Fixing the water cycle—easier said than done, with so many demands on water resources around the world. In the U.S., many areas are starting to feel the pinch of reduced water quality or quantity, or both. We are placing unprecedented demands on both surface and ground water supplies. Depleting ground water supplies is especially worrisome, because recharging water tables can be a lengthy process, depending on the type of aquafer. Underground water storage not only is much cheaper than building dams and reservoirs, evaporation is not an issue. Annual evaporation from Lake Mead and Lake Powell represents 15 percent of the annual upper basin allocation of water resources among the Colorado River basin states, and this rate may be increasing with climate change.
In many areas of the country, including the humid south, the Ogallala, and the Western U.S., we are over drafting our ground water supplies. The overdraft problem is exacerbated by the way we manage our soils. Most of the soils in our country are degraded, and are so dysfunctional that they don’t allow most water to even get past the soil surface. It runs off, carrying soil and nutrients, impacting downstream surface waters, and can contribute to flooding. The thrust of this article is how to go about changing this situation, by acknowledging that soil is a complex ecosystem and must be managed with a focus on feeding the microbes which drive much of the function of the soil ecosystem.
Worldwide, water is becoming scarcer and more expensive due to the effects of climate change. Significant adaptation will be needed to ensure adequate supply and efficient use of what is a diminishing resource. This reduction in the supply of water will affect agriculture and will require a change in focus from increasing productivity of land (yield per acre) to increasing productivity per unit of water consumed. The need for increased water use efficiency will be taking place in a changing climate that will create abrupt fluctuations of temperature, precipitation patterns, drought, heat waves, stronger storms, flooding, wild fires and pest outbreaks. Our soils, and our soil management, are not ready to meet these additional stresses. Too often, the approach of dealing with water deficits has focused on better technology—deeper wells, better drip emitters, more efficient micro-sprinklers, soil moisture monitoring devices, and variable speed drives on pumps—all of which are important. However, a different approach in dealing with the oscillation between too little and too much water uses an appropriate technology that focuses on maintaining healthy soils through following five basic principles to be discussed in detail in the following sections.
Attaining Healthy Soils
Healthy soil, with its thriving biological activity, creates a system of air and water pores that both allow water to infiltrate the soil and to hold that water in place. These pores help plant roots grow deep, holding soil in place while allowing water to infiltrate deep into the soil profile. As the amount of organic matter or carbon in the soil increases, so does the ability of the soil to hold water, release nutrients to the crop, improve soil structure, and prevent erosion.
Well aggregated, healthy soil, at left is the result of regular additions of organic matter and diverse rotations. The aggregates consist of sand, silt, and clay particles held together by fungal and bacterial glues. The soil aggregates allow water and air to infiltrate into the soil. In contrast, a soil with little or no aggregation, right, shows how the fine clay particles were not held in soil aggregates, and with rainfall (or irrigation droplets) the clay particles form a seal on the surface layer, preventing infiltration of both air and water. This farmer keeps his orchard floor clean and does not provide regular additions of organic matter.
Soil experts across the country, including land grant universities, the Natural Resources Conservation Service (NRCS), soil consultants, and farmer activists, have come to broad agreement about some general principles for restoring and maintaining soil health. These principles, when conscientiously applied to most farming systems, will improve soil health and function, and likely reduce inputs. Water infiltration into soils, as well as the soil’s water storage capacity are also improved—important qualities when considering increasingly extreme rainfall patterns. Here we present five general principles for soil management that are responsible for increasing soil health and function.
Protect the soil surface. Some folks call this “soil armor”. This includes the use of cover crops and mulch, which provide many benefits for the land, including:
- Wind and water erosion control. Cover crops and mulch protect the soil as wind or water move across the soil surface, holding the soil in place, and allowing increased water infiltration, not to mention providing organic matter and nutrients to the soil.
- Mulch reduces evaporation from the soil surface, reserving more moisture for plant use.
- Soil temperatures are moderated with cover crops and mulch, which act as a buffer, shielding the soil from extreme temperatures. The soil food web functions better when not subjected to extreme temperatures and humidity.
