csu extensions
csu nutrient

Water Quality FAQs:

What is the difference between surface water and groundwater?


Surface water is generally considered to be water on the surface of the earth, such as in rivers, streams, lakes, reservoirs, wetlands, and the ocean.

Surface water can be contrasted with groundwater, which is subsurface water that saturates underground formations and aquifers. However, the two water systems can be interrelated.

What are BMPs?


Best management practices, or BMPs, are structures, methods, and practices designed to protect water quality. Some BMPs provide economic benefits, as well as environmental benefits, through increased crop yields and decreased input expenses. Others are cost-neutral or require an investment by the landowner or public to reduce nutrient impacts to water resources. Recommended BMPs often include widely accepted and utilized agricultural practices.

How are the nitrogen and phosphorus cycles relevant to agriculture?


Nitrogen and phosphorus each naturally cycle through the environment through chemical and biological transformations into various forms. Crop growth is dependent on the forms of these nutrients that are available for uptake and use by plants. Understanding nutrient cycles can help producers more efficiently apply nutrients in time, amount and place to maximize plant growth and crop yields while minimizing harmful losses to the environment.

What is an “agronomic rate” of nutrient application?


The agronomic rate is a nutrient application rate based upon expected crop requirements. This rate takes into account residual soil nutrients that are already available as well as nutrients from all applied sources, such as commercial fertilizers, manure, and irrigation water.

What factors influence whether a field is vulnerable to nitrate leaching?


Soil type, aquifer depth and irrigation efficiency are the most important field characteristics influencing the ability of nitrate to leach below the root zone and contaminate groundwater. Fields with shallow (<25 ft) groundwater, sandy soils and furrow or flood irrigation are the most vulnerable to leaching. Producers should carefully management nitrogen applications rate, placement and timing in these situations. For more information on preventing nitrate leaching see: Nitrogen and Irrigation Management . You can also run a field scale evaluation of a site using the Nitrogen Leaching Index in the Tools section of the Clearinghouse.

What factors influence whether a field is vulnerable to phosphorus runoff?


Phosphorus can move in runoff from a field adsorbed to soil particles or as soluble forms. The same factors that affect runoff and soil erosion from irrigation or precipitation influence the amount of phosphorus that will potentially leave the field. Soil type, slope, soil test phosphorus levels, and irrigation efficiency are all important in influencing phosphorus losses from a site. Also important are the BMP’s implemented at the site such as conservation tillage and other BMPs designed to prevent erosion. For more information on reducing phosphorus runoff see: Best Management Practices for Phosphorus. You can also run a field scale evaluation of a site using the the Phosphorus Runoff Index in the Tools section of the Clearinghouse.

Nitrogen FAQs:

What forms of nitrogen are available to plants?


Plants can use nitrogen in two forms: nitrate nitrogen (NO3) and ammonium nitrogen (NH4+). Since nitrate is more mobile in soil it is usually adsorbed in higher amounts. The nitrogen forms are the same regardless of whether they come from organic sources such as manure and compost or commercial fertilizer.

What are the sources that provide nitrogen to the soil?


All of the following sources can provide nitrogen to the soil:

  • Soil organic matter
  • Crop residue
  • Organic amendments: manure, biosolids, compost
  • Irrigation water nitrate
  • Legumes such as alfalfa and beans
  • Synthetic fertilizers
  • Atmosphere: lightning, nitrogen deposition

What is nitrogen mineralization?


The process that converts organic forms of nitrogen into compounds that are available to plants.  Microbes decompose organic N from manure, organic matter and crop residues into ammonium (NH4+). Rates of mineralization vary with soil temperature, moisture, and soil aeration.

What is nitrification?


The biological process that converts ammonium (NH4+) into the nitrate nitrogen (NO3) ion.

What are the potential loss pathways of plant available nitrogen in an irrigated cropping system?

  • Leaching of nitrate (NO3) to below the rootzone
  • Immobilization of ammonium (NH4+) and nitrate (NO3) into organic forms of N
  • Denitrification of nitrate (NO3) to N gases
  • Volatilization of ammonium (NH4+) to ammonia gas (NH3)

What are slow-release and controlled-release N fertilizers?


These synthetic fertilizers are designed to delay the release of plant available N. These fertilizers typically has a special coating that slowly dissolves and makes the N more available later in the growing season.

What is the preferred depth for a soil sample to determine nitrogen availability?


The depth of a soil sample for nitrogen availability is crop dependent. For shallow rooted crops a surface sample of the plow layer (8-12 inches) may be sufficient. With deeper rooted crops a soil sample below the first foot greatly improves nitrogen recommendations. Take deep samples to 2 feet, preferably to 4 feet in irrigated systems. Sample as follows: surface to tillage depth, tillage depth to 2 feet, and 2 feet to 4 feet.

