Soil pH: Not a Useful Tool to Predict Plant Nutrient Availability

All growers are familiar with the standard chart that depicts different nutrients and how available they are to plants at different pH levels. The width of the band for each nutrient at different pH levels describes how available the nutrient is.

This diagram was first developed by Emil Truog in the 1930s and 1940s, based on earlier work, for eleven different nutrients. It has been widely disseminated and acted upon by researchers and growers over the decades. The key insight from the diagram is that a pH of about 6.5 most generally serves to make most nutrients available.

However, recent research, published in the journal Plant and Soil, urges growers to reconsider the validity and usefulness of this diagram. The authors discuss how the chart was created and its limitations. 

Unsurprisingly, given how much we’ve learned about soil biology in recent years, soil is much too complex for a single factor — pH — to be the defining lense for how nutrients become available. Biology transforms nutrients in the soil — particularly the rhizosphere — into different molecular forms. Nutrients vary in how they react with the plant due to other factors as well, such as their concentration in the soil, their ionic form, and their adsorption and mobility. 

In the words of the researchers, “The soil pH cannot be used to predict or estimate plant nutrient availability, and the diagram should not be used, as it suffers from numerous exceptions and barely represents any rules.”

Ground Beetle Gut Microbiome Influenced by Farm Management

Beetles have long been a fascination of ecological/regenerative farmers. Acres U.S.A. founder Charles Walters wrote an entire book about them (Dung Beetles). New research, published in the journal Scientific Reports, has found that the microbiome of ground beetles varies according to the system of farm management employed. The extensive use of synthetic agrochemicals appears to alter the composition of the microbial communities in the guts of these vital insects.

Intensive conventional farm management, characterized by high agrochemical input, could alter the composition of microbial communities, with potential negative effects on both functional traits and the ecosystem services provided. 

The researchers investigated the gut microbial composition of Pseudoophonus rufipes, a ground beetle that performs important ecological services on farms, to include pest predation and seed consumption. They sampled the beetles from two fields — one conventionally managed and one that used organic management practices — and then analyzed the gut microbiota via qPCR and NGS (both techniques for analyzing DNA). 

The team found significant differences between the two populations. Organic systems produced an abundance of bacteria called Tenericutes and others from the Spiroplasmataceae and Bifidobacteriaceae families; conventional fields yielded more Proteobacteria and bacteria from the Enterococcaceae, Morganellaceae and Yersiniaceae families. While we know that no bacteria are uniformly “bad” or “good,” research has shown that Spiroplasma species play important roles in protecting organisms from nematodes and wasp parasitoids. And Bifidobacterium members have been documented to provide “positive effects on immunomodulation and on the metabolism of oligosaccharides, ensuring an improved health status.”  

The researchers concluded that “The diverse gut microbial composition of insects between the two management systems is related to the pressure of environmental stressors and it may [be] representing an important bioindication of ecological functions and services provided by a carabid species.”

The question, of course, is if the microbiome of beetles is negatively affected by agrochemicals … wouldn’t it make sense that the same is true with humans?

Micronutrients Can Predict Soil Microbial Content

As highlighted constantly in this magazine, microbes — bacteria, fungi, protists, etc. — play vital roles in the health of the soil and of crops. A recent study reported in npj Biofilms and Microbiomes has shown that micronutrients — more than macronutrients — are determinative of the makeup of the microbes in the soil.

The team used data from 228 fields across a wide variety of geographic areas and climates and compared macronutrient composition of the soil, micronutrient composition (particularly iron, zinc, copper and manganese) and soil microbial communities (bacteria, fungi and protists). They found that micronutrients explained more variations in the content of microbial communities than macronutrients in soil where corn was being grown. The ability of micronutrients to predict the structure of soil microbial communities declined greatly, however, in paddy soils. The team also used machine learning to show that the addition of micronutrients substantially increased the predictive power of the type of content of soil microbial communities. 

Soil biology is important, and this research points to the fact that micronutrients are vital in cultivating microbial communities. 

