Plant Availability of Potassium in Soil Minerals: What’s Happening Near the Roots? (PART 2)

(continued): Plant roots can also exude their own acids and carbon compounds into the rhizosphere. These substances are released as the plant regulates the types and amounts of nutrients and other molecules in its roots. Finally, as roots take up K from the soil, they deplete K in the rhizosphere to concentrations much lower than the bulk soil. Very low concentrations of K near the root can trigger the release of K from mineral structures. All of these mechanisms arise from chemical and physical forces considered part of weathering. Rather than taking years, however, these processes working in the rhizosphere release K in a matter of days or weeks. Even with all these mechanisms, a plant may still not be able to access enough K from the soil to meet its nutritional requirements. Scientists have come up with a variety of measurements that differentiate conditions when plants are accessing enough K and when they are not. Soil tests and plant tissue tests that are calibrated to crop growth and development have been extremely helpful in this regard. Future improvements in the accuracy and precision of these tests will need to consider how plant varieties differ in their abilities to acquire K. How different varieties alter their rhizosphere environment will be an important consideration. [International Plant Nutrition Institute. Issue #1 2018]

Plant Availability of Potassium in Soil Minerals: What’s Happening Near the Roots? (Part 1)

Plants require a lot of potassium (K), and most of what plants need comes from the soil. Soils contain a variety of minerals that contain K in their structures, but that K is not readily available for use by the plant. The traditional view is that K in these minerals comes out in small quantities over long periods of time as soils age. Physical forces and chemical reactions slowly break down these minerals, a process called weathering, releasing K into the soil water where plants roots can take it up. While this traditional view is correct for the bulk soil, weathering can occur quickly in the zone of soil right around and influenced by the root. This zone, called the rhizosphere, has a chemical and biological composition much different from the bulk soil. Bacteria bloom in the rhizosphere. Bacterial populations are often several times those found in the bulk soil. As plant roots grow, they leave behind a lot of carbon. This carbon comes from dead cells and from chemical compounds they release, including mucilage, a gelatinous substance that lubricates the root tip as it extends into the soil. Soil bacteria need carbon, and the rich supply of it in the rhizosphere fuels their growth. Some of the bacteria blooming in the rhizosphere can solubilize a portion of the K in soil minerals. Although there is still a lot to be understood about how they do it, at least some of the bacteria that have been isolated and studied produce acids and other carbon compounds that break the chemical bonds holding mineral structures together. As bonds break, K gets released and becomes available for uptake by roots. Fungi are also abundant in the rhizosphere. They too need carbon. Like bacteria, some fungi exude acids that break down minerals and release K that can be taken up by the plant. In addition, fungi grow mycelia. As these tubular filaments grow, they exert physical forces on soil minerals that can also break their structural bonds and release K.  [International Plant Nutrition Institute. Issue #1 2018]

Mastering soil health elevates farm productivity, sustainability (PART 2)

by Dennis Pollock– Western Farm Press

Four soil health approaches

Mitchell said much has been learned from a research plot at the UC center where four approaches take place, including standard tillage and no cover crop, standard tillage and cover crops, no tillage and no cover crop, and no tillage and cover crops.

“Behind me is a very important field,” he said, referring to a research plot home to a growing number of crops in recent years, including cotton, processing tomatoes, corn, wheat, garbanzo beans, and sorghum.

Mitchell closed the meeting with the observation he believes it’s possible to grow still more crops using the no-till/cover crop approach which has gained more favor in the wake of years of drought. For one thing, he said the system of adding residue keeps the ground cooler which keeps soil organisms alive and saves water use.

Sloane Rice, a hydrology student at UC Davis, echoed the notion that cover crops are a key to improving water management by increasing water retention.

Other savings from no-till have long been known, including reduced labor costs, wear and tear on tractors, and savings on diesel fuel due to fewer passes across field. In addition, dust is reduced by nearly 70 percent.

Armchair scientist

Retired organic grower Tom Willey conceded there have been some challenges to those who grow organically. Willey said he struggled for 30 years to get more attention from scientists and researchers on organic production.

“I had to become an armchair scientist,” Willey noted.

He added that organic farmers “till and cultivate more intensively than their conventional brethren.”

