A Testimony on Linking Soil Health to Plant Nutrient Quality
By Dr. Gladis Zinati, Director of the Vegetable Systems Trial at Rodale Institute
Beginning in the early 1900s and coinciding with the Industrial Revolution, industrial farming was characterized by intensive farming of crops and animals where large-scale monoculture and high levels of chemical pesticides and synthetic fertilizers were routinely used.
After World War II, the combined effects of technological advances in large-scale machinery along with accelerated breeding programs enhanced monoculture production and accelerated farmer reliance on machinery and widespread use of chemical fertilizers and pesticides. For example, nitrogen fertilizer used for growing crops in the United States nearly doubled (about 11 million tons) between 1960 and 1983 and stabilized at that level until 2010. Similarly, there was a tenfold increase in pesticides use in U.S. agriculture during the same period, averaging 400 million pounds of pesticide active ingredient through 2010.
Soil health is an important foundation of a healthy farm ecosystem. Most of the farming techniques that have been employed in industrial crop production have contributed to soil degradation over time. Regular applications of agrochemicals in cropping systems further diminished the health of soil. Agrochemicals include herbicides, pesticides and fertilizer amendments. These chemicals interact with soil microorganisms including bacteria, fungi and earthworms which all contribute to a healthy plant rhizosphere and provide a range of benefits within cropping systems. However, these organisms are sensitive to variations in the environment, such as tillage and agrochemical inputs, leading to diminished microbial diversity, functionality, and adaptability to rapid environmental shifts.
It has been documented that soil health declines at a rapid rate when tillage practices are employed intensively and annually. Tillage and cultivation of soil are commonly used for controlling weeds, plowing under cover crop biomass, and preparing beds for seeding. Each tillage activity exposes the soil organic matter to oxygen, causing the release of carbon in the form of carbon dioxide into the atmosphere. Recent data presented at the COP26 (Conference of the Parties), held in Glasgow, Scotland, in 2021, showed that carbon dioxide concentration in the atmosphere has been continuously increasing since 1960 (when the level was 312 parts per million), reaching 411.51 parts per million as of November 5, 2021. Carbon dioxide is one of the greenhouse gases that contribute to heating up of the planet.
Intensive tillage compounded by applications of agrochemicals degrades soil health by reducing soil aggregate stability, increasing compaction, and increasing chances of soil erosion. Heavy rainstorms on degraded soils provide potential opportunities for runoff loaded with toxic chemicals and fertilizers that pollute water systems.
In the past two decades, researchers documented the negative impact of human exposure to pesticides either through direct exposure, air, water, or consumption of food contaminated with pesticide residues. Research data published between 2002 and 2005 showed that pesticide residues were detected in 73% of the 20 fruits and vegetables that were tested. Data by the Pesticide Action Network in 2012 have shown an increase in percent of mental disorder and children cancer cases since 1975 due to exposure to pesticides in foods.
Researchers have also showed that fruits and vegetables have increased in size with the introduction of breeding programs post World War II. The increase in size of fruits and vegetables has led to dilution of nutrients in harvested crops. Studies have shown that the greater the size of fruits and vegetables, the greater the dilution effect on nutrients.
Unlike agronomic crops, intensive vegetable production includes multiple cultivations and tillage of soil during the growing season. The use of moldboard plow, discs, and other cultivators are used to mix cover crops with soil, prepare seed beds, and control weeds. All these farm activities, when practiced over multiple years, lead to soil degradation and consequently negatively impact plant health and nutrient density. Impacts can be more dramatic in organic cropping systems because vegetable growers don’t use herbicides that are used by conventional growers. However, using herbicides and other agrochemicals can be detrimental to soil microorganisms, the nutrient recyclers, when used in conventional systems.
The system is broken and soils in the United States as well across the globe have degraded over the past 70 years. However, the effects of soil degradation can be mitigated, prevented, and even reversed by implementing regenerative farming techniques and land management. These techniques include cover cropping, crop rotation, reduction in soil disturbance, freeing soil from toxic chemicals, and amending soils with organic-based materials such as compost. Healthy soil is soil that is capable of sequestering carbon from the atmosphere, provides available nutrients to crops during the growing season, and is free from toxic chemicals that impact soil microbes and buildup of chemical residues in soil and harvestable crops. A healthy soil has good structure that allows easy infiltration of water and is rich in diverse living organisms that are dynamic in cycling nutrients for plant uptake. All these properties not only sustain crop productivity but also animals and humans. However, all these techniques may not contribute to production of nutrient-dense crops, sustainability, and resilience to changes in weather if they are not coherently arranged with proper farming systems and management practices.
There is a plethora of published research showing the impact of cropping systems on soil health and management practices for production of agronomic crops, suppression of root diseases, and weed control. However, there is a lack of information on how management practices may impact soil health and consequently vegetable nutrient density when grown in industrial and regenerative cropping systems.
The Vegetable Systems Trial (VST)
In 2016, the Vegetable Systems Trial (VST) was initiated at Rodale Institute in Kutztown, Pennsylvania, to assess side-by-side the impact of management practices on soil health and nutrient density in vegetable crops grown in organic and conventional cropping systems. The goal of this long-term trial is to deliver science-based information that links the soil health to plant health and human health. The vegetable growers, consumers and the public at large are all beneficiaries of such information.
In the VST, soil health and plant nutrients are evaluated in two cropping systems: the organic system that includes organic or biologically based inputs and the conventional system, representing the industrial farming system, that includes agrochemical inputs such as synthetic fertilizers and pesticides.
