The volume of soil required for experiment will depend on objective and treatments (nature and number) of experiment. You have to put all the 25 Brassica plants in one big container to make it as one treatment. I think you can use 25 X 20 X 25 = 12500 cm3 container and you will need so much volume of soil for one treatment. You can calculate the total by multiplying number of treatments X 3 (replications). If it is not moisture experiment, then you have determine moisture after regular intervals, depending upon the season (temperature, rainfall and evaporation). For fertiliser calculations, just consider the area of container 25 X 20 cm2 = 0.05 m2 (if it is not circular) and calculate the quantity by dividing the dose of fertiliser/ha by 10000 and multiplying by area of container (0.05 m2).    

Dear Lisa,

It is better to grow each of the plants in a separate pot. If you plan to grow the plants till maturity, you may use a pot of about 16-litre capacity with drainage holes at bottom. You may place 10 kg air-dry soil (thoroughly mixed with required fertilizers) in each pot, bring the soil in each pot to the field capacity moisture level by adding required amount of demineralized water. Dibble 2 to 3 seeds of Brassica sp. at about 1 cm soil depth of each pot. About 10 days after dibbling, only the most healthy seedling is kept in each pot and the remaining two seedlings are weeded out. After this, you have to maintain field capacity moisture level in your potted soil by regularly adding required amount of water. You may top-dress your plant with fertilizer N once or twice during the full growth cycle of the plant. Required amount of fertilizer material for top-dress may be incorporated in the top 2-cm of the soil, before applying water to the pots. The plants should be protected against any visible insect-pests or disease infestation by timely spraying of recommended chemicals.

Following above protocol, you may grow the Brassica plant in pots up to maturity and obtain reasonable yields (pods/grains and stover).

Hope you find this input interesting and useful.

If you are going to use Brassica plants separately in pots, that I think is the better option, you can count the total number of pots that you need and knowing the total volume of them, obtain the same volume of soil. You can apply a nutritive solution like Long-Asthon one time by week and applying water like you apply it to any plant. In any case the proper plant´s symptoms give you an idea of what they need in ezch moment. Undoubtely, the application of any mycorrhizal inoculum could be benefitial for plants, but in Brassica´s case that plants do not form normally mycorrhizas according to Mark C. Brundrett´s article: Mycorrhizal associations and other means of nutrition of vascular plants: understanding the global diversity of host plants by resolving conflicting information and developing reliable means of diagnosis. Plant Soil (2009) 320:37–77. DOI 10.1007/s11104-008-9877-9