Tillage practices linked to poor soil health and reduced soil carbon
In Boulder County, tillage practices can be grouped into three categories: conventional tillage (intensive, deep seasonal tillage like plowing or rototilling), reduced tillage (tillage that disturbs only a portion of the soil surface, like strip-till or chisel plowing), and zero tillage (no soil disturbance). Data show that Boulder farms using conventional and reduced tillage had poorer soil health and less soil carbon compared to open lands (pastures, grasslands, forests, and orchards) with no soil tillage.
A heavily plowed field ready for planting. Data show that plowing results in both lower soil health and soil carbon.
For many farms across the world, tilling soil is an essential part of farm practices. Tilling is the process of turning over old soil, in order to expose more nutritious, deeper soil for sowing new crops. For farmers, tillage is a way to solve problems. Tillage is used for seedbed preparation, weed suppression, soil aeration, turning over cover crops and forages, burying heavy crop residue, leveling the soil, incorporating manure and fertilizer into the root zone and more. However, in recent years, more and more farmers are switching to reduced-tillage practices. Since tillage fractures the soil, it disrupts soil structure, accelerating surface runoff and soil erosion. Tillage also reduces crop residue, which helps cushion the force of pounding raindrops, and disrupts the microorganisms in the soil, leading to poor soil health.
In Boulder County, tillage practices can be grouped into three categories: conventional tillage (intensive, deep seasonal tillage like plowing or rototilling), reduced tillage (tillage that disturbs only a portion of the soil surface, like strip-till or chisel plowing), and zero tillage (no soil disturbance). Data provided by the Citizen Science Soil Health Project, a grower-driven initiative that offers soil health tests and annual soil health scores for each participating grower, show that more intense tillage has a detrimental effect on soil health, because median soil health scores increase as tillage intensity decreases (Figure 1). Boulder farms using conventional tillage had the lowest soil health scores with a median score of 10. Boulder farms using reduced tillage had a better soil health score with a median of 13. However, lands with zero-tillage practices had the best soil health score with a median score of 20.
So why do farmers till the soil?
Farmers till to control weeds, loosen soil compaction, incorporate organic matter and fertilizers into the root zone, and prepare seedbeds. Organic farmers are especially dependent on plowing because they cannot use herbicides to control weeds. Most of Boulder’s agricultural water is supplied by open irrigation ditches, which are lined with plants that drop seeds into the passing water. Many Boulder farmers flood irrigate their fields with raw ditch water, a traditional low-tech economical water application method. However, flood irrigation can compact the soil and introduce weed seed from open irrigation ditches, thus forcing farmers, especially organic farmers, to plow and cultivate more.
The importance of irrigation in a semi-arid landscape
Other water application methods like drip irrigation systems, sprinklers or pivot irrigation systems can solve weed-seed and compaction problems for farmers and lead to less intensive tillage. However, these systems require large investments in holding ponds, pumps, wells, piping, electricity, control systems, and more. Boulder farmers leasing city or county agricultural lands on short-term leases are unable to afford such investments. Thus, Boulder must continue to invest in upgrading water application systems, like holding ponds, pumps, and drip/sprinkler/pivot systems for farmers, to expand options for zero-tillage practices and safeguard soil health throughout the county.
explore the data
Soil health across tillage practices
Figure 1. Soil health for conventional, reduced, and zero tillage sites. Black bars represent conventional tillage farms, gray bars represent reduced tillage farms, and gold bars represent zero tillage lands (no-till farms but also other land types that have not been plowed, including perennial pastures, horse, cow pastures, orchard, golf courses, Christmas trees, and Open Space grasslands and forests). The black horizontal lines indicate health score thresholds, where health scores below 5 represent low soil health, scores between 5-10 represent moderate soil health, 11-17 represent high soil health, and scores 18 and above represent excellent soil health. Source: Data provided by Elizabeth Black from the Citizen Science Soil Health Project. Soil health was calculated from soil health tests and annual soil health scores for each participating grower, using the Haney/Phospho-Lipid Fatty Acid (PLFA) soil tests from Ward Labs.
