You have to add water to the soil sample for saturation and leave it discharge gravimetric water and then compute water holding capacity.. If the percentage hogher or lower 55% you should add or decrease the amount of clay or you can add compost if you want to kee particle size distribution constant..

Regards

The maximum water holding capacity method is a method that doesn’t use external pressure. In this method, the soil sample is saturated with water in a cylinder. The cylinder is placed on an absorbent membrane until the excess water is drawn away by gravity. Once equilibrium is reached, the water holding capacity is calculated based on the weight of the water held in the sample vs. the sample dry weight. Then, 55% water content will be 55% of that maximum water content.

Take known amount (in your case 100g) of soil in the pot you are using during your experiment. Having a filter paper at the bottom, saturate the soil from the bottom. Allow to drain for 4-12 hours depending on the soil and the pot. At this stage all your macro-pore are drained and there is no more visible drainage. Weight the soil again ( A grams). Oven dry it at 105 degrees for 24 hours. Weight the soil again (B grams).

Now you will get 2 water contents from calcuations.

1. 100-B/B*100= Your current soil gavimetric water content (%)

2. A-B/B*100 = 100% WHC

Now you can calculate 55% WHC by dividing 100%WHC with 100/55 minus your current water content.