What Is Science Telling Us About Soil Testing and Treatment?

Soil testing is an important process for any farmer, regardless of the type of crop being grown on their land.

It is important from a quality perspective, as different crops will have different nutritional needs and testing can then allow farmers to intelligently apply fertilizers and nutrient additives to optimize their individual crop production. And it can also be economically advantageous, as regular testing means that farmers can apply these additives only as-and-when required, ensuring that money and energy is not wasted by applying these products too liberally.

Just as soil testing is important to the farmer, working to improve upon current soil testing methods is important to the agricultural scientist. By ensuring that farmers and agricultural workers have access to the best possible soil testing tools, they will then be more able to protect and improve the health of their land as needed.

New radar methodology outperforms traditional soil moisture analysis

Good soil health is usually taken to mean that a patch of soil contains a good amount of the essential nutrients needed for plant growth; which include nitrogen, phosphorus and potassium. [1] But in addition to these elemental nutrients, there are other important physical factors to consider, such as the moisture content of the soil.

Crops grown in particularly dry soil might have their growth stunted or even die completely, and those grown in very wet soil may wither and succumb to root rot [2]. By being able to effectively monitor the water content of their soil, farmers can protect their crops from these poor outcomes, while simultaneously optimizing their water usage — a perk that is particularly important in drier regions where water is a precious resource.

There are a number of different systems in existence that already allow farmers to measure the water content of their soil — such as simple probes, tensiometers, and electrical resistance blocks — but these are only capable of collecting data within a small locality, and impractical for assessing large areas of land.

“Geophysical methods, such as electrical resistivity and seismic reflection, require moving electrodes or geophones, respectively, in order to collect data over very large areas,” explains Dr Jonathan Algeo, now a project manager at Ground Penetrating Radar Systems, LLC. “Additionally, installing an electrode or geophone disturbs the soil that you are measuring.”

During his postgraduate studies at Rutgers University, Algeo worked extensively with ground penetrating radar, and quickly realized its potential as a method for measuring soil moisture content over large working areas.

“Radar is a quick, non-invasive way to look at the properties of the subsurface. It can estimate the thickness of different layers, it can measure moisture content, and it can detect buried objects, in addition to a plethora of other applications in the science, industry, and military fields,” says Algeo.

“With a radar antenna mounted on a wheeled cart, or dragged behind a vehicle, the user can collect kilometers worth of data in a single day,” says Algeo. “Additionally, radar data are collected by dragging an antenna along the ground surface, and so do not disturb the subsurface in any way. This is important when studying small-scale hydrologic or soil physics processes.”

Algeo and his colleagues evaluated how best to apply ground penetrating radar as a moisture content analysis system by comparing two pre-existing mathematical models designed to analyze the so-called “early time signal” from radar data. The results of the study were published last year in the Vadose Zone Journal [3].

“We studied two recently developed methodologies for estimating soil water content from a specific portion of the radar signal.  Pettinelli et al. (2007) and Comite et al. (2014) originally developed the methodologies — respectively, Average Envelope Amplitude (AEA) analysis and Carrier Frequency Amplitude (CFA) analysis [4, 5].”

“These methods offer a quicker way to estimate water content changes relative to traditional radar methodologies, and are applicable at field sites where the traditional methodologies cannot be applied, such as in the presence of clay-rich soils, which have a significant negative impact on the penetration depth of radar waves.” Algeo explains.

“We found that both methodologies successfully measured changes in moisture content at the field scale, though the AEA was more reliable/easier to carry out, and the CFA was sensitive to changes in moisture content over a wider range of soil moisture levels,” he adds.

For a testing methodology to become widely adopted by an industry, it first needs to be proven beyond doubt by researchers to be an effective and practical procedure. With a little more research and a little more testing, Algeo and his colleagues believe that ground penetrating radar technology could be another useful tool in the farmers' toolkit.

Are nutritional soil additives worth their salt?

Effective soil monitoring techniques give farmers a chance to take remedial action to improve soil quality and protect their crops from malnutrition.

