Soil characteristics such as moisture, density and organic-matter content play key roles in ensuring plants thrive. Soil temperature, while less obvious, is equally crucial.
Soil temperature fluctuations presents potentially costly challenges for farmers, particularly in ‘regions prone to temperature extremes, including parts of Africa, the Middle East, the US and Australia.
So anything that keeps soils from heating up and moderates soil-temperature fluctuations is a boon.
In a bid to help growers protect valuable cash crops, a group of US researchers helmed by soil scientist Samuel Haruna set out to determine whether perennial biofuel and cover crops can alter soil thermal properties in the ground they shield.
Why does soil temperature matter?
Every plant species requires a certain soil temperature to flourish. When soil temperatures rise or drop too much or too rapidly, plants fare badly: seeds fail to germinate; roots wither and die.
“Most plants are sensitive to extreme changes in soil temperature,” Dr Haruna, an Assistant Professor in the Department of Agribusiness and Agriscience at Middle Tennessee State University, told the American Society of Agronomy (ASA). “You don't want it to change too quickly because the plants can't cope with it."
Factors impacting soil’s temperature-buffering capacity
Several factors impact on soil’s ability to provide a buffer against changes in temperature, notes the ASA.
Soil compaction is one: in compacted soils, soil temperature is subject to rapid change as particles that are densely packed transfer heat or cold more quickly. Hauling weighty farm machinery results in soil-particle compaction.
Moisture also affects the temperature of soil: greater moisture content slows the rate at which soils heat up.
“Water generally has a high ability to buffer against temperature changes,” Haruna told the ASA. “So if soil has a high water content, it has a greater ability to protect the soil.”
What soil benefits can cover crops provide?
Typically, ‘cover’ or perennial crops are planted between rows of commercial crops to cover, thus protect exposed soil. Previous research has demonstrated that planting these crops can provide multiple benefits:
- they can reduce soil compaction;
- they shade the soil and help reduce soil-moisture evaporation;
- the roots add organic matter and keep soil spongy, helping it to retain water;
- the roots prevent soil erosion and reduce runoff.
The crop-soil temperature trials
Dr Haruna and his research team were keen to ascertain whether planting perennial biofuel and/or cover crops could also help soils protect themselves from temperature fluctuations, thereby improving the likelihood that crops planted in them would flourish.
In laboratory trials, the researchers grew various types of cover and perennial biofuel crops. Experimental treatments including:
- three cover crops: cereal rye (Secale cereale L.), hairy vetch (Vicia villosa subsp. villosa) and Austrian winter pea [Pisum sativum subsp. arvense (L.) Asch. & Graebn];
- two perennial biofuel crop treatments: giant miscanthus (Miscanthus × giganteus J.M. Greef & Deuter ex Hodkinson & Renvoize) and switchgrass (Panicum virgatum L.); and
- two ‘controls’: no cover crops; and row crops.
Soil sampling and analysis
Soil samples were collected from each trial plot at 10-centimetre depth increments, from the soil surface to a depth of 30 cm, then analysed in the lab to determine their respective capacities to regulate temperature.
The researchers analysed each sample to ascertain its:
- soil thermal properties, including thermal conductivity, volumetric heat capacity and thermal diffusivity;
- volumetric water content, at 0, -33, -100 and -300 kPa soil water matric potentials;
- bulk density; and
- soil organic carbon (SOC).
They found that the samples from soils used to grow perennial biofuel crops had 11% higher volumetric heat capacity at saturation, probably because they had significantly higher volumetric water content and SOC than those under conventional row-crop management.
Similarly, samples taken from the soils used to grow cover crops had 13% higher volumetric heat capacity at saturation, and significantly higher volumetric water content and SOC, than those under ‘no cover crop’ management.
Soil samples taken from the ‘row crop’ treatment plots were found to have significantly higher levels of thermal conductivity and diffusivity than those from plots used to grow perennial biofuel crops.
The results of these initial experimental trials suggest that both cover crops and perennial biofuel crops can change soil thermal properties by reducing thermal conductivity and thermal diffusivity and increasing volumetric heat capacity, at least under laboratory conditions.
The soil-data analysis revealed a number of beneficial changes in soils planted with cover crops or perennial biofuel crops. The key takeaways were that:
- both perennial biofuel and cover crops were able to help soils protect themselves against temperature extremes, essentially by slowing down the speed at which temperatures transmitted through soil;
- the roots of both cover and perennial biofuel crops were able to break up the soil, stopping soil molecules from clumping and thereby slowing the rate at which the soil heated up or cooled down;
- the roots of both types of crop also contributed organic matter to the soil, which moderated temperature fluctuations; and
- planting perennial biofuel and cover crops improved soil moisture retention, increasing its temperature-moderating capacity further.
Though Dr Haruna admitted it takes “more work, more financial investment, and more knowledge” to incorporate cover crops into a farming schedule, he’s keen to remind that “they can do much for soil health” – including, as his team’s research shows, helping protect valuable cash crops from the harm that can result from extreme temperature changes.
Dr Haruna and his team’s findings add weight to the increasingly convincing argument for investing in cover crops and perennial biofuel crops, particularly for those farming in regions prone to temperature variability.
“Climate change can cause temperature fluctuations and, if not curtailed, may affect crop productivity in the future,” he told the ASA. "And we need to buffer against these extreme changes within the soil."
Next, Dr Haruna hopes to conduct experimental field trials in which he and his team can collect more data and build on their initial findings.
Read the original article: Samuel I. Haruna et al, ‘Soil Thermal Properties Influenced by Perennial Biofuel and Cover Crop Management’. Soil Science Society of America Journal (2017). DOI: 10.2136/sssaj2016.10.0345
Contact Dr Samuel Haruna: Samuel.Haruna@mtsu.edu
Source: American Society of Agronomy (ASA)