Soil moisture can be interpreted and measured in several different ways.
Here you can read just the basic facts, but I recommend you read more at the end.
Soil moisture types and measurement methods
The gravimetric soil moisture value gives the mass of water in a soil sample of a given mass. The ratio of water to soil sample expressed as a percentage is the gravimetric soil moisture.
For example, if 100 g of soil sample contains 20 g of water, the gravimetric water content is 20%.
Measurement:the weight of the unfluffed soil sample is measured and the water content is removed in an oven. The weight of the dried soil sample is measured. The difference in mass between the wet and dry soil sample gives the mass of water that was in the original sample. It is slow enough for daily use, but gives a simple and accurate reading.
1. Figure 1 Steps for gravimetric soil moisture measurement
Volumetric soil moisture is the ratio of the volume of water in a soil sample per unit volume. For example, if you take a soil sample with a volume of one litre (1000 ml) and 250 ml of water, the volumetric water content is 25%.
Volumetric moisture is also used to measure the thickness of the water layer per unit thickness of soil (10 cm, 1 m). For example, at a volumetric soil moisture content of 25%, there is 2.5 cm of water in a 10 cm layer of soil and 25 cm of water in a 1 m thick layer of soil.
2. Figure Eijkelkamp soil sampler
Volumetric soil moisture measurement requires a soil sampler of known volume.
If the sampling device is used to take an unfluffed soil sample, volumetric soil moisture measurement can be performed together with gravimetric soil moisture testing. After the test is completed, both the gravimetric and volumetric soil moisture content of the sample can be calculated from the volume of the sampler and the weight of the wet and dried sample.
The fastest and most accurate way to measure volumetric soil moisture for practical use is with electromagnetic soil sensors(TDR, FDR, ADR). I will write about this in a later blog.
Soil moisture potential is a slightly more difficult concept to understand, but it is the most accurate way of expressing the free soil moisture available to plants.
There are several names for this concept: matrix potential, soil moisture tension, they all mean the same thing.
The word ‘potential’ here refers to the force that the root needs to exert in order to take up water from the soil. For each crop species, the root suction force required for stress-free water uptake gives the value of the soil moisture potential. The suction force (potential) exerted by a plant to absorb moisture from the soil is expressed in units of pressure (kPa, bar, mbar, cb = centibar). Since this potential value represents a suction, it is negative. The water moves from the positive to the negative potential location.
3. Figure
Plant root system soil moisture potential
Examples:
– if the root is in clean water, the potential is 0 kPa,
– in mineral-saturated (salt) water, the potential is negative, so it costs energy to absorb water,
– the soil particles attract water, a force must be created that pulls the water away from the soil,
– a 3. in the case of the root shown in Figure
100kPa = 1bar = 1000mbar = 100cb of suction force to absorb water.
Optimum soil moisture potential of some plant species:
| Plant species | Suction tension (kPa) | Water demand |
|---|---|---|
| Lucerna | 80-150 | * |
| Cabbage | 60-70 | ** |
| Cantaloupe | 35-40 | *** |
| Carrots | 55-65 | ** |
| Cauliflower | 60-70 | ** |
| Zeller | 20-30 | **** |
| Citrus varieties | 50-70 | ** |
| Cotton | 100-120 | * |
| Sweetcorn | 50-80 | ** |
| Wheat, Barley, Rye, etc. | ||
| Growth phase | 40-50 | *** |
| Maturing phase | 70-80 | ** |
| Lettuce salad | 40-60 | *** |
| Onion | 45-65 | ** |
| Potatoes | 30-50 | *** |
| Tomatoes | 60-150 | * |
Source: Hanson et al. (2000)
The soil moisture water potential value that the plant’s roots need to develop to absorb water can be measured with a tensiometer. I will write about the operation and types of tensiometers in an upcoming blog.
Soil moisture measurement based on electrical resistance measurement. This method can be fully USED for measuring soil moisture. I have included it in the blog so that it is easy to identify what NOT TO USE to measure soil moisture.
It can be misleading to find these sensors in some seemingly specialist publications. The low price may be attractive for the user, but the idea of using such devices should be immediately discarded 
because they lead to completely erroneous measurements and wrong interventions. The measurement result of these sensors depends on:
the amount of water in the soil,
the salinity of the water,
the amount of ions in the soil,
organic matter in the soil.
NOT MEASURING SUSTAINABILITY, something else, which depends on several factors!
In addition, they corrode very quickly due to direct metallic ground contact and the galvanising effect of a DC voltage measuring signal.
Similar sites:
http://talajnedvesseg.hu/
http://moe.hu/wp-content/uploads/Talajnedvess%C3%A9g-m%C3%A9r%C3%A9s-%C3%A9s-az-okos-vez%C3%A9rl%C5%91k_opt.pdf
https://agrarium7.hu/cikkek/1186-miert-fontos-a-talajnedvesseg-merese
http://www.hidrologia.hu/vandorgyules/34/dolgozatok/word/aszaly04_fiala_karoly.pdf
http://vpf.vizugy.hu/reg/ovf/doc/Az_aszalykezeles_modszertana_Fiala.pdf
https://www.itenviro.hu/forgalmazott-termekek/talaj-tulajdonsagok-talajminoseg/talajnedvesseg-meres
https://www.tankonyvtar.hu/hu/tartalom/tamop412A/2010-0019_Merestechnika/ch16.html
Next blog: Function and types of tensiometers
Article written by: Csaba Tóth, email: toth.csaba(at)tmarkt.hu
