By: Kyle Richardville, Understanding Ag, LLC
About the “Understanding” series
Agriculture isn’t rocket science. It’s much more complex than that. Farming and ranching involve the fields of biology, ecology, chemistry, botany, physics, geology, meteorology, politics, economics, psychology and mechanics, just to name a few. Companies make a fortune off many farmers and ranchers on such topics because it’s impossible to study everything and still have a life outside of work.
However, having a basic understanding of each of these topics has the potential to save a producer millions of dollars over his or her lifetime. This is why we are bringing you the “Understanding” series. The series’ purpose is to empower farmers and ranchers by helping them better understand the “why” behind regenerative practices so they can rely more on nature and less on expensive inputs.
Introduction
The previous installment of this series took an in-depth look at lime and its ability to raise pH. Soil acidification is a well-publicized topic of concern in scientific and governmental circles, but alkaline soil conditions (soils above 7 pH) are every bit as challenging to farmers and ranchers who deal with them.
Third Rock from the Sun
Excuse the pun, but geology rocks. Farmers and ranchers have a special relationship with geology whether they know it or not. The sand, silt and clay particle that make up soil are all tiny bits of weathered and degraded rock. The vast majority of nutrients their plants and animals need come from rocks, with the exception of nitrogen and carbon (from the air), as well as hydrogen and oxygen (from water). Everything else like phosphorus (P), potassium (K), boron (B) and all the rest were once wriggled free from the tight grips of rocks. This wriggling, or “weathering”, of nutrients is a never-ending process that resupplies soil with usable nutrients. Both non-living forces (like water) and living forces (like microbes) can weather nutrients out of rock, which increases the total pool size of nutrients available for living organisms to use. Old headstones are testament to this process.
There is an important opposing force that causes nutrients to exit the soil, which is called “leaching.” Water is the main culprit in this process because it has an uncanny ability to magnetically grab nutrients and take them wherever it goes, either off the surface or down through the soil into the water table where plants can no longer use them. Natural ecosystems tend to strike a balance between weathering and leaching when precipitation and plant water use are similar. However, we already know that some environments have high precipitation, which leaches away more nutrients than are weathered, causing H+ to build and soil to acidify.
In dry environments, more nutrients are weathered from rock than are leached away, causing the soil to become more alkaline. This is because calcium (Ca2+), magnesium (Mg2+), potassium (K+) and sodium (Na+) compete with H+ and aluminum (Al3+) by bumping them off negatively charged soil sites. H+ and aluminum (Al3+) are then prone to leach away or become neutralized. In addition, the positively charged nutrients listed above can react with soil water and produce the counterpart to H+, which is OH–. OH– drives pH up, making conditions more alkaline, sometimes to a level that hinders plant productivity. This is no small issue, as around 45% of Earth’s land is considered arid or semiarid, so rising soil pH is a concern for producers across the globe.
Salt of the Earth What if a producer has the capability to apply irrigation water? Will this simulate precipitation and fix the problem? The short answer is no. Irrigation exacerbates the alkaline problem in most cases because irrigation water is chock full of these weathered nutrients, particularly when it is sourced from river water, a common practice in the American West. Applied irrigation water is subsequently used by plants or evaporated away, leaving many of the nutrients behind. These positively charged nutrients can cling to negative sites in the soil, but they may also cling to negatively charged nutrients to become a salt. Salt, by definition, is the combination of a positively charged and negatively charged nutrient. Think about the salt we use to flavor our food: sodium (Na+) and chloride (Cl–) make sodium chloride(NaCl) or table salt. As a result, many irrigated landscapes are left with white, powdery salts in and on the soil.
Plants and microbes are severely hindered when excess salt levels build up, which is why ancient civilizations would “salt” the agricultural fields of their rivals as a wartime tactic.1 Problems also occur downstream, as water leaving irrigated fields is filled with unnaturally high levels of salt. For example, the Colorado River contains a very high concentration of salts by the time it reaches the U.S.-Mexico border, thanks to upstream irrigation systems and industrial use. As a result, a very large and expensive desalinization plant had to be built at the border to ensure clean water was entering Mexico.2
As mentioned in Part 1 of this series, rising water tables are another cause of rising soil pH/salt accumulation. In this case, nutrient-rich groundwater reaches the soil surface as the soil wicks it upward. This effect is first observed in low spots of the land. Loss of deep-rooted perennial species is a major cause of water table rise in these arid environments. Annual plants are simply not able to draw up and evapotranspire groundwater as efficiently as perennial species, so the water table slowly accumulates and climbs. Introducing fallow periods into a cropping rotation also speeds up water table rise in these environments.
