- Author: Ben Faber
This is an intriguing article that popped up about how to improve blueberry production in alkaline soils. High pH soils are a major issues for many of our tree crops along the coast. pH is what controls the availability of most plant nutrients and what bacteria and fungi grow in the soil, creating the biosphere. So can growing a grass cover crop in our orchards improve lemon and avocado production?
A lawn is better than fertilizer growing healthy blueberries
Intercropping with grasses is an effective and sustainable alternative to chemical treatments for maximizing blueberry yield and antioxidant content in limey soils.
Blueberries are prone to iron deficiency - and correcting it increases their health-enhancing antioxidant content, researchers have discovered.
Published in Frontiers in Plant Science, their study shows that growing grasses alongside blueberry plants corrects signs of iron deficiency, with associated improvements in berry quantity and quality. The effects are comparable to those seen following standard chemical treatment - providing a simpler, safer, cheaper and more sustainable strategy for blueberry farming on sub-optimal soils.
What do superfruits eat?
All soils are rich in iron, but nearly all of it is insoluble.
"Most plants get enough iron by secreting chemicals that make it more soluble," explains senior study author Dr José Covarrubias, Assistant Professor of Agriculture Sciences at the University of Chile. "These iron 'chelators' can be released directly from the roots, or from microbes that grow among them, and allow the iron to be absorbed."
"Blueberries, however, lack these adaptations because they evolved in uncommonly wet, acid conditions which dissolve the iron for them."
As a result, most of the world's relatively dry or alkaline ('limey') cropland is unsuitable for optimal blueberry growth.
"Iron is essential for the formation and function of plant molecules like chlorophyll that allow them to use energy," Covarrubias continues. "That's why iron deficiency shows up as yellowing leaves - and drastically reduces plant growth and yield.
"And in blueberries, iron-dependent enzymes also produce the 'superfruit' antioxidants responsible for their celebrated blue skin and health-enhancing effects."
Strong blueberries must pump iron - but at what cost?
There are two approaches to correcting iron deficiency in blueberries: acidify the soil, or add synthetic iron chelators. Each has its drawbacks, says Covarrubias.
"The commonest industrial approach is soil acidification using sulfur, which is gradually converted by soil bacteria into sulfuric acid. The effects are slow and difficult to adjust - and in waterlogged soils, hydrogen sulfide might accumulate and inhibit root growth.
"Acids can also be added directly via irrigation systems for more rapid acidification - but these are hazardous to farmers, kill beneficial soil microbes, and generate carbon dioxide emissions.
"A commoner strategy among growers is application of iron bound to synthetic chelators - often sold as 'ericaceous fertilizer' - but these are very expensive and leach potentially toxic chemicals into the water table."
A cheaper, safer alternative is needed for efficient large-scale blueberry production. Thankfully, one already exists.
"Grasses - which are well-adapted to poor soils - can provide a sustainable, natural source of iron chelators via their roots when grown alongside fruiting plants. Intercropping with grass species has been shown to improve plant growth and fruit yield in olives, grapes, citrus varieties - and most recently, in blueberries."
A grassroots approach to sustainable blueberry farming
Now, Covarrubias and colleagues have brought intercropping a step closer to the mainstream of blueberry cultivation.
For the first time, they measured the effects of different methods of iron chelation on antioxidant content and other fruit qualities in blueberries.
"In an orchard of 'Emerald' blueberry bushes cultivated in alkaline (pH 8) soil, we compared the effects of five different iron chelation treatments: a 'gold-standard' synthetic iron chelator (Fe-EDDHA), intercropping with grass (common meadow grass or red fescue), cow's blood (Fe-heme), or no treatment (control)."
"We found the association with grasses increased not only the total weight and number of blueberries per plant, but also the concentration of anthocyanins and other antioxidant compounds in their skins, compared to control. The effect sizes were comparable with the proven synthetic chelator Fe-EDDHA, whereas applications of Fe-heme from cow's blood - a fertilizer commonly used in home gardens - had no significant effect."
The beneficial effects paralleled improvement in the plants' iron status (leaf color), which was also comparable between the grass-associated and the Fe-EDDHA-treated plants. None of the treatments had a significant effect on average berry weight
Turf is ready to roll out for healthier blueberries
A potential limitation of intercropping observed in the study was a decrease in berry firmness, since firmer berries are favored by consumers.
"The association with grasses decreased berry firmness compared with control plants, whereas the berries collected from plants treated with Fe-EDDHA reached intermediate values.
"However chemical analysis showed a non-significant trend towards increased ripeness in the berries collected from the intercropped plants, which could account for this small difference."
Intercropped plants also required an additional water supply to maintain a similar soil moisture to other treatments, but plant management was otherwise straightforward and the same across groups. The grasses were kept cropped between 5 and 15cm - a typical range for an attractive mown lawn.
"Our findings validate intercropping with grasses as a simple, effective, sustainable alternative to standard iron correction strategies in blueberries," concludes Covarrubias. "Both commercial and private growers can put this strategy to use right away to boost their blueberry crop and antioxidant content."
