No Matter How You Slice It, Healthy Soil Is Important
It is by no means an accident that human civilization arose from rich agricultural regions characterized by abundant water and fertile soil. The ability to produce large quantities of food enabled people to gather together, build cities and develop culturally. Today, as the global population skyrockets, maintaining and expanding the ability to produce enough food has never been more important. And all of agriculture — field crops, vegetables, fruit and even livestock feeds — still depends on the same soil that it did 6,000 years ago.
“Soils are the foundation of crop fertility, and soil fertility is the foundation for healthy and productive crops,” said Phil Robertson, Michigan State University (MSU) AgBioResearch scientist and University Distinguished Professor in the Department of Plant, Soil and Microbial Sciences. “In order to manage for healthy soil, you have to understand what the soil is providing and how it is providing it.”
Soil is a complex medium comprising living organisms, mineral particles and organic matter, along with water and the ions needed for all the organisms that call it home, including crops.
“I think that sometimes we consider crops more as above-ground organisms than below-ground, but really they inhabit both worlds,” Robertson said. “They work in tandem with the other organisms and play a significant role in the soil environment.”
Long-term research for long-term change
Robertson and his colleagues at the W. K. Kellogg Biological Station (KBS) in Hickory Corners, Michigan, have spent the past 26 years studying long-term change in the ecology of agricultural sites under the auspices of the Long-Term Ecological Research (LTER) program. They observe the interactions between organisms and their environment. The soil environment, one of the key components of the larger ecosystem, is of high priority in Robertson’s work.
One of the most important functions of the soil microbial community is to decompose and recycle organic matter left by previous crops. Organic matter accrues in the soil in the form of aggregates, small clumps of material that build up over years into something that can be seen with the naked eye. They comprise tiny ecosystems in and of themselves. As innumerable species of bacteria and fungi consume the organic matter, nutrients are released back into the soil and taken up by the roots of crops. Given the significance of this process to crop nutrition, scientists now widely understand that soil organic matter is the basis of soil fertility.
This vital source of soil nutrients declines following conversion to conventional agriculture, however. The decline can often be attributed to increased microbial activity. As fields are tilled and soils churned, the aggregates are disturbed and exposed to the atmosphere. The sudden exposure to oxygen and the elements causes them to break apart and allows microbes instant access to the organic matter they’ve contained.
Over the course of 40 to 60 years, soil will lose between 40 and 60 percent of its original organic matter. Robertson’s team is searching for ways to reverse the deterioration scenario.
“More and more farmers are understanding how significant soil organic matter is to their crops and their livelihoods,” Robertson said. “Rebuilding soil organic matter to levels closer to that of the natural ecosystem is one of the major goals of sustainable agriculture.”
As a general rule, soils are slow-changing. Though some events, such as the outbreak of a pathogen, can generate quick change, most everyday processes take years to produce a noticeable effect. This makes long-term research projects such as those at KBS essential to understanding how soil works and how it can be enhanced.
“Soil can take more than 10 years to change, which by definition is a long-term study,” Robertson explained. “It took us 10 years to document the initial changes here at KBS, and we have some studies designed to last 20 or 30 years.”
To study soil health change, Robertson’s team oversees 2.5-acre research plots with four agricultural management scenarios: conventional, no-till, reduced-input and biological. The conventional system is managed with the standard practices of the agricultural industry; no-till is a system that does not employ plows to disturb the soil. Reduced-input plots are managed with only modest amounts of synthetic fertilizers and other chemicals, and the biological system relies totally on organic sources for soil nutrients.
The team compares the results at these plots with two natural ecosystems on-site: an area formerly used for agriculture that is now returning to a natural state, and an area of old-growth forest that has never been used for anything else. In the years since the experiments began, several means of improving soil organic matter have emerged.
“By comparing these different systems, we gain tremendous insight into the factors that underlie the overall system,” Robertson explained. “The experimental plots are our most important asset in understanding the agricultural ecosystem.”
For example, eliminating tilling preserves the existing organic matter while providing an environment in which more can form. Robertson’s team witnessed firsthand how significant this practice can be during the 2012 drought, when a severe lack of rain made crops wholly reliant on water already in the soil. Though the drought proved devastating to many Michigan farmers, the higher concentrations of soil organic matter in the KBS LTER plots under no-till management allowed the fields to store an extra three inches of water, prolonging the life of their plants.
That year, the soybean yield from the no-till fields exceeded that of the conventional ones by 50 percent. Improving soil organic matter not only has a demonstrable effect on crop yield but also helps protect the larger environment from water pollution and greenhouse gas emission. Nitrogen fixation, by which atmospheric nitrogen is converted into a form usable by plants, is an essential process fulfilled in nature largely by the bacteria that dwell in the soil.
