Page 501 - Biosystems Engineering

Agricultur e Management 473

1 mL of 1-N K Cr O = 0.003 g carbon
2 2 7
−
×
(XY × 0 .003 100
)
% of carbon in the soil =
2 × W
There is incomplete oxidation of organic matter in this procedure.
The organic carbon is multiplied by 1.3 on the assumption that there
is a 77 percent chance of recovery.
Actual organic carbon (%) = organic carbon estimated × 1.3 percent
of organic matter in soil = 2 × 1.724.

Determining Total Nitrogen in the Soil Ammonia gas that is formed due
to the reaction of soil with excess of alkaline KMnO is absorbed in a
4
known volume of standard acid. Excess acid is titrated with standard
alkali using methyl red as an indicator.
Place a 20 g of soil sample in a distillation flask and add 20 mL of
water. Then add 1 mL of liquid paraffin and a few glass beads to
avoid frothing and bumping, respectively, during distillation.
Now add 100 mL of 0.32 percent KMnO solution and 100 mL
4
of 2.5 percent NaOH solution and immediately place it in the distilla-
tion apparatus. Pipette out 20 mL of boric acid solution (with a mixed
indicator) in a conical flask and dip the end of the delivery tube in it.
Distill ammonia gas from the distillation flask and collect about 100 mL
of the filtrate in about 30 min. With the absorption of ammonia, the
pink color of the boric acid solution turns green. Titrate the contents
with 0.02-N H SO to the original shade (pink). A blank correction
2 4
(without soil) is to be made for final calculations:
Weight of soil taken = 20 g
Volume of 0.02-N H SO required for titration = R mL
2 4
1 mL 1-N H SO = 14 mg N
2 4
Available N in 20-g soil = R × 0.02 ×14 mg
Available N kg/ha = R × 31.36

Determining the Available Phosphorus To determine available phos-
phorus in soils, use the Olsen method for neutral–alkaline soils,
whereas the Bray–Kurtz P1 method is used for acidic soils.
1
Olsen’s Method This method is based on the fact that sodium bicar-
bonate solution extracts some exchangeable or surface-adsorbed
Al-P, Fe-P, calcium phosphates, and other phosphates.
Reagents of this method are prepared as follows:

1. Take sodium bicarbonate (Olsen’s reagent) 0.5-M NaHCO ,
3
pH 8.5. Dissolve 84 g of NaHCO in up to 2 liters of water.
3
Adjust pH 8.5 with 1-M NaOH (4 g/100 mL) solution. Store
in a glass or polyethylene bottle.