Terrestrial plants need nutrients in similar proportions. Lab experiments show that different plant species have similar nutrient ratios when given limited nutrients. These ratios differ from when plants have unlimited access to nutrients. By studying nutrient concentrations in ecosystems, researchers found that nitrogen, phosphorus, potassium, calcium, and magnesium are important. Nitrogen is usually the key nutrient. The ratios of these nutrients in plants closely match those found in experiments and field studies.