> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/introduction-1/databases/plants.plt/frac_p_mat.md). # frac\_p\_mat In order to calculate the plant nutrient demand throughout a plant’s growing cycle, SWAT+ needs to know the fraction of nutrient in the total plant biomass (on a dry weight basis) at different stages of crop growth. Six variables in the plant database provide this information: [frac\_n\_em](/io-docs/introduction-1/databases/plants.plt/frac_p_em.md), [frac\_n\_50](/io-docs/introduction-1/databases/plants.plt/frac_n_50.md), [frac\_n\_mat](/io-docs/introduction-1/databases/plants.plt/frac_n_mat.md), [frac\_p\_em](/io-docs/introduction-1/databases/plants.plt/frac_p_em.md), [frac\_p\_50](/io-docs/introduction-1/databases/plants.plt/frac_p_50.md), and *frac\_p\_mat*. Plant samples are analyzed for nitrogen and phosphorus content at three times during the growing season: shortly after emergence, near the middle of the season, and at maturity. Ideally, the plant samples tested for nutrient content should include the roots as well as the aboveground biomass. Differences in partitioning of nutrients to roots and shoots can cause erroneous conclusions when comparing productivity among species if only the aboveground biomass is measured. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://swatplus.gitbook.io/io-docs/introduction-1/databases/plants.plt/frac_p_mat.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.