“Until organic systems somehow learn to embrace no-till, a unique challenge in vegetable production, we’ll need to shovel lots of ‘coal’ into the firebox, possibly outstripping compost feedstock resources should acreage greatly expand,” Willey said.

He said soil-applied compost functions “like an interest-bearing savings account.” Nitrogen slowly mineralizes through secondary microbial degradation of organic matter, and nutrients are released from “predator-prey” soil interactions.

Willey said a better understanding of soil biology “is poised to take almost all of the chemicals out of agriculture. It will bring good farming and we won’t have to call it conventional or organic – just good farming.”

The new emphasis on good farming has truly moved into the soil, not just looking at yields and production of what is growing above it. It means looking at the complex interaction of microbial organisms underground and takes into carbon added to the soil.

Healthy soil = genetic diversity

Howard Ferris, UC, Davis nematologist, looked at the suppression of pest species underground, pointing out that some nematodes are beneficial in keeping problem nematode pests at bay.

He said healthy soil sustains plant and animal productivity, and provides a reservoir for genetic diversity. Organisms in the soil decompose organic materials, sequester and redistribute minerals, mineralize organic compounds, and regulate and suppress pests.

They improve soil structure and reduce soil erosion.

“With healthy soil, you can smell it,” Ferris said. “It’s healthy and alive.”

With the diversity of species in the soil comes higher levels of carbon, he said.

Radomir Schmidt, UC Davis soil microbiologist, referred to soil as part of “a solar-powered engine.” He is researching DNA sequencing of microbes, a complex process that could help in better understanding interactions of various microbes.

Mastering Soil Health Elevates Farm Productivity, Sustainability (part 1)

by Dennis Pollock– Western Farm Press

It seems in recent years it has become all the rage to make sure that the dirt under our feet – and plants or trees – is healthy in order to sustain farming.

Soil health was the chief topic at a University of California soil health field day held at Five Points, attended by about 200 people including boots-on-the-ground farmers and researchers.

Jeff Mitchell,  University of California (UC) Cooperative Extension cropping systems specialist at Fresno County, has been toiling in the trenches – literally – for some 20 years, seeking to illustrate the value of cover crops, and no or low-till agriculture.

At the workshop held at the UC West Side Field Station, Mitchell had trenches to showcase, pits that showed differences between conventional and no-till farming. Speakers on hand discussed some of those differences.

Improved soil health

Mitchell, the growers, and others emphasized that managing for better soil health was best achieved by minimizing soil disturbance, maximizing the diversity of plants in rotation or used as cover crops, keeping living roots in the soil as much as possible, and keeping the soil covered with plants and plant residue at all times.

“I have something growing in the ground 365 days a year,” said Scott Park with Park Farming in Meridian in the Sacramento Valley. “Having roots in the ground is 10,000 times better than adding biomass.”

Making money while saving the Earth

Park, a highly diversified farmer, says he moved to a no-till, cover crop approach about 30 years ago after realizing that his tillage efforts were costly. He subsequently became “sensitive to the soil” and adapted no-till and cover crops, not because he was driven to be environmentally sensitive but because his newfound way of farming “was making me a lot of money.”

His approach means using few if any nutritional inputs, he said, and he uses less water. Moreover, he shared that his insect pest problems – notably from leafhoppers in processing tomatoes – are significantly lower.

Just as Mitchell stresses that there is a learning curve to making the switch to no-till and cover crops, Park has learned he needs to “be thoughtful” in cover crop selection. “Cereals don’t break down easily,” he said.

Jesse Sanchez with Sano Farms in Firebaugh said, “Soil health is the way to go, but it takes time to learn. Cover crops benefitted our operation 100 percent.”


Yellowing of Crop Leaves? It Could Be Iron Chlorosis.

Iron chlorosis is a yellowing of plant leaves caused by iron deficiency. However it is not always a true iron deficiency but rather an iron tie-up in plants and soil. Yellow leaves indicate a lack of chlorophyll, the green pigment responsible for photosynthesis (sugar production) in plants.

