Phosphorus doesn't dissolve like nitrogen. The soil will hang onto phosphorus, not releasing it into water. If you're looking for good sources of phosphorus, check the ingredients of any plant food you buy. You should also look for ingredients like bonemeal, colloidal phosphate, or rock phosphate. You may also see superphosphates, a more soluble form of phosphorus.
Potassium promotes strong vigorous roots and resistance to disease.
Potassium is a nutrient your plants need for good internal chemistry. Plants use potassium to produce the sugars, starches, proteins and enzymes they need to grow and thrive. Potassium also helps your plants regulate their water usage, and better withstand the cold. Your plants need certain trace elements and nutrients to make the best use of soil, water, and air. An important thing to remember about trace minerals is that plants can't always use the most common forms.