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