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Iowa’s Nutrients: Agricultural Inputs and Exports

By: Aaron Lee Daigh, PhD candidate, Agronomy Department, Iowa State University

  Getting Into Soil & Water 2012

Iowa soils have given way to 92,000 farms across 48,100 square miles (86% of Iowa’s total land area) that lead the U.S. in two of the most versatile grains; corn and soybean. Meanwhile, Iowa also leads the U.S. in hog meat and egg production. Corn and soybeans, hog meat, and eggs annually produce a multi-billion dollar export industry. However, life vital nutrients from Iowa’s land are also exported in tremendous quantities with these agricultural exports. To sustain Iowa’s agriculture productivity, these nutrients, such as nitrogen (N) and phosphorus (P), must continually be replenished to the soil. To address our question regarding sustaining productivity and our natural resources we need to consider Iowa’s land as a whole including both soil and water.

In 2011, approximately 3.2 billion lbs. of N and 545 million lbs. of P were exported in corn and soybean grains from Iowa’s soil (IDALS, 2011; Libra and Wolter, 2004). For 2012, farmers will apply nearly 2.6 billion lbs. of N to 93% of corn acres and 5% of soybean acres in anticipation of sustaining production goals (USDA-NASS, 2010). Meanwhile, nearly 570 million lbs. of P will also be applied to 65% of corn acres and 10% of soybean acres (USDA-NASS, 2010). One can quickly conclude that a lot of N and P are involved with sustaining Iowa’s agriculture legacy. With common N fertilizer costs ranging from $20 to $100 per acre, these quantities translate into a several hundred-million-dollar fertilizer demand. As the U.S. currently endures its economic and environmental concerns, you might rationally ask: How efficient is our use of resources (i.e. fertilizers) for sustaining productivity of our natural resources? To address this question we need to look at all the inputs and exports of these nutrients even beyond fertilizers and budget them like a checkbook. This task was conducted and reported by Libra and Wolter (2004) and will be the bases for this article. However, soil and water are connected. In nature, nutrients jump back and forth between being adhered to soil particles and suspended in soil water. This soil water flows back and forth with groundwater. Additionally, groundwater is connected to surface waters, such as rivers. Groundwater is to rivers as the kitchen sink is to the drain. If a river is flowing, then groundwater is being drained and anything contained in that groundwater drains or moves to the river and flows with the river water. Thus, to address our question regarding sustaining productivity and our natural resources we need to consider Iowa’s land as a whole including both soil and water.

Phosphorus fertilizer recommendations are based on maintaining optimum soil-P levels, so we will assume that soil-P levels generally don’t change over time. Therefore, Iowa’s P budget generally includes inputs derived from mineral/manure fertilizers and human/industrial waste while exports of P are found in grain harvest, grazing, and flowing rivers. Generally, the budget appears to show good use of P fertilizers with 98% of P fertilizers being used and exported with harvest and grazing. Total P exports to Iowa’s rivers are 4% of P inputs where 2% is derived from fertilizers and 2% from human/industrial waste.

The N budget components include inputs derived from synthetic/manure fertilizers, legumes (plants that obtain their own N from the atmosphere and can store N in the soil), wet deposition (N contained in rainfall), dry deposition (wind deposited N and aerosols), and human/industrial waste while exports of N are found in grain harvest, grazing, crop/soil/ fertilizer gaseous losses, and lost to rivers. Similar to soil P, we can assume soil-N levels generally don’t change over time. The budget shows fair use of N with 78% of agricultural inputs (synthetic fertilizer, manure and legume N) being used and exported with harvest and grazing and leaving 22% of agricultural N inputs lost to the atmosphere and rivers. Total N exports to Iowa’s rivers are 7% of that applied, of which only 0.7% results from point source human and industrial waste.

Although the use efficiency of N and P in Iowa generally tend to be favorable, the use efficiency of N needs improvement. An estimated loss of 22% of agricultural N inputs can be translated into a $70 million to $240 million loss in N fertilizer for Iowa farmers, depending on N-fertilizer costs. Additionally, N loss to Iowa’s rivers appears to be small until you consider how many lbs. equivalent (400 million lbs. of N) equals the 7% N loss. This 7% N loss from the 55,800 square miles of Iowa results in 20% of the N exports out of the 1.15 million square mile Mississippi River Basin to the Gulf of Mexico (Libra and Wolter, 2004). Additionally, research at Iowa State University indicates an increased frequency of intense rainfall events and high rainfall years for Iowa. Studies also report that though the quantity of agricultural water drainage many vary substantially from year to year, the water drainage’s N and P concentrations stay the same (Daigh et al.,2011). This means those years with greater rainfall and drainage will increase the amount of N and P loss to rivers. Nitrogen fertilizer losses in agricultural drainage water reached 50% in traditional corn-soybean rotations during the high rainfall year 2010 (Daigh et al., 2011).

Fertilizer use efficiency can be increased by using best management practices. These include reduced tillage to decrease runoff/erosion (i.e. N and P adhered to soil particles), split spring N applications and use of the Late Spring Soil Nitrate Test to identify appropriate N plant needs, cover crops to retain extra soil N in the root zone, incorporating longer alternative crop rotations, and incorporating perennials into the landscape to keep sediments and nutrients from entering drainage ways and rivers (a priority along major rivers with fast-flowing cold waters that can transport nutrients farthest downriver. These efforts will help to increase fertilizer use efficiency, improve on farm economics, and increase water quality for the 50% of Iowa’s 3 million residents that rely on well water.


Daigh, A.L., M. Helmers, X. Zhou, and R. Horton. 2011b. Tile-drainage of bioenergy cropping systems. In Annual Meetings Abstracts. Bioenergy Crops and Their Impacts on Crops Production, Soil and Environmental Quality: II. ASA, CSSA, and SSSA, Madison, WI. Iowa Department of Agriculture and Land … (IDALS), 2011. Iowa Agriculture Quick Facts [Online]. Available at (verified on Dec. 5th, 2012). Libra, R.D. and C.F. Wolter. 2004. Nitrogen and phosphorus budgets for Iowa and Iowa watersheds. Iowa Department of Natural Resources-Geological Survey. Iowa Geological Survey Technical Information Series 47. United States Department of Agriculture – National Agricultural Statistics Service (USDA-NASS). 2010. Statistics of Fertilizers and Pesticides [Online]. Available at Publications/Ag_Statistics/2010/Chapter14.pdf (verified Dec. 5th, 2012).

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