Monday, May 30, 2016

Year of Soils: Soil Degradation Destroys Productivity

Who cares about dirt? Soil is the fragile skin on the earth that provides more than 95% of our food. Soil also provides an essential life-sustaining role in cleaning air and water.

When we lose our soil, many vital functions are also lost. It has been estimated that over 40%
of the soil used for agriculture around the world is already degraded or seriously degraded and that half of the topsoil on the earth has been lost during the last 150 years. Soil degradation is the slow decline in land quality caused by human activity. We have plenty of reasons to be concerned with this growing threat to food security.

Soils become degraded from both man-made activities and accelerated natural processes. Some impacts of soil mismanagement and degradation include compaction and poor drainage, depletion of essential plant nutrients, rapid loss of organic matter, accumulation of salts, and acidification. Soil degradation frequently accelerates soil erosion and may result in permanent loss of a soils productive capacity.

Soil degradation is a severe challenge that threatens the sustainability of crop and livestock
production worldwide. For example, in sub-Saharan Africa, about 65% of the land area is degraded, with devastating economic and human impacts.

Some major constraints to agricultural productivity in sub-Saharan Africa resulting from soil
degradation include soil acidity and aluminum toxicity, nutrient depletion, and soil erosion with resulting shallow soils. The slow process of restoring these soils begins by balanced addition of crop nutrients and lime, adjusting cropping rotations to include cover crops, and adopting practices to halt soil erosion.
Soil degradation in Sub-Saharan Africa (IPNI)

A major step in preventing soil degradation is proper use of plant nutrients. Fertilizers replace
essential plant nutrients removed in harvested crops, preventing nutrient exhaustion of the soil. Several recent studies show that proper fertilizer use maintains or improves soil microbial activity, boosts inputs of crop residue returned to the soil, and can maintain soil organic matter...all while enhancing crop yields.

The damaging effects of soil erosion are also felt off of the farm. Streams and lakes can
become clogged with sediment and nutrients lost from agricultural fields, damaging fish and aquatic life.

Erosion and soil degradation is usually a slow process, easily escaping our attention at first
glance. However, their cumulative effects are devastating on many levels. Farmers everywhere should consider how they can protect their precious soil resources. Their livelihood and their neighbors depend on careful stewardship of the soil beneath our feet.
Sediment-choked stream (Cornell Univ)

This article originally appeared in the IPNI quarterly publication: Plant Nutrition Today

International Year of Soils: Nutrients and Soil Biology

It is a tendency of some people to only think of plant nutrition in terms of how much fertilizer to add. This simplification may be understandable since a healthy crop reveals only the above ground plant; the roots that support the visible plant are seldom seen without further exploration. Plant roots grow in an incredibly complex soil environment, teeming with billions of organisms, particularly bacteria and fungi, which play a crucial role maintaining an adequate supply of plant nutrients for crop growth.
Complex interactions occur between plant roots and microorganisms (Haichar et al., 2014)
 There is still much to learn about the complex interaction between microorganisms and plant nutrition, but the importance of these relationships is clearly recognized. Living organisms have a crucial role in controlling the transformations of plant nutrients. In most soils, nitrogen (N), phosphorus (P) and sulfur (S) are mainly present in various organic compounds that are unavailable for plant uptake. Understanding the role of microorganisms in regulating the conversion of these organic pools into plant-available forms has received considerable attention from soil scientists and agronomists

The microbial conversions of nutrients into soluble forms take place through numerous mechanisms. Extracellular enzymes and organic compounds are excreted to solubilize nutrients from soil organic matter, crop residues, or manures. Organic acids released by microbes can dissolve precipitated nutrients on soil minerals and speed mineral weathering. Some nutrients become more soluble as microbes derive energy from oxidation and reduction reactions.

Mycorrhizal fungi are found in symbiotic association with the roots of most plants. These soil fungi can increase the supply of various nutrients to plants in exchange for plant carbon. The boost in P uptake provided by mycorrhizal fungi is especially important for crops with high P requirements or growing in soil with low concentrations of soluble P. Mycorrhizal fungi release various enzymes to solubilize organic P and they can extract soluble P from the soil at lower concentrations than plant roots are able to do alone.

Well-nodulated soybean root (Pioneer)
Biological N fixation is another essential contribution of microbes to plant nutrition. Specialized symbiotic bacteria living in root nodules can fix atmospheric N into ammonium-based compounds for plant nutrition. The most important of these organisms for agricultural plants are from the species Rhizobium and Bradyrhizobium. There are symbiotic N2-fixing bacteria that infect woody shrubs, and asymbiotic bacteria, such as Azospirillum, that provide N to the roots of grasses such as sugarcane.

An often-overlooked contribution of soil microorganisms to plant nutrition is their benefit to improving soil physical properties. Good soil structure enhances plant root growth, resulting in greater water and nutrient extraction. Individual soil particles are bound into aggregates by various organic compounds such as polysaccharides and glomalin. The small hyphal strands of mycorrhizal fungi also contribute to improved soil aggregation by binding small particles together.

A better understanding of the essential link between soil microbes and plant nutrition allows more informed management decisions to be made for proper stewardship of soil resources and for sustaining crop productivity.

This article originally appeared as part of the IPNI quarterly update: Plant Nutrition Today which can be accessed here

Wednesday, December 9, 2015

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.

Tuesday, March 31, 2015

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.