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.

International Year of Soils: Modifying soil to improve crop productivity

The essential link between productive soil and humans has been clear since the beginning of agriculture. However, our food crops come from only a sliver of the world’s land area (12%).

There is room for limited expansion of crop production in some countries, but much of the earth is covered by urban settlements, forests, and environmentally protected areas that are not appropriate for agricultural expansion. Proper stewardship of our current agricultural land is vital to long-term food sustainability.

Improving soils is an ancient practice

The earliest recorded agriculture describes attempts to improve soil properties and crop productivity through application of manures, ash, minerals, and other amendments. Our understanding of the scientific principles underlying plant growth has greatly improved, but the fundamental effort to eliminate soil constraints to food production remains the same after thousands of years.

Soil physical properties have a major impact on root growth and development. Compacted soils have reduced water-holding capacity and can form a brick-like barrier that roots cannot penetrate.

Soil crusts prevent rainfall from entering the soil and crusty soils are prone to excessive water runoff. Coarse-textured and low-organic matter soils generally retain less plant-available water, and crops growing in these soils may be more susceptible to drought stress.

Compacted soil reduces wheat root growth (R)

www.agric.wa.gov.au

Soil degradation causes pollution

Soil chemical properties often are a key factor in determining crop productivity. There are very few soils in the world that contain all of the essential plant nutrients in the proper concentration. The modern fertilizer industry helps farmers to identify and eliminate any limiting nutritional factors. Addition of plant nutrients prevents the depletion and degradation of agricultural land that occurs when crops are repeatedly harvested without replacing the nutrients back into the soil.

Soil degradation impairs productivity

Many soil chemical properties can limit plant growth if left unmanaged. Soil acidity is one of the largest global constraints to plant growth. Although soil acidity is relatively simple to remedy, it remains untreated in vast areas of the world. Other soil chemical issues that hinder plant growth include excessive salinity, and pollution from poor municipal and industrial waste management.

Soil acidity stunts root growth

 The importance of biological activity in supporting crop productivity is too often overlooked. Soil microorganisms are responsible for regulating the availability of many of the essential plant nutrients. Nitrogen fixation by bacteria living within the roots of some plants provides vital support to important cropping systems and rotations. The contribution of mycorrhizal fungi to root health is clear, but not fully understood. Similarly, the intricate exchange of chemical signals between plant roots and soil microbes plays an important role in supporting plant growth.

Nodules on soybean roots host N-fixers

 We are fortunate to live in an age when we understand these soil factors that limit plant growth and have the ability to manage them. Nutrient limitations can be eliminated through proper fertilization. Soil acidity or alkalinity is easily modified through use of appropriate amendments. Converting to no-tillage practices, or sub-soil tillage can often help improve soil physical properties. Conservation of soil organic matter and crop rotation may improve soil biological activity.

Soil stewardship is fundamental to modern agriculture.  Every farming decision should be one that maintains or improves essential soil resources.

This article originally appeared as a contribution to the IPNI series: Plant Nutrition Today

Nitrophosphate - making phosphate fertilizer using nitric acid

The production and application of nitrophosphate fertilizers is largely regional, its use centered where this technology is advantageous. The process uses nitric acid instead of sulfuric acid for treating phosphate rock and does not result in gypsum byproducts.

   

21-7-14 Nitrophosphate

Production

The majority of commercial P fertilizer is made by reacting raw phosphate rock with sulfuric or phosphoric acid. The sulfuric acid method of producing P fertilizer results in large amounts of calcium sulfate (gypsum) by-product that incurs additional disposal costs. 

Nitrophosphate differs because it involves reacting phosphate rock with nitric acid. Nitric acid is made by oxidizing ammonia with air at high temperatures. A primary advantage of this method is that little or no S inputs are required. With the nitrophosphate process, excess Ca from the phosphate rock is converted to valuable calcium nitrate fertilizer instead of gypsum. The nitrophosphate method was first developed in Norway and much of the global production still occurs in Europe.

The general reaction is: Phosphate rock + Nitric acidgPhosphoric acid + Calcium nitrate + Hydrofluoric acid. The resulting phosphoric acid is often mixed with other nutrients to form compound fertilizers containing several nutrients in a single pellet. The co-generated calcium nitrate or calcium ammonium nitrate is sold separately.

 Chemical Properties

The chemical composition will vary depending on the combinations of nutrients used to make the final granule. Popular grades of fertilizer made with the nitrophosphate method include:

N & P:     ranging from 20-20-0 to 25-25-0, 28-14-0, 20-30-0

N-P-K:     ranging from 15-15-15 to 17-17-17, 21-7-14, 10-20-20, 15-20-15, 12-24-12

  

Agricultural Use

Nitrophosphate fertilizers can have a wide range in nutrient composition depending on their intended use. It is important to select the proper composition for each specific crop and soil requirement. Nitrophosphate fertilizer is sold in granular form to be used for direct application to soil. It is commonly spread on the soil surface, mixed within the rootzone, or applied as a concentrated band beneath the soil surface prior to planting.

Management Practices

Nitrophosphate fertilizer contains varying amounts of ammonium nitrate, which attracts moisture. To prevent clumping or caking, nitrophosphate fertilizers are generally packed in water-tight bags and protected from moisture before delivery to the farmer.

This fertilizer fact sheet originally appeared in the series of IPNI Nutrient Source Specifics.  Additional information on fertilizer materials can be found at the IPNI website.