Sufficient levels of soil moisture are an important condition for proper plant formation and high crop yields. For the plant, water serves not only as an agent of moisture restoration, but also as a temperature regulator. In the process of thermoregulation, the plant evaporates up to 99% of all water obtained, utilizing only 0.2% to 0.5% for the formation of vegetative mass. Therefore it is easy to understand that the plant has different needs for moisture depending on the weather conditions and growth stages.
A specific amount of water comes in the form of condensed water vapor and depends on the climate, topography, type of vegetation, as well as the hydrogeological conditions. The water layer is measured in mm (millimeters): 1 mm of rainfall per 1 ha (hectare) corresponds to 10 tons of water. As a result a farmer’s key task is to maximize the accumulation, storage, and efficient usage of moisture. Of course, it is still challenging and thus requires extra effort and labor.
The Importance of Soil Moisture for Efficient Farming
None of the physical processes in the atmosphere and environment can occur without water, here on planet Earth. Soil moisture depends on the amount of precipitation, intensity of water consumption by plants, air temperature, among other factors. Ample moisture levels are of high importance to yields, thus, plants will not grow and develop with inadequate soil moisture. Water has other purposes, which are as follows:
- Soil moisture levels affect air content, salinity, and the presence of toxic substances
- Regulates soil structure, ductility, and density
- Influences soil temperature and heat capacity
- Prevents soil from weathering
- Determines the readiness of fields to be worked upon
Key Parameters of Soil Moisture
The level of water in the soil is expressed as the ratio of the amount of water to the weight of dry soil (or as a percentage). In 2010, the World Meteorological Organization added Soil Moisture to the list of 50 Essential Climatic Variables that are recommended for systematic observation.
Soil moisture content is a value that determines the amount of water in a certain known amount of soil; it can be expressed as a percentage, water by the weight or volume of soil, or inches of water per foot of soil.
Soil moisture potential or soil moisture tension shows the degree to which water clings to the soil. It is expressed in units of pressure called bars. Generally, the drier the soil, the more water it will absorb.
Plant available water (PAW) is the amount of water in the soil available to the plant at a point in time. Available water is the difference between the maximum amount of water the soil can hold and the wilting point where the plant can no longer extract water from the soil. It is expressed in inches of available water per foot of soil.
The relation between content and potential is not universal and depends on the characteristics of the local soil, such as soil density and texture. Based on PAW value, the farmer can work out the irrigation plan. But how can one measure the moisture in soil?
Ways to Determine the Sufficient Level of Soil Moisture
Available methods for soil moisture determination are based on the preliminary selection of soil samples, followed by their analysis directly in the field or in the laboratory.
Soil moisture measurement approaches are typically categorized into direct, indirect, and remote. Direct methods include the extraction of water from a soil sample by means of evaporation, washing, and chemical reaction. The soil moisture calculation is based on the weight of water extracted and the lеvel of dryness. Indirect methods include measuring soil characteristics depending on the water content. They can also measure the characteristics of a particular soil object, usually like that of a porous absorber.
Unfortunately, the relationship between the physical and chemical properties of soil and soil moisture has not been fully understood. Remote soil moisture measurements are based on satellite data that use the reflection of electromagnetic radiation of a specific spectrum from the soil surface.
The most popular direct methods are gravimetric and volumetric.
Gravimetric soil water content (%) = [mass of moist soil (g) − mass of oven-dried soil (g)/mass of oven-dried soil (g)] × 100;
Volumetric soil water content (%) = [volume of water (cm3)/volume of soil (cm3)] × 100;
Other Methods for Calculating Soil Moisture Include:
- Radioactive – a measurement of radioactive substances on the ground
- Electric – presupposes determination of soil resistance, conductivity, inductance, as well as capacity
- Tensometric – based on the difference in the voltage of water between the phase boundaries
- Optical – characterized by the reflectivity of light fluxes
- Express methods – mainly organoleptic
Soil Moisture in the Context of Field Monitoring
The ability to predict soil moisture helps to efficiently plan field works at any stage of crop growth that include the following:
- Soil Moisture Level Defines the Appropriate Time for Sowing
Farmers should carefully evaluate the water level before beginning to sow. Optimal moisture level depends on the crop, soil type, region, and other variables. For example, rice grows well in wetlands but the highland crops like wheat, mustard, potato, pulses, etc. are sensitive to excessive water levels and cannot survive under waterlogging for long durations.
