Plant Available Calcium: Using Calcium Fertilizers in Calcareous Soils in the Desert Southwest

Alberto Diaz
July 11, 2023

In desert Southwest agriculture, the debate surrounding calcium fertilizers on calcareous soils has long been settled - or so it seems. The traditional belief is that calcium-rich soils render additional supplementation unnecessary. Yet, as we dig into the depths of this technical subject, we uncover a different narrative, one that challenges conventional thinking. Join us as we explore the intriguing dynamics of calcium deficiency. We also explore the limitations of traditional practices and the compelling reasons behind the growing adoption of calcium fertilizers in forward-thinking farmers' nutrition programs.

Unveiling the Challenges

Arizona's arid landscapes have witnessed a clash of conservative mindsets and calcium-related crop issues. Despite high soil calcium levels, blossom end rot and salt damage in crops like watermelon, chili pepper, and leafy greens cannot be overlooked. A number of progressive farms in the region have begun integrating calcium fertilizers into their nutrition programs, driven by compelling reasons and real-world experiences.

Redefining Best Management Practices

Traditionally, countering calcium deficiencies in crops grown in calcareous soils revolves around acidifying irrigation water, the precise timing of irrigation, and, in the presence of sodium, the use of gypsum. These practices have been deeply ingrained and proven effective over time. However, emerging challenges, such as impending water allocation reductions, necessitate a fresh perspective on calcium dynamics. With limited access to high-quality water, farmers face tough decisions, including growing less profitable, salt-tolerant crops or even ceasing production altogether. Economic implications are significant, prompting innovative solutions.

A Case Study

Several years ago, a major watermelon grower in Arizona approached us with a calcium deficiency issue. Despite their extensive knowledge and adherence to established best management practices, a sensitive and economically significant watermelon variety was plagued by Blossom End Rot. With marginal water quality, conventional methods fell short. In a bold move, we recommended a product based on micronized calcium carbonate and algae extract, even against skepticism. The grower agreed to a no-cost field demonstration, and the initial results were encouraging. The grower subsequently incorporated calcium carbonate fertilizers into their nutrition programs, a decision echoed by numerous respected growers in Arizona.

Understanding Plant-Calcium-Soil-Water Dynamics: 

To understand why this seemingly simplistic and unconventional solution works, we must explore the intricate dynamics between plants, soil, and water that influence calcium uptake. When irrigation water evaporates, dissolved calcium cations react with bicarbonates and phosphates in the soil, precipitating as low-solubility calcium carbonate or insoluble calcium phosphate crystals. Consequently, calcium availability in calcareous soils rapidly diminishes. Another critical factor is the presence of antagonistic cations, particularly sodium, in soils and irrigation water. Sodium, being less strongly bonded than calcium, readily dissociates from the exchange sites in the presence of water and weak root exudates. This makes sodium highly soluble and easily absorbed compared to other nutrients. Acidifying soils to make calcium available necessitates considering the relationship between sodium and other cations, as sodium becomes more readily available than calcium, magnesium, and potassium.

Calcium is primarily absorbed through new root hairs, transported as dissolved cations in the water moving between root cell walls, known as the apoplastic pathway. However, as roots mature, they develop the Casparian Strip, a non-calcium permeable biopolymer membrane that effectively blocks calcium from traveling through the apoplastic pathway. Once this membrane forms, calcium must be actively transported through root cell walls, (the symplastic pathway), limiting its availability inside the plant. This challenge to plant available calcium is highly independent of how much of this nutrient it is available in the soil solution. The key takeaway is, new root hairs (pre-Casparian strip, a.k.a non-suberized roots) are required for effective calcium absorption.

The Enigma of Foliar Calcium Applications: 

Can foliar applications of calcium increase its amount inside the fruit? This is a technical and important question, whose common answer is no. It cannot. Calcium travels upward through the xylem and is largely immobile in the phloem, therefore it is unavailable to the fruit when foliar is applied. However, there are related dynamics that may result in the desired increase of calcium in the fruit.  

Since most calcium reaching the fruit occurs during the initial growth stage, and negligible amounts enter through the phloem, it is true that foliar-applied calcium will not reach the fruit. However, it may indirectly favor the fruit by reducing calcium demands on leafy structures, allowing a greater portion of xylem-available calcium to reach the developing fruit. It is important to note reducing the calcium sink on leaf structures by itself is not enough, and root-absorbed calcium must be available for a foliar strategy to be effective. Foliar strategies can influence calcium partitioning in the fruit, but the nutrient reaching the fruit is always absorbed by the roots.

Key Considerations for Optimal Calcium Strategy: 

Achieving the optimal calcium solution requires a comprehensive approach that addresses potential limiting factors. The ideal solution should:

  • Provide readily available calcium in a fully soluble or easily dissolved form
  • Have a low salt index to prevent damage to newly formed root hairs
  • Be adequately balanced against other soil solution bases, particularly sodium
  • Be timed appropriately to coincide with fruit growth's initial stages
  • Include strategies to promote growth and minimize damage to new roots
  • Include a foliar strategy to support xylem available calcium allocations to the fruit

When used as part of a comprehensive strategy to address calcium-related disorders, calcium fertilizers can help enhance the overall performance of crops grown in the desert southwest.

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