Synthetic Fertilizers

Synthetic Fertilizers

The chemical compounds which are obtained by the artificial methods which are added to the soil to increase the fertility of the soil and the agricultural production are known as synthetic fertilizers.To increase the production of the crops,the use of chemical fertilizers ie essential.In Nepalese lands,due to the rainfall,the nutrients for plants are washed away and fertilizers are essential for the increment of the plant products.To increase the agricultural products,three chemical fertilizers are very essential.They are Urea,DAP and Potash fertilizers.

Essential Qualities Of a Good Fertilizer

N,P and K are the essential nutrients for a plant.But every compound of N,P or K cannot be used as a fertilizer.A compound containing N,P and K can act as a fertilizer only when it has the following condition:

i)It should be soluble in the water.

ii)N,P and K must be present in the compound,in such a form,that they can easily be assimilated by the plants.

iii)By rain or water,the compound must be changed into the form which the plant can take up easily.

iv)The compounds should be stable,so that it may be available for a long time for the growing plant.

v)It should not be injurious to the plants.

vi)It should be able to correct the acidity of the soil.

vii)It should be cheap.

Problems caused by the improper use of the nitrate fertilizers

The widespread use of nitrate fertilizers greatly increases the crop yields.Since nitrate fertilizers are highly soluble in water,the run off water into lakes,river and sea also contain nitates.The problem caused by the improper use of the nitrate fertilizers are given below:

i)It produces increased growth of algae and other aquatic plants which may clod off river and lakes(Eutrophication).

ii)It produces NO₃^- ions which makes the soil acidic.

iii)Higher level of nitrates in drinking water would result many disease.It cause methaglobonemia which reduce the amount of oxygen and haemoglobin in babies blood.

iv)Denitrification of nitrates into oxides of nitrogen particularly N₂O may harm the ozone layer.

Urea

Urea is widely used as a nitrogenous fertilizer.It is very soluble and hence quick acting but it is easily washed away.It has very high nitrogen content(46.6 %).The nitrogenous component that is made in the largest volume in the world is urea.It is prefered nitrogenous fertilizer worldwide and its demand is incresing day by day.Urea is also used for the production of the industrial product.The main reason for the increasing demand of the urea is due to the high nitrogen content fertilizer provided by urea in solid or in aqueous form.Urea is also replacing the highly ammonium salts in region where soil is already too acidic and lime(CaO) is also not present in the soil.Urea can also be easily granulated and provide medium for slow release type of fertilizer by forming low molecular weight urea formaldehyde resins.

Manufacture

Urea is manufactured by liquid NH₃ with CO₂ at 180–200°C under 100–300 aatmospheric pressure.Urea formation takes place in the two stages,ammonium carbamate(H₂NCOONH₄) is obtained first which is unstable and decomposes to give urea.

$$2NH_3+CO_2 \xrightarrow[100–300atm]{180–200°C} NH_2COONH_4 \rightarrow NH_2CONH_2+H_2O$$

In the soil,urea slowly hydrolyses to ammonium carbonate.

$$NH_2CONH_2+2H_2O \rightarrow (NH_4)_2CO_3$$

Advantages of urea as a fertilizer

i)Urea hs the highest nitrogen content(46.6%).Higher nitrogen content in urea results in freight and packing.

ii)The cost of production of ammonia is relatively low.

iii)Urea is not subject to fire on explosion hazards,as in NH₄NO₃ and hence there is no risk in the storage of urea.

iv)It does not change the pH of the soil.

v)Urea cab be used for all types of crops and soils.

Disadvantages of urea as a fertilizer

i)Urea is very soluble in water and hygroscopic and hence requires better quality package than (NH₄)₂SO₄.

ii)It is not so stable as other solid nitrogenous fertilizers,since urea decompose even at lower temperature releasing NH₃ and CO₂.The formation of NH₃ and CO₂ results in serious loss.

iii)If urea contains impurities more than 2%,it cannot be used as fertilizer since the impurities are toxic to certain crops,particularly to the citrus crops.

Phosphate Fertilizers

Phosphorus is important nutrient for the plants.Phosphorus is essential for life both as structural material in higher plants and essential for metabolism of both plants and animals.Large amount of phosphates are used as fertilizers. Superphosphate and triple superphosphate are well known phosphate fertilizer.About 90% of the phosphate rocks are directly used as fertilizers and remainders are used to make elemental phosphorus and phosphoric acid.Phosphorus for root growth used as slightly soluble form of phosphate 'Superphosphate" and "Triple superphosphate". Phosphate rocks such as fluoroapatite[3Ca₃(PO₄)₂CaF₂] are very insoluble,and thus are of no use to the plants.Superphosphate is made by treating phosphate rock with the concentrated H₂SO₄.The acid salt Ca(H₂PO₄)₂ is more soluble,over a period of weeks the superphosphate will dissolve in the soil water.

$$[3Ca(PO_4)_2CaF_2]+7H_2SO_4 \rightarrow 3Ca(H_2SO_4)_2+7CaSO_4+2HF$$

The CaSO₄ is an insoluble waste product,and is of no value to plants,but is not removed from the product sold.

"Triple superphosphate" is made in a similar way,using H₃PO₄ to avoid the formation of the waste product CaSO₄.

$$[3(Ca_3(PO_4)_2CaF_2]+14H_3PO_4 \rightarrow 10Ca(H_2PO_4)_2+2HF$$

References

Poudel, Bhoj Raj Poudel and Dr.Megh Raj. A Text Book Of Inorganic Chemistry. Kathmandu: National Book Centre, 2012.

Sons, John Wiley and. J.D Lee Concise Inorganic Chemistry. 2007.