How much nitrogen is released annually from soil organic matter?


Approximately 20-30 lbs of nitrogen per acre is released or mineralized for every one percent of soil organic matter.

What is Nitrogen Use Efficiency and why is it important?


Nitrogen Use Efficiency is the percent of applied nitrogen that is recovered in the harvested portion of the crop. Unrecovered nitrogen has the potential to be lost to the environment and is an economic loss for the producer.

How do I convert a soil test nitrate result from parts per million (ppm) to pounds per acre?


For a one foot sample in soils with normal bulk densities the conversion factor is 3.6. To convert soil samples taken at different depths the result is adjusted by the ratio of the actual soil depth to 12 inches. For example, an eight inch soil sample contains 10 ppm nitrate-nitrogen. The lbs per acre are 10 ppm x 3.6 x 8/12 = 24 lbs/acre

How much nitrogen can you expect following a legume crop in a rotation?


Legume crops can be a very significant source of plant available nitrogen due to bacterial Nfixation in root nodules. Plowing down a full stand of alfalfa will release as much as 100 pounds of nitrogen per acre in the first year after termination. The amount of nitrogen for legumes depends upon the crop, stand, and degree of nodulation. See the Legume Nitrogen Credits fact sheet and Best Management Practices for Nitrogen Management  in the BMP Library for more information on nitrogen management and legumes.

Phosphorus FAQs:

What forms of phosphorus are available to plants?


The inorganic ions, (H2PO4) and (HPO4-2), are the primary forms of P taken up by plants.

What is phosphorus mineralization?


The process that converts phosphorus into forms that are available to plants. Microbes break down organic phosphorus and release inorganic phosphates that are available to plants.

Is phosphorus mobile in the soil environment?


In most Colorado soils, phosphorus is not mobile. Placement of P in subsurface bands within the crop rootzone will optimize plant uptake and minimize potential to movement to surface water bodies.

Why is phosphorus less plant available in soils with high pH (>7.5)?


Soluble forms of plant available P react with calcium in high pH soils to form insoluble minerals. This P is then largely unavailable for future plant uptake. These soils typically have a high calcium carbonate or lime (CaCO3) content. Soil labs typically report lime content as estimated lime (EL). Soils with EL above two percent are considered high and most likely to cause plant available forms of P to become unavailable.

Why are there several different soil tests for measuring phosphorus availability in soils?


Different soil tests dissolve different fractions of mineral soil P. Depending upon the soil pH and lime content some soil tests are more appropriate for measuring potential plant available P. The most commonly used tests are the Olsen (neutral to high pH) , Bray (acidic to neutral), and Mehlich (used in a range of soil pH).

Can I convert my phosphorus soil test analyses results to pounds per acre of plant available P?


No. Most common soil tests measure the relatively availability of P for plant uptake. These tests are then calibrated in field trials for their ability to predict a yield response in different crops. Low soil test P has a high probability of a yield response to added P and soils testing high in P have a low probability that P application will produce higher yields.

What is the best depth for a soil sample to determine P availability?


The plow layer or top six to eight inches of soil should be sampled for fields under tillage systems that mix the top soil. In hay, pasture, or no-till systems the top three to four inches is recommended.

How can I enhance P availability in calcareous soils?


Phosphorus availability in calcareous soils can be enhanced by:

  • Application of acid forming fertilizers containing ammonia-N
  • Co-placement of nitrogen and phosphorus in a band near the plant roots
  • Increasing soil organic matter content with amendments such as manure, composts, and biosolids or planting cover crops
  • Application of Mycorrhizal inoculum

Soil Erosion FAQs:

Why is soil erosion management important to managing nutrients and other water quality issues?


Soil erosion can result in the loss of soil sediments from cropland to surface waterways. Soil sediments are a contaminant in addition to the nutrients and other chemicals that can be adsorbed to these soil sediments. In addition, BMPs that can reduce loss such as utilizing better tillage and residue management practices can improve soil structure to more efficiently use nutrients and water.

What is conservation tillage?


Conservation tillage refers to tillage and planting systems that maintain at least 30% of the soil surface covered by residue following planting, or a system that maintains at least 1,000 pounds per acre of small grain residue on the surface. This residue should be present during the the time of the season when erosion potential is highest.

What are the drawbacks of conventional tillage?


Conventional tillage (or clean tillage) often results in over-tilled soils, which causes the loss of organic matter and the breakdown of soil structure. Clean tillage can also promote soil erosion, create soil compaction, increase soil moisture loss, as well as increase labor and production costs.

What are the benefits of conservation tillage?

  1. Reduce/eliminate field operations, which reduces production costs and soil compaction
  2. Reduces soil erosion caused by wind and water, which maintains crop production sustainability and protects water quality
  3. Provides wildlife shelter and habitat