- Soil aggregation is preserved when rainfall hits the cover crop or mulch, dissipating the raindrop’s energy. When rainfall hits bare soil, soil aggregates are destroyed, erosion by wind and water is increased and the soil is starved of oxygen and water. Fine clay particles seal the soil surface, dramatically reducing water infiltration and oxygen exchange into the soil.
- Weed growth is suppressed through competition with the cover crop and/or smothered with mulch.
- Habitat is provided by cover crops for beneficial insects and pollinators. Biological mulches/plant residue provides habitat for spiders, an important predator of agricultural pests.
Minimizing soil disturbance of all kinds. Both physical (tillage) and chemical (overuse of fertilizers and pesticides) can disrupt the soil food web. Continuous tillage over time, without regular and significant additions of organic matter to the soil, degrades soil function and reduces soil pore space, which in turn restricts water infiltration and destroys the biological glues that hold soil together. Tillage in combination with overuse of fertilizers is like throwing gas on a fire. The excess nitrogen feeds bacterial populations, which explode when exposed to oxygen through tillage.
The problem is, the bacteria are feeding on the organic matter, which reduces organic matter levels unless significant crop residues, compost, or cover crops are added to the soil on a regular basis. Repeated tillage and overuse of chemical N, season after season, degrades soil structure and causes soil aggregates, which hold sand, silt and clay together, to fall apart, for lack of biological glues. This makes the soil an easy target for both water and wind erosion. Clay particles, released from the soil aggregates by rainfall or irrigation droplets, will form an effective seal on the soil surface, preventing water infiltration to the root zone (or water table), increasing run-off, and also creating anaerobic conditions in the root zone.
Plant diversity. Original landscapes in which soils were built over geological time consisted of a varied plant diversity which was largely replaced by an annual (or perennial) monoculture when Europeans arrived. The soil food web used to receive carbon exudates (food) from the roots of a diverse group of perennial and annual plants. Each species of plant provides a unique set of root exudates, which in turn host a microbial community with some unique members, so a diverse plant community above ground provides for a very diverse microbial community in the soil. In most cases, soils now receive root exudates from only one species of annual or perennial plant at a time. By using crop rotation, or rotating alley crops in orchards, we can start to better mimic the original plant diversity that benefits the soil food web. This in turn improves rainfall and irrigation water infiltration and nutrient cycling, while reducing disease and pests. Diverse rotations in annual crops, which provides plant diversity over time, can keep soil healthy. For perennial crops, it’s important to rotate your cover crops in alleys, as that will help ensure a healthy soil ecology, and help prevent the build-up of soil pathogens. In pasture and rangeland, carefully managed grazing encourages plant diversity.
Continual live plants/roots in the soil. The native vegetation in converted agricultural areas consisted of continuous stands of perennial and annual grasses and broadleaves providing carbon exudates to the soil food web during most of the growing season.
Today’s croplands typically grow annual crops with an extended crop-free period of bare soil before planting or after harvest. It is extremely rare in nature to see vast expanses of bare soil. Bare soil does not receive any root exudates, which starves the soil microbial community. Cover crops are able to fill in this crop-free period, providing cover to the soil and root exudates to the soil’s food web. Cover crops address a number of resource concerns already listed, and also provide an opportunity for livestock integration into cropping systems. In pasture systems, a diverse mix of warm and cool season forage plants lengthens plant productivity over the course of the year, maximizing root exudation.
Livestock integration. Animals, plants and soil have played a synergistic role together through geological time. Animal roles have been reduced recently due to fewer farms including animals as part of the operation and the development of confined animal operations.
Returning animals to the agricultural landscape can contribute to soil health by adding some biology to the soil, especially if the land hasn’t had grazing animals on it. Livestock also convert high carbon annual crop residue to low carbon, high nitrogen organic material, i.e., manure, which is beneficial to the soil. Some cover crops can be grazed without damage. Conversely, livestock can be used to manage an overly vigorous cover crop. Thoughtful integration of livestock onto cropping land can reduce weed pressure, reduce herbicide use and reduce livestock waste associated with confinement thereby improving water quality and nutrient management concerns.
