Fertilizer


Soil can provide plants with nitrogen through organic matter mineralization, deposition during rainfall events, and residual inorganic nitrogen in soil. However, plants often need more nitrogen than soil can provide to produce a crop yield of desired quality. As such, nitrogen fertilizers need to be applied.

When you purchase nitrogen fertilizers, you will notice that the guaranteed analysis for essential nutrients contained is provided on the bags or containers. For nitrogen, usually guaranteed analysis for percent of total nitrogen, urea nitrogen, nitrate nitrogen, ammoniacal nitrogen, water soluble nitrogen, water insoluble nitrogen, or slowly available nitrogen is provided. Depending on the fertilizer product, one or more of these forms may be contained. It is important that you read the guaranteed analysis label to understand the percentage and forms of nitrogen fertilizer because different forms of fertilizers behave differently in the soil and are lost from soils in different ways.

Ammoniacal nitrogen undergoes a series of transformations after being applied to soils. For example, ammoniacal nitrogen can be converted to a nitrate form, gaseous form (ammonia), or organic form. If you apply ammoniacal nitrogen on the soil surface when the soil is dry or high in pH, you will lose a significant portion of nitrogen to the atmosphere in the form of ammonia. Windy conditions also favor such loss. With dry or high pH soil conditions, the nitrate, slow release, or controlled release forms of fertilizers are better choices. If you inject ammoniacal fertilizers deep into the soil, then nitrogen loss to the atmosphere in the form of gaseous ammonia is very little. Under optimum soil temperature and moisture conditions, ammoniacal nitrogen can be converted to nitrate nitrogen quickly.

Injecting anhydrous ammonia into the soil. Photo credit: Haiying Tao

Nitrate nitrogen, either converted from ammoniacal nitrogen or being applied as a nitrate form of fertilizer, also undergoes a series of transformations in soil. Nitrate is water soluble, therefore, leaching is the major loss pathway – often during heavy irrigation, heavy rainfall, and snowmelt events. If your fertilizer product contains mainly a nitrate form of nitrogen, you should avoid applying the fertilizer before heavy rainfall to avoid leaching loss. If you have soil with poor drainage, applying a nitrate form of fertilizer in water logging condition should be avoided because significant portion of nitrate nitrogen can be converted to other gaseous forms of nitrogen and be lost (e.g. nitrous oxide or molecular nitrogen which makes up most of earth’s atmosphere).

Liquid fertilizer application equipment setup. Photo credit: Haiying Tao

Once applied in soils, urea is converted to ammoniacal nitrogen. If urea is applied on the soil surface, it is best if you apply urea fertilizer before rain or irrigation, so that urea is carried into the soils by water. By doing this, you can avoid nitrogen loss to the atmosphere in the form of ammonia gas. In general, it takes about 0.5 inch of rainfall within 24 to 48 hours after surface application to transport urea to the depth that will minimize volatilization loss.

Slow release or controlled release fertilizers reduce nitrogen loss by delaying nitrogen release into the soil. They gradually feed crops during the growth period. Generally, slow-release nitrogen is most beneficial when fertilizer is applied in cold temperatures, on the soil surface, in soil with high leaching potential, or at hillslope locations with poor drainage/high water ponding risk. Carefully managing application rate is still necessary for these fertilizers because excessive application can cause nitrogen loss. The portion of nitrogen that is converted to ammoniacal and nitrate forms but not taken up by plants can still be lost to the environment.

Knowing that nitrogen is not held by soil and can be lost if not taken up by plants, the timing of nitrogen application is important to minimize nitrogen loss from your soil. It is best that you synchronize nitrogen availability with plant requirements and look at weather forecasts before fertilizer application.

For questions on fertilizers, contact the UConn Soil Nutrient Analysis Laboratory (email soiltest@uconn.edu or call 860-486-4274) or contact your local Cooperative Extension Center. For any other gardening questions, contact the UConn Home & Garden Education Center (email ladybug@uconn.edu or call 877-486-6271).

