Soil Temperature As A Factor Of Crops Development

What Is Soil Temperature, And Why Is It Important?

Soil temperature refers to the measurement of the ground’s inherent warmth. It controls the chemistry and biology of the ground and the atmospheric-ground gas exchange. You may also encounter the term “soil surface temperature,” which is the measurement of warmth/coldness in the top 4 inches (10 cm) of the ground. Seasonal and daily changes in the land’s warmth degrees may cause variations in radiant energy and energy shifts at the ground surface.

The importance of soil temperature in agriculture is due to its impact on the effectiveness of many farming procedures. The soil solarization process relies on the ground’s warmth and moisture levels. The success of fertilizing and weed management also heavily depends on the ground’s thermal conditions.

The ground’s warmth affects various plant processes, such as nutrient and water uptake and root growth. It has also been shown that nitrogen uptake varies both in quantity and form depending on the thermal conditions of the ground ^{Chatterjee, A., de Jesus, A. F., Goyal, D., Sigdel, S., Cihacek, L. J., Farmaha, B. S., Jagadamma, S., Sharma, L., Long, D. S. (2020). Temperature Sensitivity of Nitrogen Dynamics of Agricultural Soils of the United States. Open Journal of Soil Science, 10(7), 298-305. https://doi.org/10.4236/ojss.2020.107016.}. Therefore, the ground’s temperature is even more influential on plant growth than air warmth.

Factors Affecting Soil Temperature

Soil temperature is not a universal value and depends on several constituents, including its color, slope, vegetation cover, compaction, moisture, and, naturally, the amount of sunlight available.

Understanding physical and chemical properties, such as correlations between soil moisture and temperature, allows successful crop yield forecasting.

• Amount of solar radiation. It is the primary source of land heating. This is why ground warmth at different depths varies, and the upper layers are usually warmer than the deeper ones.
• Season and atmospheric conditions. The distribution of solar energy depends on the season and the absence/presence of sunlight, clouds, and air warmth/coldness. Naturally, the warmer the day is, the warmer the earth is.
• Soil color. Physics proves that darker objects absorb more sunlight, and the earth is no exception. So, the darker it is, the faster it warms up.
• Ground cover. Bare lands heat up faster, while any additional layer on the earth that prevents evaporation cools them down ^{Mark Longstroth. Bare ground is warmer (2008) Michigan State University Extension.}. It refers to mulch, cover crops, crop residue, vegetation canopy, etc.
• Organic matter. It increases water retention and darkens the earth. For these two reasons, organic matter content also increases the temperature of the 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. Terrace farming is a great agricultural practice to control the intensity of solar radiation on hilly farmlands.
• Compost and manure. Decomposition is a chemical process with a certain volume release. In this regard, it raises soil temperature.
• Soil moisture. Wet grounds conduct heat vertically better than dry ones ^{Daniel E. Haskell, David J. Flaspohler, Christopher R. Webster, and Michael W. Meyer. Variation in Soil Temperature, Moisture, and Plant Growth with the Addition of Downed Woody Material on Lakeshore Restoration Sites. 2012. Restoration Ecology Vol. 20, No. 1, pp. 113–121.}. It means dry earth heats up faster during the day and cools down faster at night. However, the 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 precipitation cools the earth.
• Soil composition and texture. Clay usually has a higher heat capacity than sand, with equal water content and density ^{Abu-Hamdeh, Nidal. (2003). Thermal Properties of Soils as affected by Density and Water Content. Biosystems Engineering. 86. 97-102. 10.1016/S1537-5110(03)00112-0.}. However, sand heats more quickly than clay due to the lower volume of water (lower porosity). Thermal conductivity increases in finer grounds. Nonetheless, factors that influence soil temperature are complex and depend on how they interact. For example, water produces a reverse effect on thermal conductivity.

Effects Of Soil Temperature On Plant Growth

The influence of soil temperature on plant growth is related to the fact that warmth promotes crop development through increased water and nutrient uptake, while cold inhibits water uptake due to lower water viscosity and slows down the process of photosynthesis.

Furthermore, a lack of warmth is unfavorable for earth-dwelling microorganisms since their metabolism slows down, leading to less nutrient release and less nutrient dissolution. Thereby, plant growth is stunted in colder climates. As for shoot growth, the cold ground and air are slowing it down because of inhibited cell duplication.

Even though the warmer ground is beneficial for root development, excessive heat reduces land quality by speeding up the decomposition of organic matter and the evaporation of moisture. So it’s crucial to keep the ground at an optimal level of warmth for healthy plant growth.

Impact Of Soil Temperature On Soil Properties

The ground’s thermal conditions can either decrease or increase the biological, chemical, and physical characteristics of various types of soil. To effectively control thermal conditions in light of your objectives, you must have a firm grasp of these influences on the following properties.

