Year Of Soils: 4R Nutrient Stewardship and Soil Management

This year was designated as the International Year of Soil. This recognition gives us an extra opportunity to reflect on the importance of soil as the basis for plant growth, healthy animals, clean water, and maintaining life on earth. 

In recent years, much of the fertilizer industry has embraced the principles of 4R Nutrient Stewardship as a way that farmers can maximize their yields, improve nutrient efficiency, and reduce environmental impacts. This involves selecting the right source of nutrient, added at the right rate, applied at the right time, and put in the right place. Adopting the correct set of 4R principles requires planning, management, and flexibility to meet local challenges.

It is important to remember that 4R Nutrient Stewardship is not a single set of practices that stand alone in achieving these economic, environmental, and social goals. Careful nutrient management must be accompanied by a package of other production and conservation techniques to be successful.

A sophisticated jet airplane cannot launch into flight if it lacks an engine or is missing the jet fuel. Similarly, successful modern crop production requires all the components to work together to be successful. Modern nutrient management practices must be accompanied by other locally appropriate conservation approaches.

The concept of “Soil Fertility” integrates many factors such as soil physical properties (e.g., soil texture, structure, water, and air), biological properties (microorganisms and organic matter), and chemical properties (nutrient availability, pH). Clearly the 14 essential plant nutrients supplied from the soil are a vital part of growing a healthy plant that produces high yields. Despite their irreplaceable nature, the presence of an adequate nutrient supply does not alone make a fertile soil.

4R practices are not confined to only inorganic fertilizer, but they are applicable for both inorganic and organic nutrient sources. Organic and mineral fertilizers complement each other and best results for both crops and soil commonly occur when they are used together. For example, there is plenty of evidence that proper fertilization will commonly increase soil organic matter or at least slow its loss in cultivated soils compared with using no fertilizer.

As the end of the International Year of Soil draws near, remember the essential role that plant nutrients play in sustaining soil productivity. Proper 4R-based nutrient stewardship clearly has a positive contribution in this effort. But nutrient management is only one piece of the solution to maintaining our precious and irreplaceable soil resource.

Let’s make 4R Nutrient Stewardship more than a slogan. It needs to be implemented into a complex and continually changing conservation-based farming landscape that wisely preserves soil for generations to come. The conclusion of the International Year of Soil prompts a renewed reflection of the fundamental role of soil and the need for wise nutrient management.

Urease Inhibitors

Some compounds added to urea or urea-containing fertilizers can reduce the rate of the first “hydrolysis” step, and slow the rate of ammonia production. Under certain conditions, this can help reduce ammonia loss to the atmosphere.

Urease Enzymes and Nitrogen Loss from Urea

Urea is the most widely used form of N fertilizer, and can be formulated as dry granules, prills, or as a fluid alone or mixed with ammonium nitrate (UAN). Urea is also present in animal manures. All these forms of urea have the disadvantage of undergoing considerable losses as ammonia gas if not incorporated into soil soon after application.

Once dissolved in water, urea is converted to ammonium bicarbonate within a few days following application by the naturally occurring enzyme, urease. Urease is produced by many soil microorganisms and plants, and is present in nearly all soils.

When urea is hydrolyzed by urease, much of the resulting ammonium is held on soil cation exchange sites. During the conversion, the pH temporarily rises and ammonia gas is produced.  The loss of ammonia, termed volatilization, can be from nil to over 50%.

(NH2)2CO + 2H2O      =>      2 NH4HCO3        =>     2 NH3  +  H2O + CO2

                                               Ammonium carbonate     Ammonia gas

The factors conducive to N loss as ammonia from urea are: surface application, less than10 mm (0.4 in.) of rainfall and/or irrigation in the first few days after application, presence of crop residues, open crop canopies, high temperatures, high soil pH and low cation exchange capacity soils. Moving the applied urea below the soil surface with tillage or through rainfall and irrigation also effectively minimizes ammonia loss from urea.

Reducing Urease Activity

Urease inhibitors are used to temporarily reduce the activity of the enzyme and slow the rate at which urea is hydrolyzed. There are many compounds that can inhibit urease, but only a few that are non-toxic, effective at low concentrations, chemically stable and able to be mixed with or coated onto urea-containing fertilizers.

The most widely used urease inhibitor is N-(n-Butyl) triphosphoric triamide (NBTPT), which converts to active NBPT (N-(n-Butyl) phosphoric triamide). Other widely studied urease inhibitors include phenylphosphorodiamidate (PPD/PPDA) and hydroquinone. Ammonium thiosulfate and some metals can also inhibit urea hydrolysis. There are many other organic compounds, especially structural analogues of urea, capable of inhibiting urease.

Management Practices

Urease inhibitors are potentially useful tools for controlling or reducing gaseous  losses of ammonia following fertilization with urea. They can restrict urea hydrolysis for up to 7 to 14 days, after which rain, irrigation, or soil mixing would be required to further restrict ammonia losses.

Because the magnitude of ammonia loss varies with soil type, climate and crop cover, the reduction due to the use of a urease inhibitor can also be variable. Research suggests NBTPT-treated urea use can reduce ammonia loss by 50% to 90% when compared to untreated urea.

The potential boost in crop yield from the preserved N will depend on the nutrient demand of the crop, the indigenous soil N supply, and other management practices.

Urease inhibitors provide farmers with an additional tool to keep applied N in the root zone, which can have agronomic and environmental benefits.

This article originally appeared as #25 Nutrient Source Specifics, a series published by the International Plant Nutrition Institute.