“This is a process that has been ongoing for eons,” Robertson said. “All life depends on these bacteria, flat out.”
As soil organic matter declines, the nitrogen originally captured by these bacteria similarly declines, leaving many farmers with little recourse but to add nitrogen in the form of chemical fertilizers to fields. Applying too much fertilizer, however, or applying it at the wrong time, can cause it to leach into the groundwater and/or escape into the atmosphere, contributing to both damaging algal blooms in lakes and rivers and to climate change. Nitrous oxide that escapes into the atmosphere is approximately 300 times more effective at trapping heat than the most common greenhouse gas, carbon dioxide.
“As much as possible, we want nitrogen to originate from and stay in the cropping system,” Robertson said. “Creating an environment that encourages nitrogen conservation is beneficial not only to the plants that live there but to the planet as a whole.”
Across the pond
The LTER lessons also hold import for regions far removed from Hickory Corners, Michigan. Sieg Snapp, MSU AgBioResearch agronomist and associate director of the Center for Global Change, has been working to improve soil health in the African nation of Malawi since before she came to MSU in 1999.
“The issue of sustainability, of whether soils can continue to produce food in the future, is very stark and clear in Malawi,” Snapp said. “The need there is very urgent.”
In Malawi, a high population density and small land holdings equate to people farming 2-acre plots to feed their families. Decades of intensive use have resulted in a severe decline in soil organic matter. Over 20 years ago, the Food and Agriculture Organization of the United Nations (FAO) conducted a soil mapping project across the African continent. Soil pits were dug throughout Malawi, and soil samples were taken at each site to record the soil type and its attributes. The FAO scientists carefully georeferenced these pits, so Snapp and her team could revisit the sites decades later to measure changes that took place in the intervening years. Snapp said that what they found was shocking.
“We were wondering if these soils had gotten worse,” Snapp explained. “I thought they might have hit a plateau, where they reached a minimal level of quality and remained there, but that did not happen. The long-term data showed they had continued to decline, which is quite worrisome.”
Finding ways to restore Malawi’s soils and, therefore, its food security became paramount. As chairwoman of the KBS LTER agronomy committee, Snapp has taken many of the lessons learned at LTER and applied them to the situation in Malawi. The utility of cover crops in the project has taken on particular importance because not only do they improve soil organic matter but the right cover crop can also yield a profit for the farmer.
Snapp’s team has explored a number of cover crops with beneficial properties for farmer and soil both. Pigeonpea, a short, leafy shrub native to South Asia but imported to Africa from India around 3,000 years ago, grows for one to two years and produces peas that can be sold or consumed by the family. The plant is also useful as a forage for livestock and is optimal for recycling both soil nitrogen and phosphorus.
“We need cover crops that can be harvested or grazed, something that generates a product farmers can sell,” Snapp said. “It offsets the economic impact of not growing a cash crop on their field while helping to remediate soil quality.”
Snapp has unearthed important agricultural insights from her work in Malawi and applied them to the rest of the world.
“One of the things we learned in Malawi is that there is a minimum amount of soil carbon that you need to have for productive soils,” Snapp said. “We’re getting close to finding out what that tipping point is so that we can recommend a basic level at which farmers need to keep their organic matter.”
Putting marginal lands to use
One of the greatest concerns about the decline of soil health in places such as Malawi is the subsequent increase in marginal agricultural landscapes, areas where the soil is too poor or the terrain too hostile for healthy crop growth. These lands are far from unique to southeast Africa. Anywhere the land is steep or marshy or the soil is rocky or has poor drainage can be considered marginal, as well as sites with poor soil nutrient availability. Helping farmers make use of these less fertile lands has been a major focus of MSU AgBioResearch forage specialist Kim Cassida.
As part of a team alongside fellow MSU AgBioResearch scientists Jason Rowntree and Lisa Tiemann, Cassida has found that marginal landscapes are particularly useful for grazed livestock forage crops, which remove fewer nutrients from the soil than traditional cash crops.
“We look at a particular plot of land and determine the plants that are best suited for it,” Cassida explained. “At the same time, the plants have to have the right nutrient profile for the animals because we’re trying to produce the highest quality beef for the market.”
The challenge with forage crops stems from the fact that their quality is neither uniform nor static. As a particular shoot ages, its nutrient levels decline, making a compromise between crop yield and nutritional value inevitable. One of the ways to offset this issue is by growing mixtures of pasture species that complement one another in timing of nutritional value and yield. In addition to improving nutritional value, nitrogen-fixing legumes also improve soil fertility.
