Maui County Unversity of Hawaii at Manoa UH Seal Soil Nutrient Management for Maui County College of Tropical Agriculture and Human Resources (CTAHR)
Skip Navigation LinksHome > Nutrient Management > Essential Nutrients > Nitrogen


Of all the essential nutrients, nitrogen is required by plants in the largest quantity and is most frequently the limiting factor in crop productivity.

  • In plant tissue, the nitrogen content ranges from 1 and 6%.
  • Proper management of nitrogen is important because it is often the most limiting nutrient in crop production and easily lost from the soil system.

Nitrogen Forms and Function

Forms of nitrogen available for plant uptake
  • Ammonium
  • Nitrate
Functions of nitrogen in plants
  • Nitrogen is an essential element of all amino acids. Amino acids are the building blocks of proteins.
  • Nitrogen is also a component of nucleic acids, which form the DNA of all living things and holds the genetic code.
  • Nitrogen is a component of chlorophyll, which is the site of carbohydrate formation (photosynthesis). Chlorophyll is also the substance that gives plants their green color.
    • Photosynthesis occurs at high rates when there is sufficient nitrogen.
    • A plant receiving sufficient nitrogen will typically exhibit vigorous plant growth. Leaves will also develop a dark green color.

The Nitrogen Cycle

The Nitrogen Cycle
Figure 5. The nitrogen cycle

Gains of Nitrogen to the Soil

  • Biological and Atmospheric Fixation: Conversion of atmospheric nitrogen to ammonium which is subsequently available for plant uptake
  • Direct additions of commercial and organic fertilizers

Transformations in the Soil

  • Mineralization: Conversion of organic nitrogen to ammonium
  • Nitrification: Conversion of ammonium to nitrate

Losses of Nitrogen from the Soil

  • Denitrification: Conversion of nitrate to atmospheric forms of nitrogen
  • Volatilization: Loss of gaseous ammonia to the atmosphere
  • Run-off
  • Leaching
  • Consumption by plants and other organisms

Nitrogen is a very dynamic element. It not only exists on Earth in many forms, but also undergoes many transformations in and out of the soil. The sum of these transformations is known as the nitrogen cycle.

Table 5. Various forms of nitrogen

Form of Nitrogen Formula Availability for plant uptake
Nitrogen gas N2 Although 78% of our atmosphere is nitrogen gas, this form of nitrogen must be transformed to usable forms before it is available for plant uptake.
Ammonia NH3 Ammonia is a gas. Ammonium can escape from the surface of the soil under certain conditions and is harmful to plants in high quantities. Ammonium is the basic building block of commercial nitrogen fertilizers.
Ammonium NH4+ Soil particles attract and retain ammonium on cation exchange complexes. This form may be directly taken up by plants.
Nitrate NO3- Nitrate is the second form of nitrogen which is available for plant uptake. In most soils, nitrate is highly mobile. However, in the highly weathered soils of Hawaii, nitrate is stored in soils with ‘anion exchange capacity’ and becomes less mobile.
Nitrite NO2- Nitrite is an intermediate product in the conversion of ammonium to nitrate (nitrification). It is usually present in low quantities, but is toxic to plants.
Organic Nitrogen Various compounds Organic nitrogen must be converted to ammonium before it is used by plants. This conversion occurs with time and is known as mineralization.

Though complex, the nitrogen cycle:

  • Helps us to understand the complex relationships that exist between the many forms of nitrogen
  • Provides us with insight pertaining to the availability of ammonium and nitrate, which are the only nitrogen forms usable by plants
  • To understand the many ways in which N may be lost from the soil

In this section, we will discuss eight major transformations of nitrogen in the soil: nitrogen fixation, mineralization, immobilization, nitrification, denitrification, volatilization, and leaching.

Nitrogen fixation

Although atmospheric nitrogen gas(N2) makes up approximately 78% of the air, it cannot be directly used by plants. Instead, atmospheric N2 only becomes available to plants through three unique processes. The final product of each of these processes is ammonium, which is then available for plant uptake.

The three processes which convert atmospheric nitrogen to ammonium

  • biological nitrogen fixation
  • chemical nitrogen fixation
  • atmospheric addition

Biological nitrogen fixation

Certain soil organisms have the special ability to convert atmospheric nitrogen to ammonium. These organisms include several species of bacteria, actinomycetes, and cyanobacteria.

