Dear Abis,

A similar question to yours was posted before on ResearchGate. Here is the link . You may find the answer to your question.

https://www.researchgate.net/post/How_can_I_determine_the_field_capacity_FC_and_the_permanent_wilting_point_PWP_of_soil

With my best regards

Prof. Bachir ACHOUR

Well suggested by Prof. Bachir.

In field method it can be estimated through a protocol which is often used by the researchers. Take a sizable amount of soil aggregates in a cloth bag. Now dipped  the soil with bag in water. Allow to soak fully. now hang the samples so that the excess of water will be drained out. When there will be no draining of water estimate the moisture of the soil aggregates through gravimetric method.

Abis letizia

You can use the methodology van Genuchten. Below has a summary of it.

Samples are collected in metal cylinder with measures: diameter of 5 cm, height of 5 cm.

ABSTRACT

This work proposes to adjust mathematical models to predict the parameters of van Genuchten equation (qR,qS, a and n), besides a model to predict the soil moisture corresponding to the field capacity. The models express the equation parameters as dependent variables and sand, clay, organic matter, bulk density and depth as independent variables. All independent variables can be easily and routinely determined in laboratory. The samples were collected at 12 points in and three layers (0-30; 30-60; 60-90 cm) of a Cambisol, using a Uhland sampler. The soil moisture at matric potentials of 2, 6 and 10 kPa were determined using a porous plate apparatus and those at matric potentials of 33, 100, 500 and 1500 kPa were determined using a pressure plate apparatus. The models were adjusted with the SAS for Windows program, ProcReg routine and Backward procedure to select the variables significantly different from zero at the probability level adopted. The coefficient of determination and prediction errors for the parameters predicted and soil moisture generated by van Genuchten equation adjusted with parameters predicted by models, were analyzed to evaluate the models. The coefficient of determination for models was high and the errors were small justifying the application of models.

van Genuchten, M.T. van. A closed form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Science Society of America Journal, Madison, v.44, p.892-898, 1980.

It is quite well elaborated for adoption.

Dear Abis

You can read this paper.

Best Regards

The paper provided by Ahmad Solgi is pedo transfer functions approach, determination of field capacity through soil textural components. This approach is generally employed when percent sand, silt, clat, OC are given, but first we derive the functional parameters from the actual observations of texture, oc, moisture retention and moisture transmission. if we do not have the set of values, then from literature we have to choosethe coefficients as per our situation. Dr Pal has also worked on this aspect for hydraulic conductivity measurements. You can simply put percent sand, silt, clay, oc of the sample and you can get the value of FC, WP, HC etc

but these are indirect methods

thanks Ahmad for providing this paper, which is very useful

regards

Hi, I have a doubt, I am making my thesis project for an agricultural area, I want to calculate field capacity and wilting point, It´s an area of 17E6 squared meters, but I know if I only have to take one sample, or should I take more soils samples? best regards.