Each question should be answered in your own words. Direct copying from other students, the textbook, or other sources is strictly prohibited; you will at a minimum receive 0 points credit for the entire problem set in the case of copying, with possible additional consequences, such as an F for the course.
- (A) Write out the equation for total water potential and each of its components as described in the book and your lecture notes. Assume your soil is unsaturated and above the water table, so you do not include hydrostatic potential.
(B) Explain the physico-chemical forces that each component potential you listed in part A accounts for.
(C) List the net magnitude (whether it is positive or negative) of each component potential;
(D) Indicate the two most common units (standard and metric) used to express water potential;
3. You are given the following data set for the water potential components of three soils with similar texture, structure and organic matter content (water potential given in kPa):
ψm ψo ψg
Soil A -100 -5 +50
Soil B -800 -15 +33
Soil C -0.0 -70 +75
A. Which soil is the driest?
B. Which most likely recently received additions of a synthetic fertilizer?
C. Which soil is at highest elevation above its local water table or reference point?
D. Which soil is most likely saturated?
E. If you placed soils A and B directly next to each other which way would water flow between them and what type of flow (saturated or unsaturated) would occur?
4. Coach Franklin has been testing the turf in Beaver Stadium and needs some help interpreting the data. Use the following total water potential data set for the sandy loam soil in Beaver Stadium to answer the following questions. Give your answer in percent or g/100g soil rounded to the nearest whole number.
Potential (kpa) % water (g/100 g soil)
– 33 29
A. At saturation what is the gravimetric water content?
B. After the gravitational water has drained away, what would be the gravimetric water content?
C. What would be the gravimetric water content of the soil when you observed permanent wilting in crops grown in the field?
D. What is the total plant available water content on a gravimetric basis?
5. A new client namedElon Musk is interested in transporting some soil to Mars with his new Falcon Heavy rocket to set up a vegetable growth experiment. As part of the schematics he needs you (NASA’s top soil scientist) to calculate some of the physical properties of the soil his team has selected. Elon has collected the following data for you based on a subsample of the soil:
The subsample is 100 cm3 in total volume. When it is oven dry the sample weighs 140g. After completely saturating the sample with water it weighs 185g. After allowing the sample to drain overnight it weighed 170g. Adding the oven dry sample causes the water level in a graduated cylinder to rise 55mL.
- What is the bulk density of the soil in g/cm3? (rounded to two decimal places)
- What is the %porosity of the soil? (rounded to the nearest whole number)
- What is the % gravimetric water content of the soil at field capacity? (rounded to the nearest whole number)
- What is the % volumetric water content of the soil at field capacity? (rounded to the nearest whole number)
- After arriving on Mars, how many kilograms of water would need to be added to 1.5 cubic meters of oven dry soil to bring the moisture content up to field capacity in order to start the vegetable experiment? (rounded to the nearest whole number)
- After arriving on Mars, how many liters of water would need to be added to 1.5 cubic meters of oven dry soil to bring the moisture content up to field capacity in order to start the vegetable experiment? (rounded to the nearest whole number)
6. You just purchased a new home and decided to build your hot tub behind it. After several hours of careful work in the field, you figure out that they will need to excavate exactly 5.0 m3 of soil. You go out and sub-sample representative spots in the soil as it is excavated and find that it weighed 125 g right after you pulled it out of the hole, and then weighed 95 g after you dried it out in the oven. Your contractor excavates the site and informs you that exactly 8.7 Mg of soil (as is/wet) were hauled away from your site. Based on this information, answer the following questions:
- What was the gravimetric water content of the soil sub-sample? (rounded to the nearest whole number)
- What was the bulk density of the soil in Mg/m3 rounded to two decimal places? Hint: you will first need to find the dry weight of the entire soil excavated by using the wet mass of the entire soil and the %gravimetric water you calculated for the subsample in part A. above using the formula:
- Assuming a standard soil particle density as discussed in lecture, what was the % solid space of this soil (rounded to nearest whole %)?
- What was the % pore space of this soil (rounded to nearest whole %)??
- What was the volumetric water % of this soil (rounded to nearest whole %)?
- If this soil were completely saturated with water, how many Mg of wet soil would the contractor have to haul away? (rounded to 2 decimal places)
- If you could compact the soil that was hauled away to its maximum dry density (2.20 Mg/m3) how many m3 of disposal area would be required for it? (rounded to 2 decimal places)
- What was the % air filled pore space in the intact soil as it was excavated to the nearest whole percent? (rounded to the nearest whole number)
7. A soil sample was found to have a CEC of 12.0 cmolc kg-1. Important: note that a cmol is not the same thing as a cmolc for divalent and trivalent ions! (rounded to the nearest whole number)
A. How many cmolc kg-1 of Al3+ would be required to satisfy this negative charge?
B. How many cmol kg-1 of Al3+ would be required to satisfy this negative charge?
C. How many cmolc kg-1 of Mg2+ would be required to satisfy this negative charge?
D. How many cmol kg-1 of K+ would be required to satisfy this negative charge?
E. How many mg kg-1 of Mg2+ would be required to satisfy this negative charge?
8. Would the following soils be dominated by 1:1, 2:1, or Fe/Al oxides in the colloidal fraction? Would they be dominated by pH-dependent charge or permanent charge?
|Order||Location||1:1 / 2:1 / Oxides||Permanent or pH Dep.|