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Please link to the original research article in your reporting: https://www.frontiersin.org/articles/10.3389/fpls.2019.00255/full
Frontiers is an award-winning Open Science platform and leading Open Access scholarly publisher. Our mission is to make research results openly available to the world, thereby accelerating scientific and technological innovation, societal progress and economic growth. We empower scientists with innovative Open Science solutions that radically improve how science is published, evaluated and disseminated to researchers, innovators and the public. Access to research results and data is open, free and customized through Internet Technology, thereby enabling rapid solutions to the critical challenges we face as humanity. For more information, visit http://www.frontiersin.org and follow @FrontiersIn on Twitter.
- Author: Ben Faber
Carbonateceous? Gypsum? Read on.
Soil pH or the “acidity” or lack of acidity of a soil can be confused by the different uses and chemistries that surround the term pH, or the power of hydrogen. This can be further confused by the terms “alkalinity” or “basicity” of the soil or the soil solution which can further confuse the situation by whether the solid or liquid phase of the soil is being measured. So, in short, a soil is acid if it has a pH below 7 (more hydrogen ions) and basic when above 7 (fewer hydrogen ions). Big numbers are more basic, small numbers more acidic.
The natural world has a pH scale of 1 to 14. Knowing soil pH is important because it can tell you the inherent fertility of a soil. Usually the higher the number between 5 and 8, the more “basic” nutrients are present, like calcium, magnesium and potassium. When the numbers get lower than 5 and larger than 8, the nutrients may not be there or they may be tied up. Changing the pH can often release nutrients that are not available. Iron and zinc plant deficiencies are most often controlled by soil pH and once pH is neutralized or made acid, the deficiency disappears. In a way, pH is one of the most important nutrient indicators of a soil's fertility and managing should be an essential practice.
Soils that have an elevated pH, those above 7, are usually dominated by carbonate and most commonly this is calcium carbonate. As a rock, we call this limestone which is derived from sea shells or coral. As a mineral, it is called calcite. And in various manipulated forms it is called,calx, lime, calcium hydroxide, calcium oxide,calcined lime, quicklime. Maybe other names, as well, depending on how it is made and used. Mixed with quartz sand it is made into glass. When an acid is added to calcium carbonate, like citric acid, sulfuric acid or rain water (yes pure rain water is acidic), the reaction gives off carbon dioxide and water. (When you respire, burning sugar which is a form of carbonate, you do the same thing, giving off CO2and H2O). When calcium oxide or lime is mixed with water and left to harden, it forms cement when it absorbs carbon dioxide from the air. So, lime, limestone and calcium carbonate are not very soluble. But it does dissolve. Calcium carbonate is a salt and dissociates into calcium ions and when surrounded by water molecules, the carbonate becomes bicarbonate ions. When the water dries up, the bicarbonate becomes carbonate again. A soils report may refer to this cement as “free” or “diffuse lime”, indicating that there is a lot present, and it may even be possible to see the old shells there.
A soil that is dominated by calcium carbonate is called a calcareous soil. It is the carbonate that defines the soil, it has an elevated pH, usually between 7.5 and 8, depending on other minerals in the soil (minerals are naturally occurring chemicals). A high pH leads to plant nutrition problems. It's not until the soil is acidified to drive off the carbonate as carbon dioxide that the pH will drop and the nutrient deficiency disappears.
One of the problems with the word “calcareous” is that it can be interpreted as meaning dominated by calcium. A soil or water analysis may reveal high levels of calcium and this can lead to concern. Calcium is a base cation, necessary for plant and human growth. In western states, water and soil tend to have calcium as the dominant cation, balancing anions like sulfate and carbonate and in some cases chloride. The important character to look for, though, is the carbonate or bicarbonate in the soil or water. It's not the calcium that is controlling pH, it's the carbonate. Very commonly calcium is said “to reduce the acidity of soil.” But it's not the calcium, it's the carbonate.
So, does this seem like a big semantic problem, how many dancing on the head of the pin sort of debate? No, because one of the most common recommendations for correcting an alkaline water/soil is to add gypsum – calcium sulfate. In a calcareous soil, calcium is already present, so adding more calcium is not going to change it, but rather increase it. Sulfate does not displace carbonate. Carbonate just stays there because calcium carbonate is not very soluble. Remember it is cement. Actually, adding gypsum is adding more salt which has its own problems - leading to saline soils.
So, this blog all came about because two different people asked me about correcting iron deficient avocado trees in calcareous soils (or a carbonateceous soil as I call it), with gypsum. pH correction is not going to occur with gypsum. An acid needs to be added to drive the carbonate off as carbon monoxide and then the pH comes down. So, you use acids or elemental sulfur that converts to sulfuric acid or urea-sulfuric acid fertilizer, or long-term use of acid fertilizers like ammonium sulfate or organic mulches that gradually create acid conditions.
Soil acidification as a practice is a separate subject all together that needs to be discussed, but gypsum is not normally a part of the process of field acidification. Does that mean gypsum does not have a place in soil management? It does, just not in the case of calcareous soils. And even though you can see bags of lime and dolomite (calcium-magnesium carbonate) on store shelves, for sure don't use that in the calcareous soils of California.
So, what about gypsum and sodic soils – soils dominated by sodium. And what about serpentine soils – soils dominated by magnesium? There is more to tell about gypsum.
Photo: old shells in the soil