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Soil Analysis-Determination of Available Phosphorus content in the Soil by Bray's method
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Objectives:

 

  • To determine the available phosphorus in the soil sample by Bray’s method.

 

Introduction:

A soil analysis is a process by which elements such as P, K, Ca, Mg, Na, S, Mn, Cu and Zn are chemically extracted from the soil and measured for their “plant available” content within the soil sample.

 

 

Significance of Soil Analysis:

 

  • It increases the knowledge of what nutrients are especially available in a given soil.
  • It reduces the environmental impacts due to soil amendments. 
  • It increases the efficiency of resource inputs such as fertilisers and water.
  • It helps to predict the nutritional values needed for crop production.
  • It helps to evaluate the fertility status of soils of a country, state or district.

 

Procedure for Taking Good Soil Samples:

 

  • Determine the soil unit (or plot).
  • Make a traverse over the soil unit (or plot).
  • Clean the site (with spade) from where soil sample is to be collected.
  • Insert the spade into soil.
  • Standing on the opposite side, again insert the spade into soil.
  • A lump of soil is removed.
  • A pit of ‘V’ shape is formed. Its depth should be 0-6" or 0-9" or 0-12" (i.e., Depth of tillage).
  • Remove the soil-slice (like a bread slice) of ½ inch thick from both the exposed surface of the pit from top to bottom. This slice is also termed furrow-slice. To collect the soil-slice spade may be used. Collect the soil samples in a polyethylene bucket.
  • Collect furrow-slices from 8-10 or sometimes 20-30 sites. Select the sites at random in a zigzag (or criss-cross) manner. Distribute the sites throughout the entire soil unit (plot). In lieu of spade auger may be used. Do not take the prohibited samples and local problem soils.
  • Furnish the following information on two sheets of thick paper with the sample. One sheet is folded and kept inside the bag. Another sheet is folded and attached to the bag.

 

 

Hydrangeas...Blue or Pink?

 

Hydrangeas respond to the soil pH where they are planted. They produce blue flowers in acid soil (pH 5.5 & <5.5), if the soil is highly acidic then the colour become the bluest of blue. Pink flowers are produced in alkaline soil (pH 7 & >7), if the soil highly alkaline then the colour of the flower become white such as ‘Lanarth White’.

 

           

 

            

 

 

 

Available Phosphorus in Soil:

 

The term available phosphorus refers to the inorganic form occurring in soil solution which is almost exclusively ‘Orthophosphate’. This Orthophosphate occurs in several forms and combinations. The phosphate contributions in soil are governed by hydrogenous equilibria in which it takes part. This can be represented as,

 

Phosphorus absorbed in soil phase ⇌ P in soil solution ⇌ Precipitated P

 

The phosphorus absorbed by plants from soil comes from the soil solution in which it exists as an inorganic Orthophosphate ion H2PO4-, HPO42- and PO43-. The most accessible ion is H2PO4-. The quality of phosphorus accessible to the plants is influenced by a series of soil properties.

 

 

Apparatus:

 

Spectrophotometer, Whatman No: 42 filter paper, Beakers, Standard Flasks, Pipettes, Funnels, etc.

 

Principle of Dickman and Bray’s Method:

 

The underlying principle of this method is that the soil is shaken with an extracting solution of 0.03 N NH4F in 0.025 N HCl, which dissolves the fraction of Phosphorus roots. Ammonium Fluoride complexes with Al and Fe ions in the acid solution with the consequent release of Phosphorus held by the soil, by these trivalent ions. Thus the combination of HCl and NH4F helps in removing easily acid soluble forms of Phosphorus, largely Calcium Phosphates and a portion of Aluminium and iron phosphates. The reactions in acid solution may be represented as follows,

 

3 NH4F + 3 HF + AlPO4 → H3PO4 + (NH4)3AlF6

3 NH4F + 3 HF + FePO4 → H3PO4 + (NH4)3FeF6

 

In the above reactions AlPO4 and FePO4 represents various hydrated and hydroxyl phosphates of Al + Fe including any absorbed or precipitated surface layer on oxides of Al, Aluminosilicates and an oxide of Fe.

 

HCl results in the dissociation of more active Calcium Phosphate and in addition to this, it prevents the precipitation of Phosphorus (as Calcium Phosphate) which has been dissolved by Ammonium Fluoride.

 

In the filtered extract Phosphorus is estimated calorimetrically by adding Ammonium Molybdate and thereafter, reducing the Molybdenum Phosphate complexes with Stannous Chloride in the acidic medium. The heteropoly complex (Phosphor Molybdate) is formed by coordination of Molybdate ions with Phosphorus as the central coordinating atom, the Oxygen of the molybdate being substituted for that of PO4. The reaction may be represented as follows.

 

H3PO4 + 12 H2MoO4 → H3P(Mo3O10)4 + 2H2O

 

The heteropoly complexes, before reduction gives a yellow colour to their water solution. In the solution of low Phosphorus concentration, the Molybdate is partially reduced to a characteristic blue colour. The intensity, a measure of concentration of Phosphorus in the test sample is read on colorimeter.

 

Significance:

 

  • The availability of P is considered to be a fairly good indicator/ measure of the P supplying capacity of soil.
  • Knowledge of the available phosphorus content of soil is important for determining the critical limit based on soil-test-crop response calibration study.
  • The evaluation of a soil critical limit of P would help in developing P fertility classes for effective fertilizer recommending schedule.

 

 

 

 

 

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