7c: Water Management
Written by Jenna West
Introduction
The movement and function of water in soil create a base for understanding how the addition of inputs in organic agricultural systems affects crops and soil. Crops, soil, and farming practices also influence the hydraulic processes, like water infiltration and runoff, that occur in organic systems. Solute transport relates to how roots access nutrients in soil water and how leaching and runoff remove nutrients and particles from the agricultural system. Both aspects of solute transport also tie into organic management choices. Knowing the effect that negative aspects of hydraulic processes can have on the environment if water is not well-managed gives rise to practices to minimize externalities and resource waste. Managing water during times of excess and shortage is also an important concept in organic agriculture. All management methods come with trade-offs that include energy, financial, environmental, and yield costs. The underlying theme of water management is soil management, because practices promote good soil structure and high soil organic matter (SOM) levels, resulting in soils that are beneficial in several aspects of water management.
Water Properties in Soil
To understand how water moves in the soil, one must first understand the properties that allow water to stay in the soil. The three main factors are bonding, adhesion and cohesion forces, and capillary rise (Brown et al. 2021). Hydrogen bonding occurs because water molecules are polar so water molecules are attracted to each other to form hydrogen bonds (Brown et al. 2021). Hydrogen bonding leads to the second factor – the forces of adhesion and cohesion. Adhesion is the force of attraction between water and solid material particles, while cohesion is the force of attraction between water molecules (Brown et al. 2021). Capillary rise is the third factor that allows for water to be retained in the soil. This force refers to the “pull” of water that causes it to rise against gravity in a small tube, or soil pore (Brown et al. 2021).
The actual amount of water that soil contains is referred to as the soil water content which is highest when at the saturation point as shown in Figure 1 where all the soil pores are filled with water (Gregorich et al. 2001). When rapidly draining water leaves all the macropores in the soil due to gravity, the soil is then at field capacity (Gregorich et al. 2001; Eden et al. 2017). The water remaining in the micropores for plant uptake is largely plant-available water (PAW) (Eden et al. 2017). When there is no plant available water left, the soil water content is at the permanent wilting point (Eden et al. 2017).
Water availability depends not only on the water properties mentioned above but also on the type of soil. Silt loam soils allow for the highest amount of soil available water, sandy soils hold the least water overall, and clay soils hold the most unavailable water in many small pores (Weil and Brady 2016).
Organic agriculture has been shown to improve soil structure which has positive effects on water in soil and crop access to water. Organic practices, like adding organic matter through compost and cover cropping, tend to increase the amount of soil organic matter (SOM) in fields (Fess and Benedito 2018). High SOM levels are associated with numerous benefits – one of the primary ones being better soil structure (Bai et al. 2018; Fess and Benedito 2018; Pimentel and Burgess 2014; King et al. 2020). Soil structure refers to the distribution and connectivity of pores, the texture refers to the size of soil aggregates, bulk density, and organic carbon (Eden et al. 2017). Soils with relatively high levels of SOM tend to aggregate better and those aggregates are more water-stable (Tisdall and Oades 1982; King et al. 2020). One of the ways that high levels of SOM are thought to be beneficial in soils is its positive effect on a soil’s water-holding capacity, although the degree of its effect is still under debate in the scientific literature (King et al. 2020). However, the point made by King et al. (2020), is that even if the water holding capacity changes only marginally with SOM, as was found by Minasny and McBratney (2018), SOM provides other benefits in agriculture that allow plants better water access. Namely, water-stable aggregates are more resistant to compaction which allows for better aeration and drainage – both of which are required for plants to take up water and nutrients (King et al. 2020).
Water Movement in Soil
Water movement is important because it affects the hydraulic processes shown below in Figure 2, like infiltration, percolation, and runoff in soils. Infiltration refers to how water enters pore spaces at the soil surface, and percolation refers to the downward movement of soil water through pores (Weil and Brady 2016). The improvement of soil structure and soil aggregation by organic practices also enhances water infiltration and percolation, since compaction is less prevalent and pore space and connectivity are better (King et al. 2020). When transitioning from conventional to organic agriculture, soil improvements can take time, which is a reason for some of the yield lag associated with new organic operations. Morvan et al. (2018) discovered that substantial improvement of soil properties in silty soil can be achieved in three years. This is important because the slower the infiltration and percolation in a given soil, the faster that soil becomes saturated during heavy rain events. When soil is saturated, no more water can enter soil pores, which is when runoff occurs.
