Nitrification Inhibitors

Some compounds added to nitrogen (N) fertilizers can reduce the rate at which ammonium is converted to nitrate. Under appropriate conditions, this can help reduce N losses through denitrification and leaching. Nitrification in Soil Nitrification is a natural process in soils that converts ammonium to nitrite and then to nitrate. The soil bacteria Nitrosomonas spp. extract energy from ammonium by converting it to nitrite. A second group of bacteria, Nitrobacter spp. then convert nitrite to nitrate. Both types of bacteria are common in soil and it is the first reaction that limits the overall rate of nitrate production. 

  

With moderate temperature and soil water content nitrification occurs on most soils within a few days or weeks after application of ammonium sources. The ammonium can come from urine, manures, composts, decomposing crop residues, or fertilizer containing ammonium or urea.

Nitrate is the dominant form of plant available N in soil and unless taken up by roots, it can be transferred to either water or the atmosphere. Nitrate can leach below the root zone with the potential to be transferred to surface or sub-surface waters. Under waterlogged conditions, nitrate can be denitrified to form nitrous oxides and dinitrogen by other soil bacteria. While dinitrogen is the most common gas in the atmosphere and inert, nitrous oxide is a powerful greenhouse gas. Some nitrous oxide can also be produced during nitrite decomposition during nitrification. 

Nitrification is rapid in warm (>25^o C) soils, and it largely ceases below 5^o C. It occurs most rapidly where the soils are well aerated and near field capacity, and decreases with higher or lower moisture contents. In saturated soils nitrification nearly stops because of the lack of oxygen. 

The nitrogen cycle involves many biological processes

Regulating Nitrification

The rate of nitrification can be controlled by preserving N as ammonium, which can be held on soil colloids rather than leached. Nitrification inhibitors are compounds that delay nitrate production by depressing the activity of Nitrosomonas bacteria.

There are at least eight compounds recognized commercially as nitrification inhibitors although the most commonly used and best understood are 2-chloro-6-(trichloromethyl)-pyridine (Nitrapyrin), dicycandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP). These compounds suppress microbial activity for a few days to weeks depending on soil moisture and soil type although there are differences between them in the way they are deployed. In general, nitrification inhibitors are more effective in sandy soils, or soil low in organic matter and exposed to low temperatures. 

Management Practices 

Nitrapyrin can be injected directly into the soil with anhydrous ammonia or coated onto solid N fertilizers or mixed with manures. Because nitrapyrin is volatile it needs to be incorporated into the soil. Nitrapyrin is usually broken down within 30 days in warm soils. 

DMPP is usually supplied pre-blended with fertilizers. It is considered a highly specific nitrification inhibitor active for 25 to 70 days, but its effect is reduced at higher temperatures. It is relatively immobile in the soil. 

DCD can be coated on solid fertilizers, and is also used where manures are surface applied, and can be used post-grazing to reduce nitrate leaching from urine patches. The inhibitory effect of DCD usually lasts between 25 to 55 days, and it is readily leached, lowering its effectiveness.

A study in New Zealand showed that DCD applied to grazed pastures reduced nitrate leaching from urine patches by 59% and nitrous oxide emissions by 82%, as well as increasing herbage production by 30%. 

Because there are many factors that control both the rate of nitrification and the activity of the inhibitor, there is considerable variability in the reduction in the amount of nitrate leached, the reduction in nitrous oxide produced, economic return and the potential benefits associated with their use. So, the yield benefits occur when sufficient N is preserved to provide additional growth.

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This overview of nitrification inhibitors originally appeared as part of the IPNI series "Nutrient Source Specifics".  They can be viewed at  https://www.ipni.net/specifics-en

Phosphate Rock

Phosphorus (P) additions are needed in most areas of the world to improve soil fertility and crop production. Direct application of unprocessed phosphate rock (PR) to soil may provide a valuable source of plant nutrients in specific conditions, but there are several factors and limitations to consider. 

Rock phosphate production and final product

  

Production Phosphate rock is obtained from geologic deposits located around the world. Apatite, a calcium phosphate mineral, is the primary constituent of PR. It is primarily extracted from sedimentary marine deposits, with a small amount obtained from igneous sources. Most PR is recovered through surface mining, although some is extracted from underground mines. 

North Carolina, USA phosphate mining

The ore is first screened and some of the impurities removed near the mine site. Most PR is used to produce soluble phosphate (P) fertilizers, but some is used for direct application to soil. While PR can be a valuable source of P for plants, it is not always appropriate for direct application. Its suitability depends partly on naturally occurring mineral impurities, such as clay, carbonate, iron, and aluminum (Al). The effectiveness of PR for direct application is estimated in the laboratory by dissolving rock in a solution containing a dilute acid to simulate soil conditions. Sources classified as “highly reactive” are the most suitable for direct soil application.

 

Florida, USA phosphate mining

Direct use of PR avoids the extra processing associated with converting apatite to a soluble form. The minimal processing may result in a lower-cost nutrient source and make it acceptable for organic crop production systems. 