It may seem that canola is an simple culture, but it’s sowing dates must be carefully planned. Since rapeseed is a moisture-loving crop, the amount of rainfall during the growing season should be at least 280-300 mm. Although, the seedlings will not root without moisture, the depth of sowing should not be increased in order to reach moisture. When the effective moisture is expected to be at a depth of at least 50 mm, it is better to wait for rain or to sow the seeds at a depth of less than 50 mm in the hope of rain. However, if the soil is sufficiently warm, in-depth sowing of rapeseed can be carried out. It allows sowing the seeds in the moist soil layer.
With in-depth canola sowing, the seeding rate should be increased by at least 10%. In any case, rain is needed soon for sowing to be successful.
- Drought Forecasting and Monitoring
Atmospheric drought, i.e. a hot period without rains with air humidity of less than 30-35%, is accompanied by soil drought. It is manifested as a decrease in moisture reserves in the soil to wilting moisture, soil overheating, and an increase in the concentration of soil solution to toxic levels.
By using the soil moisture index (SMI) obtained through satellite monitoring, farmers may ensure the crops are successful. This index was developed by Bergman at the National Meteorological service in the US in the mid-1980s as a way of assessing aridity conditions on a global scale. It characterizes the degree of aridity or saturation of the soil demonstrating how a lack of soil moisture affects crop productivity.
- Machinery Impact on Soil at High Moisture Level
Efficient farming practices presuppose avoiding tractor trafficking and cultivating or sowing on excessively wet soil to prevent compaction and other structural damage. Farmer assessment may be visual when they evaluate the water saturation. A handful of soil is rolled into a ball in the farmer’s hands to ensure there is a sufficient amount of water. As an alternative, more technologically advanced methods can be used for the estimation of soil moisture; Crop Monitoring is one of them.
- The Significance of Soil Moisture Monitoring
Crop monitoring is integral in agriculture as it facilitates all of the related stages in the process of farming. Information about each field zone, the state of crops, quality of operations, weather conditions, and equipment operation allows specialists to make informed and reliable decisions. For farmers to obtain sufficient information on the above-listed aspects, remote sensing is an effective tool.
Soil moisture mapping helps with obtaining all of the necessary data with just a few clicks. To accomplish this, save the field and wait for the map to be built. As a result, you will get a map with a legend, which describes the values of each pixel, as well as a graph (a curve of moisture to soil ratio). The user then can analyze this curve at different depths (soil layers), see flood zones (in cavings), process 5-year historical data in various regions Thus, farmers can make comparisons. The field map looks like this:
Soil moisture control and prediction is critical for optimum crop growth. Accurate monitoring of soil moisture enables efficient control of nutrients and other inputs. Modern crop monitoring technology provides automated tracking of soil moisture. The obtained data assists in maintaining necessary soil moisture throughout irrigation, and as such, leads to efficient crop results.
How Soil Moisture is used in Monitoring Crops
- Determine the sowing dates, e.g. the rapeseeds will not be sown (the dates of sowing are ignored) in dry (if insufficiently moisturized) soil
- Forecast and monitor the periods of droughts (the indicators of atmospheric humidity along with soil moisture)
- Farmers do not put equipment into the field as it might get stuck in the mud if the soil is excessively moist
- Coverage – the level of water in the soil is indicated in the moisture index. When the moisture is below a particular index, insurers must be warned