Runoff refers to water that flows overland rather than infiltrating the soil (Weil and Brady 2016). Surface runoff is a problem in agriculture because the water flowing on the surface carries away soil particles and small aggregates (Weil and Brady 2016) causing soil erosion. This soil loss is unsustainable since the rate of soil formation cannot replace what is being lost (Morvan et al. 2018). Besides soil particles, runoff also removes water-soluble nutrients from agricultural systems and pesticides from conventional systems (Weil and Brady 2016). Even with good soil structure, runoff can still occur during heavy rain events. Good management practices that are often associated with organic agriculture can help further reduce runoff and erosion.
No-till is one such practice as it is associated with better aggregate stability, higher SOM, increased biological activity in the soil near the surface and decreased bulk density (Bai et al. 2018). These factors result in a better soil structure that allows for improved infiltration and water use efficiency as well as decreased runoff (Bai et al. 2018). Crop rotation is another management practice that increases biological activity and SOM (Bai et al. 2018) and therefore influences soil structure and the hydraulic processes shown above in Figure 2. Growing cover crops in the off-season or between plantings also helps reduce runoff since having stalks and foliage as opposed to bare soil helps slow down the velocity of water and enhance infiltration, as mentioned in Chapter 3c on cover crops. If cover crops cannot be planted during a period between crops, leaving residues from the previous crop on top of the soil can also be used to slow water movement. With no-till, this residue is left on top of the soil during the seeding of the following crop, which ensures the water-slowing ability of the residues is maximized.
The above practices are often associated with organic agriculture, although they are not often required for organic certification and can be implemented into conventional systems as well. The application of compost as a source of nutrients, rather than synthetic fertilizer, is one practice required in organic agriculture that benefits the soil and hydraulic processes associated with the system. A long-term study at the Rodale Institute comparing the effects of applying compost, manure, and synthetic fertilizer highlights these benefits (Hepperly et al. 2009). These nutrient application methods were tested on plots with maize, wheat, and peppers in combination with a few types of cover crops (Hepperly et al. 2009). The results showed that yields were similar with the various nutrient applications, but only the compost methods had a long-term positive effect on C and N soil levels (Hepperly et al. 2009). It was concluded that adding compost is the most effective way to maintain and build soil structure and enhance infiltration (Hepperly et al. 2009).
As mentioned above, the good soil structure facilitated by ecological organic farming practices leads to macropores that are well interconnected. The benefit of this extends to crops as it allows plants to maximize their root system and water access (King et al. 2020). Better water infiltration also prevents waterlogging of roots which can cause damage or prevent efficient root extension (King et al. 2020). Damage to roots occurs without oxygen as the plant root cannot produce enough ATP, or energy, to take up nutrients (King et al. 2020). Additionally, waterlogging can kill the apical meristem, or tip of roots, which permanently prevents that root from expanding further and forces the plant to put energy into producing new roots (King et al. 2020).
As opposed to runoff which refers to particles moving in water over the soil surface, leaching refers to soluble materials moving in water through the soil profile (Weil and Brady 2016). Some leaching will always happen as water infiltrating into the soil causes the downward movement of soluble materials through the soil profile via water over time (Gregorich et al. 2001). Leaching becomes a problem when mobile nutrients and pesticides are lost from an agricultural system. They then become a waste of resources for the farmer and cause problems like eutrophication or contamination of groundwater in the environment (Weil and Brady 2016). Nitrogen, one of the limiting nutrients for plant growth, is also very mobile and therefore susceptible to leaching when surplus is added to the system (Hepperly et al. 2009). All methods of adding nutrients to agricultural systems, be it compost, manure, or synthetic fertilizer, provide nutrients that can be leached (Hepperly et al. 2009). In the long-term study at the Rodale Institute, leaching from systems with broiler litter leaf compost was less than manure and synthetic fertilizer (Hepperly et al. 2009). Dairy manure leaf compost was also lower in six of nine years of the study (Hepperly et al. 2009). Herai et al. (2006) also found that the sawdust compost trials saw significantly less nitrogen leaching than the synthetic nitrogen fertilizer, despite the maize crops taking up similar levels of nitrogen in the plants with both nutrient methods. Although nitrogen still needs to be managed to minimize leaching in organic systems, this is less of an issue since synthetic nitrogen fertilizers are not being used.