Biological Properties

The average soil temperatures for bioactivity range from 50 to 75°F (10-24°C). These values are favorable for the normal life functions of the ground biota that ensure proper organic matter decomposition, increased nitrogen mineralization, uptake of soluble substances, and metabolism. On the contrary, conditions near freezing slow down the activities of soil-dwelling microorganisms, while macroorganisms can’t survive below freezing points. Decreased microbial activities are the reason for reduced organic matter decomposition and its excessive accumulation.

Chemical Properties

Due to decomposed organic matter, high soil temperature regimes show higher cation exchange capacity. The warmer the ground, 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 to a greater degree due to organic acid denaturation.

Physical Properties

Increased soil temperatures induce the dehydration of clay and cracking of sand particles, eventually reducing their content and increasing the concentration of silt. The warmer the earth is, the more carbon dioxide it releases. Heat is the reason for land cracking due to evaporation and, thus, insufficient water penetration into the ground profile.

Ideal Soil Temperature For Planting

We already know that plants cannot grow normally at low temperatures because the intensity of the biological and chemical processes in the ground is reduced. Furthermore, these processes cease when thermal readings reach freezing points.

Considering this, it becomes vital to know the ideal soil temperature for plant growth and certain beneficial conditions for crop germination and development. The key aspects contributing to success are the analysis of historical soil temperature data for a specific region, monitoring the current thermal conditions and vegetation, and weather forecasting.

In the extreme dependence of harvest prognosis on soil temperature ranges, the secret of high yields lies, to a great extent, in the successful match of cultures planted, the timing of their seeding, and further weather conditions to ensure their sufficient performance ^{Eric Anderson, Bruce MacKellar. Soil temperatures and planting timing – Part 1 of 3: Optimal soil temperature for planting. 2019. Michigan State University Extension.}.

Keep in mind that the optimal ground’s warmth varies depending on the stage of plant growth. For instance, pre-emergent herbicides work best under 50-55°F (10-13°C). The optimum soil temperature for seed germination ranges between 68 and 86°F (20-30°C). Additionally, various plant species have different thermal requirements for planting and throughout their development.

The minimum soil temperatures for planting common crops are the following:

• spring wheat – 37°F (3°C);
• soybeans – 59°F (15°C);
• spring canola and sugar beat – 50°F (10°C);
• sunflower and millet – 60°F (16°C);
• dry beans are the most demanding, requiring 70°F (21°C) for their successful germination and rooting.

The optimal soil temperature for growing vegetables varies from 65 to 75°F (18-24°C). For example:

• tomatoes and cucumbers – 60°F (16°C);
• sweet corn – 65°F (18°C);
• watermelons, peppers, and okra are the last ones to sow at 70°F (21°C).

When deciding on 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 also get strong plants vigorously competing with weeds.

How To Measure Soil Temperature

Once farmers noticed the correlation between the ground’s temperature while planting and crop productivity, they started to follow certain seeding rules, such as waiting for the earth to get warm enough.

The first primitive method of measuring the ground’s warmth was manual (through palpation). Subsequently, soil temperature sensors and thermometers, accurate over a wide thermal range, were developed. To ensure the ground is warm enough for your crops, take a soil temperature probe at night or in the early morning.

Remote sensing and satellite monitoring are the latest and most convenient scientific findings to define the ground’s warmth. 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 checking soil temperature, allowing farmland owners to keep ahead of the game at affordable costs when they have an idea of what is happening in their fields, even without getting there personally. Soil temperature data is also useful for other agribusiness stakeholders, e.g., insurance agents and traders.

EOSDA Crop Monitoring And Soil Temperature Measuring

Since most plants can’t efficiently grow in the cool ground, soil temperature monitoring is a significant aspect of farming. Its assessment and soil temperature forecast are possible by analyzing vegetation indices provided by online tools like EOSDA Crop Monitoring.

Because of the earth-cooling effect of vegetation cover, we can measure soil temperature using GIS data that enables inspection of vegetation in the fields. In this context, EOSDA Crop Monitoring is an efficient tool that elaborates four vegetation indices, namely NDVI, MSAVI, NDRE, and ReCl. Each index is best applied 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, and stems), assessed with the NDMI index. The normalized differentiated moisture index is available at EOSDA Crop Monitoring and shows whether water content is sufficient for proper plant development. As suitable water saturation is possible under certain thermal conditions (at low degrees, it decreases), the vegetation’s water content allows the ground’s warmth/temperature to be judged.

If irrigation or moisture is abundant, but plants suffer from stress due to water deficiency, it means that the earth’s warmth degree is still critically low. A decrease in soil temperature causes a decrease in water uptake. However, optimal warmth for root and shoot growth is different and varies not only in different plants but at different growth stages. This is the case when different vegetation indices are useful.

Knowing the expected levels of solar radiation, cloud cover, and precipitation is also crucial because weather conditions impact the ground’s warmth. EOSDA Crop Monitoring provides up to 14-day forecasts as well as historical weather, allowing farmers to schedule their field activities and prepare optimal growth conditions for crops.

This way, online software can provide valuable information for the most accurate planning and estimations. With access to soil temperature historical data and current readings, you can create a thermal regime best suited for crop growth and development.