or more nutrients eg: CAN, ammonium super phosphate II : 1) Solid fertilizers : Produced in the form of powders and granules. Granular type is superior to powder type. 2) Liquid: Liquid ammonia, aqueous ammonia, ammonium nitrate

Nitrogen cycle and its fixation: • Nitrogen is the essential constituent of vegetable and animal tissues, but both cannot absorb nitrogen directly, except leguminous plants (these uses symbiotic bacteria in their roots). During electric discharge in atmosphere, nitrogen is converted to nitric oxide. This on oxidation by excess oxygen gives nitrogen dioxide. This reacts with rain water and forms nitric acid which is washed down by rain to soil. In the soil it is converted to nitrates by bases.

• This fixed nitrates and nitrites which are soluble are absorbed by plants through their roots. Plants convert them into complex nitrogenous organic substance called proteins. 250000 tones of nitric acid are produced daily by electric discharge. Animals gain nitrogen from the protein present in plants by eating them. Herbivorous animals gain nitrogen directly from plants and carnivorous by consuming other animals.

• Due to this nitrogen content of air gradually decreases. But much of the nitrogen consumed by animals is resorted to the soil in the form of urea in the excreta. Urea is readily hydrolyzed to ammonia and carbon dioxide. Plant or animal body decay and the organic matter liberates nitrogen to the soil as ammonia, this is then oxidized in soil by nitrosofying bacteria and nitrifying bacteria into nitrates, which again can be assimilated by plants.

• The fixed nitrogen can be set free by denitrifying bacteria in the soil. So nature maintains the nitrogen amount. This process of fixing and setting free nitrogen is called nitrogen cycle.

• During fixation most of nitrogen is lost in sea and other water bodies. So ammonia formed during fixation is made into fertilizers as • Ammonium sulphate , prepared by passing ammonia gas into sulphuric acid.

• Ammonium sulphate and calcium carbonate mixture prepared from the reaction between gypsum, carbon dioxide and ammonia. • Double salt of ammonium sulphate and ammonium nitrate

• Urea from synthetic ammonia • Manufacture of nitric acid by ammonia • Manufacture of calcium cyanamide or nitrolim.

Source of fertilizer: I) Natural a) Plant matter- oil cakes b) Farm yard manures-Cow dung, sheep dung and human excretions c) Animal matter- Powdered dry fish, red dry blood from slaughter house. d) Guano – Bird’s excrement, fish refuge, fish hones

II) Natural Organic Fertilizers Chile salt peter : Na. NO 3 Rock phosphates : Insoluble in water, finely divided, weathers rapidly Potassium salts : Wood ash and waste materials of sugar beet crops.

Nitrogenous fertilizers Ammonium sulphate: (NH 4)2 SO 4 This is manufactured by passing ammonia gas into 60% H 2 SO 4 solution. On large scale it is prepared by passing ammonia into a suspension of gypsum powder in water and then passing CO 2 through the liquid. Then separate Ca. CO 3 formed. Ammonium sulphate solution on evaporation gives needle shaped crystal.

Action as fertilizer • It reacts with lime of the soil to form ammonium hydroxide, this then gets oxidized by air with the help of nitrolifying bacteria into nitrous acid and then to nitrites. And these are then oxidized to nitric acid and nitrates. Nitric acid is converted to calcium nitrate and potassium nitrate in the soil. Most plants takes up nitrogen in the form of soluble nitrates.

2) Double salt of ammonium sulphate and ammonium nitrate • Ammonium sulphate is manufactured as above. Ammonium nitrate is prepared from the reaction between ammonia and nitric acid. Mix these two we get the valuable double salt fertilizer. Ammonium nitrate is and explosive so if used as a single fertilizer add calcium carbonate (this mixture is called nitro chalk) or ammonium sulphate.

3) Urea • This is manufactured by the action of ammonia with carbon dioxide under high pressure and at a temperature 200 -210 o. C. This is an exothermic reaction. At ordinary pressure the reaction is slow and in the presence of water, ammonium carbonate is formed. At high temperatures ammonium carbonate formed decomposes to urea and water. This method gives a high yield.

• It is also manufactured by the action of liquid ammonia with carbon dioxide at 180 o. C. The yield by this method is only 37%. So ammonia and carbon dioxide are left out, and these two are converted to ammonium carbonate. This on treatment with dilute nitric acid gives ammonium nitrate and carbon dioxide. Carbon dioxide formed can be recycled.

• This fertilizer is less hygroscopic than ammonium nitrate. If biuret (NH 2 CONH 2) is a component of urea, it is toxic to plants like citrus crops. This fertilizer is easily hydrolysed and loses ammonia to atmosphere, generally at high temperature. It is slow in conversion to nitrates in soils compared to ammonium nitrate and ammonium sulphate.

Calcium cyanamide : Ca(CN)2 • This has 21% of nitrogen. This is manufactured by combining lime and coke in an electric furnace, gives calcium carbide. Treat this calcium carbide with nitrogen to produce calcium cyanamide.