One common way that farmers do this is by using biofertilizers designed to help plants’ uptake of nutrients [6]. Biofertilizers contain specific living microorganisms that colonize the soil, in theory creating a healthier growing environment for crops without the need for synthetic chemical fertilizers.

Arbuscular mycorrhizal (AM) fungi are natural biofertilizers; they live around the roots of plants and provide those plants with water, nutrients, and pathogen protection in exchange for photosynthetic products [7].

AM fungal inoculants are increasingly being used as biofertilizers by farmers who are looking to improve the health of their soil and crops. But do they actually work?

Dr Vasilis Kokkoris has examined just how effective these AM fungal inoculants really are. While working as a part of the research team led by Professor Miranda Hart at the University of British Columbia Okanagan Campus, Kokkoris authored a paper reviewing how these AM inoculants function under an array of different conditions. The paper was published earlier this year in the journal Science of the Total Environment [8].

“Agricultural soils are not as healthy as natural soils,” Kokkoris explains. “Common farming practices — such as tillage, fertilization, use of pesticides and herbicides etc. — are causing a significant amount of chemical and physical disturbance to the soil. This can reduce the abundance of AM fungi and subsequently the interaction of plants with AM fungi.”

“The physicochemical properties of the soil can vary incredibly between sites. Some soils can be very sandy while others might have more silt or clay, for example. Factors like this will eventually affect nutrient levels and nutrient availability and have a direct impact on plant health but also on microbial communities in the soil,” he continues.

“AM fungi, similar to any other living organism, interact with a diverse set of organisms that share the same environment. It is therefore very hard to predict how an introduced AM fungal species will respond to this combination of factors, and whether it will be able to establish and colonize the target plants.”

The research team looked at four fields with different crop systems over the course of two growing seasons in Saskatchewan and Alberta, Canada. These fields had been treated with a typical commercially sold AM fungal inoculant for the study.

The results showed dramatic variation. In one field, the AM fungi became invasive and disrupted other local fungi, taking over the resident fungal community in less than a year. In another, the AM fungi failed to establish at all.

“From our research, we found that it is still very hard to use a universal inoculation recipe in order to ensure establishment and successful interaction with the targeted crops,” says Kokkoris. “This is due to the variation seen between sites, and due to multiple other factors that have be researched further.”

So, are farmers wasting their money, and maybe even risking their crops, by using these remedial soil treatments on their unhealthy soil? It is hard to say for sure. But Dr Kokkoris believes that further scientific research will be key in answering these questions for good.

“There is no doubt that AM fungi can work miracles when applied on sterile soils or soil-like material as usually happens in greenhouses and plant nurseries. Results following inoculation of agricultural fields though, are not consistent,” Kokkoris says.

“I believe that there is merit in using AM fungi in agriculture, but we need additional research to understand further how to make such [biofertilizer] products consistently effective and safe.”

References

1.       J. A. Silva and R. Uchida, eds., Plant Nutrient Management in Hawaii’s Soils, Approaches for Tropical and Subtropical Agriculture, University of Hawaii at Manoa, (2000), Link: https://bit.ly/2Pr0nXx

2.       Missouri Botanical Garden; Overwatering, Link: https://bit.ly/2mnsU02

3.       J. L. Algeo et al., Vadose Zone J., 17, (2018), Link: https://bit.ly/2zz3yBr

4.       E. Pettinelli et al., Geophysics., 72,  (2007), Link: https://bit.ly/2Zz1S9T

5.       D. Comite et al., Proceedings of the 15th International Conference on Ground Penetrating Radar, Brussels, (2014), Link: https://bit.ly/32cFeSg

6.       European Biomass Industry Association; Biofertilizers, Link: https://bit.ly/2PoNFIR

7.       A. Berruti et al., Front Microbiol., 6,  (2016), Link: https://bit.ly/2NBiJCy

8.       V. Kokkoris et al., Science of The Total Environment, 660, (2019), Link: https://bit.ly/2Zok86x