In the soil, nitrogen fixating organisms can form special relationships with plants, called “symbiotic” associations. Symbiotic is a term that means “living together.” Although a symbiotic relationship can be antagonistic, the symbiosis that occurs during biological nitrogen fixation is generally mutual and beneficial.

Legume-Rhizobium Symbiosis

The most abundant symbiotic relationship in nitrogen fixation forms between legumes (i.e. alfalfa, soybeans, etc.) and the Rhizobia bacteria species.

  • As the roots of legumes grow, Rhizobium bacteria infect the root hairs where they begin to multiply.
  • As a response to this colonization, the legume forms nodules, which are structures that form around the Rhizobia.
  • Within these nodules, Rhizobia bacteria are able to continue multiplying and converting the N2 from the soil air to ammonium.
    • However, the presence of nodules is not a sufficient indicator that nitrogen is being converted to ammonium. Active and effective nodules are generally greater than 2 mm, have pink to red interiors, and concentrate around the tap root.
    • On the other hand, non-effective nodules are generally smaller in size with white, green or brown interiors.

The image below shows cross sections of soybean (Glycine max) nodules. Nodules in the first row are highly effective at converting N2 from the atmosphere into ammonium. The nodules in the second row are moderately effective at biological nitrogen fixation, while the bottom row of nodules do not fix any nitrogen at all. Notice the lack of color in the interior of these nodules, which indicates that they do not have an active ‘nitrogenase system’. A ‘nitrogenase system’ is the bacterial enzyme that is necessary to convert N2 gas into ammonium through this biological process.
Relationship between color and effectivity of nodulation in soybean.
Figure 6. Relationship between color and effectivity of nodulation in soybean.
Source: Legumes Inoculants and Their Use, 1984. University of Hawaii NifTAL Project and FAO.

The next image shows how a well nodulated soybean root system looks when it contains many highly effective rhizobia in the soil or is applied as an inoculant.

Depiction of effective nodulation of soybean by rhizobia.
Figure 7. Depiction of effective nodulation of soybean by rhizobia.
Source: J. Burton. Legumes Inoculants and their Use, 1984. University of Hawaii NifTAL Project and FAO


Some Rhizobium species are only capable of nodulating a particular legume species and cannot successfully nodulate other legumes.

  • For example, the Rhizobium that nodulates alfalfa is a different species from the Rhizobium that nodulates soybean.

This phenomenon is known as Rhizobium specificity. However, not all Rhizobium are legume-specific. Thus, some may nodulate a number of different legumes.

The figure below gives examples of some common cross-inoculation groups that may assist you in selecting the proper rhizobial inoculant for a particular legume host. The proper combination of rhizobia and legume will result in the optimal nodulation and most nitrogen fixation. From this figure, we see that using soybean rhizobia with a soybean plant forms an effective symbiosis, while using a soybean rhizobia with a leucaena plant does not. However, cowpea rhizobia is capable of nodulating both mungbean and peanut.

Specificity of Rhizobia for successful nodulation of certain legumes.
Figure 8. Specificity of Rhizobia for successful nodulation of certain legumes.
Source: Singleton et al. 1994. BNF Technology for Extension Specialists. NifTAL Project. College of Tropical Agriculutre and Human Resources.

Biological Nitrogen Fixation Management Program

The occurrence of the symbiotic relationship is heavily dependent upon a variety of soil conditions. If your program incorporates nitrogen fixation, the following considerations can determine your success.

  • First and foremost, the Rhizobium must be compatible with the legume. If your crop is Rhizobium specific, you must use the correct Rhizobium species.
  • If your inoculum (which contains the Rhizobium bacteria) is applied to seeds, the procedures must be properly followed.
  • Nitrogen fixation takes place when total soil nitrogen is insufficient. When sufficiently present, the plant will instead rely on the nitrogen available from the soil.
  • Rhizobia are sensitive to any growth factor that limits root development. Such conditions as aluminum and manganese toxicities will limit inoculation.
  • Rhizobia are influenced by mineral nutrient imbalances.
    • Low levels of calcium, phosphate, molybdenum under acidic conditions will limit nitrogen fixation.
    • Under alkaline conditions, phosphate, cobalt, boron, iron, and copper levels become a concern.
  • Any growth factor (such as light, water temperature stresses or soil compaction) and any management factor (such as nutrient management, salinity) that detrimentally affects growth of the legume will detrimentally impact nitrogen fixation.

