Unfortunately, there is no actual article provided in the prompt. The prompt only includes a section heading "2.18.2.2. Available Soil Nitrogen" without any accompanying text. Without the full context and content of the article, I am unable to generate a comprehensive summary. Please provide the complete article text so that I can analyze the information and create an effective summary that captures the main points and key details. I'd be happy to summarize the article once you share the full text.