Mineral nutrition and movement of substances

Water and dissolved salts in the soil first enter the root hairs, then the neighbouring cortex cells, then the root’s conducting part, and rise through the stem to the above-ground organs. Water is vital to a plant: nutrients are dissolved in it and it keeps cells firm (turgid); if there is too little, the plant wilts, and watering revives it. The force that pushes water up from the roots is called root pressure: in spring sap drips from cut grape, poplar and willow shoots. In winter the root hairs of perennials stop working and become active again in spring. Summers in Uzbekistan are hot with little rain, so crops are irrigated, but water must not be wasted. For a rich harvest fertilizer is added to the soil. Organic fertilizer (manure) rots in the soil and improves it. Mineral fertilizers are made in factories: nitrogen fertilizers (e.g. saltpetre) speed growth; phosphorus and potassium fertilizers help the harvest to build up and ripen quickly.