Table 6. A summary of biological nitrogen fixation measurements by different legumes.
A summary of biological nitrogen fixation measurements by different legumes.
Source: Singleton et al., 1993. The importance of legume-based BNF in world agriculture - an examination of the major commercial and environmental issues, IFDC Muscle Shoals Alabama.

When does inoculation of legumes with rhizobia increase yield and biological nitrogen fixation?

This question is commonly posed by farmers who are deciding when to apply an inoculant to their crops. Since inoculation is relatively inexpensive (less than $5.00/acre), growers should error on the side of caution and inoculate their legume crops unless they have good evidence that inoculation is not needed.

The figure below is a conceptual model which integrates all the factors controlling biological nitrogen fixation. Additionally, it explains when the inoculation of legumes with rhizobia will result in an increase in plant growth and biological nitrogen fixation activity. The model is based on the fact that if the plant’s need for nitrogen is greater than the nitrogen that is supplied by both the existing soil nitrogen and the rhizobia already present in the soil, the inoculation of supplementary effective rhizobia will result in increased yield and biological nitrogen fixation.

The various Factors that control nitrogen fixation.
Figure 9. The various Factors that control nitrogen fixation.
Source: Legume response to inoculation in the tropics: Myths and realities, 1992, p135-155. In R. Lal and P. Sanchez (eds) Myths and Science of Soils of the Tropics. SSSA, Madison.

Amount of Nitrogen Fixed by Legume

When nitrogen is converted to ammonium during biological nitrogen fixation, ammonium becomes available to the legume and the microorganism that fixes it. Typically, the bacteria can fix anywhere between 20 and 80% of the total legume N.

  • Perennial legumes may fix 100 to 200 lb/a/yr
  • Annual legumes fix 50 to 100 lb/a/yr.

Small amounts of ammonium can also be released by roots of the legume into the rhizosphere, or the surrounding soil.

Availability of Nitrogen in Subsequent Cropping Systems

What about subsequent cropping systems? Can a legume rotation benefit later plantings of a nonlegume crop?

Research shows that yields of nonlegume crops can increase when following a legume rotation. It is believed that the legume rotation increases the N content of soil, thus making it an effective nutrient management strategy.

However, when the legume is incorporated into the soil, the major benefit of the legume rotation lasts only during the first year following the legume rotation.

Other symbiotic relationships

Legumes and Rhizobia are not the only species that can establish a mutual symbiotic relationship needed for nitrogen fixation to oocur.

  • In wetland rice production, a symbiotic relationship may form between Anabaena azolla (a blue green algae) and the Azolla fern. As a result, wetland plants can benefit by incorporations of Azolla as a green manure.
  • Certain tree species (i.e Causarina sp.) can form symbiotic relationships with certain species of Actinomycetes and the Frankia bacteria. Though this relationship has lesser agricultural importance, it may gain significance in forestry, or wood production.
Free-Living Nitrogen Fixation

“Free-living” nitrogen fixating organisms are also capable of nitrogen fixation, but are not associated with any plant species.

  • Examples of these organisms are azotobacteria, azolospirillum, and clostridium. However, free-living species do not contribute largely to agricultural production

Chemical nitrogen fixation

Since the 1950s, ammonium-based fertilizers have been manufactured using the Haber-Bosch technique. In this catalytic process, N2 reacts with hydrogen under 1,200 degrees Celsius and 500 atm.

Since the production of chemical fertilizers requires large inputs of fossil fuel, chemical fertilizers can be relatively expensive.

The impact of the Haber-Bosch technology on agriculture has been very dramatic. The Haber-Bosch technology enables high-analysis ammonium fertilizers to be produced quickly. As a result, the reliance on biological N fixation and manures as N sources has declined.

Atmospheric nitrogen additions

Nitrogen is deposited onto the earth’s surface by:

  • Rain
    • In the form of ammonium, nitrate, nitrite
  • Finely divided organic N swept along the earth’s surface
  • Lightning
    • Responsible for approximately 10-20 % of soil nitrate (114, Fertilizers)
  • Industrial wastes
    • In Hawaii, as compared to the major industrial regions of the Mainland, industrial wastes do not significantly contribute to atmospheric N.