Haiying Tao, PhD

Nitrogen is an essential nutrient required for the production and growth of all plants, vegetation, and living organisms. It makes up 78% of our atmosphere; however, that only accounts for 2% of the Nitrogen on our planet. The remaining 98% can be found within the Earth’s lithosphere; the crust and outer mantel. The Nitrogen found within the nonliving and living fractions of soil represents an unimaginably low fraction of a percentage of all the Nitrogen on our planet. That tiny percent of all total Nitrogen found in our soils is what we can interact with to help or hinder plant production.

To be considered an essential nutrient, an element must satisfy certain criteria:

  1. Plants cannot complete their life cycles without it.
  2. Its role must be specific and defined, with no other element being able to completely substitute for it.
  3. It must be directly involved in the nutrition of the plant, meaning that it is a constituent of a metabolic pathway of an essential enzyme.

In plants, Nitrogen is necessary in the formation of amino acids, nucleic acids (DNA and RNA), proteins, chlorophyll, and coenzymes. Nitrogen gives plants their lush, green color while promoting succulent growth and hastens maturity. When plants do not receive adequate Nitrogen, the leaves and tissues develop chlorosis. However, over-application of Nitrogen can cause even more problems, including delayed maturity, higher disease indigence, lower tolerance to environmental stresses, reduced carbohydrate reserves, and poor root development.

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Chlorotic corn. Image provided by T. Morris, 2018

The Nitrogen Cycle describes the movement of Nitrogen through a landscape. Nitrogen undergoes numerous changes that affect its availability to certain plants and organisms.

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The Nitrogen Cycle. Image provided by T. Morris, 2018

Nitrogen undergoes numerous transformations within a landscape; each transformation represents a distinct chemical reaction or process that acts to further Nitrogen within the cycle. The different transformations are shown in the image provided, but some important ones to keep in mind are Mineralization (organic N -> NH4+), Immobilization (microbial), Denitrification (NO3 to a gaseous form), and Leaching (the loss of dissolved Nitrate into groundwater). There are factors that determine the rates and occurrences of all Nitrogen transformations including pH, temperature, saturation, etc… All of these transformations determine how much Nitrogen is available in your soil for plant uptake. Leaching poses a big problem, when too much Nitrogen is applied via fertilizer, NO3 can be transported in the soil water. Excess leaching can lead to Eutrophication.

Most plants take in Nitrogen as Nitrate, NO3, and Ammonium, NH4+. Generally, Nitrate is absorbed much more than Ammonium, but it is all plant-specific. The combination of both of these forms of Nitrogen can help to improve over-all plant growth when compared to intake of just one. Some plants use symbiotic N2 fixation, where they supply C for fixed Nitrogen from bacteria, actinomycetes, and cyano-bacteria (blue-green algae). This process involves the transformation of N2 to NH3. For instance, Legumes use Rhizobia inside their root nodules to convert N2 to NH4.

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Nitrogen Fixing Nodules. Image from NC State University

Applying the correct amount of Nitrogen is key in reducing leaching, and ensuring your plants are getting the perfect amount for maximum yields. Nitrogen testing proves to be difficult because of the constant transformations it undergoes. Getting your soil tested for other micro and macro nutrients can help provide information on overall soil health, and from there, proper Nitrogen fertilizer recommendations can be made. Talk to anyone from the UConn Soil Nutrient Lab or Home & Garden Education Center for more information on Nitrogen fertilizers and soil testing.

An alarming piece of new research shows decreasing Nitrogen availability with continued global warming. As CO2 levels increase in the atmosphere, essential nutrients are becoming less available to plants. As essential nutrients become less available, forests and ecosystems that usually absorb CO2 would be unable to do so, further increasing the CO2 in the atmosphere. Oligotrophication is the term coined to describe the decreasing productivity of a forest due to the unavailability of Nitrogen. You can read more about this process in the paper “Isotopic evidence for oligotrophication of terrestrial ecosystems” in Nature Ecology & Evolution by Andrew Elmore and David Nelson from the University of Maryland Center for Environmental Science and Joseph Crain of Jonah Ventures.

Joe C.

This year I had the opportunity to work in the UConn Soil and Nutrient Analysis Laboratory during the ‘spring rush’. During this time the Soil lab can get up to hundreds of samples a day. These samples may come in one at a time from homeowners with established lawns or garden beds who are looking to maintain their plantings or from new homeowners who have never planted or cared for a landscape before, or dozens of samples from commercial landscapers on behalf of their clients, or from commercial growers.

