You can find the amount of water of your soil at 100% and 0% of the field capacity. Then you may easily calculate the amount of water required to your soil at 80% of field capacity.

Dear Imtiaz,

1.Obtain soil moisture (M) by suitable method such as Oven method.

2. Obtain Field Capacity (FC) of soil by field or laboratory method.

3.Calculate 0.80*FC

4.Obtain weight of water to reach from moisture of M to 0.80 FC.

5. Test your estimation by measuring soil moisture once more.

Dear Imtiaz,

You need to first obtain soil moisture at the field capacity (FC).

FC is defined as the soil moisture at the matric potential of -33 cm (assumed to be the rooting depth of many crops).

This means you need to apply a suction of -33 cm to your soil (pre-saturated with water) and wait till there is not out flow from your soil. This can be done by pressure plate apparatus or a hanging column method.

If none of these methods are available to you, I can propose you an alternative method to estimate FC?

How do I determine field capacity?

Dear Mohsen Zarebanadkouki,

Please let me know if you have any alternative and easy method compared to this one?

- Dry a quantity of soil in an oven at 60 ° C until a constant weight ;

- Weigh a quantity p1 (1 kg for example) of dry soil in a pot (the pot must be perforated below);

- Water the pot to saturation and place it in the dark to avoid evaporation;

- After 48 hours, weigh the pot again (p2);

FC = p2-p1

ex: if p2 = 1.2 kg ie FC = 200 g (100% FC)

you must add 200 g of water to your dry soil to have 100% FC and therefore 160 g to have the soil at 80% FC.

NB: Make at least 3 repetitions

If you do not want to dry all of your soil, once the FC determined. Calculate the amount of water present in your soil by making the difference between the weight of the soil and the weight of the dry soil. You will have the amount of water already present in your soil. Complete this quantity to achieve the desired FC.

Thank you Carine for your answer. can you please give me the reference?

I totally agree with Carine Temegne Nono . For reference, you can see any reference about how to calculate the field capacity and follow the calculation method mentioned above to calculate how to reach the required ratio .

yes as per given by carine

Yes i also agree with Mme Carine.

BUT i want to know WHY DRY THE SOIL at 60°C instead of 105°C as usuall?

Also what about the INTERNE DRAINAGE or REDISTRIBUTION Method when determining FC in filed (NOT In LAB)?