मिट्टी में उर्वरकों का प्रबंधन

The fertilizers are becoming costly input day by day. Hence it is felt necessary to study the efficient use of this input. So some important points are given here to increase fertilizer use efficiency and to get higher yield.

1. Management of Nitrogen fertilizers in soil

Nitrogen is lost from fertilizers by ways as follows:

Volatilization

Ammonia volatilization is an economic loss to the farmer in terms of both drained fertilizer prices and decreased crop yields. Some important facts are as follows:

• The ammonia volatilizing capacities of the fertilizers are graded as urea > ammonium sulphate > ammonium chloride > ammonium nitrate.
• The ammonia volatilizing capacities of the soils are graded as calcareous soil > alkaline soil > neutral soil > acid soil.
• In neutral and acid soils, ammonia volatilization occurs from urea only, not from other ammonium fertilizer viz. ammonium sulphate, ammonium chloride, ammonium nitrate.
• Surface application of fertilizers increases ammonia volatilization.
• Ammonia volatilization from urea added to the surface of warm and moist alkaline or calcareous soils can be completed within a week.

To reduce volatilization loss of nitrogen, few recommendations are enlisted hereunder.

• Immediately after application (within a few hours), urea must be incorporated with soil by tillage or washed into the soil. Urea should never be left on the soil surface without incorporation. 
• In coarse-textured soils (e.g., sandy or sandy loam soils) urea may be applied on the surface followed by irrigation so that urea is washed into the soil with percolating water. Care must be taken so that urea does not wash away with runoff.
• In case of acid or neutral soil, ammonium fertilizer, e.g. ammonium sulphate or ammonium nitrate may be applied on the surface, but urea must be incorporated with soil.
• If the soil is highly alkaline or highly calcareous, then deep incorporation of urea should be made.
• Slow release nitrogen fertilizers or sparingly soluble nitrogen fertilizers, e.g., coated urea, urea form, IBDU etc. may be applied.
• Foliar applications of urea may be adopted.
• Split applications of fertilizers should be adopted.

Denitrification

Some important facts are as follows:

• Denitrification occurs mostly in submerged rice soils.
• Nitrate is denitrified.
• The denitrification loss of nitrogen is as high as 87% of added nitrogen. 

To reduce denitrification loss of nitrogen, some recommendations are as follows:

• Nitrate fertilizer application should be avoided.
• Nonnitrate fertilizers, e.g. ammonium sulphate, urea etc. should be applied.
• The fertilizer should be placed in reduced zone of sumberged rice soils.
• Slow release nitrogen fertilizers (i.e., sparingly soluble nitrogen fertilizers), e.g. coated urea, urea form, IBDU etc. or nitrification inhibitors, e.g., N-serve, DCD, A.M. etc. may be used. Also urea mud balls may be used.
• Split applications of fertilizers should be adopted.
• Foliar application of urea may be adopted.
• Anhydrous ammonia, NH3, or aqua ammonia, NH4OH, may be applied before submerging and puddling.

Leaching

Some important facts are as follows:

• Nitrate is leached.
• Leaching occurs greatly in submerged rice soil, coarsed textured soils, soil under high rainfall. 
• The unhydrolyzed urea may wash out in submerged rice soil.
• The leaching loss of nitrogen is as high as 70% of added nitrogen.

The followings are some recommendations to reduce leaching loss of nitrogen:

• Nitrate fertilizer application should be avoided.
• Nonnitrate fertilizers, e.g. ammonium sulphate should be recommended.
• The fertilizer should be placed in reduced zone of submerged rice soils.
• Slow release nitrogen fertilizers (i.e., sparingly soluble nitrogen fertilizers), e.g., coated urea, urea form, IBDU etc. or nitrification inhibitors, e.g., N-serve, DCD, A.M. etc. may be used. Also, urea mud balls may be used.
• Split applications of fertilizers should be adopted.
• Foliar application of fertilizer may be adopted.

2. Management of Phosphorus fertilizers in soil

• Phosphatic fertilizers should be applied at high rate (dose) to make the soil supersaturated with phosphate (H2PO4–). Once, the adsorption sites are saturated with phosphate (H2PO4–), further adsorption will not occur. Thus, excess phosphate (H2PO4–.) is left in the soil solution, which is readily available (accessible) to crops.
• In lieu of broadcast application, localized application (e.g., band placement, drill placement, plough-sole placement etc.) of phosphatic fertilizers near the seed or seedling roots should be followed.
• For high phosphorus fixing soils, the phosphatic fertilizers should be applied frequently in a crop rotation, because phosphate is rapidly fixed (adsorbed/precipitated/sorbed) in this soil and thus, plants get a very short period to assimilate (adsorb) phosphorus.
• The acid soil may be limed to raise its pH near neutrality (pH 6.5). Over liming should be avoided. With rise of pH near (upto 6.5) the activity of iron and aluminium decreases and thus their reaction with phosphate (H2PO4–) decrease which results in lower phosphorus fixation (adsorption/precipitation) and higher phosphate concentration in soil solution.
• The Pelleted or aggregated phosphatic fertilizers may be used. When applied to soil, the contact between fertilizer and soil decreases which results in less phosphorus fixation. (adsorption/precipitation/sorption).

3. Management of Potassium Fertilizers in soil

• Plants’ need for potassium is high during early growth. Hence, adequate potassic fertilizer should be applied to seedlings. Care must be taken so that the fertilizer salt cannot damage the seedlings.
• For crop which need high potassium such as sugar producing crops (e.g., sugar cane, potato, tobacco banana etc.) the dose of potassic fertilizer should be high.
• Among potassic fertilizers muriate of potash is cheapest. Hence, its use is wide.
• For potato and tobacco muriate of potash (KCI) should be replaced at least partly if not fully by nonchloride potassic fertilizers such as sulphate of potash (K2SO4).
• Under prolonged submerged condition of soil, high dose of sulphate of potash (K2SO4) should not be applied. Otherwise it will induce hydrogen sulphide (H2S) toxicity and unavailability of cationic micronutrients such as iron, manganese, zinc and copper (Fe, Mn, Zn and Cu).