Slightly dark, 1.2 forms weak ball. 1.2 Very slight color will flatten out rather due to moisture. than crumble. Lightly colored, 1.4 small clods crumble Slightly dark, clods fairly easily. 1.6 1.6 Slight color due to moisture, small 1.8 clods are hard. 2.0 Field Method of Approximating Soil Moisture for Irrigation, from Am.
Matric potential is analogous to the effort required to draw fluid through a straw.
It is the force that must be exerted by the roots to remove water from the soil. Tensiometers measure the matric potential directly. Tensiometers are likely to break tension or suction beyond -1.0 bar, hence, tensiometers are best suited to systems and irrigation management that maintain a high moisture level in the soil. They are typically placed at several depths in the root zone with the difference in readings monitored to initiate the irrigation event. For routine maintenance, a vacuum pump is used to evacuate air from the tensiometer and, when equipped with a vacuum gauge, enables the user to check the accuracy of the vacuum gauge on the tensiometer.