Soils, Organic Matter and Water
Organic matter in the soil is made up living, dead and decomposed organisms. The living organisms in the soil, which represent roughly 15 percent of the total organic matter in the soil, vary from microorganisms like fungi, bacteria and viruses to insects, plant roots, earthworms and mammals. The dead organisms are made up of recently deceased microbes, insects, earthworms, decaying plant material and animals.
The living organisms feed on both the living and the dead organisms, releasing proteins, sugars, and amino acids that feed plants and decomposers. The decomposition process and its various by-products also produce substances that hold sand, silt and clay particles together to form aggregates and give it structure. This structure allows for efficient infiltration of rain and irrigation water into the root zone, and ultimately, into the water table. The smallest organic matter particles in the soil are called humus. Humus is a relatively stable part of the soil, a complex component that can buffer the plant from exposure to harmful chemicals, reduce the effect of compaction, improve drainage in clay soils and improve water retention in sandy soils. This stable organic matter has surface charges that allows water to adhere to the surface. In addition, organic matter, being generally negatively charged, attracts positively charged ions (cations), many of which are important plant nutrients.
Earlier research demonstrated that a silt loam soil with 4 percent organic matter holds more than twice the water of a silt loam with 1 percent organic matter (Hudson 1994). Further recent research has shown that there have been overestimations on the relative contribution of soil organic matter to water holding capacity and it is influenced greatly by the soil physical properties (particle size, texture, and bulk density) and mineralogy. The increase of water holding capacity as levels of organic matter increase was more pronounced for sandy soils than for loam and clay soils. This more recent research still suggests that for every 1 percent of soil organic matter (SOM) in the top six inches, the soil will be able to store an additional 10,800 liters of water in the top 6 inches. But regardless of the soil type, adding organic matter to soil is beneficial for the numerous functions it provides besides increasing the soil’s water holding capacity. Farmers investing in their soils by increasing organic matter and improving soil health will find that their soils will better support plant health, especially during times of drought and flooding.
You Can’t Manage What You Can’t Measure
Measuring irrigation distribution is important, and especially effective when used in combination with practices that support a healthy soil.
The moisture content of the soil regulates the moisture levels in the plant. Overly dry soil, or overly moist soil stresses the plant and can induce diseases and reduce future seasons’ yields. This is why it is important to monitor soil moisture in order to schedule irrigation and provide the crop with adequate water to achieve ideal growth and yields. Soil moisture monitoring devices use sensors and probes located in the soil root zone. Combined with information about temperature, evapotranspiration (evaporation from the soil and transpiration from the plant), and water requirements of the crop, these devices are able to provide the farmer with information that can be used to properly schedule irrigation.
Another important component in managing soil moisture is irrigation distribution uniformity. This measures how evenly water is applied across a field to a crop during irrigation. Micro-sprinklers often get plugged, as do drip emitters. Sprinkler heads get worn, and leaks in the system all effect distribution uniformity, not to mention human error (a worker forgot to turn a valve, etc.) All these can significantly affect water distribution, and fertilizer distribution if the farmer is fertigating. If water distribution is uneven in a field, it will negatively affect yields. Inspecting and performing distribution evaluation in your irrigation system will identify the causes, and corrections can be made to eliminate plugging, minimize variation in pressure, thereby correcting flow rate, infiltration time, spacing, set duration and land grading.
Soil Health and the Future of Farming
Farmers across the country are operating in an era of uncertain weather and uncertain markets. Many farmers have reduced their input costs and increased their bottom line by choosing to invest in their soil health, just as they would in new machinery and maintaining farm structures. Healthy, living soils can better sustain the increased demands we’re placing on them to grow healthy food and maintain clean water and air. It is important to build and maintain soil health before drought or flood conditions appear. Healthy soils can better withstand climatic stresses of drought and floods and in some cases, can help mitigate these stresses. All this requires an increased understanding of how to manage the soil as an ecology. Investments, such as adding organic amendments, practicing no- or reduced tillage, leaving crop residue, planting cover crops, and diverse crop rotations—these practices will help the soil to efficiently cycle both water and nutrients to sustain plant and animal productivity, and maintain or improve water quality. The return on soil health investments will pay off year after year after year.