How to Determine Soil Moisture: Methods and Optimal Values

The harvest depends directly on how accurately an agronomist or farmer understands the land’s moisture state. Soil moisture is one of the key indicators, without which it is impossible to properly manage irrigation and crop nutrition. And, so that crops do not suffer from drought or rot due to waterlogging, it is worth understanding the specifics of moisture.

What is soil moisture?

In general, this term refers to the amount of water contained in the ground at a specific point in time. More precisely, soil moisture is an indicator expressed as a percentage of the mass or volume of a sample. It is divided into absolute and relative moisture depending on the calculation method.

It reflects the upper layers’ capacity to provide water to plants. The terms “moisture capacity” and “water-holding capacity” are also encountered – they focus on the soil’s ability to retain water rather than its current water content. That is why determining soil moisture is a mandatory element of competent agronomic control.

Types of soil moisture

There are four main types of soil moisture, each of which has its own role for plants:

• Available – the moisture that plant roots can actually absorb.
• Capillary – held in the narrow pores of the upper layers due to surface tension forces.
• Gravitational – excess moisture that freely flows down under the influence of gravity.
• Hygroscopic – tightly bound to soil particles and unavailable to plants.

This is exactly the principle by which an agricultural laboratory operates when interpreting analysis results.

What factors affect soil moisture?

The moisture indicator is not a constant value. It is influenced by several groups of factors at once:

• The first – climatic conditions of the fields: air temperature, precipitation amount and pattern, and evaporation level.
• The second – terrain relief. On slopes, water runs off quickly, while in lowlands it accumulates.
• The third – agrotechnical measures. Mulching slows down evaporation, and high-quality drainage prevents waterlogging.

Soil structure also matters – a granular composition retains water better than a compacted or silty one. That is why soils enriched with organic matter tolerate dry periods more easily.

Optimal and threshold values of soil moisture

When a specific method of determining soil moisture is used, the soil type and the crop being grown are considered. The moisture norm for sandy soils is 10-20%; for loamy soils, 20-30%; and for clay soils, it can reach 35% or higher. Depending on the field’s scale, available equipment, and the required accuracy of the results, the appropriate norm is selected.

How to determine soil moisture: main methods

Today, there are several proven approaches. Soil moisture measurement methods differ in accuracy, cost, and ease of use.

Gravimetric method

This is a classic of laboratory analysis. A soil sample is weighed before and after drying in an oven at about 105°C. The difference between the initial and final mass is the amount of water that evaporated. The method is considered the gold standard for accuracy, but it takes time. Laboratory-determined field moisture provides the most reliable result, although portable devices are more commonly used in field conditions.

Tensiometric method

A tensiometer measures not the amount of water, but the force with which it is held in the soil, that is, the matric potential. The device is buried in the ground, and in real time, it shows how easily plants can absorb water.

Electromagnetic methods (TDR, FDR, capacitive)

TDR (Time-Domain Reflectometry) and FDR (Frequency-Domain Reflectometry) measure the soil’s dielectric permittivity. Capacitive sensors work on a similar principle but are cheaper and easier to maintain. All three varieties are popular in precision agriculture due to their speed and the possibility of continuous monitoring.

Microwave method

Microwave sensors measure how soil particles absorb or reflect electromagnetic radiation at a specific frequency. The method is effective for large agricultural areas but is technically complex and expensive.

Infrared spectroscopy and remote sensing

If the question arises of how to check soil moisture across hundreds of hectares at once, the answer lies in infrared spectroscopy and satellite remote sensing. The method is based on the analysis of the reflection of infrared radiation from soil particles: wet soil and dry soil reflect rays differently.

Neutron scattering

The method is based on the physical interaction of neutrons with hydrogen atoms in water. A special probe lowered into the soil emits neutrons and measures their reflection. The more moisture, the more hydrogen, the more neutrons return to the sensor.

Thermal methods

The thermal conductivity of soil depends directly on water content: wet soil conducts heat better than dry soil. Thermal sensors measure this characteristic and convert it into a moisture percentage.

Soil moisture – calculation formula

Knowing the principles of gravimetric analysis makes it easier to understand and verify the calculations’ logic. This method served as the basis for the reference mathematical model. To measure soil moisture using the gravimetric method, you only need to know the sample’s mass before and after drying. The classic formula for determining moisture looks like this:

W = (M₁ − M₂) / M₂ × 100%

where W is the moisture in percent, M₁ is the mass of the wet sample, and M₂ is the mass of the dried sample.

The result depends on the accuracy of weighing at each stage. Even a slight error will give false data. Therefore, the soil moisture formula is effective only in combination with high-quality laboratory equipment. If you need to quickly assess soil moisture at home without a device, you can use a simple test: squeeze a handful of earth in your palm – the moisture will leave a mark on the paper.

What to do if the moisture is insufficient or excessive?

The measurement result is not the end of the work, but its beginning. Soil analysis provides guidelines for specific agrotechnical actions.

With a moisture deficit, irrigation is the first priority. Drip irrigation is the most effective option: it delivers water directly to the root system, reducing evaporation. With excess moisture, the main task is to drain the water and improve aeration. Drainage systems (open ditches or closed pipes) lower the groundwater level.

Methods for determining soil moisture are tools, not ends in themselves. Regular monitoring lets you react in time rather than trying to save the crop after the fact. The accumulated data reveal another advantage: predicting irrigation needs and detecting stress even before plants react externally.