Potassium in the Earth’s crust

2.6% of the weight of the Earth’s crust is Potassium (K). 

By the way, it is the second most abundant nutrient in plant leaves just behind nitrogen (N). N, P (phosphorus) and K are the three valuable nutrients the body disposes in our urine. 

Source: Potassium: a neglected nutrient in global change – Sardans – 2015 – Global Ecology and Biogeography – Wiley Online Library