Agricultural Use When a water-soluble P fertilizer is added to soil, it quickly dissolves and reacts to form low solubility compounds. When PR is added to soil, it slowly dissolves to gradually release nutrients, but the rate of dissolution may be too slow to support healthy plant growth in some soils. To optimize the effectiveness of PR, these factors should be considered:

 •Soil pH: PR requires acid soil conditions to be an effective nutrient source. Use of PR is not usually recommended when the soil pH exceeds 5.5. Adding lime to raise soil pH and decrease Al toxicity may slow PR dissolution. 

• Soil P-fixing capacity: The dissolution of PR increases with a greater P-fixing capacity of soil (such as high clay content). 

• Soil properties: Low calcium and high organic matter in the soil tend to speed PR dissolution. 

• Placement: Broadcasting PR and incorporation with tillage speeds the reaction with the soil. 

• Species: Some plant species can better utilize PR due to their excretion of organic acids from the roots into the surrounding soil. 

• Timing: The time required for the dissolution of PR necessitates its application in advance of the plant demand. 

 

Global P Resources Map

Management Practices Not all sources of unprocessed PR are suitable for direct application to soil. Additionally, many soils are not suitable for PR use. The total P content of a material is not a good predictor of the potential reactivity in the soil. For example, many igneous PR sources are high in total P, but are of low reactivity and provide minimal plant nutrition because they dissolve so slowly. However, mycorrhizal fungi may aid in the acquisition of P from low-solubility materials in some environments. Over 90% of PR is converted into soluble P fertilizer through reaction with acid. This is similar to the chemical reaction that PR undergoes when it reacts with soil acidity. The agronomic and economic effectiveness of PR can be equivalent to water-soluble P fertilizers in some circumstances, but the specific conditions should be considered when making this choice.

Rob Mikkelsen examines phosphorus fertilizer response of wheat

Good Nutrition: Key to Plant Health

Getting crops off to a good start is critical for achieving high yields. During this early stage of growth, seedlings are especially vulnerable to many environmental and biological stresses. Protecting plants from stress  and disease begins with providing balanced nutrition from planting through harvest. The critical link between plant nutrition and disease resistance has become apparent
as the frontiers of plant health are better understood. A few of these examples are explained here:

How potassium protects plants; Wang, M. et al. 2013.

Potassium
Potassium plays an essential role in many well-recognized metabolic processes for plants. Potassium’s contribution to sustaining high yielding crops with top quality is well understood. However, the role of potassium in plant stress resistance is less known and appreciated. Potassium is unique among the essential mineral nutrients in its role for plant survival against environmental stress, pests, and diseases.

Supplying adequate potassium to crops through proper fertilization is a simple way to lower the requirement for pest-control treatments that may be costly, time-consuming, and troublesome. The frequently observed benefits of potassium on plant health were reviewed by Wang et al. (2013), which summarizes many recent scientific studies.

Potassium deficiency symptoms on squash (normal on left, severe deficiency on right. Photo by D. Pitchay

When there is a lack of sufficient potassium in plants, low molecular weight compounds begin to accumulate. This build-up of soluble nitrogen-containing compounds (such as amino acids and asparagine) and sugars (such as sucrose) makes a particularly favorable environment for numerous pathogens and insects. For example, aphids are severely nitrogen limited, making potassium-stressed plants an attractive host as an abundant nitrogen source. The presence of sufficient potassium also promotes the production of defensive compounds (such as phenols) which are an important component in plant pest resistance.

An adequate potassium concentration within the plant decreases the internal competition with various pests and pathogens for resources. This results in more resources available for hardening cell walls and tissues to better resist penetration of pathogens and insect pests, and to repair any damaged tissue. Air-borne pathogens are more rapidly shut out from stomatal invasion when adequate potassium is present.
Phosphate
The link between adequate phosphate and plant health is also well known, but perhaps less understood than the association with potassium. Phosphorus is involved in the synthesis of many organic molecules and complex metabolic functions within plants. Crop growth and yields will be significantly reduced when phosphorus is deficient in soil or when plant roots cannot access it.

Pythium root rot on melon

A shortage of phosphorus frequently leads to more disease for many crops. Some of the protective response occurs because healthy plants with sufficient phosphorus have vigorous root growth which allows them to outgrow and escape disease. More specifically, an adequate phosphorus supply has been linked with decreased incidence of Pythium root rot for wheat, leaf blight for rice, numerous tobacco diseases, blight in soybean, and many other diseases. Foliar application of phosphorus-containing sprays is reported to induce protection against powdery mildew.

Chloride

Severe wheat chloride deficiency (L) and moderate (R)

The important role of chloride as a nutrient is often overlooked, especially in regions where soil salinity is a  concern. However in many areas, the addition of chloride results in increased plant vigor and disease resistance. The occurrence and severity of a number of plant diseases have been documented to be reduced following the application of chloride. This includes take-all, stripe rust, and Septoria in wheat, and stalk rot in corn. Promoting plant health clearly includes a solid foundation in proper nutrition. Strong and vigorous crops are able to produce abundant yields of high quality, while better resisting diseases and pests.

Rob Mikkelsen, IPNI, discusses plant health

References
Wang, M. et al. 2013. Internat. J. Molec. Sci. 14:7370-7390.
Available at: http://www.mdpi.com/1422-0067/14/4/7370