Within each cropping system, two tillage practices are implemented. The intensive tillage which entails the use of a moldboard plow and discing for mixing the cover crop biomass with the soil and preparing beds laid with black plastic mulch. On the other hand, reduced tillage is implemented to minimize soil disturbance and increase buildup of soil organic matter over time. Regenerative techniques are mainly practiced in organic systems, where the cover crop biomass composed of hairy vetch (legume) and cereal rye (grass) is roll-crimped in spring to form a mulch and vegetable crops are either seeded using no-till seeders (e.g., snap beans) or transplanted using no-till transplanters (e.g., butternut winter squash). In comparison, the chemical conventional system includes the burndown of cover crop biomass (cereal rye) using herbicides in spring and before planting vegetable crops. As for potato production with reduced tillage practice, the chisel plow is considered a less aggressive implement to use for plowing and mixing the cover crop biomass with soil compared to the moldboard plow.
Collection and Interpretation of Data from VST
The data collected from the VST is used to assess the changes in soil health and crop nutrient quality caused by the implementation of management practices within cropping systems over multiple years. While some soil health indicators may change over a short period of time, others may take longer to change. The multi-year data collected from the VST trial will aid in defining the link between soil health and crop nutrient density and subsequently their impact on human health.
Throughout the growing season, soil health is assessed for physical, chemical, and biological properties in samples collected from the plow layer (8 inches). In addition, every three years, marking the completion of a crop rotation in the trial, deep soil cores are collected in the soil profile (up to 40 inches deep) and assessed for changes in soil health indicators when compared to baseline reference data collected in 2016. For plant health and nutrient quality, all crops are monitored for pests and diseases and the harvested crops are graded for quality and quantity. Ground freeze-dried samples of harvested vegetable crops are analyzed for various vitamins, phytonutrients and mineral nutrients.
Soil Health Indicators
Two major soil health indicators will be discussed in this article. These include soil protein and labile soil carbon. Soil protein is an organic form of soil nitrogen from plants and microbes. It is an active pool for readily available nitrogen that is recycled and taken up by plants. This pool of proteins acts as a reservoir for the microbial community. Thus, since this pool depends on inputs of nitrogenous sources and availability to crops, we expect that protein levels may change over time. Labile organic carbon (also known as POXC) is another measurement of the active pool of soil organic matter. Simply, it is the measurement of how much soil organic matter is utilized by soil microorganisms by oxidizing it (use of oxygen) to get their energy for various functions. Published research has shown that this indicator can be more sensitive to management changes over time.
Recent and Major Findings from the VST
Preliminary and major findings on soil health in the VSTinclude those related to soil protein and labile organic carbon. Between 2016 and 2019, soil protein levels in the top 8 inches of soil increased by 37% in the organic cropping system. Interestingly, there was no change in soil protein levels in the conventional system between 2016 and 2019. The management practices did not influence soil protein levels. As time elapses, changes in the conventional system may become evident especially because the nitrogen source is urea, a synthetic non-organic fertilizer. In the next few years, these measurements will be repeated to learn more about impact of cropping systems and management practices on soil protein over time.
Similarly, labile organic carbon increased by 25% in the organic system and significantly more than in the conventional system (which had a 15% increase) between 2016 and 2019 in the top 8 inches of soil. Unlike soil protein, an increase in labile organic carbon levels was expected. Such increases can be attributed to inclusion of cereal rye, a cover crop with a high carbon-to-nitrogen ratio, in both systems. The labile organic carbon in both systems ranged between 700 and 1,100 parts per million. According to Cornell Soil Health Laboratory, the soil in the VST assessed in the top 8-inch plow layer is considered healthy (scored 80 to 90%) after three growing seasons from inception.
How Do Soil Health Results Link to Vegetable Nutrient Quality?
Total protein levels in sweet corn and potato followed a similar trend of soil protein in that organic sweet corn had significantly greater total protein levels (10.6%) than conventional levels (9.6%) in 2019. Organic Lehigh and Purple Majesty potato tubers had significantly greater total proteins (11.5% and 10.5% respectively) when compared to conventional (10.5% and 9.4% respectively).
On the other hand, total phenols in potato tubers varied with potato cultivar and tillage practice. Phenols in vegetables and fruits have anti-inflammatory, anti-viral and anti-bacterial properties and boost the human immune system. Purple Majesty tubers (purple flesh and skin) were three times greater in total phenols (5 milligrams GAE/g [gallic acid equivalents per gram] dry weight) when compared to Lehigh. Purple Majesty is more sensitive to management practices than Lehigh, and potato tubers harvested from plots managed with intensive tillage showed a slight increase in total phenols over those managed with reduced tillage. In comparison, total phenols in Lehigh potato did not differ with management practice.
The data presented here from the Vegetable Systems Trial show that certain soil health indicators are impacted by the cropping system regardless of the management practice. Soil protein and labile carbon levels mirrored sweet corn and potato protein levels and varied with cropping systems. However, management practices were found to impact total phenols in one of two tested potato cultivars. Additional data collected in the next five to ten years will verify the factors that contribute to short- and long-term changes in soil health and crop nutrient density.
Gladis Zinati was the keynote speaker at the 2021 Farmer to Farmer Conference. Her address, entitled “Linking Soil Health to Plant Health,” is archived on MOFGA’s YouTube channel. It was also broadcast on Common Ground Radio on WERU Community Radio and is available to listen to as a podcast at weru.org.