Molecular Mechanism behind Nutrient-Induced Plant Disease Resistance Discovered

Just as humans can’t subsist on a diet of only French fries and brownies, plants must also consume a balanced diet to maintain optimal health and bolster their immune responses. Nutrient element uptake is necessary for plant growth, development and reproduction. In some cases, treatment with essential elements has been shown to induce plant disease resistance, but conclusive research on the molecular basis of this remedy has been limited.

In one of the few studies to directly investigate the mechanism underlying the effect of essential elements on plant disease resistance, Rupali Gupta of Volcani Institute and colleagues have demonstrated that nutrient elements activate immune responses in tomato plants through different defense signaling pathways.

Their paper, recently published in Phytopathology, outlines the molecular mode of action that potassium, calcium, magnesium and sodium take to minimize both fungal and bacterial plant diseases. Using straightforward laboratory methods, the authors demonstrate that essential element spray treatment sufficiently activates immune responses in tomato — including defense gene expression, cellular leakage, reactive oxygen species production and ethylene production — leading to disease resistance. Their results suggest that different defense signaling pathways are required for induction of immunity in response to different elements.

Understanding the genetic mechanism underlying this process may provide new insights into crop improvement. Corresponding author Maya Bar commented, “We are excited to probe the molecular basis of this phenomenon, define another facet of induced resistance, and provide data that will assist in applying this principle to agricultural systems in a more purposeful, reproducible manner.”

The tenets of mineral nutrient-induced disease resistance discovered in this study can be exploited in agricultural practices — benefiting growers/farmers and protecting valuable crops.

Organic Agriculture Produces Higher Crop Yields during Periods of Extreme Weather   

For more than 40 years, the Farming Systems Trial (FST) at Rodale Institute has combined real-world practices and rigorous scientific analysis to document the different impacts of organic and conventional grain cropping systems. The scientific data gathered from this research has established that organic management matches or outperforms conventional agriculture in ways that benefit farmers and lays a strong foundation for designing and refining agricultural systems that can improve the health of people and the planet.  

New FST research has shed light on how regenerative organic agriculture is an effective and resilient farming model in an era of extreme weather. This is a significant finding as farmers around the world contend with the devastating effects of crop losses stemming from droughts and floods. 

“Regenerative organic farming builds healthy soil through enhancing soil organic carbon,” said Rodale Institute Chief Scientist Dr. Reza Afshar. “This allows the soil to absorb more rainfall during periods of flooding and retain moisture for longer periods during droughts.”  

Here are several key takeaways from the 40-year report: 

• Yields: Organic systems produce yields of cash crops equal to conventional systems. However, in extreme weather conditions, such as drought, the organic plots sustained their yields while the conventional plots declined. Overall, organic corn yields have been 31 percent higher than conventional production in drought years. 
• Carbon capture: Healthy soil holds carbon and keeps it out of the atmosphere. Organic systems usually have much more diverse carbon inputs going into the soil, so microbial biomass is significantly higher than in the conventional system, leading to higher soil organic matter over time. 
• Water: Water infiltration is significantly faster under long-term organic management compared to conventional practices. 
• Soils: FST data has established that soil health in the organic systems has continued to increase over time while the soil in the conventional systems has remained essentially unchanged. 

“The Farming Systems Trial is one of our most significant research projects,” said Rodale Institute Chief Executive Officer Jeff Moyer. “In fact, with FST’s now 40 years of accumulated data and findings, it is fair to say that it is the most consequential study of organic agriculture anywhere.” 

The Farming Systems Trial (FST) compares three core farming systems: a chemical input-based conventional system, a legume-based organic system, and a manure-based organic system. Corn and soybean production is the focus of each system because 70 percent of U.S. acreage is devoted to growing grain. In 2008, each core system was further divided to compare standard full-tillage and emerging reduced-tillage practices. At that time, genetically modified corn and soybeans were also introduced to the FST’s conventional system to mirror common practices. For more findings from the Rodale Institute Farming Systems Trial 40-year report, please visit rodaleinstitute.org/FST.