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Worksheet Grade 11 Science

Oshton's Biosolids: Nitrogen, Phosphorus, and the Agronomic Rate

A grade 11 worksheet turning a plant's daily solids production into an application area, then showing why phosphorus rather than nitrogen limits how many acres it covers.

Objectives

  1. Students will convert a plant's daily dry solids production into an annual nutrient mass.
  2. Students will calculate the acreage a nitrogen-based and a phosphorus-based agronomic rate each require.
  3. Students will explain why exceeding an agronomic rate moves nutrients into groundwater or surface water.

Vocabulary

Biosolids
Biosolids are the nutrient-rich solid material left over after sewage sludge has been treated and made safe. Many farms use treated biosolids as fertilizer for their fields.

Also: dry solids, agronomic rate, plant-available nitrogen, mineralization, crop removal

Teacher overview

The solids a wastewater plant removes carry much of the nitrogen and phosphorus that arrived dissolved in the sewage, and once those solids are digested and dewatered they can go to farmland as biosolids. The amount a field can accept is set by the agronomic rate, which is the nutrient supply a crop will actually take up in a season. This worksheet has students carry the fictional Oshton plant's daily solids production out to an annual nitrogen and phosphorus mass, then find how many acres each nutrient would require. Two misconceptions show up here. Students assume all the nitrogen in biosolids is available the year it is applied, when only part of the organic nitrogen mineralizes in the first season, and they assume nitrogen sets the limit, when phosphorus often forces a far lower application rate than nitrogen does.

Oshton's Biosolids: Nitrogen, Phosphorus, and the Agronomic Rate — Grade 11 Science
Name: ____________________________ Date: ______________

Student Worksheet

Every item describes Oshton, a fictional town whose water resource recovery facility sends digested biosolids to a nearby grain farm. Use these values throughout.