Table 7. This table presents estimates of the different sources of atmospherically fixed nitrogen that was deposited onto the earth in the latter half of the twentieth century. Biological sources account for around 20% of total nitrogen deposition.
estimates of the different sources of atmospherically fixed nitrogen that was deposited onto the earth in the latter half of the twentieth century.
Source: Singleton et al., 1993. The importance of legume-based BNF in world agriculture - an examination of the major commercial and environmental issues, IFDC Muscle Shoals Alabama.

Nitrogen Mineralization in Soils

When absorbed by plants, ammonium and nitrate are incorporated into plant cells as organic, or living, forms of nitrogen. When plants die, microorganisms break down, or decompose, dead plant cells. During the decomposition about plant matter, organic nitrogen is once again converted to inorganic ammonium and released into the soil.

The process that converts organic N to ammonium is called mineralization and plays a significant role in the management of nitrogen.

To calculate the amount of nitrogen which can potentially be mineralized from your organic fertilizer source, click on the link below:

Conditions affecting N mineralization

The amount of ammonium that is released to the soil through mineralization depends on several factors:

  • Quantity of Organic Nitrogen: The amount of organic nitrogen originally present in the organic matter determines the amount of N that can ultimately be mineralized.
  • Temperature: The optimal range for mineralization to occur is between 77-95 degrees Fahrenheit.
  • Oxygen: Microorganisms need oxygen and since microorganisms mediate mineralization, sufficient oxygen must be available in the soil.
  • Moisture content: Ideally, water should fill 15 – 70 % of pore space for maximum mineralization. This roughly corresponds to field capacity.
  • Ratio of carbon to nitrogen (C:N): The C:N ratio is a term used to describe the relative amount of total carbon in comparison the amount of total nitrogen present in the soil and/or organic matter.
    • This ratio is very important in determining the rate of mineralization that should occur for a given type of organic matter.

Since the microorganisms living in the soil need both carbon and nitrogen, net mineralization occurs when C:N ratio is less than 20:1. This means for every two parts of carbon, there should be 1 part nitrogen for net mineralization. If you are applying organic amendments to your soil, it is important to become familiar with the C:N ratio to ensure N availability.

After mineralization of N occurs, ammonium can be:

  • Taken up by the plants
  • Consumed by other organisms
  • Nitrified
  • Volatilized


Immobilization is process that converts inorganic nitrogen to organic nitrogen. It is the reverse reaction of mineralization.

Immobilization occurs when decomposing organic matter contains low amounts of nitrogen. Thus, immobilization occurs if the source of organic matter has a high C:N ratio. Microorganisms, who also need nitrogen to live, scavenge the soil for nitrogen when plant residues contain inadequate amounts of nitrogen. As inorganic ammonium and nitrate are incorporated into the cells of living microorganisms, the total N levels in the soil are reduced. Immobilization can ultimately result in nitrogen deficiencies.

When nitrogen is immobilized in the soil, there may be little nitrogen available for crop growth. As a result, plants can suffer from nitrogen deficiency and develop a yellow coloration. This is the reason why organic materials with a high C:N ratios, such as grass clippings and grain stover, are usually composted before they are incorporated into the soil or planting. This allows time for soil microorganisms to decompose the materials and begin to release nitrogen and other nutrients back into the soil

Mineralization or Immobilization?

The processes of mineralization and immobilization are constantly occurring simultaneously. As organic matter decomposes, inorganic nitrogen will be released into the soil. As both plants and microorganisms grow, they utilize the nitrogen in the soil. Once plants and microorganisms die, they decompose and release inorganic nitrogen to the soil through mineralization.

Even though mineralization and immobilization are both occurring, we can determine which process, mineralization or immobilization, predominates.

  • When mineralization occurs at a greater rate, we say that there is net mineralization.
  • Likewise, when immobilization occurs to a greater extent, there is net immobilization.

What determines whether there is net mineralization or net immobilization?

The answer is the C:N ratio of the decomposable organic matter.

