Soil Moisture Content Calculator

The method

Weigh the sample moist, dry it, and weigh it again, subtracting the container tare from both weighings. To dry it, NRCS microwaves a subsample in 4-minute cycles at medium power until the weight stops dropping^[1].

Divide the water lost by the oven-dry weight, not the wet weight; Kansas State University’s soils lab manual calls the result the gravimetric water content^[2].

Gravimetric vs volumetric

Gravimetric moisture is the mass of water per mass of dry soil. Volumetric moisture is the volume of water per volume of soil. The two aren’t equal, and bulk density converts one to the other because a gram of water takes up one cubic centimeter: a soil with a bulk density of 1.5 g/cm³ and 30% water by weight holds 45% water by volume^[2].

Volumetric moisture times the layer’s depth is the depth of water it holds: Kansas State’s example is a 25 cm layer at 20% by volume holding 5 cm of water^[2]. That is the number to compare with rain or irrigation.

Reading it against field capacity and wilting point

Field capacity is the water a soil holds after it drains, the wilting point is the water too tightly held for plants, and plants draw on the water between them. With Use my soil, the calculator shows USDA’s values for your address^[3][4].

• Above field capacity: the extra is gravitational water, which drains from the large pores too fast for plants to use^[2].
• Between field capacity and the wilting point: plant-available water, held in the medium-sized pores^[2].
• Below the wilting point: water held too tightly in the smallest pores for plants to take up^[2].

Clay soils read wetter at the same tension: as clay content rises, soil holds more water^[2]. So compare a reading with your own soil’s values, not with a number from a different soil.

How much water to add

USDA’s irrigation handbook calls the shortfall the soil moisture deficiency: the water needed to refill the root zone to field capacity^[5]. Field capacity minus your volumetric reading, times the layer depth, is that amount in inches (calculated). The water holding capacity calculator covers when to water, which NRCS puts before about half of the available water is used.

NRCS also estimates available moisture in the field by comparing how a sample feels and looks with a table or a guide^[5]. It’s a different test from the ribbon test, which judges texture.

Worked example

A 150 g sample that dries to 120 g over a 20 g tare has gravimetric moisture (130 − 100) ÷ 100 = 30%^[1]. At a bulk density of 1.30 that is 39% by volume, and across a 6-inch layer that is about 2.3 inches of water^[2].

Engineers mean something else by “optimum moisture”: the Proctor compaction test finds the moisture at which a soil compacts to its maximum dry density, for fill and foundations^[6]. This garden method doesn’t give that number.

How this calculator works

Gravimetric moisture is (wet − dry) ÷ dry, taken after the container tare is subtracted from both weighings^[1]. Volumetric moisture is that figure times your bulk density, and multiplying it by a layer thickness gives the inches of water in the layer^[2]. With Use my soil, the calculator fills in the bulk density USDA maps at your address and shows its field capacity and wilting point for comparison^[3]. See the bulk density calculator to measure that density, and the water holding capacity calculator for the whole root zone.

Frequently asked questions