For over 50 years farmers, greenhouse growers, and homeowners have been served by the UConn Soil Lab. With more than 14,000 samples coming in on an annual basis, that is a lot of soil! Soil fertility is the first building block of plant health. If a plant is not growing in soil that has the proper proportion of available nutrients then it will not grow as well as it could. Poor soil health leads to stressed plants with stunted growth and stressed plants are vulnerable to insect and disease issues.

Iron deficiency on buddleia

Buddleia with iron deficiency

There are a minimum of 16 elements that have been deemed necessary to vigorous plant health. In order by atomic weight they are: hydrogen, boron, carbon, nitrogen, oxygen, magnesium, phosphorus, sulfur, chlorine, potassium, calcium, manganese, iron, copper, zinc, and molybdenum. Some other elements that may not be used by all plants are sodium, silicon, vanadium, and cobalt. The big 3 are, of course, nitrogen, phosphorus, and potassium. Represented by their symbols from the periodic table as N-P-K, they are the prime ingredients in most fertilizers. The seedlings below show signs of nutrient deficiency and are in need of a weak solution of a balanced fertilizer.

 

Also essential to healthy plant growth is the pH of the soil. It won’t matter how much fertilizer is applied if the soil pH is not in the correct range for the host plant. pH stands for potential of Hydrogen and is represented by a scale that runs from 0-7 for acidic solutions and from 7-14 for the alkalis. The higher the concentration of hydrogen ions, the more acidic the sample is. All soil test results will recommend the addition of either limestone to raise the pH, sulfur to lower the pH, or no action required if the pH falls into the acceptable range for the plant/crop.

All standard nutrient analysis tests begin their journey in the same way. For each area to be tested one cup of soil is sent or brought to the lab along with the soil sample questionnaire. The standard test will provide soil pH, the macro and micro nutrients, the total estimated soil lead, and basic texture and organic matter content. Many homeowners and growers request additional tests or only require specific information in the form of textural analysis, organic matter content (measured by Joe in the images below), soluble salts, a pH only test, saturated media analysis (for soil-less potting media for greenhouses), or nitrate testing (for commercial growers).

 

This spring was very cool and wet, as we all know. Many samples were sent in later than usual and a good many were very much wetter than usual. It is important then that the first step requires that soils be spread onto paper toweling and allowed to dry.

1. Spread soils on drying rack

Once the soil has adequately dried out it must be sieved so that any rocks or bits of organic matter are removed. This step may also involve some pounding to break up any chunks of soil as shown by Skyley.

 

From there a small amount of each sample is placed in a paper cup by Louise to be tested for its pH. It is mixed into a slurry with a small amount of distilled water, the calibrated testing meter probe is placed in the mixture and the pH level is stored in the computer program for later retrieval.

 

In a manner similar to a coffee pour over, some of the soil is placed in filter paper that is resting in a test tube in preparation for the nutrient analysis. A Modified Morgan solution is the liquid used for this extraction method.

 

The nutrient analysis is done by a machine called the ICP which stands for Inductively Coupled Plasma. This machine would be right at home in Abby’s lab on NCIS! When I was in school back in the 70’s we were taught that matter existed in three states: solid, liquid, and gas. But matter has a fourth state and it is plasma. It doesn’t exist on Earth under normal conditions but we do witness it every time we see a lightning strike.  Plasma can be generated by using energy to ionize argon gas.

The plasma flame is hot. Really hot.  6000 Kelvin.  For some perspective, the surface of the sun is approximately 5,800 K.  The solution from the individual tube samples is passed through a nebulizer where it is changed to a mist that is introduced directly to the plasma flame. A spectrometer is then able to detect the elements that are present in the soil sample.

 

Additionally, the testing for phosphorus is done with this machine shown below, the Discreet Analyzer.

 

Some soil samples come from outside of CT and those may present a particular set of problems. The USDA has quarantines in several states to limit the spread of certain invasive insect pest species such as the imported fire ant, golden nematodes, and even a few plant species. For more information visit the Federal Domestic Soil Quarantines site.

Working at the UConn Soil Lab has been a great experience and quite an eye-opener. Who knew that there was so much behind a soil test?

Susan Pelton

All images by S. Pelton, 2108