Agriculture for Engineers 4 (3+1)

Lesson 17. Fertilizer Reaction in Soil and Use Efficiency

17.1 Nitrogenous Fertilizers

17.1.1 Fate of Nitrogen in the Soil

The nitrate-nitrogen of the soil, whether added in form of fertilizer or formed by nitrification, may loss in four ways: (1) volatilization (gaseous loss), (2) leaching loss (3) denitrification loss (4) used by microorganism and weeds.

(1) Volatilization loss. In this chemical reduction process, nitrogen is lost in the gaseous form when urea or ammonium fertilizers are applied on the soil surface. Loss of nitrogen as ammonia is occurred especially in alkaline soils. High concentration of ammonia (high dose of ammonical or amide fertilizers) is toxic to the nitrification process, resulting in an unusual build-up of nitrates. Under acid conditions these nitrites are converted to gaseous elemental nitrogen or nitrous oxide, when brought in contact with certain ammonium salts or urea. It may be represented in the following reaction:

(2) Denitrification loss. The nitrates may change to gaseous form in the lack of air or by poor drainage. The biochemical reduction of nitrate-nitrogen to gaseous compounds by microorganism is called denitrification. The microorganisms involved are common anaerobic forms. Under field conditions, nitrous oxide is the gas lost in largest quantities, although elemental nitrogen is also lost under some conditions. Nitric oxide loss occurs most readily under acid conditions.

(3) Leaching loss. The nitrate-nitrogen is lost in drainage or with percolating water. The amount of nitrogen lost depends upon the climate and cultural conditions in humid region or a water-logged condition, losses of nitrate by leaching are significant. In arid and semi-arid region, such losses are minimum.

(4) Used by soil microorganisms and weeds. Soil microorganisms readily assimilate nitrate-nitrogen. If microbes have a ready food supply (organic matter) they utilize the nitrates more readily. This is one of the reasons, crops get about one-half the nitrogen added in forms of nitrogenous fertilizer.

Weeds may also utilize the nitrate-nitrogen added to the soil (or present in the soil). Therefore, crops may not get nitrogen in fully quantity.

17.2 WAYS TO INCREASE NITROGEN USE EFFICIENCY

Management of nitrogen in the soil to improve the nitrogen fertilizer economy is given below:

1. Minimizing nitrogen loss with proper management of Nitrogen

(a)  Deep placement of N fertilizers in a field before transplanting/sowing increases the nitrogen use efficiency. The loss would be minimum.

(b) Ammonia volatilization losses in flooded soils range from negligible to almost 60% of applied nitrogen. In recent studies NH₃ loss from ammonium sulphate (NH₄)₂ SO₄ was 0.2-6.8 per cent and that from urea 1-20 per cent. Through incorporation of ammonium fertilizers like ammonium sulphate into the soil minimizes NH₃ volatilization loss.

(c)  The loss of NH₃ varies from 27 to 47% when urea is applied on surface as top dressing at 14 to 27 days after transplanting of rice crop. Rates of NH₃ loss were limited to 10-15% when urea was applied.

(d) Nitrogenous fertilizers should be applied in split dose (or instalment) to minimize the losses.

(e)  Use of slow and controlled-release fertilizers. Fertilizers that release – their plant nutrients slowly throughout the growing season (or for longer period), have a number of potential advantages. Slow release volatilization loss and sustain the crop with adequate N nutrition throughout the growing season.

(f)   Use of nitrification inhibitors increase the controlled availability of plant nutrients. The inhibitor is block the conversion of ammonium to nitrate-nitrogen by inhibiting Nitrosomonas (bacteria) growth or activity. The inhibitors are dicyandiamide (DCD), acetone extract of neem etc.

A group of organic compounds in neem seed known as meliacins are responsible for inhibiting nitrification. Urea treated with neem cake inhibited nitrification by 40 and 74 per cent at the end of 1 and 2 weeks of incubation, respectively. The coating technique used coal tar solution in kerosene oil (1kg/2 litres, enough for 100 kg urea) as sticker to hold the finely produced neem cake. One quintal urea is transferred to a seed treatment drum and coal tar-kerosene solution is added in parts while rotating the drum. Neem cake (15-20 kg) is then added and the content of the drum is rotated and thus the neem cake is ready for use. The technique is simple and being used only on individual farmer’s level. This technique could not go to the industrial level.

