Lacking nutrients that plants need to thrive, not only causes them to falter, but also makes them more susceptible to pests and diseases. The three nutrients most vital to plants are nitrogen (N), phosphorus (P), and potassium (K). Because many parts of the west are deficient in both decomposing plant material and rainfall (two primary sources of nitrogen), nitrogen is often lacking in the soil. Without sufficient nitrogen, plant leaves yellow, from their tips toward the stem and from the bottom of the plant upward. The growth of the plant is also stunted by nitrogen deficiency. Phosphourus and potassium are needed to maintain healthy root systens and plant structure. They also can affect flower production.