# frac\_p\_em 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*, [frac\_p\_50](/io-docs/introduction-1/databases/plants.plt/frac_p_50.md), and [frac\_p\_mat](/io-docs/introduction-1/databases/plants.plt/frac_p_mat.md). 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: 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: ``` GET https://swatplus.gitbook.io/io-docs/introduction-1/databases/plants.plt/frac_p_em.md?ask= ``` The question should be specific, self-contained, and written in natural language. 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.