The C:N ratio is a characteristic of all organic matter, which includes:

  • Crop residues
  • Soil organic matter (including humus)
  • Soil microorganisms (remember microorganism have both carbon and nitrogen)

Rule of thumb

  • When the C:N ratio of decomposing organic residues is between 20:1 and 30:1, mineralization and immobilization occur at fairly equal rates.
  • Net mineralization occurs at C:N ratios less than 20:1.
  • Net immobilization occurs at C:N ratios greater than above 30:1.
  • Most well decomposed organic matter in soils have a C:N ration near 10:1

Management of organic residues

If your program involves the addition of organic residues, it is important to know its C:N ratio. This knowledge allows you to predict whether net mineralization or net immobilization will occur. If the residue has a wide C:N ratio range, it may be necessary to apply additional amounts of nitrogen to your soil or choose a residue with a narrower range.


In most aerobic soils under optimal soil conditions, ammonium is rapidly converted to nitrate by soil bacteria through a process known as nitrification.

  • Nitrification involves two steps:
    • First, ammonium is converted to nitrite
    • Then, nitrite is converted to nitrate.

As you can see from the outline of steps above, the intermediate product of nitrification is nitrite. If conditions are unfavorable to undergo the second step of nitrification, nitrite can leach into the ground water and pose as a health risk.

The process of nitrification produces hydrogen ions. When large quantities of ammonium-containing fertilizers are applied to soil over time, this process can acidify the soil. See figure below for a simplified presentation of the nitrification process.

Basic process that causes soil acidity by ammonium fertilizers.
Figure 10. Basic process that causes soil acidity by ammonium fertilizers.
Source: Singleton, P. Nutrient Management Concepts: pH & Nutrient Formulation, University of Hawaii Cooperative Extension Service, Hilo Jul 25 2006.

Factors affecting nitrification

There are many factors that affect nitrification. Since nitrification is mediated by microorganisms, environmental factors that affect biological life will also influence nitrification. In general, the optimal conditions for most plant growth are also the optimal conditions for nitrification:

  • Presence of ammonium in the soil: In order for nitrification to occur, there must be a source of ammonium in the soil. Sources include mineralized ammonium or additions of ammonium-containing synthetic fertilizers
  • Presence of microorganisms: Microorganisms that carry out nitrification must be present in the soil.
  • Soil pH: The optimal pH for nitrification is 8.5, but it may occur over a fairly wide pH range. However, acidity (less than 5.5) has a detrimental effect on the nitrifying bacteria, thus reducing nitrification.
  • Soil moisture: Nitrification is optimal at the field capacity of the soil. Nitrification is reduced at moisture levels greater and below field capacity.
    • Field capacity is the amount of water that remains in the soil after free drainage in a saturated soil ceases.
    • Field capacity is also the optimal soil moisture for most plant growth.
  • Soil aeration: Nitrification requires oxygen. Any management factor that improves soil aeration, such as adding organic matter, will help optimize nitrification.
  • Soil temperature: Nitrifying bacteria are sensitive to temperature. The optimal temperature range for nitrification is between 77 and 95 degrees Fahrenheit. However, nitrification can occur between 41 and 95 degrees Fahrenheit.

Environmental Considerations

Nitrate is generally a very mobile in most soils. Excessive amounts of nitrate that are not taken up by plants is subject to leaching. Nitrate leaching can have an adverse effect on the environment.

Gaseous Losses of Nitrogen


Denitrification is the biological process in which nitrate is converted to atmospheric N2.

  • It is one source of N loss from the soil.
  • Like other N processes, denitrification is a biological process that is mediated by denitrifying bacteria.
Soil conditions that lead to denitrification:
  • Waterlogged soils: In waterlogged soils, the flow of air is poor. Even in aerated soils, small, localized areas in the soil (microsites) can lack oxygen. Any microenvironment within the soil that lacks oxygen is referred to as anaerobic. In contrast, nitrification requires oxygen and occurs under aerobic conditions.
  • Presence of nitrate: Nitrate must be present for denitrification to occur. Nitrification provides a source of nitrate, as well as certain synthetic fertilizers.
    • The greater the amount of nitrate present, the greater the denitrification potential.
  • Presence of decomposable organic matter: Decomposing organic matter yields is a source of carbon. In return, carbon is the source of energy for denitrifying microorganisms.
  • Oxygen: As stated earlier, denitrification occurs only when oxygen is absent. In aerated soils, denitrification can occur but is limited to those microsites that lack xygen.
  • Soil pH: Denitrifying microorganisms are sensitive to low pH.
    • Generally, denitrification is severely reduced at pH less than 5.0.
    • The optimal range for denitrification is between 6.0 and 6.5.
  • Soil Temperature: Denitrification will occur between 35 and 77 degrees Fahrenheit.


