Measuring Soil Water
There are several ways of estimating soil moisture from the simplest and subjective to advanced technology. A compromise between practicality and accuracy needs to be made. How close to actual field capacity is needed? For most situation in scheduling irrigation in potato fields, an accuracy of 5% is acceptable. The keys are how much water is in the soil and where in the soil profile, in other words, the depth of water penetration from irrigation and rain. Soil texture or the relative amounts of sand, silt and clay in the soil are the critical factors influencing the amount of available water to the plant (Table 1).
Table 1: Available water capacity of soils
| Soil Texture | Available water, in/ft |
| fine sand | 1.0 |
| loamy sand | 1.1 |
| sandy loam | 1.4 |
| fine sandy loam | 1.8 |
| silty clay loam | 1.8 |
| clay loam | 1.8 |
| loam | 2.0-2.5 |
| silt loam | 2.0-2.5 |
(Modified from Klocke and Fischbach, 1984, and Kranz et al., 1989)
Below is outlined three common methods for estimating soil moisture during the season: “appearance and feel,” tensiometer and moisture blocks.
1. Appearance and feel (Klocke and Fischbach, 1984).
Different soil types respond in different appearance and feel using this method so it is necessary to know the soil texture. This can be identified from county soil maps at Cooperative Extension offices.
A soil sample needs to be extracted from the potato root zone using either a soil probe for up to 24 inches (bottom of root zone), a soil auger or, at minimum, hand-dug to the depth around the growing tubers. Using the appearance and feel method requires some experience and observation. When first learning it, start using it in the spring after a heavy rainfall when the soil should be saturated (field capacity, FC) and take samples every so often as the field dries until it is quite dry. Record your observations per field for reference. Table 2 lists the appearance and feel that represent different field capacity ranges for various soils. Representative pictures are shown in the NebGuide.
2. Tensiometer (Kranz et al., 1989).
Tensiometers are a common and simple objective way to monitor available soil water. It measures the force that a potato root needs to overcome to absorb water from the soil, and consists of a tube with a porous ceramic tip at the bottom, a vacuum gauge near the top and a scaling cup. The tube is filled with water before inserting it into the soil. Upon insertion of the tensiometer, water can move between the tube and the soil through the ceramic tip. During the drying of the surrounding soil, water leaves the tube creating a vacuum measured by the gauge, calibrated in negative centibars (cb), until an equilibium is reached. The final gauge reading is reached when water no longer flows out of the tube and measures the actual force required to remove water from the soil at that time and location. Therefore, suction pressure is actually measured. The wetter the soil :: the lower the value (less cb needed to remove water; and visa versa. Plants extract water easier from sandier soils than heavier ones due to the larger air spaces in sandy soils. But, note that sandier soils also hold less water.
For an outline on preparation, procedures and data interpretation of tensiometer use, refer to Kranz et al., 1989.
3. Moisture Block (Kranz and Eisenhauer, 1989).
For fine-textured soils such as loam and silt loam, a moisture block is recommended by UNL for measuring soil moisture; they are not recommended for sandy soils because of the large soil particles of these soils. The moisture block is made up of gypsum and is about an inch in size. The gypsum is attached via a wire to a measuring meter. Being measured is the electrical resistance between two wires imbedded into the gypsum block. Water from the soil enters the block saturating the gypsum. Electrical resistance through the gypsum decreases allowing more electrical flow between the two imbedded wires. This electrical flow is measured by the meter. A high reading indicates a moist soil while a low one indicates a dry soil. For details on preparation and use of moisture blocks, refer to Kranz and Eisenhauer, 1989).