Water is an important part of plants; When there is enough water in the plant, the plant can stand upright and keep upright posture, and the leaves can stretch, which is beneficial to photosynthesis; Inorganic salts can only be absorbed by plants and transported to various organs of plants if they are dissolved in water.

Plants need a variety of inorganic salts, among which inorganic salts containing nitrogen, phosphorus and potassium are the most needed. Nitrogen fertilizer can promote the vigorous growth of plant stems and leaves, phosphorus fertilizer can make plants blossom and bear fruit, and potassium fertilizer is beneficial to the transportation of organic matter to plant storage organs.

Plants need organic matter for their growth. First, organic matter is used to build plants themselves. The cell wall, cell membrane and DNA in the nucleus of a cell are all composed of organic substances. Second, organic matter can provide energy for the life activities of plants. Organic matter decomposes into carbon dioxide and water in the mitochondria of cells, and at the same time releases energy for the life activities of plants.

As can be seen from the above, the nutrients needed for plant growth are mainly water, inorganic salts and organic matter.

So the answer is: water.