Salty landscapes also form naturally when bodies of water slowly dry up and leave their salts behind, such as at the Bonneville salt flats in Utah. In total, salt-affected soils cover approximately 7% of the total land on Earth, 23% of cultivated agricultural land and nearly 50% of irrigated land.3
Lowering pH
Removing excess H+ in an acidic soil is fairly straightforward with the availability and action of lime. Removing excess salts in an alkaline soil is a little more challenging. The good news is that salts dissolve in water, ie. they are “water soluble”. This means they will readily hitch a ride in the water and go wherever it goes. Here lies a root cause of high pH. Many agricultural soils are compacted and have poor drainage, so water and the excess salt do not flow through properly. This is why tile drainage is pushed by conventional agronomists in these situations. Perforated tile speeds up the movement of water out of the soil, taking the salts with it.
Oftentimes, nutrients like magnesium (Mg2+) and sodium (Na+) are not in the form of leachable salts and need to be transformed into one so water can carry them away. This is why another common piece of advice is to apply some form of sulfur amendment. Common choices are gypsum (calcium sulfate) and elemental sulfur. Gypsum is used because it combines excess magnesium (Mg2+) with sulfate, making a salt called magnesium sulfate (a.k.a. epsom salt). In a similar way, excess sodium (Na+) becomes a salt called sodium sulfate. Once again, this strategy works best in soils that receive adequate moisture and exhibit good drainage to flush the newly formed salts away. Finally, elemental sulfur is recommended because it turns into sulfuric acid, a very strong acid with a pH around 2-3. This means sulfuric acid readily donates H+ to the soil, which lowers pH.
Organic Matter
One often overlooked strategy to bring down high soil pH is to build organic matter levels. As you might recall, soil organic matter behaves as a powerful pH buffer due to the high number of sites that can bind with nutrients like calcium (Ca2+), magnesium (Mg2+), H+ and all the others. This characteristic positively influences soil nutrient balance over time. In addition, organic matter decomposition by microbes can help lower pH thanks to the weak acids they directly exude, as well as the carbonic acid created from the carbon dioxide they breathe out. (See Part 2 for more on this process.) Scientists don’t have a full explanation of how these processes play out, but they do know that soil nutrients and pH move toward a healthy range when organic matter levels and the richness and diversity of soil life increase. When allowed, nature is self-healing, self-organizing and self-regulating.
Higher organic matter levels in the soil also fight against high pH because organic matter, and the soil organisms that it feeds and houses, actively improve drainage and a soil’s water holding capacity. In other words, a good balance is struck between water infiltration, water holding capacity and water percolation down the soil profile. Drainage is improved through the action of microbes that create proper soil aggregation, as well as increased earthworm and insect tunnels that water containing excess salts can use to percolate downward. Water holding capacity is increased because organic matter behaves like a sponge, magnetically biding to polar water molecules in a way that plants can access. This reduces the need for irrigation water when conditions start to turn dry.
An important point to always remember about organic matter is that its accumulation and decomposition happen at the same time, sort of like a person’s monthly income brings in money and monthly expenses cost money. This means building organic matter levels requires putting in a little more organic matter than is decomposed every year. Tips and strategies to increase organic matter levels will be discussed in Part 4 of this series.
Conclusion
Farming and ranching on soil with a high pH can be challenging, particularly when excess salts have accumulated. Installing drainage tile and applying gypsum are often recommended to rectify the situation for their ability to improve drainage and flush away salts. It’s easy to see where advocates for these practices are coming from and, in fact, many farmers and ranchers have lowered soil pH by implementing such practices. But these amendments and practices are tools, not magic bullets. Soil pH will revert back to undesirable levels if they aren’t accompanied with systematic changes as well. The fourth and final installment of “Understanding pH” will look at soil pH management from a regenerative perspective, highlighting farmers and ranchers who have significantly reduced or eliminated the need for inputs that temporarily raise or lower pH.
References
1 https://en.wikipedia.org/wiki/Salting_the_earth
2 https://en.wikipedia.org/wiki/Yuma_Desalting_Plant
The post Understanding pH Part Three: Salt of the Earth: Addressing Alkalinity appeared first on Understanding Ag.
