Soil Science & PQNK System
Understanding Soil Water Absorption: Composition, Gusra Soil, and a Field Test for Field Capacity
Explains how soil composition and pore size govern water infiltration and holding capacity, defines the locally significant 'Gusra' soil fraction, and provides a simple steel-pipe field test farmers can use to measure their own field capacity.
Abstract
This paper explains soil water absorption as a function of the balance among sand, silt, clay, and organic matter, and specifically of pore size, which determines field capacity, the amount of water soil can hold. It stresses that this balance breaks down when soil is compacted or when mineral particles are poorly distributed, regardless of the underlying texture.
A locally significant fraction called 'Gusra,' a mineral particle size smaller than silt but larger than clay, is defined and flagged as a common regional factor that reduces water infiltration capacity, particularly when combined with low organic matter, even in soil that isn't compacted.
Diverse cover cropping (legumes, grasses, radishes) is presented as the structural fix: varied root architectures at different soil depths enlarge pores, increase field capacity, and reduce standing water, improving both absorption and holding capacity simultaneously.
The paper provides a specific, reproducible field-capacity test: press a 6-inch diameter, 12-inch long steel pipe vertically into the soil, pour in one liter of water, and time its absorption with a stopwatch, then repeat with a second liter to confirm the reading. This is offered as a practical, equipment-light way for farmers to benchmark their own soil rather than relying on generic guidance.
Key Takeaways
- Water absorption is governed by the balance of sand, silt, clay, and organic matter and by resulting pore size (field capacity), which collapses under compaction or poor particle distribution regardless of soil texture.
- 'Gusra,' a local term for particles smaller than silt but larger than clay, is identified as a common regional cause of reduced infiltration, especially when organic matter is low.
- Diverse cover crops (legumes, grasses, radishes) improve both infiltration and holding capacity by opening pores at multiple soil depths simultaneously.
- A simple field test is specified: a 6-inch diameter, 12-inch long steel pipe pressed into the soil, with 1 liter of water timed for absorption and a second liter to confirm the field-capacity reading.
- Hardpan is identified as the primary blocker of water flow regardless of composition; diverse cover crops and added organic matter are the corrective levers.