Distribution of Soil Health Scores across Boulder County farms and Open Space lands
Figure 2. Percentages of sites on farms and open space lands with soils in each soil health score category. 5.3% of sites had low soil health scores (light grey); 20.2% of sites had moderate soil health scores (dark grey); 38.3% of sites had high soil health scores (blue); and 36.2% of sites had excellent soil health scores (gold). Source: Data provided by Elizabeth Black from the Citizen Science Soil Health Project. Soil health was calculated from soil health tests and annual soil health scores for each participating grower, using the Haney/Phospho-Lipid Fatty Acid (PLFA) soil tests from Ward Labs.
A little background on the ‘Haney Test’, which we use to calculate soil score:
All soil contains many small, easily-absorbed, plant-available molecules of minerals like soluble salts, trace minerals, inorganic nitrogen (nitrates), inorganic phosphorus (orthophosphates), and potassium. Soil has an inherent pH (acid/base balance) that determines how well some of these minerals are absorbed by plant roots.
Soil also contains soil organic matter, which is where soil microbes live, eat and die. Soil organic matter contains organic carbon, which microbes eat. Soil organic matter also stores organic nitrogen and organic phosphorus in large complex organic molecules which are not readily plant-available.
Soil microbes break down ("mineralize") some of these large organic molecules into small easily-absorbed plant-available molecules of nitrate and orthophosphate. As they work, soil microbes exhale CO2, measured as soil respiration.
Haney’s Soil Health Score is a weighted sum of a soil’s respiration plus the organic carbon and organic nitrogen that is available in the soil to feed both soil microbes and plants. A higher Soil Health Score means that there are more soil microbes, there is more food for them, and there are more soil nutrients available for them to break down and supply to plants.
Table 1. Variables contributing to the Haney Soil Health Score. Source: table built with explanation from Midwest Laboratories.
explore the data
Soil carbon in grassland sites with and without historic tillage
Figure 3. Percentage of organic carbon in the soil sample as determined by difference between the total carbon and inorganic carbon percentage. Blue bars represent grassland sites without historic tilling and gray bars represent grassland sites with historic tilling. Note that grassland sites without historic tilling tend to have higher percentages of organic carbon. Source: Data was collected as part of the City of Boulder's Open Space and Mountain Parks grassland monitoring program and 2018-2019 externally funded soil monitoring. Data represents the 07/17/20 version of the data set with a sample size of 141. Total percentage of carbon in the soil sample was determined using LECO furnace combustion CN analyses at the CSU Soil, Water, and Plant Testing Laboratory. Inorganic C was determined by Tracy Halward working at CSU laboratories by pressure transducer analyses. Tillage data was derived by intersecting the transect locations with OSMP's 2018 Agricultural Layer depicting historic tilling or agricultural conversion. Note that some tilled sites have also received additional treatments to accelerate conversion from agriculture to native grassland such as seeding, planting and erosion protection.
Recommendations
Continue to upgrade water systems, so farmers have options for zero-tillage practices.
A pivot system is a movable pipe structure that rotates around a central pivot point connected to a water supply, pump, and electricity. For large farms, pivot irrigation systems are the most popular sprinkler irrigation systems in the world because of their high efficiency, high uniformity, ability to irrigate uneven terrain, and low capital, maintenance, and management costs. And because pivot irrigation does not compact the soil, unlike flood irrigation systems, pivot systems allow farmers to consider reduced and zero-tillage practices.
Boulder County provides some incentives for farms that are seeking access to improved water systems, enabling more of these pivot systems to be installed within Boulder County. However, pivots are often more appropriate for large-scale growers.
Smaller farms need smaller more complex systems and have less capital to allocate to upgrades, often leaving them with fewer solutions that don’t involve tilling. Organic growers in particular have fewer alternatives available because tilling is a main source of weed control without the use of chemical herbicides.
With that in mind, a broader allocation of resources such as holding ponds, filtration systems, pumps, sprinkler/drip systems and electricity, to small and medium-sized farms that could benefit from these upgrades would improve the outlook for heavily tilled land within Boulder County. While there are many options to consider, pivot systems in particular would provide large farms with an effective management system to let their over-tilled soil recover.
Resources:
Waller P., Yitayew M. (2016) Center Pivot Irrigation Systems. In: Irrigation and Drainage Engineering. Springer, Cham. https://doi.org/10.1007/978-3-319-05699-9_12
