Tissue testing is the quantitative measurement of the essential elements in plant tissue. Plants require 17 elements for normal vegetative growth and reproduction. These elements fulfill a variety of functions in plants and are required at varying levels by different plant species. Carbon, hydrogen and oxygen are not analyzed because they come from the air or water and virtually are never limiting to plant growth. Of the remaining 13 elements that come from the soil, chlorine is normally not analyzed because it is always sufficient under Wisconsin conditions. As a result, tissue testing or plant analysis, usually refers to the analysis for nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), and molybdenum (Mo). Aluminum (Al) and sodium (Na) are normally included even though they are not essential elements. Aluminum can be toxic in acid soils, and sodium improves the quality of some crops.