Zeolites have a long history of enriching soils and enhancing plant and crop yields. The renowned ability of many volcanic soils to bring better harvests is due in large part to the presence of natural zeolites in these soils, created from the intense interaction between lava and the environment. Natural zeolites have long been added to soil to moderate moisture availability in arid environments, and to hold and gradually release nutrient supplementation in less fertile soils. Zeolites used in soil amendment work in two complimentary ways: by adsorbing nutrients and micronutrients, and by absorbing water. By holding nutrients and water when these are abundant – such as when fertilizer or irrigation are applied – and gradually releasing these when they start to become scarcer, zeolites even out water and nutrient availability over time. This greater and steadier availability over time leads to more consistent plant growth, more reliable and abundant yields, and healthier soil systems.
Let’s dive into how zeolites accomplish these things!
Plant roots send cations – positively charged hydrogen ions – out into the surrounding soil when they are nutrient-deficient. These cations travel via the moisture in the soil and interact with clay particles and other particles that hold various other cations. Clays in soil contain collections of aluminum and silicon molecules, where the aluminum molecules have negative charge sites to which cations such as calcium, magnesium and copper can bind. When plants increase the concentration of hydrogen ions through signalling, some of these hydrogen cations displace some of the other nutrient cations that are held on these sites. Breakdown of organic matter or application of fertilizer will, in turn, cause the relative concentration of nutrient cations to rise relative to the hydrogen ion concentration in the soil. When this happens, nutrient cations displace some of the hydrogen ions and bind once again to the soil particles.
This holding of cations by these negative charge sites is referred to as adsorption. Adsorption is different from absorption, in that adsorption only happens on the surface of a material: adsorption is only skin deep, so to speak. The ability for soils to hold cations for later release is referred to as the soil’s Cation Exchange Capacity (CEC). Higher CEC values indicate soil that can hold larger quantities of cations in reserve through adsorption.
How zeolites enhance cation exchange capacity
Zeolites are similar to clays as they are collections of aluminum and silicon with negatively charged areas able to hold cations. However, zeolites contain huge amounts of surface area relative to their mass. Zeolites contain many different folds and features that are invisible to the naked eye but that dramatically increase their surface area, providing much more surface for cation interaction than meets the eye.
But surface area is not the only factor in determining cation exchange capacity. The specific proportion of aluminum to silicon and other elements determines the number of negative charge sites for cation retention. Greater proportions of negative charge sites bring greater cation holding abilities for the same amount of surface area than lesser proportions. Zeolites contain many more charged sites for cation retention than the clay typically found in soil, on a weight-proportional basis.
Different zeolites have different types of folds and features – different structures – and vary significantly in their surface area and in the quantity of available charged sites. Just like the pages of a book pack a tremendous amount of writing area into a small package, “platey” zeolites containing microscopic stacked sheets pack much more surface area into a space than other configurations. Thus, different zeolites enhance the ability for soil to hold nutrients in different amounts. But all zeolites used for soil amendment feature a vastly greater cation exchange capacity than the typical clays in soil. For bulk mined zeolite, this may be as much as a 10x increase in CEC. But for manufactured zeolites specifically engineered to increase CEC, these can bring as much as a 70x increase or more in CEC vs. soil clays. Therefore, supplementing soil with zeolite can significantly increase soil CEC and therefore increase soil nutrient retention and availability.
Moisture retention enhancement
Zeolites take up moisture in the soil via absorption, in a manner very much like a molecular sponge. Unlike adsorption, absorption takes advantage of the volume made by the empty spaces created in the microscopic folds and features of zeolites. Like a sponge, water comes into zeolites and is absorbed when the soil is sufficiently saturated. As the soil dries out and becomes less water-saturated, the water contained in the zeolite seeks to bring the moisture content of the soil closer into equilibrium with that of the zeolite. This causes the retained water to leave the zeolite and enter the surrounding soil, becoming available again, until the zeolite and soil are closer to the same moisture content.
While zeolites have a greater ability to absorb and release moisture than the clay in soil, different zeolites with different microscopic features have different abilities to absorb moisture. But all zeolites share the unique ability to absorb moisture without dissolving or compacting. Unlike clay, zeolites do not swell or change shape at all when they absorb water. This means that zeolites help to reduce soil compaction and keep moist soil aerated and looser, leading to better plant rooting and root growth.
Moisture retention is important not just so that plant roots can obtain moisture! Having adequate soil moisture is also vital for letting hydrogen cations diffuse from plant roots into the soil, and nutrient cations travel from adsorption sites back to the plant roots. Without adequate wetting, these cations will not be able to travel and nutrients remain locked within soil, inaccessible to the plants signalling for these.
What’s right for me?
Across a wide variety of applications, the nutrient and moisture retention and release capabilities of zeolites make them highly effective as soil amendments for increasing plant growth and yields. Zeolites do so by improving the cation exchange capacity (CEC) of soil, which in turn leads to greater nutrient and micronutrient availability. Zeolites also increase the moisture retention of soil, releasing moisture as prompted by the drying out of the surrounding soil.
Different zeolites have different levels of soil amendment ability. Bulk mined zeolites may be less effective than purpose-engineered zeolites, but they are affordable and ideal for high-volume applications such as outdoor produce and tree farms. For higher-value indoor agriculture, where each plant is grown in a relatively low amount of soil, engineered zeolites such as the ZK GrowFactor family of zeolite soil amendments bring a higher level of soil amendment and a greater increase in yields over commodity zeolites.