The temperature of soil is a significant parameter in agriculture since proper warmth at proper depths not only conditions efficient plant growing. It also determines the time for sowing, due to the importance of soil temperature for seed germination.
Soil temperature regimes vary monthly, seasonally, and daily, and since the main source of earth heating is solar radiation, farmers have to manage the hottest peaks during the day, with ultimate sun activity. Studying soil temperatures, earth heat fluxes, and, in particular, correlations between wet and dry lands and their heat absorbing capacities helps agriculturalists to productively schedule field events.
Factors Affecting Soil Temperature
Soil temperature is not a universal value and depends on a number of constituents, including its color, slope, vegetation cover, compaction, moisture, and naturally, the sunlight available.
Understanding physical and chemical properties, for example, correlations between soil moisture and its temperature, allows successful yield forecasting.
- Amount of solar radiation. It is the main source of land heating. This is why soil temperature at different depths varies, and the upper layers are usually warmer than deeper ones.
- Season and atmospheric conditions. The distribution of solar energy depends on the season and absence/presence of sunlight, clouds, and air temperature. Naturally, the warmer the day, the warmer the earth is.
Soil color. Physics proves that darker objects absorb more sunlight, and earth is no exception. So, the darker it is, the faster it warms up.
- Soil cover. Bare lands heat faster while any additional layer on the earth that prevents evaporation, reduces its temperature. It refers to mulch, cover crops, crop residue, vegetation canopy, etc.
- Organic matter. It raises water retention and darkens the earth. For these two reasons, organic matter content also increases the temperature of soil.
- Angle of slope. Solar radiation penetrates the ground more intensively when the angle is around 90 degrees and disseminates more if the field is on a hill. To control the intensity of solar radiation on hilly farmlands, terrace farming is great agricultural practice.
- Compost and manure. Decomposition is a chemical process with a certain volume released. In this regard, it raises soil temperature.
- Soil moisture. Wet soils conduct heat vertically better than dry ones. It means that dry earth heats up faster during the daytime and cools down faster at night. However, water content may affect double ways depending on the earth’s compaction and density – either evaporating from the surface or dissipating in the profile underneath. Cold precipitations cool the earth.
- Soil composition and texture. Clay usually shows higher heat capacity compared to sand with equal water content and density. However, sand heats quicker than clay due to less volume of water (lower porosity). Thermal conductivity increases in finer grounds. Nonetheless, factors that affect soil temperature are complex and depend on the way they match. For example, water produces a reversed effect on thermal conductivity.
Why Is Soil Temperature Important?
Temperatures affect biological, chemical, and physical features of soils either decreasing or increasing them. This is why soil temperature importance is the object of keen studies in many scientific fields, especially in biology, physics, chemistry, ecology, agriculture, and economics.
Biological Properties
The average soil temperatures for bioactivity range from 50 to 75F. These values are favorable for normal life functions of earth biota that ensure proper organic matter decomposition, increased nitrogen mineralization, uptake of soluble substances, and metabolism. On the contrary, conditions next to freezing slow down activities of soil-dwelling microorganisms, while macroorganisms can’t survive below freezing points at all. Decreased microbial activities are the reason for reduced organic matter decomposition and its excessive accumulation.
Chemical Properties
High soil temperature regimes show higher cation exchange capacity due to decomposed organic matter. The warmer the soil, the more water-soluble phosphorus it contains for plants. Vice versa, low-heated earth is poor in phosphorus available for vegetation. As to pH-levels, the acidity rises with a higher degree as well due to organic acid denaturation.
Physical Properties
High soil temperatures induce dehydration of clay and cracking of sand particles, eventually reducing their content and increasing the concentration of silt. The warmer is the earth, the more carbon dioxide it releases. Heat is the reason for land cracking due to evaporation and thus, insufficient water penetration to the ground profile.
Effects Of Soil Temperature On Plant Growth
Soil temperature and plant growth strongly relate. Warmth induces vegetation development in terms of water and nutrient uptake and overall plant growth. Low temperatures inhibit water uptakes due to lower water viscosity and slow down the process of photosynthesis.
Besides, lack of warmth is an unfavorable condition for the activities of earth-dwelling microorganisms since their low metabolism means low nutrient release and also its low dissolution. So, the cooler the land is, the fewer nutrients and water plants can get.
As for roots and shoots growth, cold conditions hinder cell reduplications and thus slow down the overall growth. It refers both to cool air and earth.
Ideal Soil Temperature For Planting
It becomes clear that when the ground is not warm enough, plants cannot develop properly since the biological and chemical processes in the ground are not intense enough. Furthermore, they are impossible when temperatures reach freezing points.
Taking this into consideration, it becomes vital to know optimum values for growing particular agro cultures and secure ultimately beneficial conditions for their germination and development. The analysis of historical soil temperature data for a specific region, monitoring the current state of things, and soil temperature and weather forecasting are the key aspects contributing to success.
