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How much water and nutrients can your soil hold?

Your soil is a unique mixture of sand, silt, clay, and organic matter. The particular make-up of your soil determines its color, texture, and nutrient storage capacity. Knowing your soil’s texture and nutrient storage capacity is important when deciding how much and how often to feed and water your plants. Some nutrients are more easily stored and attached to soil particles compared to others due to the strength of their electrostatic bond. As the famous saying goes, opposites attract – and this holds true in soil as well.

Mineral nutrients such as calcium, potassium, ammonium, and magnesium are called cations because they have positively charged ions. The ability to attract and hold onto these positive cations comes from negatively charged soil particles, called colloids, found in organic matter and clay. It’s important for these nutrient cations to attach to the soil colloids so that they can be supplied to the plant when needed. If the nutrient cations don’t attach, they’ll easily leach out during a time of rain.

However, like in most fair economic systems, the plant can’t just take these nutrients from the soil without giving something in return. For example, if a plant needs some potassium, it will have to exchange one of its cations for the soil’s potassium cation. Thankfully, plants produce hydrogen cations that they can use for this exchange. The soil accepts these hydrogen cations because they’ll be used in photosynthesis and respiration.

This exchange is easier than others because both hydrogen and potassium have a positive charge of +1. Calcium, on the other hand, has a positive charge of +2 and therefore requires two hydrogen cations for its exchange, making the process a bit harder. The higher the positive charge on the cation, the harder it becomes to exchange between the soil and plant. However, the bond between the higher charged cations and the soil is stronger than that of the lower charged cations. This exchange process occurs on the plant’s root hairs, which is why it’s important to have a strong, healthy root system for your plants. The amount of cations that can be retained within the soil is called Cation Exchange Capacity (CEC) Source: Jagdish Patel.

Understanding the CEC of your soil is important due to its strong influence on nutrient and water retention and availability, soil structure stability, and soil pH and fertility. Adding organic matter to your soil is one of the most effective ways of increasing your soil’s CEC and increasing the amount of exchange sites. The more exchange sites, the greater the ability for nutrients to be retained within the soil. Having a high CEC not only reduces leaching of nutrients, but also helps buffer your soil against pH changes. 

While it’s very beneficial to have a high CEC in your soil, soils with a low CEC can still be managed successfully – they just have different requirements than soils with a high CEC. Low CEC soils need small, but frequent intakes of nutrients and water, rather than large, infrequent intakes due to their fewer exchange sites. Less exchange sites means less space to hold onto the incoming nutrients. And as microbes are actively involved in transforming nutrients to plant-available forms, it’s imperative to maintain suitable soil conditions for optimal microbial activity.

Many soil testing labs will provide you with your CEC levels which are reported  in units of milli-equivalents per 100 grams of soil (meq/100 g). Average levels range from less than 10 for sandy soils and 50-100 for organic rich soils. Pure organic matter has a level of 200-400.  Generally, 1-10 is considered low while 10-50 is considered moderate to high.