Quantity Value
Dry solids produced 2,150 pounds per day
Total nitrogen content 4.2 percent of dry weight
Nitrogen available to the crop in year one 35 percent of total nitrogen
Total phosphorus content 2.8 percent of dry weight
Corn nitrogen requirement 160 pounds per acre per year
Corn phosphorus removal 60 pounds per acre per year
Price of fertilizer nitrogen $0.55 per pound
  1. Oshton's facility produces 2,150 pounds of dry solids per day. Calculate the pounds of dry solids produced in a 365-day year.
  2. Convert your item 1 answer to dry tons, using 2,000 pounds per ton. Round to the nearest tenth.
  3. The biosolids are 4.2 percent nitrogen by dry weight. Using your item 1 answer, calculate the total pounds of nitrogen produced in a year.
  4. Only 35 percent of that nitrogen mineralizes into a form the crop can use during the first season. Using your item 3 answer, calculate the pounds of plant-available nitrogen in year one.
  5. The farm's corn needs 160 pounds of nitrogen per acre each year. Using your item 4 answer, calculate the acres a nitrogen-based agronomic rate would cover. Round to the nearest tenth of an acre.
  6. Using your item 2 dry tons and your item 5 acreage, calculate the application rate in dry tons per acre, rounded to the nearest hundredth.
  7. The biosolids are 2.8 percent phosphorus by dry weight. Using your item 1 answer, calculate the total pounds of phosphorus produced in a year.
  8. The corn crop removes 60 pounds of phosphorus per acre each year. Using your item 7 answer, calculate the acres a phosphorus-based agronomic rate would cover. Round to the nearest tenth of an acre.
  9. Compare your item 5 and item 8 answers. State which nutrient limits how many acres the farm needs, and explain what would build up in the soil, and eventually reach surface water, if the farm applied at the other nutrient's rate year after year.
  10. Using your item 2 dry tons and your item 8 acreage, calculate the application rate in dry tons per acre under the phosphorus limit, rounded to the nearest hundredth.
  11. Fertilizer nitrogen costs $0.55 per pound. Using your item 4 answer, calculate the value of the nitrogen the farm does not have to buy in year one, rounded to the nearest dollar.
  12. Oshton could instead send the solids to a landfill. Phosphate fertilizer is made from phosphate rock, which is mined from a finite deposit. Give one benefit and one drawback of land application compared with landfilling, and explain what land application does to the demand for mined phosphate.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 784,750 pounds per year.
    2,150 pounds per day x 365 days = 784,750 pounds of dry solids per year.
  2. Answer: 392.4 dry tons.
    784,750 pounds / 2,000 pounds per ton = 392.375, which rounds to 392.4 dry tons per year.
  3. Answer: 32,959.5 pounds of nitrogen.
    784,750 x 0.042 = 32,959.5 pounds of total nitrogen in a year's biosolids.
  4. Answer: 11,535.8 pounds of nitrogen.
    32,959.5 x 0.35 = 11,535.825, which rounds to 11,535.8 pounds available to the crop in the first season. The rest stays in organic forms that mineralize over later years.
  5. Answer: 72.1 acres.
    11,535.825 pounds / 160 pounds per acre = 72.099, which rounds to 72.1 acres.
  6. Answer: 5.44 dry tons per acre.
    392.375 dry tons / 72.1 acres = 5.4421, which rounds to 5.44 dry tons per acre.
  7. Answer: 21,973 pounds of phosphorus.
    784,750 x 0.028 = 21,973 pounds of total phosphorus in a year's biosolids.
  8. Answer: 366.2 acres.
    21,973 pounds / 60 pounds per acre = 366.217, which rounds to 366.2 acres.
  9. Answer: Phosphorus limits the acreage, at 366.2 acres against 72.1.
    Meeting the crop's nitrogen need on 72.1 acres would deliver about five times the phosphorus that crop removes each year, so phosphorus would accumulate in the soil. Once soil phosphorus is high, storm runoff and eroded soil carry it to streams and lakes, where it feeds algae growth. Spreading the same biosolids across 366.2 acres keeps phosphorus near what the crop removes, at the cost of hauling to a much larger area.
  10. Answer: 1.07 dry tons per acre.
    392.375 dry tons / 366.2 acres = 1.0715, which rounds to 1.07 dry tons per acre, roughly a fifth of the nitrogen-based rate.
  11. Answer: 6,345 dollars.
    11,535.8 pounds x 0.55 dollars per pound = 6,344.69, which rounds to 6,345 dollars of fertilizer nitrogen the farm does not purchase in year one.
  12. Answer: It returns nutrients to soil but requires more hauling and monitoring.
    Accept any sound pair. Land application returns nitrogen, phosphorus, and organic matter to farmland and keeps the material out of a landfill, where those nutrients are simply buried. The drawbacks are the hauling distance a phosphorus-based rate requires, the need for soil testing and record keeping, and public concern about what else the solids carry. Because the phosphorus applied substitutes for purchased fertilizer, land application lowers the demand for phosphate rock, which is mined from deposits that cannot be replaced.

Differentiation

Support

Provide a flow chart tracing pounds per day to pounds per year to pounds of nutrient to acres, with the operation written on each arrow, so students can see the whole chain before starting item 1. Give the item 1 and item 2 results as a worked example for a plant of a different size, and mark on the data table which two rows item 5 needs.

Extension

Have students calculate how many years of application at the nitrogen-based rate it would take to deliver a decade's worth of the crop's phosphorus removal to the same field, then explain what a soil test would show and what the farm would have to change. Ask them to state one reason a utility might still prefer the higher rate.

ELL supports

Pre-teach biosolids, dry solids, agronomic rate, and mineralize with a picture sequence showing sewage solids, a digester, a dewatering step, and a field, labeling where nutrients travel. Provide the frame "Phosphorus limits the acreage because the crop removes only ___ pounds per acre, while the biosolids supply ___" for item 9.