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Exercise 10: Analysis of Soil Properties

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Part 2: Analysis and Interpretation of Soil Texture

1. Rank the soils from coarsest to finest.

• Soil A: very coarse sand
• Soil B: coarse sand
• Soil C: medium sand
• Soil D: find sand
• Soil E: very fine sand

2. Problems may have included:

• Not starting or stopping the stopwatch at the right time.
• Forgetting to record water amounts before emptying graduated cylinder.
• The stopwatch malfunctioning.
• For soil A the infiltration rate is more a function of how fast the water was poured on the soil and not the rate at which it soaked in. This is not a problem with the folks who had soil A, but is just due to the fact that soil A infiltrates water at a higher rate than we can accurately measure.
• Not allowing all the gravity water to drip out the bottom.
• Water being absorbed by the filters, not the soil.
• The soil samples not being 100% within the specified particle size range.
• There are a variety of things that could have gone wrong. There is no one correct answer for this question.

3. Each group's field capacity is different.

4.

Table 10.3 Field Capacity Summary Spring 2009(ml water per 100 g soil)
	Soil A	Soil B	Soil C	Soil D	Soil E
Average	28.5	31.8	31.3	32.3	42.8

Table 10.3 Field Capacity Summary Fall 2008 (ml water per 100 g soil)
	Soil A	Soil B	Soil C	Soil D	Soil E
Average	27.3	27.6	28.0	32.3	42.0

5. Rank the soils from lowest to highest field capacity.

lowest:	A
	C
	B
	D
highest:	E

6. a. Four of the soils are in the right order.

6. b. Soils B and C are reversed relative to each other, but are in the correct order relative to the other three soils.

7. Hypothesis 1 stated that coarse textured soils have lower field capacities than fine textured soils. Based on our results, this hypothesis is most likely true.

8. Each group's infiltration rate is different.

9. Table 10.5

Table 10.5 Summary of Infiltration Rates (ml/min)
	Soil A	Soil B	Soil C	Soil D	Soil E
Pour 1	809	524	58	80	160
Pour 2	175	180	60	16	14
Pour 3	438	210	73	20	8
Pour 4	556	238	76	21	8
Pour 5	606	239	78	22
Pour 6	644	235	80	22
Pour 7	645	252	83	24
Pour 8	698	247	84	25
Pour 9	736	251	83	25

10. Graph from Spring 2009

Graph from Fall 2008

11. a. Hypothesis 2 is not completely true.

11. b. The infiltration rates for soils B, D, and E start high, quickly drop and then level off - they do not steadily drop which is what the hypothesis says. Soil A starts high, drop quickly, but then increases. Soil B is relatively consistent for all pours.

11. c. There is no one right answer to this question. Here are a few possible hypotheses:

• Very coarse soils show no clear pattern of change in infiltration rate over time.
• The infiltration rate is high for dry soils, and as soil moisture increases, the infiltration rate decreases and then levels off to a steady rate.
• The infiltration rate is high for dry soils, drops quickly, then increases and eventually levels off.

12. The infiltration capacity is the infiltration rate where the line levels off.

Soil A: doesn't level off, so I picked 650 ml/min.
Soil B: approximately 250 ml/min
Soil C: approximately 80 ml/min
Soil D: approximately 25 ml/min
Soil E: approximately 8 ml/min

13. Rank the soils from highest to lowest infiltration capacity.

highest:	A
	B
	C
	D
lowest:	E

14. a. All the soils are in the correct order; the coarsest soil has the highest infiltration capacity while the finest soil has the lowest infiltration capacity.

14. b. No soils are in the incorrect position.

15. We should accept hypothesis 3 as true. Despite the fact that we may have made some errors in our measurements, all of our results support the hypothesis that coarse textured soils have high infiltration capacities and fine textured soils have low infiltration capacities.

16. Soil texture and soil moisture characteristics are clearly related. Coarse textured soils have low field capacities and high infiltration capacities, while fine textured soils have high field capacities and low infiltration capacities.

17. a. Hypothesis 2 was not completely true, and this was most likely not due to mistakes or errors. If hypothesis 2 was true and infiltration rates steadily declined as soil moisture increased, we would not be able to define infiltration capacities for soils.

17. b. There is no one right answer to this question, but here are some possible changes:

• Have more people test each soil texture.
• Use more than 200 grams of soil for each sample.
• Use more (or less) than 50 ml of water.
• Use a filter that is less likely to retain water.
• Use a filter that won't allow any sediment to get washed out of the container with the gravity water.
• Find a device that will regulate the rate at which water is poured on to the soil so that water is always poured at the same rate (particularly for the very coarse sand).

 

Part 3: The Soil Texture Triangle

1. Use the texture triangle and Figure 10.3 to determine the texture and moisture characteristics of the soils listed below.

	% Sand	% Silt	% Clay	Texture	Field Capacity	Wilting Point	Available Water
Soil T	10	20	70	clay	31	23	8
Soil V	90	5	5	sand	10	4	6
Soil W	60	25	15	sandy loam	18	5	13
Soil X	40	40	20	loam	23	7	16
Soil Y	30	40	30	clay loam	30	14	16
Soil Z	20	70	10	silt loam	28	10	18

2. Silt loam has the highest available water capacity.

3. Clay and sand have the lowest available water capacity.

4. Sand has a low available water capacity primarily because it has a low field capacity; there just isn't much water for plants to extract. Clay on the other hand has a high field capacity, but a low available water capacity. This results because the clay particles hold the water so tightly that plants can't extract the water. Silt loam falls in the middle of the texture range. Silt loam has sufficient clay that it has a fairly high field capacity, yet the clay is mixed with enough sand and silt that the soil doesn't hold the water very tightly, thus plants can extract a lot of the water.