The causes of iron chlorosis are complex and not completely understood. It could be a deficiency in the soil or the plant or an iron tie-up in the soil or plant. Many reactions govern iron availability and make iron chemistry in the soil complex. Iron chlorosis generally occurs in calcareous soils with a high pH. Even though these soils have plenty of iron, the high pH causes chemical reactions that make the iron unavailable to plant roots

Plants that are native to high pH soils don’t illustrate symptoms of iron chlorosis, because they have evolved to use iron efficiently and can obtain iron from the soil. Recent research has discovered that bicarbonates (HCO3) play a major role since they are readily produced in high pH soils, especially when moist conditions limit iron availability. Even in alkaline soils, bicarbonate is much higher in the calcareous soil than in the non-calcareous soil.The causes of iron chlorosis are complex and not completely understood. It could be a deficiency in the soil or the plant or an iron tie-up in the soil or plant. Many reactions govern iron availability and make iron chemistry in the soil complex. Iron chlorosis generally occurs in calcareous soils with a high pH. Even though these soils have plenty of iron, the high pH causes chemical reactions that make the iron unavailable to plant roots

In calcareous soils, bicarbonate inhibits mobilization of accumulated irons from roots to foliage and affects availability of iron in the soil by buffering soil pH. When high bicarbonate irrigation water is applied, iron deficiency is enhanced because bicarbonate is supplied to the soil. The adverse effects of high bicarbonate levels are exacerbated in saturated soils, very dry soils, or compacted soils, where bicarbonate levels increase, leading to diminished root growth and nutrient uptake.

Gypsum can help remediate iron chlorosis. This is because gypsum reacts with bicarbonate to form calcium carbonate (CaCO, or lime) decreasing the amount of bicarbonate in the soil that affects iron uptake and iron availability.

Daniel Davidson, Ph.D. Agronomist


By Mahdi Al-Kaisi

We need to figure out how to reward farmers for using conservation practices to build soil health.

The term “economics of soil health” has been used frequently in an attempt to quantify and validate the value of improving soil health. The traditional thinking about assigning dollar values to soil health metrics, which are many, can be very challenging, and it is easier said than done.

One of the challenges in putting a dollar value on soil health is the improvement in soil health is a long-term process. Realizing an immediate economic return can defeat the purpose for those who are looking for a quick fix of the long-term sustainability of soil health and its cumulative effects on soil productivity.

It is imperative to view soil health economics in two ways: include the system role in reducing operational costs along with the system effects on improving the soil’s biological, physical and chemical attributes as main components of soil health metrics. These two benefits are mutually interconnected.

Practices improve soil health
The drivers for improving soil health are many, but we can name a few including the conservation systems illustrated in the chart below. These drivers are no-till, the use of cover crops, crop residue management, buffer strips, etc. Therefore, to quantify the economic value of improving soil health, the focus should be on the outcomes and impacts of these practices used in a system approach for return on investment (ROI).

It has been documented by many long-term studies that conservation systems such as no-till, strip till, cover crops, and filter strips (i.e., prairie strips) have significantly reduced input cost per acre and provided ecological services, such as the reduction in soil erosion and soil nutrient loss. In a long-term tillage and crop rotation study in Iowa, over 10 years of results suggest that input cost with conventional tillage for corn production was 7.5% and 5.7% greater than that with no-till and strip till, respectively.

In the meantime, no-till and strip till practices have been documented to have a greater impact on improving the soil health metrics of soil biology, soil water processing (i.e., infiltration rate), and the soil physical attributes of aggregate stability and water-holding capacity, to name a few.

Sustainability is profitable
Essential to soil health and crop productivity is a shift to a system that provides economic incentives in terms of change in the cost of operations, such as labor, fuel, machinery, chemicals, etc., yet provides ecological services reflected in the improvement of soil health metrics. This approach will potentially minimize risks during severe climate events (both dry and wet).

The link between different components of the system highlights the economic value of improving soil health through improvement in soil carbon sequestration and water quality. Thus, improving the soil ecological system, reducing input costs and stabilizing crop productivity by moderating the severe impact of climate variability events during a growing season are the best economic indicators for improving soil health with a lasting economic impact on sustaining a system’s productivity.