Either too low or too high degrees kill both soil organisms and plants. In particular, agricultures develop slowly at 90F, while 140F is critical as soil bacteria can’t survive the heat. At 100F, vegetation cannot absorb enough moisture since as much as 85% is lost due to evaporation and transpiration. Irrigation in exceeding atmospheric temperatures is extremely undesired as most water inputs would turn into waste due to extreme evaporation rates. Besides, refracted water drops acting as magnifying glasses will burn vegetation.
In the extreme dependence of harvest prognosis on soil temperature, the secret of high yields to great extent hides in the successful match of cultures planted, time of their seeding, and further weather conditions to ensure their sufficient performance.
For example, the minimum planting temperature for spring wheat is 37F, soybean – 59F, spring canola and sugar beat – 50F, sunflower and millet – 60F. Dry bean is the most demanding in this regard, requiring 70F-warm ground for successful germination and rooting.
As to soil temperature for growing vegetables, farmers should remember that tomatoes and cucumbers need 60F, while sweet corn will benefit from at least 65F. The last ones to sow are watermelons, peppers, and okra (70F).
When deciding the ideal planting time, it is also important not to put seeds too deep to reach enough moisturized layers since shallow seeding means quick sprouts. Also, with quick sprouts, farmers not only save time but get strong plants vigorously competing with weeds.
How To Measure Soil Temperature
Once farmers noticed the correlations between soil temperatures for planting and cultures’ productivity, they started to follow certain seeding rules waiting for the earth to get warm enough.
The first primitive method was manual (through palpation). Later, special thermometers and in-field soil temperature sensors were introduced. The recent and the most convenient scientific finding to determine soil temperature is remote sensing and satellite monitoring. These soil temperature measurement methods are based on assessing the reflectance properties of our planet’s surface either by active or passive remote sensing.
Online platforms made a huge step forward in measuring soil temperature, allowing farmland owners to keep ahead of the game at affordable costs when they can have a certain idea of what is happening on their fields even without getting there personally. The information is also useful for other agribusiness stakeholders, e.g., insurance agents and traders.
EOSDA Crop Monitoring And Soil Temperature
Since most plants can’t efficiently grow in the cool earth, soil temperature monitoring is a significant aspect of the farming business. Its assessment and soil temperature forecast are possible with the analysis of vegetation indices provided by online tools like EOSDA Crop Monitoring.
Vegetation cover cools the earth, and this effect allows determining soil temperature via inspecting vegetation in the fields. In this context, EOSDA Crop Monitoring is an efficient tool that elaborates four vegetation indices, namely NDVI, MSAVI, NDRE, ReCl. Each index is best to apply at particular stages of crop development. Reports derived can help agriculturalists in decision making.
Another important correlation is the one between soil moisture and water content in plants (their leaves, buds, stems), assessed with the NDMI index. The normalized differentiated moisture index is available at EOSDA Crop Monitoring and shows if the water content is sufficient for proper plant development. As proper water saturation is possible under certain temperature conditions (at low degrees it is low), the water content in vegetation allows judging about soil warmth/temperature as well.
If irrigation/moisture is abundant but plants suffer from stress due to water deficiency, it means that the earth’s temperature is still critically low. A decrease in soil temperature causes a decrease in water uptake. However, optimal warmth for root and shoot growth are different and vary not only in different plants but at different growth stages. This is the case when different vegetation indices are useful.
Furthermore, since the temperature of soil strongly depends on weather, and solar radiation, clouds, precipitations in particular, knowing them in advance becomes vitally important. EOSDA Crop Monitoring provides up to 14 days forecasts as well as historical weather, allowing farmers to schedule their field events and prepare optimal growth conditions for crops.
This way online software can provide precious information for the most accurate planning and estimations.
Vasyl Cherlinka has over 30 years of experience in agronomy and pedology (soil science). He is a Doctor of Biosciences with a specialization in soil science.
Dr. Cherlinka attended the engineering college in Ukraine (1989-1993), went on to deepen his expertise in agrochemistry and agronomy in the Chernivtsi National University in the specialty, “Agrochemistry and soil science”.
In 2001, he successfully defended a thesis, “Substantiation of Agroecological Conformity of Models of Soil Fertility and its Factors to the Requirements of Field Cultures” and obtained the degree of Biosciences Candidate with a special emphasis on soil science from the NSC “Institute for Soil Science and Agrochemistry Research named after O.N. Sokolovsky”.
In 2019, Dr. Cherlinka successfully defended a thesis, “Digital Elevation Models in Soil Science: Theoretical and Methodological Foundations and Practical Use” and obtained the PhD in Biosciences with a specialization in soil science.
Vasyl is married, has two children (son and daughter). He has a lifelong passion for sports (he’s a candidate for Master of Sports of Ukraine in powerlifting and has even taken part in Strongman competitions).
Since 2018, Dr. Cherlinka has been advising EOSDA on problems in soil science, agronomy, and agrochemistry.