Working with alfalfa, the most common legume forage crop in Michigan, and other nitrogen-fixing legumes, grasses and annual forages, Cassida’s team is pioneering forage mixes that not only provide excellent nutrition for livestock but also provide nitrogen and other nutrients for the entire soil ecosystem.
Soil is home to far more than crops and bacteria. Among its most influential denizens, to both the benefit and detriment of farmers, are nematodes — roundworms. Nematodes have adapted to nearly every ecosystem on the planet, marine and terrestrial, but by far their most important role in human health and nutrition comes from their interactions with plants. Those interactions have been a major focus of MSU AgBioResearch scientist and Department of Horticulture associate professor Haddish Melakeberhan.
“Nematodes live in the soil and water and can be either beneficial or not, depending on the species,” Melakeberhan said. “About 10 percent of the species are harmful parasites of plants and humans. The rest play an important role in nutrient cycling and the biological control of pests.”
Melakeberhan came to MSU as a postdoctoral researcher in 1990 to study cherry tree decline in western Michigan. Suspecting bacteria and parasitic nematodes as the culprits, Melakeberhan and his colleagues took samples from the trees back to the greenhouse but were unable to recreate the disease that was killing the trees. That was when he had an epiphany.
“I said, ‘I think we need to scrap everything and go back and look at the soil,’” Melakeberhan said.
Returning to the cherry orchards, the MSU team took soil samples across the root zone down to 6 feet below the surface and discovered that the soil pH there was significantly lower than what was normal for the area. They reproduced these conditions in the lab, and, in combination with the nematodes and bacteria, the disease manifested. They traced the low soil pH to the use of ammonium fertilizers instead of nitrate, and when they advised growers to change fertilizers and add lime, the disease was mitigated.
“This stood as a case study for how to do science in the field for many years,” Melakeberhan said.
In addition to helping growers fight off detrimental nematodes, Melakeberhan’s research also helps them take advantage of the beneficial species. The beneficial nematodes that feed on bacteria, fungi and other nematodes are part of the soil food web, driving nutrient cycling and soil organic matter accumulation. Using community structure analyses models, Melakeberhan’s team can help growers evaluate their soil health and identify necessary changes to improve it as habitat for helpful organisms.
Managing harmful and beneficial nematodes in a diverse agricultural state such as Michigan becomes a complex juggling act. Melakeberhan’s team has developed a fertilizer use efficiency-based model that separates nutrient deficiency and toxicity from harmful and beneficial nematode suppression, and yield and soil quality benefits.
“You need the kind of biological activity that nematodes provide for nitrogen to be released while suppressing harmful nematodes,” Melakeberhan said. “That’s where the sustainability of soil health starts. Different soils behave differently, and you need to understand those differences in order to effectively manage them for beneficial organisms.”
Conducting a study spanning Michigan’s Huron and Saginaw counties, which feature loam and sandy clay loam soils, respectively, Melakeberhan studied soil differences in radish, mustard, sugarbeet, soybean and corn fields. His team found that different soils react to cropping systems in different ways. For example, corn crops showed more nitrogen availability in Huron County’s loam soil than in the sandy clay loam of Saginaw County. The cause of these disparities is the subject of future research.
“We have to go to the next level to identify the causes and how to adjust the soil,” Melakeberhan said. “The way to do that is to find out which organisms are driving the food web in those soils.”
In systems dominated by bacteria-feeding nematodes, which have a much shorter life cycle than other species, the soil tends to see nitrogen become available in brief, nutrient-rich bursts. Melakeberhan would like to see growing populations of longer lived nematodes to stabilize nitrogen availability.
“The impact for the grower from this work is that now we can test soil treatments to make these changes,” Melakeberhan said. “This is a significant step in managing soil health.”
Better soil, better food
Tackling the complex issues surrounding soil health is helping farmers around the world produce food with higher yields more reliably and with reduced impact on the environment.
“The most fulfilling part of this work is answering questions that matter and being able to ask them and search for answers in an environment where everyone else is just as excited about it as you are,” Robertson said. “Soils are the key to productive croplands and a healthy environment, and we’re working to ensure we have those in the future.”
This will only prove more important as time and the human population march on.
“Having soil with richer organic matter means you can get through a longer dry spell and rely less on irrigation and fertilizer, and as we move toward a more weather-inconsistent future, we’re going to need more of those buffers,” Snapp said. “We’re going to have to feed 9 billion people in the future, and we have to know if we can do that.”
More from 2015: International Year of Soils
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