Gunz And Rows – What You Need to Know About Soils and Moisture
August 8, 2019
We have thermometers and rain gauges to measure the weather, and we can tell when the sun is shining or not, but what about soil? How do we measure it, and perhaps more importantly, measure it -in relation to water? How much water can it hold, and how well can water infiltrate and move through it?
In this Gunz and Rows we’ll explore how soil properties related to water are measured and estimated and why it matters to producing your crops.
Texture and Organic Matter
As you split apart soil, you start to find that a certain portion of it will be from organic sources, and in most cases, some mineral sources as well (the exceptions are truly organic soils like mucks and peats). Generally, soils are believed to have developed through years and years of weathering of minerals. Through this weathering process of various parent material, different sizes of particles developed.
In order to classify these mineral particle sizes, the USDA developed these “separates” and the defined diameter limits of each:
|Soil Separate||Diameter limit (mm)|
|Sand||0.05 to 2.0 mm|
|Silt||0.002 to 0.05 mm|
|Clay||< 0.002 mm|
Because we don’t often deal with sizes this small, let’s put them into context with the help of this illustration:
While there are techniques to measure texture by feel (ribbon test), modern soil labs will use the hydrometer method. This utilizes a mix of the soil sample and a dispersing agent to separate the soil particles. Mixed together and shaken overnight, the solution is then transferred to a 1 liter graduated cylinder and mixed with water. Larger particles (sand) will sink to the bottom first, and a hydrometer is used to measure the water density after 45 seconds for sand. At 1-½ hours the hydrometer is used again to measure density for silt, and at 6 to 24 hours to measure density for clay. Based on these densities, calculated percent sand, silt, and clay are made.
These percentages of sand, silt and clay are further used to determine the soil texture classification. Using the Soil Textural Triangle below, one can determine the classification based on a texture analysis from a lab. For example, a 0-12” depth sample from my home farm in southern Iowa on a slight ridge came back at 29% sand, 56% silt, and 15% clay, and according to the Soil Textural Triangle, this would be considered a silt loam.
Organic Matter consists of plant and animal residues, cells and tissues of soil organisms, and substances synthesized by soil organisms. Generally, it is made up of the organic carbon-based materials found in the soil. As you can imagine, not all organic matter is created equally; depending on if the field had a history of manure, cover crops, and/or crop rotation different quantities of organic matter sources will exist. Initial efforts in quantifying Soil Health are attempting to explain this variability and effects on productivity and sustainability.
A commonly promoted factoid is that a 1% increase in soil organic matter helps the soil to hold 20,000 more gallons of water per acre. Also, organic matter can hold 10 times its weight in water. These statements come with a few assumptions, and shouldn’t be held as absolute constants for all situations, but it does indicate the importance of organic matter and water in the soil.
Because of soil organic matter complexity, the go-to method for quantifying OM has been the Loss On Ignition test, or LOI After a soil sample has been heated to remove all water, it is weighed, then put into a blast furnace-type oven for a set time and temperature (ex. 4 hours at 550 C) to essentially “burn off” the organic matter. The sample is weighed again, and the difference calculates the portion considered organic matter.
Water Holding Capacity and Saturated Drainage Flow
Once we have our soil texture and organic matter values, how we do we know how much water our soil will hold, make available to the plant, and how fast does water drain through the soil once it becomes saturated?
Referring back to the illustration of relative fraction size above, a single sand particle has a larger outside surface area than a clay particle, and therefore has more sites available for water and nutrients to attach on to. However, if you had a volume of sand particles, there’d also be a great deal of open space between them, and the distance at some points would be so great that water and nutrients could not be retained far enough to span the gaps.
Likewise, if that same volume contained clay particles, there would be a great deal of surface area available for water and nutrients to attach, but little space between the particles. Clay has the capacity to hold on to a great deal of water and nutrients, but doesn’t A) give it up easily and B) doesn’t have a great deal of open space for them to move through.
There are (4) key measurements for soil-water interactions:
- Lower Limit, also known as Wilting Point – Soil water content of the soil at which a plant cannot generally extract any more water.
- Drained Upper Limit, or Field Capacity – Soil water content at which all of the surface area around soil particles have water attached to them, being held on by the natural bonding attraction. Space exists between particles and provides an ideal combination of soil, water, and air.
- The difference between Drained Upper Limit and Lower Limit is considered to be Available Water Capacity to the plant.
- Saturated, also known as Satiated – Soil water content at which all the available space in the soil is filled with water.
- Saturated Flow, also known as K Sat – Speed at which water flows downward through a saturated soil.
When characterizing soils from across the nation, the USDA National Resources Conservation Service (NRCS) performed specialized lab tests to measure these 4 properties; they are not something typically performed at your favorite local soil testing lab. Based on measurements taken on key representative soils, NRCS is able to model water characteristics for its nearly 300,000 soil profiles across the United States.
In general, soils towards the lower left of the Textural Triangle (sand, sandy loam, loamy sand, etc) will have relatively lower values for Lower Limit and Drained Upper Limit, but will have high Saturated Flow values. There will be a large gap between Drained Upper Limit and Saturated levels, which means that it has the ability to hold a lot of water, albeit for not a long time.
Soils towards the upper corner of the Textural Triangle (clay loam, silty clay loam, silty clay, clay) will generally have higher Lower Limit and Drained Upper Limit values, and in turn will have lower Available Water Capacity (AWC). These soils will also have decidedly lower Saturated Flow values – little space between particles for water to move downward.
Finally, soils in the middle to lower right corner of the Textural Triangle (loam, silt loam) have higher Lower Limit values than the sands, about the same Drained Upper Limit values as both the Sands and Clays, and will have moderate Saturated Flow values. These are the “Goldilocks” soils, with water holding capacity being maximized and Saturated Flow being sufficient to not create anaerobic conditions for long periods of time.
For all of these, increased Organic Matter will typically increase water holding capacity, but its effects are not equal across all soil textures (increased OM in clay soils do not have as great effect in AWC as in loamier and sandier soils). Increased organic matter also affects the overall bulk density of the soil, which in turn has some effects on water holding capacity.
Other factors, such as presence and quantity of gravel, compaction and salinity will have effects on water holding capacity, but the big drivers in general are soil texture and organic matter.
One caveat on the estimation of soil water content and saturated flows: The NRCS performed this work on soils found in the United States and developed under the environmental conditions therein. However, water holding capacity and saturated flow may be quite different for soils of the same texture and organic matter in other regions, especially in tropical soils. This may be due to the extended weathering of those soils, the parent material, and the forms of clay found there.
If you want to learn more about the estimation of soil water capacity and the effects of varying levels of sand, silt, clay, organic matter, and other factors, a great tool would be to download the Soil Water Characteristics software that accompanies the SPAW software available from the USDA. Researchers (Saxton and Rawls) in the mid 2000’s developed models to estimate water holding capacity and saturated flow given various soil parameters, and a simple program was developed to calculate and display these values. Click here to download the SPAW and Soil Water Characteristics software.
Soil is one of the greatest resources we have in agriculture, and understanding how it holds and moves water (another key resource) is important. By having a better knowledge of soil water capacity and saturated flow, we can start to understand the performance of our crops based on the environmental conditions thrown at us. Not much can be done to change soil texture and therefore water capacity, but we can manage it better once we understand the dynamics.
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