A second loss of nitrogen to the atmosphere is due to volatilization. By definition, volatilization is the loss of gaseous ammonia to the atmosphere.

Note the distinction between ammonia and ammonium. Although similar in form, ammonia is a gas that can escape from the soil into the atmosphere. You may be familiar with ammonia, which is characterized by its pungent smell.

Factors affecting volatilization

There are several factors that affect volatilization:

  • Soil pH: At a soil pH of 9.3, half of the ammonium in the soil is converted to ammonia and subject to volatilization loss. Generally, a pH greater than 7.5 allows for considerable loss of ammonia due to volatilization.
  • Type of fertilizer: Urea fertilizers experience greater losses due to volatilization than ammonium fertilizers.
    • However, if an ammonium fertilizer forms insoluble calcium compounds in the soil, the ammonium fertilizer will have greater volatilization losses than urea.
  • Method of fertilizer placement: Broadcasting the fertilizer over the surface of the soil increases the losses due to volatilization. Incorporation into the soil reduces losses.
  • Soil Temperature: The occurrence of volatilization increases as soil temperatures increase to 113 degrees Fahrenheit
  • Soil Moisture: Evaporation promotes volatilization. Thus, volatilization is greatest as the soil dries after reaching field capacity.
  • Buffering Capacity: Volatilization is less in well-buffered soils.
  • Crop Residues: Crop residues that are not incorporated into the soil may increase the rate of volatilization.
  • Manure: If not incorporated, nitrogen from manure sources can undergo volatilization.

Nitrogen Exchange and Nitrate Leaching

In our discussion on cation and anion exchange, we mentioned that cations, such as ammonium, are attracted to soil particles that have a cation exchange capacity. Since most surface soils have a cation exchange capacity, ammonium is retained by soil particles.

  • Ammonium is largely immobile
  • Losses of ammonium due to leaching are minimal

In contrast, nitrate is not retained by cation exchange capacity.

  • Nitrate is highly immobile
  • Losses of nitrate due to leaching can potentially be high

Not only is nitrate leaching an economic loss to the farmer, it is also an environmental concern. The conditions that lead to nitrate leaching follow:

  • High rainfall intensity and distribution
  • Highly irrigated fields
  • Coarsely textured soils

Nitrate Leaching in Maui soils

In Hawaii, there are soils that can have a high anion exchange capacity under acidic conditions, which reduces nitrate leaching. This phenomenon primarily occurs in the acidic subsurface soil layers that have the greatest anion exchange capacity. As a result, researchers have found that subsurface soil layers in Hawaii can retain nitrate and may prevent it from leaching into the ground water.

Acidification: Management of Nitrogen Fertilizers

The fertilizer formulation can alter the pH of soil. Generally, fertilizers with high proportions of total nitrogen and are derived from ammonium sources (such as urea, ammonium sulfate, ammonium phosphate or ammonium nitrate) can acidify soils with repeated applications. Most fertilizers provide the “Lime Equivalent” on the bag’s label. The lime equivalent is the amount of limestone (calcium carbonate) it takes to neutralize the acidifying effects of using one ton of a particular fertilizer.

In contrast, other fertilizers can increase soil pH. These fertilizers are usually low in ammonium, but high in nitrate. Additionally, these fertilizers sometimes contain calcium from calcium nitrate. The lime equivalent is also given for these fertilizers, but it indicates the equivalent liming effect rather than the lime needed to offset acidity.

The lime equivalent is only an estimate. The actual acidifying effect of the fertilizer is influenced, to some degree, by soil conditions that affect the transformation of the ammonium to nitrate and also by how much ammonium the plant assimilates before this transformation occurs. If growers know the history of their fertilizer applications over time, they can use the lime equivalent to predict when lime additions are likely needed.

See the table below for some fertilizer formulations and note the relationship between the percentage of nitrogen in the fertilizer that is derived from ammonium and the lime equivalent needed to counter this acidifying effect.

Table 8. Fertilizer composition can change the pH of soil


Ammonium nitrogen

Fertilizer’s Potential:





lbs lime/ton fertilizer

























--data derived from various commercial fertilizer labels