• Calcium cyanamide mixed with carbon is called nitrolim, which is used as a nitrogenous fertilizer. Calcium cyanamide is a good fertilizer since it slowly converts to ammonia and nitrates, so prolonged action.

• In soil it first changes into calcium carbonate and cyanamide. The cyanamide formed reacts with water to form urea. Urea is hydrolyzed by water by urase enzyme and forms ammonia. Ammonia formed is converted into nitrates by nitrifying bacteria.

5) Calcium ammonium nitrate (CAN) • This is manufactured by reacting lime stone with concentrated nitric acid. Ammonia is added to neutralize excess acid, and evaporate the resulting solution. Then convert the melt into flakes, the double salt 5 Ca(NO 3)2. NH 4 NO 3 is formed. This is more useful than calcium nitrate since this has only 15. 5% 0 f nitrogen but CAN has 19% of nitrogen.

• Ammonium nitrate is first prepared by reacting ammonia and nitric acid. Neutralize the un reacted nitric acid by adding calcium carbonate and cool , CAN separates out. Granules of CAN are coated with a thin layer of soap stone which acts as a protective coating and prevents absorption of moisture during storage and transportation.

6) Sodium nitrate: Na. NO 3 • This has 16% of nitrogen, rarely used and mined from Chile. This if used as fertilizer reduces the acidity of the soil.

7) Ammonium chloride: NH 4 Cl • This has 26% of nitrogen, and prepared by reacting ammonia with hydrochloric acid.

Phosphate fertilizers 1) Super phosphates: These are soluble phosphates of calcium. There are three types namely; mono, di and tri super phosphates. Among these only mono calcium phosphate is soluble.

• Phosphatic fertilizers are taken up by the plants in the soluble form. When tri calcium phosphate in the form of bone-meal or rock phosphate powder is applied to the soil it is slowly transformed into mono calcium phosphate and then the soluble phosphate is taken by the plant.

• To avoid this slow process rock phosphate is transformed quickly into soluble phosphate when applied to the soil. This artificially prepared mono calcium phosphate is called as super phosphate. Commercial super phosphate is a mixture of mono calcium super phosphate and calcium sulphate. It contains 16% of P 2 O 5.

2) Normal super phosphate: • The soluble calcium phosphate prepared by reacting rock phosphate with sulphuric acid is called super phosphate. Normal super phosphate is manufactured by mixing equal quantities of powdered phosphate rock and sulphuric acid in a cast iron mixer.

• Mix the contents for five minutes and leave for a day, a solid mass is formed. The phosphate rock used should contain 20% P 2 O 5. Normally the rock containing 33. 5% P 2 O 5 is used, and 93 -98% sulphuric acid is used.

3) Triple super phosphate or concentrated super phosphate Ca (H 2 PO 4)2 This has 44 -47% of P 2 O 5, three times higher than normal super phosphate. This is manufactured by the action of 78% phosphoric acid (has 5254% P 2 O 5) on finely divided calcium phosphate or phosphate rock containing 32. 5 -33% of P 2 O 5. In an impure mono calcium phosphate form it is obtained from the following reaction

Ca 10 (PO 4)6 F 2 + 14 H 3 PO 4 + 10 H 2 O → 10 Ca. H 4(PO 4)2. H 2 O + 2 HF Its property depends on the type of rock and acid used for the production and on granular of non granular nature of the product.

4) Ammonium phosphate 1. Mono ammonium phosphate: This is prepared by the action of sulphuric acid on a mixture of calcium phosphate and ammonium sulphate. This has 12% of nitrogen and 50% of P 2 O 5.

2. Di ammonium phosphate: This is prepared by passing anhydrous ammonia gas and pure phosphoric acid into a saturated mother liquor containing mono ammonium phosphate.

Potassium fertilisers Potassium nitrate This is an important potassium fertilizer, also called as Chile salt peter (K 2 O. KNO 3) and is obtained from nature.

Potassium chloride This prepared by evaporating Dead Sea brine by solar energy. By this method 97% pure KCl is obtained. This has 63. 17% K 2 O and 52. 44% of potassium.

Potassium sulphate This is prepared by the action of sulphuric acid on KCl, or by the action of KCl and sulphur or from burkeite(Na 2 CO 3. 2 Na 2 SO 4) by reacting this with KCl or from langbeinite(K 2 SO 4. 2 Mg. SO 4) by ion exchange to remove magnesium sulphate. This has 54% of K 2 O.

Natural organic materials: • Besides synthetic N, P, K fertilizers natural organic materials are also used as fertilizers. Commonly used of these types is cow dung and horse dung. Nitrogen present in Humous and humic acid in cow dung by the use of bacteria is fixed easily in the soil. Other materials used are dried blood, castor pomace, mustard pomace, compost, cotton seed meal, fish scrap, sewage sludge etc.