Worldwide, water is becoming scarcer and more expensive due to the effects of climate change. Significant adaptation will be necessary to ensure adequate supply and efficient use of 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 to increasing productivity per unit of water consumed.
The need for increased water-use efficiency will be arising 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 to dealing with water deficits has focused on better technology: deeper wells, better drip emitters, more efficient micro-sprinklers, and variable-speed drives on pumps – all of which are important. However, a different approach to dealing with the oscillation between too little and too much water uses an appropriate technology that focuses on maintaining healthy soils by following five basic principles discussed in detail in the following sections.
Healthy soil, with its thriving biological activity, creates a system of air and water pores that both allow water to infiltrate the soil and 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 that soil to hold water, release nutrients to the crop, and prevent erosion.
Attaining Healthy Soils
Soil experts across the country, including land grant universities, the USDA 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 is also improved, as well as the soil’s water storage capacity – 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.
The first principle
Protect the soil surface. Some people call this “soil armour.”
This includes the use of cover crops and mulch, which provide many benefits for the land, including the following:
- Wind and water erosion are brought under control. Cover crops and mulch protect the soil as wind or water move across the soil surface. This holds the soil in place and allows 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. Th e 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.
The second principle
The second soil health principle is to minimise soil disturbance of all kinds. Both physical (tillage) and chemical (overuse of fertilisers and pesticides) disturbance 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 fertilisers is like throwing gas on a fire. The excess nitrogen feeds bacterial populations, which explode when exposed to oxygen through tillage. Raised beds with vetch cover crop, which protects the soil and provides N.
On some California farms, the farmer protects his soil from heavy winter rains by planting vetch cover crops on raised beds. In the spring, he’ll mow the cover crop, lightly incorporate the residue, and transplant processing-tomato seedlings into the beds. At a Georgia cotton farm, the farmer chem-killed a small-grain cover crop and no-tilled cotton into it. The mulch adds organic matter, protects the soil from rains, and reduces water usage.
The problem is, these 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 the soil aggregates that 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 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 runoff and also creating anaerobic conditions in the root zone.
The third principle
The third soil health principle is 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 aboveground plant community 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 provide plant diversity over time, can keep soil healthy. For perennial crops, it’s important to rotate 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.
The fourth principle
The fourth soil health principle is the concept of continual live plants/roots in the soil. Th e 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 Soil physical disturbance – tillage – is hard on the soil ecosystem. Farmers who minimise tillage not only save money on labour and equipment wear and tear, but they’re also taking a step toward healthier soil.
Chemical disturbance can be equally detrimental to soil health. A diverse cover crop of more than a dozen species of grasses, legumes, and mustards helped the farmer at a Northern California walnut farm reduce his lesion nematode population from a count of more than 5,000 to “undetectable” over five years.
Having a diversity of crops on a field or a diverse rotation of different crops from different plant families both support a diverse soil ecology. 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, and this 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 in Principle 1 and also provide an opportunity for livestock integration into cropping systems.
In pasture systems, a diverse mix of warm-season and cool-season forage plants lengthens plant productivity over the course of the year, maximising root exudation.
The fifth principle
The fifth principle of soil health is the concept of livestock integration. Animals, plants, and soil have played a synergistic role together through geological time. Fewer farms are including animals as part of their operations, due to increasing specialisation in growing only crops, combined with an increase in the number 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, herbicide use, and livestock waste associated with confinement, thereby improving water quality and addressing nutrient-management concerns.
Organic matter in the soil is made up of living, dead, and decomposed organisms. The living organisms in the soil, which represent roughly 15% 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 recently deceased microbes, insects, earthworms, animals, and decaying plant material. 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 them 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 a 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 allow 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.
Does soil type matter?
Earlier research demonstrated that a silt loam soil with 4% organic matter holds more than twice the water of a silt loam with 1% 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 are increased was more pronounced for sandy soils than for loam and clay soils. This more recent research still suggests that for every per cent of soil organic matter (SOM) in the top six inches, the soil will be able to store an additional 10,800 liters of water.
But regardless of the soil type, adding organic matter to soil is beneficial for the numerous functions it provides besides increasing the soil’s waterholding 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.
Increasing levels of soil organic matter can increase the cation exchange capacity (CEC) of soils, providing a reservoir of nutrients and micronutrients (calcium, potassium, magnesium, iron, manganese, ammonium, and others) especially needed in sandy soils with very low CEC levels. In fact, organic matter can have four to 50 times higher CEC per given weight than clay.
Source: https://attra.ncat.org