New thinking needed
New thinking needs to emerge to address the economic value of soil health by protecting and sustaining the soil system from any long-term degradation resulting from destructive and imbalanced management practices such as intensive tillage, which is the leading cause of soil and water quality degradation in the Midwest. This new thinking for soil health economics needs to be linked to the system effects on the ecological level. Furthermore, those who have been pioneering conservation systems for many decades need to be recognized in the agriculture marketplace, such as crop insurance.

To encourage the adoption of soil health practices, there needs to be a mechanism, which reflects the economic benefits or incentives of soil health by linking crop insurance rates, for example, to the improvement in soil health as currently advocated by some economists. It has been documented that conservation systems improve soil health and reduce input cost. Crop insurance can be used as a tool to provide incentive to encourage emerging adopters and existing practitioners of conservation systems to improve and sustain soil health and ROI.

Soil health is an indicator of system performance in rejuvenating and enhancing the soil’s ecological, environmental and economic services. Its economic value is an integral part of the system’s returns and benefits. In conclusion, the case for soil health economics has to be articulated in a system approach that reflects the mechanism by which we develop and maintain healthy soil as the foundation for a productive and sustainable agriculture.

Al-Kaisi is a professor in the agronomy department at Iowa State University.






Agricultural Limestone

(also called agricultural lime, ag lime, biolime, garden lime, or simply “liming”)





Chemical names:


Calcium carbonate

Calcium magnesium carbonate



Calcium sulfate dihydrate


Chemical formulas:









Common names4 used in agriculture, and purity or % calcium carbonate equivalent (CCE):

Ground limestone

(up to 100% CCE)



(80-110% CCE)



(up to 100% gypsum equivalent)

Uses in agriculture: 1. Raises pH of acidic soils (generally when pH  6) by increasing exchangeable calcium and neutralizing hydrogen ions.


2.  As a source of calcium and/or magnesium in low pH and low Ca and Mg areas.  Note: agricultural limestone is always 150 times less soluble than gypsum.


3.  May slightly improve water penetration in acidic soils (pH  6) but the improvement decreases as pH rises. No improvement at pH  7.

1.               1. Amends and reclaims soils high in destructive sodium and magnesium.


2. Improves water penetration at the soil surface, and infiltration through the entire profile.


3. Is necessary when irrigation water and soils are low in total salts.


4. Counteracts acidity in subsurface soils.


5. An excellent fertilizer source for calcium and sulfur.

1  Dr. Arthur Wallace, former Professor of Plant Physiology at UCLA, founder of Wallace Laboratories, and one of the most respected Soil Scientists of the 20th century, listed forty benefits for using gypsum with agricultural soils.  http://us.wlabs com/

2  dolomite can have a CCE greater than 100 percent since each molecule of magnesium carbonate is lighter than calcium carbonate

3 note: the proper name for dolomite is “dolomite (or dolomitic) limestone”

4 other less commonly used liming materials include sugar beet lime, hydrated lime, burned lime, shell meal, calcium silicate, power plant ash, and cement kiln dust


By Brent Rouppet, Ph.D.

(Part 2 of a 3 Part Series)

Gypsum. Gypsum is invaluable is since it the only natural product on Earth that serves as an (1) amendment, (2) a soil conditioner, and (3) a fertilizer; and this unique and essential product should not be confused with limestone, dolomite, and other liming materials.  Gypsum typically is neutral with a pH.

  • Improves water use efficiency (WUE). Twenty-five to 100% more water is available in gypsum treated soils vs. untreated soils; therefore, less irrigation water is required to achieve the same results.
  • Improves soil structure and loosens compacted soils. Reduced water infiltration causes ponding and runoff and can waste irrigation water. Improved soil structure reduces erosion and soil crusting.
  • Amends and reclaims soils high in destructive sodium and magnesium. Sodium and magnesium (to a lesser extent) act the opposite of calcium in soils by deflocculating structure and reducing water and air movement, and root growth. Preferably, here should to be 16 times more calcium in the soil than sodium, and 8 times more calcium than magnesium.
  • Counteracts acidity in subsurface soils. Calcium moves into the subsoil displacing aluminum and other acidifying ions from the exchange complex.
  • Improves water penetration at the soil surface. Gypsum is necessary when irrigation water doesn’t contain many minerals and when soils are low in total salts.
  • An excellent fertilizer source for calcium and sulfur. There are 16 nutrients required or essential for plants. Calcium and sulfur are two of the essential nutrients.