Leaching can further be reduced by proper nutrient management practices as well as those that facilitate good soil structure. The 4Rs of nutrient stewardship are often referred to for fertilizer application but can also be used with compost application. The 4R’s are “right source, right rate, right time, and right place” and are used to govern nutrient application for the most efficient, safe, and environmentally friendly application (International Plant Nutrition Institute (IPNI) 2017). The right source refers to an organic source in organic agriculture, as well as knowing the nutrient levels and availability of the compost being applied. The right rate refers to matching the number of nutrients being applied according to the needs of the crop to prevent a surplus that is susceptible to runoff and leaching. Right time refers to applying compost during weather and seasonal conditions that minimize the risk of runoff and leaching (IPNI 2017). For example, precipitation levels are higher and plant growth is lower in the winter so compost application is not recommended in that season. Applying compost before a heavy rainfall event also increases the potential for runoff and leaching (IPNI 2017).
Drainage
Excess water is largely an issue in the spring and fall when precipitation levels are higher, so having a drainage system in a field can allow farmers to extend their growing season (Magdoff and van Es 2021). Digging ditches around the edges of fields was one of the first strategies employed to reduce waterlogging by lowering the water table as shown below in section “b” in Figure 3 (Magdoff and van Es 2021). Tile drainage also referred to as subsurface drain lines and shown in “c”, is now one of the more common strategies in Canada (Magdoff and van Es 2021). This system is set up to only remove excess water from the field and does not remove water that plants would have otherwise used as it only removes gravitational water. Removing excess water and preventing waterlogging allow plants to access more water as their roots can grow in soil that would otherwise be waterlogged. This can be seen with the larger root systems in the Figure 3 field with tile drainage as opposed to the field without that has a higher water table. Mole drainage, shown below in “d”, is used more often in clay soils with smaller pipes placed closer to the soil surface than with tile drainage (Magdoff and van Es 2021). Placing the pipes closer to the surface encourages the clay soil to crack above and create channels for water to drain through (Magdoff and van Es 2021).
Environmental issues have been related to tile drainage, as the excess water that leaves the system and flows into rivers or lakes is often contaminated with nutrients and pesticides (Madramootoo et al. 2007). Nutrients like nitrogen and phosphorous, as well as bacteria like E. coli from manure, are all examples of agricultural inputs that cause problems in the environment when they leave the system through drainage water (Madramootoo et al. 2007). Various efforts to control this include constructing wetlands to filter out the contaminants, with the option of reusing the water for irrigation on the fields in dry months (Madramootoo et al. 2007). As mentioned above, the 4Rs of nutrient management are also useful for reducing the leaching potential from compost application (IPNI 2017).
Irrigation is one strategy to deal with water shortages, however, there are numerous problems associated with it. As mentioned above, problems are associated with financial and energy costs (Magdoff and van Es 2016). Adding large amounts of water to the system can increase runoff and leaching from the system, which can contaminate nearby drinking and natural water sources (Magdoff and van Es 2016). Using water for irrigation also competes with urban and other use of water. If recycled water is used as an alternative, salt accumulation in fields becomes an issue which affects crop growth and development in areas of low precipitation levels (Rahman et al. 2016).
Organic agriculture has been shown to be better suited to drought conditions than intensive conventional agriculture. Although yield lag is an ongoing point of contention in the sustainability of organic agriculture, organic yields during drought conditions often match or exceed conventional yields as was seen in the study by Pimentel and Burgess (2014) where the corn and soybean production yields were higher with compost than in the conventional system. This resilience is due to the ecological farming methods that have been mentioned above that lead to good soil structure, enhance plant water access, and increase soil water capacity.
Conclusion
Along with sunlight, water is one of the primary factors in plant growth. Waterlogged plant roots become damaged due to a lack of oxygen which, in turn, results in the plants becoming wilted and stunted. Stress caused by the wrong amount of water is responsible for a significant amount of yield loss in field crops. Understanding how water behaves and moves in the soil is helpful when trying to manage water for crops in organic systems. This chapter covered how organic practices affect soil structure which in turn affects soil water content and plant available water. The connection between nutrients and soil water was made, as water-soluble nutrients participate in hydraulic processes including runoff, infiltration, and leaching. Practices to minimize runoff and leaching, as well as those to manage excess and the lack of water, were then introduced. Considering the environment that surrounds and contributes to farm systems and the delicate balance that is needed to preserve and utilize water, organic farming practices that promote good soil structure are a worthy investment in an agricultural system.
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