Why plants grown in water only are smaller and weaker than plants grown in water containing some dissolved fertiliser.
Plants need water, but plants grown in only water will not be healthy and will soon die. Plants also need nutrients. We sometimes call the nutrients plants to absorb ‘plant food’. Food is eaten by living things to provide energy and nutrition. But plants need nutrients to make food and other molecules that build their structure; they can combine the nutrients with carbon from carbon dioxide and hydrogen and oxygen from water to build the complex molecules (proteins, carbohydrates and fats) using the energy from the sunlight. Animals need food because they do not use the energy of sunlight to build food molecules and must get those molecules from eating plants or other animals.
Plants are usually grown in soil which contains the nutrients they need; the nutrients slowly dissolve from minerals in the water within the soil. But many plants can grow in water without soil, so long as the nutrients they require are also in the water. This is known as hydroponics and is a common way of growing tomatoes, peppers, cucumbers and lettuces commercially.
The main chemical elements in nutrients plants need are discussed here. It is also beneficial, particularly in soils, for the organisms in the soil close to the plant roots (rhizosphere) to have access to organic matter, humus, humates and humic acid molecules that provide food for the micro-organisms. Plant roots are not always capable of absorbing the available nutrients in the soil on their own. The soil micro-organisms are better able to absorb the nutrients and convert them to a form the plants can use. The organisms pass the nutrients to the plant roots, often in return for sugars (food) made by the plant in its leaves.
In soils, there is often enough of the nutrient plants require dissolving from the minerals in the soil, the breakdown and composting of dead organic material and in the rain that falls. But also, in many cases, the soil lacks some nutrients; the nutrients are leached away, or the plants are gross feeders and need higher levels of nutrients. In these cases, the addition of nutrients in the form of fertilisers, composts, manures, etc. will give optimal growth conditions for the plants. If one or more essential nutrients are lacking, then this will limit the healthy growth of the plant. Providing those lacking nutrients allows the plants to grow to their full potential.
Identical plants are grown side-by-side, one in water alone and the other in water containing the nutrients it requires in sufficient (but not too high) quantities will have very significantly different growth. In time the water only plant will die, and the nutrient-rich plant will flourish.
If the experiment is done with one plant in the soil which is only watered and the other in the nutrient-rich water, there may be little or no difference in size and health if the soil is good. Indeed, because the soil will contain the beneficial micro-organisms around the roots it is likely that the plant in the soil will outperform the hydroponically grown plant.
So, water alone is never enough!