Nitrogen is quickly taken up because it's key to the production of chlorophyll. Period. The plant can take up large amounts in exess before toxicity appears. Chlorophyll in photosynthesis. Absolutely necessary.
"The most important chemicals that plants need, without which they won't thrive, are the three macronutrients: potassium, nitrogen and phosphorus. The reason for this is that the basic building blocks of every plant are ATP, cell membranes and amino acids. Nitrogen is an element in every amino acid; ATP, which is the primary source of energy for all cells, contains phosphorus. Potassium is essential to a plant's ability to metabolize. This element also comprises up to two percent of a plant's weight."
So again which element is taken up most quickly? Nitrogen because it's the one most needed. Not surprising when atmosphere contains 78% nitrogen. It's abundance is utilized.
"Primary nutrients, also known as macronutrients, are those usually required in the largest amounts. They are carbon, hydrogen, nitrogen, oxygen, phosphorus, and potassium. Secondary nutrients are those usually needed in moderate amounts compared to the primary essential nutrients. The secondary nutrients are calcium, magnesium, and sulfur."
And with that I'm out. I'm not good at arguing with fenceposts.