Water-soluble 13-40-13 is a type of fertilizer that contains a high percentage of phosphorus (40%) and a moderate amount of nitrogen (13%) and potassium (13%). The numbers represent the percentage by weight of each of these three essential plant nutrients in the fertilizer, with nitrogen-phosphorus-potassium (N-P-K) being the standard order.

Phosphorus is an essential nutrient that is important for root development, flowering, and fruiting in plants. It also helps plants withstand stress and disease. Nitrogen is important for plant growth and foliage development, while potassium helps plants to resist disease and stress, as well as promoting fruit and flower development.

Water-soluble fertilizers can be dissolved in water and applied to plants through watering or foliar spraying. They are often used in commercial agriculture and horticulture, as well as for home gardening. The advantage of water-soluble fertilizers is that they can provide a quick and efficient source of nutrients to plants, and they can be adjusted to meet specific plant needs.