Growers must be aware of which soil amendment product is needed for application for each specific use.  Limestone and dolomite are used to amend soils that are too acidic for optimum plant growth and production; and gypsum is used primarily to correct soil structure-related problems, and to help with any water penetration problems.

From the standpoint of plant nutrition and as a soil conditioner or soil amendment, gypsum uniquely helps soils be more productive and more fruitful than any other single product on earth. Worldwide gypsum’s usage is largely underutilized, yet routine and frequent application of this essential amendment is required for the sustainability of all irrigated soils.


 By Brent Rouppet, Ph.D.

(Part 1 of a 3 Part Series)

There are two key soil amendments in agriculture: (1) gypsum (calcium sulfate dihydrate [CaSO4 H2O]), and (2) liming materials such as limestone (CaCO3) and, dolomite ((CaMg(CO3)2). Liming materials are applied to neutralize soil acidity, but in situations where Ca is required without the need for correcting soil acidity, then gypsum is necessary.

These soil amendments should never be confused. Routine and frequent application of gypsum is required for the sustainability of all irrigated soils1; and while limestone, dolomite and other liming materials are necessary to neutralize soil acidity problems, improper usage of these liming amendments will actually result in harm to crops and plants.

The differences between gypsum and liming materials, and their uses in agriculture: 1. Limestone, dolomite, and other liming materials. Liming products are used and necessary in agriculture when the pH of the soil becomes too acidic for optimal plant growth and production. For most soils worldwide optimum pH is 6.2. At this pH the essential plant nutrients are most available; so in most cases in agriculture we strive to maintain a pH as close to 6.2 as practical.

If the soils become too acidic (have pH values lower than 6.2) then limestone, dolomite, or other liming products are applied to bring the pH back to the optimal range for crop/plant growth and production.

An example of the chemical reaction:

CaCO3 (limestone) + CO2 + H2O + 2H+ (acid) Ca2+ + CO2 + H2O

Acidic Soils:

• Soils become acidic when they are leached, especially in areas of higher rainfall. The more leached the soils are, the more strongly acidic they are.

• Strongly acidic soils have (1) few basic cations (calcium, potassium, magnesium and sodium) available in the root zone; (2) higher amounts of aluminum, hydrogen, and manganese; and (3) have lower amounts of more easily leached nutrients: sulfur, boron, zinc, molybdenum, and chlorine.

• Levels of toxic aluminum and manganese iron increase as pH levels do down.

• Most microbial processes, including nitrogen fixation, are slowed down by strong acidity.

Limestone, dolomite and other liming materials should never be applied to soils when the pH levels are above 7.0. At higher soil pH values the carbonate in limestone and dolomite will actually burn the crops, and even cause plant death in more severe cases.

The grower must be aware of which soil amendment products are needed (and how much) for application for specific uses to maximize crop production, and not harm plant growth and production.

Note: Dolomite is also used worldwide as a fertilizer source for magnesium (and calcium), but should only be used when soil pH values are less than 7.0.

Again, at pH values higher than 7.0 the carbonate in dolomite will actually burn the crops. Since limestone and dolomite are 150 times less soluble than gypsum, they generally are not a preferred source of calcium as a fertilizer.


Western Farm Press. 15 February 2016.  U.S. and Chinese officials appear to have reached an agreement on a phytosanitary protocol for the shipment of American-grown milled rice into Chinese ports, according to a statement by the USA Rice Federation.

Most trade exports believe China has been importing more rice than it reports with most of the foreign-grown rice coming from Cambodia, Thailand and other countries along its southern border, but none from the U.S.

“The challenge now is to move from agreement to shipments,” said USA Rice Federation CEO Betsy Ward, who, in the past, has described the long-running negotiations as “complex,” and involving issues that seemed to have little to do with actual pest issues faced by rice producers in the U.S.

“This extraordinary agreement has been a long time coming, and I commend the U.S. negotiators and USA Rice for sticking to it and getting us a phytosanitary protocol that while more complicated and detailed than any other rice protocol in the world, is something both industries appear able to make work,” said Dow Brantley, USA Rice chairman.  read more