Soil Physics — Soil Science Reviewer Questions

20 board-style Soil Physics items for the Agriculturist Licensure Examination, free and open to every examinee. Texture drives water, water drives aeration, and aeration drives nutrient availability. Reason down that chain and most items unlock.

20 questions in this topic · part of Soil Science · every answer explained

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Sample Soil Physics questions with answers and explanations

Board-style items taken from the Soil Science bank. Every answer is explained, which is the part that makes a review question worth doing twice.

A core sampler of exactly 250 cm3 is pushed into a field, and the soil recovered weighs 412 g moist and 330 g after oven-drying. What is the bulk density?
- A. 1.32 g/cm3 correct
- B. 1.65 g/cm3
- C. 0.76 g/cm3
- D. 1.25 g/cm3
Why: Bulk density uses OVEN-DRY mass over total core volume: 330 g / 250 cm3 = 1.32 g/cm3. Using the moist mass (412/250) gives 1.65 and is the commonest error in this calculation, because bulk density must not vary with how wet the soil happened to be when sampled.
A soil has a bulk density of 1.35 g/cm3 and a particle density of 2.65 g/cm3. What is its total porosity?
- A. 50.9%
- B. 49.1% correct
- C. 51.0%
- D. 39.2%
Why: Porosity = (1 - BD/PD) x 100 = (1 - 1.35/2.65) x 100 = (1 - 0.509) x 100 = 49.1%. Reporting 50.9% means the ratio was subtracted the wrong way round -- that figure is the proportion of the volume occupied by SOLIDS, not by pores.
A field sample weighs 145 g before drying and 120 g after. What is the gravimetric moisture content?
- A. 17.2%
- B. 25.0%
- C. 20.8% correct
- D. 82.8%
Why: Gravimetric moisture is water mass over OVEN-DRY mass: (145 - 120) / 120 x 100 = 20.8%. Dividing by the wet mass instead gives 17.2%, which is the error the alternative is there to catch; the oven-dry mass is the only stable reference because it does not change with wetting.
A soil is at 20% gravimetric moisture and has a bulk density of 1.30 g/cm3. What is the volumetric moisture content?
- A. 15.4%
- B. 20.0%
- C. 33.8%
- D. 26.0% correct
Why: Volumetric = gravimetric x (BD / density of water) = 20 x (1.30 / 1.00) = 26.0%. Dividing rather than multiplying by bulk density gives 15.4%. The conversion matters because irrigation is scheduled on volume of water per volume of soil, not on mass.
A soil layer 30 cm thick holds water at 26% by volume. What equivalent depth of water does that layer contain?
- A. 7.8 cm correct
- B. 0.78 cm
- C. 26.0 cm
- D. 3.9 cm
Why: Depth of water = volumetric moisture x depth of soil = 0.26 x 30 cm = 7.8 cm. This is the step that converts a laboratory percentage into something an irrigator can act on, because irrigation is applied and measured as a depth.
A soil holds 32% water at field capacity and 14% at permanent wilting point, both by volume. How much available water is stored in the top 40 cm?
- A. 12.8 cm
- B. 7.2 cm correct
- C. 5.6 cm
- D. 18.0 cm
Why: Available water is the difference between the two limits: (32 - 14) = 18% by volume, and 0.18 x 40 cm = 7.2 cm. Using field capacity alone (12.8 cm) counts water the crop can never extract, because water held below the wilting point is not available however much of it there is.
A hectare of soil is sampled to a depth of 20 cm and has a bulk density of 1.25 g/cm3. What is the mass of that furrow slice?
- A. 2,000,000 kg
- B. 250,000 kg
- C. 2,500,000 kg correct
- D. 1,250,000 kg
Why: Volume = 10,000 m2 x 0.20 m = 2,000 m3; mass = 2,000 m3 x 1,250 kg/m3 = 2,500,000 kg. The often-quoted 2 million kg assumes a bulk density of 1.0, which almost no field soil has -- the figure has to be recomputed from the measured density, not recalled.
Traffic compacts a soil so that bulk density rises from 1.30 to 1.56 g/cm3. Assuming particle density stays at 2.65 g/cm3, by how many percentage points did total porosity fall?
- A. 20.0 points
- B. 26.0 points
- C. 4.9 points
- D. 9.8 points correct
Why: Porosity before = (1 - 1.30/2.65) = 50.9%; after = (1 - 1.56/2.65) = 41.1%. The loss is 9.8 percentage points. Note the porosity loss is not the same as the 20% rise in bulk density -- the relationship is not proportional, which is why compaction damage must be computed rather than estimated.

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Soil Physics — Soil Science Reviewer Questions

Sample Soil Physics questions with answers and explanations

A core sampler of exactly 250 cm3 is pushed into a field, and the soil recovered weighs 412 g moist and 330 g after oven-drying. What is the bulk density?

A soil has a bulk density of 1.35 g/cm3 and a particle density of 2.65 g/cm3. What is its total porosity?

A field sample weighs 145 g before drying and 120 g after. What is the gravimetric moisture content?

A soil is at 20% gravimetric moisture and has a bulk density of 1.30 g/cm3. What is the volumetric moisture content?

A soil layer 30 cm thick holds water at 26% by volume. What equivalent depth of water does that layer contain?

A soil holds 32% water at field capacity and 14% at permanent wilting point, both by volume. How much available water is stored in the top 40 cm?

A hectare of soil is sampled to a depth of 20 cm and has a bulk density of 1.25 g/cm3. What is the mass of that furrow slice?

Traffic compacts a soil so that bulk density rises from 1.30 to 1.56 g/cm3. Assuming particle density stays at 2.65 g/cm3, by how many percentage points did total porosity fall?

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