Soil is a complex substrate which holds the water and minerals that a plant needs in order to grow. The rest of the dry matter is synthesised from thin air and sunlight, by the process of photosynthesis. This is a truly remarkable process in which carbon dioxide from the surrounding air is transformed into organic compounds within the plant leaf in a matter of seconds. Plants in a properly managed water culture are able to develop in an entirely normal way. The water contains the nutrients normally found in the soil, and a strong root system is able to take up water, or nutrients, as required.
Photosynthesis proceeds as normal.
The plants are no more 'watery' than a plant grown in soil. On the other hand the soil is not simply a passive substrate. It is also teeming with microbial activity which allows for close symbiotic relationships to develop between the plant roots and the surrounding nutrient supplies. Nutrients are often available in forms that first require to be broken down by bacteria before the plant can take them up, and in general this symbiosis is of great value for plants. In hydroponic systems it is now possible to introduce microbes to the root zone so that this activity, not normally associated with water cultures, can proceed as normal.