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Treebeard's Stumper Answer
2 November 2001

Soil Matters

We had a good soaking rain this week in central California, our first in many months! Where does all that water go when it soaks into the ground? It must seep into the pore spaces between the soil particles, but how does the makeup of the soil effect this? Does fine or coarse soil have more pore space, and which holds the most water for growing plants? I've heard it said that cultivating or breaking up the dry crust of soil actually conserves soil moisture better than a solid crust. Can that be? Doesn't breaking up the crust just make it easier for the soil moisture to evaporate?

A worm's eye view of soil? Nah, this is a photo of basalt boulders used to fill a slide area on Boundry Road along the North Umpqua River in Oregon. Those "particles" are really a foot or two across, but they show pore space just as well as a microphotograph. How does the percent pore space of these boulders compare with sand? How would a real microphotograph of soil look different?
Fractals are mathematical objects that are self-similar when magnified. Nature seems to have a fractal quality, as shown in this classic Ansel Adams photograph of Mount Williamson: Clearing Storm. Soil particles also have this fractal quality. Do the physical properties of soil change with the particle size despite the similar appearance?


In my DMS science class, we've been using marbles and BBs to investigate pore space and soil. BBs pack together with smaller gaps than marbles but more of them, so the total pore space is (nearly) the same for both. But the smaller BBs have more surface area, so they hold more water, just like your wet hair holds more water than your skin after a swim. More water might evaporate from packed soil with smaller pores for the same reason. It's possible that cultivating the soil in your garden can actually conserve water. These are our ongoing experiments, and we still have lots to learn!

Notes:

Einstein is quoted as saying "Everything should be made as simple as possible, but not simpler." That's good advice for this stumper. Real soil is complicated, and we depend on it for life. Marbles and BBs greatly over-simplify soil, but that's OK as long as we remember it.

We measured the pore space of marbles and BBs by adding enough water to more than cover the balls in a graduated cylinder. The water within the (non-soluable) balls gives the pore space. We also weighed the balls dry and wet (after draining) to calculate water retention. This is an easy procedure that gives surprising results. Who would guess that a bowl of marbles is almost half empty space! Here is our step-by-step procedure for marbles or BBs or anything else. The numbers refer to the spreadsheet that follows.

I repeated the measurements with different quantities in each of my four morning classes with these results:

Marbles (1.6 cm diameter) Trial:1234 Average:
 1. Dry mass of particles (grams)525.22404.98452.12545.48 
 2. Wet mass after shaking (grams)531.18407.05456.50549.65 
 3. Mass of water retained(2) - (1)(grams)5.952.074.384.17 
 4. Volume of dry particles (cm3)400300350400 
 5. Volume of water added (cm3)300250300250 
 6. Level of water after mixing (cm3)515417485470 
 7. Volume of water NOT in mix(6) - (4)(cm3)11511713570 
 8. Volume of pore space(5) - (7)(cm3)185133165180 
 9. Volume of solid particles(4) - (8)(cm3)215167185220 
 12. Bulk Density(1) / (4)(g/cm3)1.311.351.291.36 1.33 g/cm3
 13. Particle Density(1) / (9)(g/cm3)2.442.432.442.48 2.45 g/cm3
 10. Percent pore space(8) / (4) x 100 46%44%47%45% 46%
 11. Percent water retained(3) / (8) x 100 3.2%1.6%2.7%2.3% 2.4%
 
BBs (0.45 cm diameter) Trial:1234 Average:
 1. Dry mass of particles (grams)914.10271.78222.55305.90 
 2. Wet mass after shaking (grams)927.45274.92224.85309.10 
 3. Mass of water retained(2) - (1)(grams)13.353.142.303.20 
 4. Volume of dry particles (cm3)200605070 
 5. Volume of water added (cm3)200505050 
 6. Level of water after mixing(cm3)320867990 
 7. Volume of water NOT in mix(6) - (4)(cm3)120262920 
 8. Volume of pore space(5) - (7)(cm3)80242130 
 9. Volume of solid particles(4) - (8)(cm3)120362940 
 12. Bulk Density(1) / (4)(g/cm3)4.574.534.454.37 4.48 g/cm3
 13. Particle Density(1) / (9)(g/cm3)7.627.557.677.65 7.62 g/cm3
 10. Percent pore space(8) / (4) x 10040%40%42%43% 41%
 11. Percent water retained(3) / (8) x 10016.7%13.1%11.0%10.7% 12.8%

The larger marbles had slightly more pore space than the BBs. That caught me by surprise since it's a geometry matter, so size shouldn't matter. But the larger marbles take up extra space on the sides and top of the graduated cylinder we used to measure them. I don't think that's signicant, but it is significent that the BBs held 12.8 / 2.4 = 5+ times more water (figured as percent of total pore space) after two shakes in a kitchen strainer. The smaller particles have about the same pore space, but they hold more water because of surface tension and capillary forces. It's just what I expected.

Real soil is more complicated than marbles and BBs. It contains a mix of particles, and it's alive!

Following Einstein's advice, I won't try to over-simplify. Particle size matters, but so do many other factors.

Soil cultiivation and water content is a different question. "If a footprint in cultivated soil is left undisturbed, the soil inside the footprint will become hard and dry." That's a quote from Jearl Walker's The Flying Circus of Physics (3.101), one of my trusted stumper sources. He goes on to say:

The soil must be broken up in order to retain its moisture. A packed ground has many small openings that will act as capillary tubes. As the water climbs to the surface, it is lost to evaporation. Cultivated ground has much larger openings and thus less capillary action.

This sounds reasonable, though I've noticed that the loose soil in gopher mounds usually looks drier than surrounding soil, and that certainly makes sense too.

I found an interesting book on the Web on Dry-Farming by John A. Widtsoe, published in 1920. It's based on the author's real experiments in Utah, as dry as it gets. His Chapter 8 ends with these stirring words:

The conservation of soil-moisture depends upon the vigorous, unremitting, continuous stirring of the topsoil. Cultivation! cultivation! and more cultivation! must be the war-cry of the dry-farmer who battles against the water thieves of an arid climate.

Another source says just the opposite,

Consider the impact of cultivation on soil moisture loss... Under dry conditions and when a soil crust has formed, very little soil evaporation occurs, and cultivation disturbs the soil surface and increases soil moisture loss... If weeds or crusting are not a concern, producers should probably leave the cultivator in the shed, regardless of soil moisture status.
Here's a real science stumper with practical consequences. We set up an experiment before Thanksgiving vacation, but we've had regular rain since then. Everything is soaked, so our experiment will have to wait.

Here are some links for further research:

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Copyright © 2001 by Marc Kummel / mkummel@rain.org