According to information provided by the USDA Economic Research Service (2013), the national average price per pound of N has increased between 2000 and 2012 by a factor of 2.6. Over the same time period, phosphate price increased by a factor of 2.8, and potassium price increased by a factor of 4.0. As a result, fertilizer costs now contribute 30% to 40% of the annual variable costs to grow many cereal grains.