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Soil Health
Introduction
Soil health is the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans. Oftentimes, the features of a healthy soil are related to the organic matter content.
The soil carbon present in this organic matter is integral to providing the fuel and structure needed to keep a soil healthy. Similar to human health, soil health will vary widely, with different types of challenges arising in different soil types. These challenges are going to differ largely due to the specific mineral composition of the soil,[1], [2], [3], [4], [5] which functions as the “genetics” of a given soil.
What Does a Healthy Soil Do? [6]
The definition of a healthy soil will vary according to the soil’s desired use, but they often incorporate facets of each of the following functions.
- Productivity/yield: healthy soils should be able to maintain yields or, in extreme weather years, minimize yield losses.
- Nutrient cycling: a tight cycling of nutrients, especially fertilizer-derived nitrogen and phosphorus, entails making nutrients available to plants and other organisms as needed, while reducing excess mineralization and nitrification, which can lead to loss of nutrients and adverse environmental effects.
- Holding water for plant uptake: healthy soils have strong soil structure and aggregation, which results in an increased ability of soils to hold plant-available water.
- Filtering contaminants: healthy soils can reduce the amount of contaminants, (e.g. antibiotics or heavy metals) that enter the freshwater supply.[7]
- Withstanding erosion: the increased aggregate stability of healthy soils can help soils to resist erosion and compaction.
- Mitigate crop diseases: healthy soils can lessen the incidence of soil pathogens that might affect crop performance.
Soil Aggregation: the Basis of Soil Health
Many of these methods of improving soil health pertain to the formation and retention of soil aggregates, or clumps of soil particles. As soils receive carbon input, which can be from crop residues, root exudates from live roots, dead roots, or other organic sources (e.g. compost or manure), soil microorganisms work to decompose these materials. In the process, they produce compounds that begin to bind soil mineral particles together into microaggregates. With increasing organic matter inputs these microaggregates begin to bind to one another, forming larger and larger aggregates[8]. Soil carbon within these aggregates is considered “protected” from decomposition from biological activity[9]. Due to changes in the chemical structure of soil organic matter that occurs in microaggregates (aggregates smaller than 250 microns in diameter), the carbon in these microaggregates is generally more protected from decomposition than the carbon in macroaggregates (those larger than 250 microns)[10].
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| As roots decompose, soil microorganisms bind soil mineral particles together. |
Increased soil aggregation from soil organic matter inputs can result in improved soil structure. This improved soil structure results in greater resistance to compaction[11] and increased water retention (especially in sandy and silty soils).[12]
In addition to improving structure, organic matter can increase the stability of soil aggregates when exposed to water[13],[14]. This stability can result in increased water infiltration[15], which means less runoff and erosion[16]. See image below.
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| Improved soil structure (top) increases infiltration and water holding capacity while decreasing runoff. |
Enhancing Soil Health
1) Use plant diversity to increase soil diversity
As plant roots grow, they exude a variety of carbon-rich compounds that will alter the microbial community composition[17],[18]. Some research suggests that increasing the number of plant species may increase the diversity of the microbial community, which may enhance the soil’s ability to cycle nutrients and supply plant-available N [19],[20].
Additionally, the use of crop rotations has been shown to decrease soilborne pathogens and root diseases. A decrease in pathogens has been linked to increases in soil microbial diversity [21],[22], which can lead to concurrent increases in yield [23]. Currently, it is unclear if increased diversity, absent any pathogen suppression, will increase yields. One recent study showed increased diversity to be associated with higher yields in unfertilized rice fields [24], but it is unclear if this diversity and yield increase persists with fertilization and under other crops.
2) Keep plants growing throughout the year
Plant roots are thought to contribute more carbon to soil than the above-ground biomass [25], although the amount will vary depending on crop and tillage practices [26]. In no-till systems, root-derived carbon is more important than residue-derived carbon [27]. Cover crops are a vital tool to increase both root-derived carbon and total carbon input into a soil. For more information on cover crops, visit our focus topic page or the cover crops database.
Continuous plant growth throughout the year can help reduce water contamination issues, a vital function for healthy soils to perform. The use of a winter cover crop allows for increased nutrient retention during the rainy months when leaching risk is greatest in California [28]. Perennial crops have also been shown to improve water quality by combating nutrient leaching [29].
3) Manage soils by disturbing them less
Decreasing tillage can increase overall organic matter content [30], although it is often constrained to the top several inches[31],[32]. Tillage decreases soil organic matter by exposing the carbon protected within aggregates to decomposition and loss[33],[34]. This loss of soil carbon can result in increased erosion from water [35] and from wind [36], which is especially important in maintaining air quality in the southern San Joaquin Valley of California.
While no-till practices can result in initial yield reductions, not all studies show this[37], and in some studies yields have generally recovered after a few years and could be mitigated by increasing carbon inputs (through residues, etc.) [38],[39]. Fields that had been cover cropped for 15+ years in California have been shown to have stable or even increased yields from reducing tillage operations [32]. It should also be noted that some California studies have shown that cover crops increase organic matter content more than reducing tillage operations [32],[40].
Tillage, or other similarly disruptive field operations, has also been shown to more strongly alter soil microbial community composition than other variables such as organic versus conventional management [41]. However, most research so far has not shown any definitive effect of tillage on other biological indicators typically used to measure soil health (microbial biomass, respiration, potentially mineralizable N, etc.) [41],[42],[43].
4) Keep the soil covered as much as possible. Soil cover can take several forms, including cover crops and crop residues.
Crop residues can help to maintain yields in no-till systems in dry climates by reducing evaporation and increasing water retention [39]. Soil cover can also help combat erosion [44] in a number of ways. Soil cover, whether from crop residue or other sources of mulch, helps to absorb the impact energy of raindrops falling on soil, reducing soil particle detachment, as well as crusting and sealing of the soil surface.
It also slows the speed of water moving across the surface, reducing the amount of soil particle detachment and transport[45]. Soil cover, whether as standing vegetation or residue, also reduces wind speed at the soil surface, preventing wind erosion. Even a soil cover of only 30% of the soil’s surface can reduce soil loss from wind erosion by 70% [46]. Standing vegetation with some height is more effective at reducing wind speeds than residue lying flat on the surface [46].
Mulch has also been shown to improve biological properties of soil, including earthworm populations and fungal biomass, as well as carbon content[47],[48],[49].
References
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3. Six, J., Paustian, K., Elliott, E.T. and Combrink, C., 2000a. Soil structure and organic matter I. Distribution of aggregate-size classes and aggregate-associated carbon. Soil Science Society of America Journal, 64(2), pp.681-689.
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