by Albert Diaz
For many farmers, agronomists, and soil scientists, the question of using calcium supplements on calcareous soils is simply mute. “Calcareous soils of the desert southwest have a lot of calcium, therefore Ca supplements are not necessary”, end of story. This has been the prevailing mindset in Arizona for many years, and anyone trying to promote the use of Ca inputs will likely face strong, and sometimes hot, headwinds. Mindset and high soil calcium lab tests notwithstanding, blossom end rot (BER) on watermelon and chili pepper, and salt damage to borders of leafy greens, are not uncommon in AZ. Some well-known growers in AZ now include Ca supplements in their nutrition programs, and the reasoning is solid.
The time-proven technique in calcareous soils involves acidifying irrigation water, proper timing of irrigation, and when sodium is present, the use of gypsum. While many farmers make go-no go decisions based on their farm’s ability to grow a profitable crop, new challenges require new perspectives on these Ca dynamics. Of great importance to Arizona and California desert farmers are the imminent reductions of water allocations from lake Mead (Salt River Project). For instance, Yuma growers depend on availability of good quality water to grow the fresh lettuce most of the nation consumes during winter. When surface water is limited, growers must supplement irrigation with lesser quality ground water. In some cases, farmers may have to grow less profitable, albeit more salt tolerant crops, or even cease production in those fields. The economic impact can be significant.
Some years ago, I had the opportunity to address a Ca deficiency issue for a major watermelon grower in Arizona. The problem; a sensitive but economically important variety of watermelon was showing Blossom End Rot. While grower and company experience were high, the water quality was marginal. Established BMP’s, management practices developed for the region and validated for many years, were not yielding satisfactory results.
The company I worked for at the time recommended the use of a product based on micronized calcium carbonate and algae extract. As expected, we faced the hot headwinds. However, the grower was open enough to allow us to perform a no-cost field demo. After encouraging initial results, and independently trying the method and material, he included the use of calcium carbonate supplements into his nutrition programs. This one example is not in any way unique in Arizona, as many well-respected growers now include Ca supplements in their programs.
Why and how this apparently simplistic and counter-culture solution works, can may help us expand our understanding of the plant-soil-water dynamics that affect calcium uptake and give new insights on the management of this important nutrient.
In the soil, as irrigation water dries, the dissolved calcium cations react with the bicarbonates contained in well water, and the phosphates in the soil, and precipitate in the form of low solubility calcium carbonate, or mostly insoluble calcium phosphate crystals. These soil chemistries quickly reduce calcium availability in calcareous soils.
Another consideration is the presence of antagonistic cations in soils and irrigation water, especially sodium (Na+). Na+ has much lower bonding force than Ca++. In the presence of water, and the weak acids exudates produced by roots, Na+ easily disassociates from the CEC at the exchange sites and is the last one to be adsorbed back as soils dry out. This makes sodium highly soluble and easier to absorb than other nutrients. This is especially significant with Ca++, as it has the strongest adsorption strength. (The order of the strength of adsorption of cations is known as “lyotropic series”, where Al+3 > H+ > Ca+2 > Mg+ 2 > K+ >= NH4+> Na+). When we acidify soils to make soil Ca available, it’s important to consider the relationship between Na+ and other cations present in the soil solution, as it will become more easily available than Ca++, Mg++ and K+. A paste extract analysis using irrigation water can provide insights on these relationships and the nutrient’s expected availability.
Calcium is mostly absorbed through new root hairs, moving into the plant as a dissolved cation in the water mass traveling between the root’s cell walls, (the apoplastic pathway). As roots mature, a membrane called the Casparian Strip, forms between the cell walls on the outside perimeter of the roots, effectively forming a casing separating the inner root from it’s cortex. The Casparian strip, while permeable to water and other nutrients, is impermeable to calcium. After the Casparian Strip has formed, any further calcium coming into the plant must be transported across the cell walls, the simplastic pathway. Regardless of how much Ca++ is dissolved in the water column, its uptake will be very limited without new roots.
Here’s a trick question I’ve come across: Can foliar applications increase fruit available calcium? We know that calcium travels up through xylem, is phloem immobile, most of the calcium coming into the fruit is allocated during the first stages of fruit growth, and, that negligible amounts of calcium get into the fruit through phloem. This effectively means that foliar calcium will not travel into the fruit. However, it may help (vagueness intended) to reduce the Ca sink on leafy structures, allowing for a more generous partitioning of the xylem available calcium into the early growing fruit. Root calcium must be available for the foliar to be effective. Foliar applications can only help allocate root absorbed Ca++.
The optimal calcium solution will take into consideration all possible limiting factors, and provide methods and materials that;
- include a root available calcium in a fully soluble or easily dissolved form,
- that does not contain antagonistic nutrients
- has low salt index to prevent burning new root hairs
- Is balanced against other bases present in soil solution, especially Na+
- is timed appropriately to coincide with the fruit’s initial stages of growth
- includes a strategy to promote new root growth
includes a foliar strategy to increase amount partitioned into the fruit