Fact Sheets And Publications

Nitrogen Removal by Delaware Crops

Effective nutrient management minimizes nutrient losses to the environment while maximizing the crop nutrient uptake. Valid numbers for all nutrient inputs and outputs are required to most effectively manage nutrients in agronomic and vegetable crop systems. The amount of nitrogen (N) removed by the harvested portion of the crop is needed to develop nutrient balances. However, N removal by crops can vary considerably from field-to-field and year-to-year. The purpose of this document is to provide average values of N uptake for common Delaware grain, forage, and vegetable crops for use in nutrient management planning activities.

What is “Crop Nutrient Removal”?

Crop nutrient removal is defined as the total amount of nutrients removed from the field in the harvested portion of the crop (e.g., grain, silage, hay). The term crop nutrient removal should not be confused with crop nutrient uptake, which is defined as the total amount of nutrients contained in the entire crop at maturity. For example, this would include N in the grain, stover, and roots of a corn crop. Crop nutrient removal is lower than crop nutrient uptake because a significant percentage of the nutrients taken up by a crop are returned to the soil in the form of crop residues. Nutrients remaining in crop residues are subsequently available for uptake by crops planted in the next season.

Determining Crop Nitrogen Removal for Delaware Crops

Removal of N by crops can be estimated using standard values for the N content in the harvested portion of the crop and crop yield. One reliable source of information about the N content of most Delaware crops is available in the Plant Nutrient Content Database, which was developed by the USDA Natural Resources Conservation Service (NRCS). However, the most accurate way to determine crop N removal is to have a representative sample of the harvested portion of the crop analyzed for N content. Analyzing crop samples for nutrient removal is different than routine plant tissue analysis, which is used to monitor the nutrient content of a crop or to identify nutrient deficiency or toxicity. For example, a subsample of the harvested grain that is collected from the combine or weigh wagon after yield determination is analyzed for N content to quantify N removal at harvest. In contrast, analysis of ear leaf samples collected at initial silking is useful to monitor corn nutrient content during the growing season. For vegetable crops, the use of standard values for N content may be preferable to harvested tissue analysis due to the difficulty encountered when trying to dry vegetables. Because of their high water content, a freeze drier is often required to prevent the vegetable samples from rotting during drying.

Laboratory nutrient analysis reports usually provide nutrient content of plant tissue samples on a dry weight basis (i.e., units of nutrient per unit of dry plant tissue). To determine crop N removal, these dry weight values must be adjusted to account for the moisture content of the crop. In addition, for crops where yield is reported in units other than pounds per acre, the N content of the harvested tissue must be adjusted based on the weight per unit (such as pounds per bushel).

The following example illustrates how to determine crop N removal for corn grain containing 1.45% N based on results of lab analysis:

This value corresponds to 1.45 pounds (lbs) N per 100 dry pounds of corn grain. Because this value is listed on a dry weight basis, it must be adjusted to account for the moisture content of the crop. For corn grain, if we assume a moisture content of 15.5%, which is equivalent to 84.5% dry matter or 0.845 lb dry corn grain per lb corn grain:
 

Crop N removal must then be adjusted (when applicable) based on the standard test weight. The standard test weight for corn
grain is 56 lb/bushel (bu):
 

Therefore, the actual nutrient removal for corn grain in this example would be 0.69 lb N per bushel.

Nitrogen Removal by Typical Delaware Crops

The N removal rates for typical Delaware grain crops listed in this document were determined by analyzing the harvested portion of selected crops in Delaware (Binford, 2008). Between 2003 and 2007, a total of 668 corn grain samples, 175 soybean samples, 322 winter wheat samples, and 117 winter barley samples were collected at harvest from locations throughout Delaware (with a small number of samples collected from farms located on the eastern shore of Maryland that share similar climate, soil, and cropping conditions as Delaware) and analyzed for N content (Binford, 2008). Nitrogen content of harvested grain samples in Delaware reported by Binford (2008) was comparable to standard values reported in the USDA-NRCS Crop Nutrient Removal database.

Only a small number of vegetable and forage harvest samples were collected from Delaware fields in 2004 due to issues related to drying samples for analysis. Nitrogen values in harvested tissue of the vegetable and forage crops data presented by Binford (2008) were in good agreement with values obtained from the USDA Plant Nutrient Content Database. Therefore, standard values for crop N removal of selected vegetable and forage crops from the USDA Plant Nutrient Content Database are reported in this publication. We then calculated the amount of N removed per acre by grain and forage crops (Table 1) and vegetable crops (Table 2) over a range of realistic yield goals for major Delaware crops, where removal is the product of N content and crop yield.