Soil Physics — Soil Science Reviewer Questions

20 board-style Soil Physics items for the Agriculturist Licensure Examination. Try 40 questions free; lifetime access is ₱49. Texture drives water, water drives aeration, and aeration drives nutrient availability. Reason down that chain and most items unlock.

20 built-in questions in this topic · approved additions may publish live · part of Soil Science

Try Soil Physics

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.

Try Soil Science — first 40 free

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?

Other Soil Science topics