A new technique involving neem oil micro-emulsion has been developed at IARI, New delhi for coating urea. Urea coated with micro-emulsion has shown definite advantage in nitrogen use efficiency. The technology has two major advantages (i) only 0.5 kg neem oil is needed  per tone of urea and (ii) the product is also eco-friendly.

(g)  Application of fertilizers at proper time.

(h) Use of soil amendments as corrective methods.

(i) Volatilization losses from nitrogenous fertilizer may be minimized by mixing the urea and soil in 1:5 ratio. The mixture should be used after drying in under shade. Likewise, nitrogenous fertilizer may be mixed with oilseed or neem cake before broadcasting.

2. Minimizing nitrogen loss with proper water management

(a) For minimizing the denitrification loss in the field arrangement for proper drainage should be done. Proper aeration would decrease the denitrication loss.

(b) Optimum use of irrigation water would reduce the leaching loss.

(c) Applying the first dose of nitrogen to a puddle field  without any standing water and then introducing water at 4 days transplanting.

3. Weed Control:  Loss of N can be minimized by removal of weeds from the   field.

4. Varietal Differences in Nitrogen-use Efficiency. Rice variety IR 42 uses N more efficiently than IR 36 and IR 8. Variety like IR 42 should be used for increasing N-efficiency.

17.3 PHOSPHATIC FERTILIZERS

17.3.1 Superphosphate and soil reaction

When superphosphate is applied to neutral or alkaline, the soluble monocalcic phosphates are produced. The phosphates are then said to be reverted. Since the reverted phosphates are chemical precipitates, they are in a fine state of division and offer a large surface for the soil solution to act. As a result, even tricalcic phosphate is rendered soluble slowly and gradually, and made available for plants. When the soil is acidic and the pH is 5.5 or lower, the iron and aluminium in the soil become soluble and combine with the soluble phosphates in superphosphate. Iron and aluminium phosphates that are formed, as a result, are very insoluble and do not become available for the use of plants ordinarily. The phosphates are then said to be fixed or tied up in the soil.

The organic matter in the soil effectively prevents applied phosphates from being tied up (fixed), in the soil, in insoluble forms and even releases phosphates that had already been fixed, is likely to influence phosphate application practice in the future.

17.3.2 Rock phosphate

For effective results, it should be used in heavy application, particularly with organic matter. Its availability is increased by the presence of decaying organic matter. Rock phosphate must be finely ground before application in acid soils.

17.3.3 Phosphorus Use Efficiency

In the case of phosphorus fertilization, fixation of phosphate is the main problem. Water soluble phosphatic fertilizers soon after application to the soil react preferably, called the initial phosphate reaction products.

• Measures to improve phosphorus use efficiency are as follow:
• To improve phosphorous use efficiency, phosphatic fertilizers should have minimum contact with the soil.
• In acid soil, phosphorus use efficiency can be improved by raising pH with the application of lime.
• Surface broadcast, following by mixing during puddling, has the highest phosphorus use efficiency for rice, being even better than placement at various depths.
• Phosphate placement is more beneficial than broadcast application in wheat. Application in seed furrows or by drilling just below the seeds is quite efficient methods for wheat.
• Recovery of fertilizer phosphorus in single season is very low. Residual effect of phosphatic  fertilizer is quite high in the next succeeding crop.
• Phosphorus use efficiency is increased by application of phosphatic fertilizers with organic manures.
• Because of fixation, crop may not use more than 10 percent of phosphorus in the fertilizers applied broadcast and incorporated to the soil. However, up to 30 percent or higher may be used when applied as concentrated band along the plant row. While mixing with the soil increases fixation, localized placement of banding allows the phosphatic fertilizer to react with only a much smaller portion of the soil in the immediate vicinity of the band. Clay soils have greater phosphate fixing capacity than sand soils.
• Fixation of P in citrate soluble form in less that of water soluble form. Hence, phosphatic fertilizers in citrate soluble form may be broadcast.
• Ground rock phosphate which is neither water not citrate soluble should preferably be applied to acid soils and thoroughly mixed with the soil. This ensures reaction with the soil acids which bring phosphorus into available form.

17.4 POTASSIC FERTILIZERS

Potassium sulphate

This fertilizer is more effective on light and medium soils with high pH (alkaline and calcareous soils). Under wet conditions, it is preferable to apply potassium sulphate than potassium chloride. The SO₄ (sulphate) ions are retained by soils more strongly than Cl (Chlorine) ions in the heavy soils. Excess of sulphate ion in the heavy soil develops toxicity.