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Soil-nutrient relationships

Understanding CEC and why it matters for Soil Health.

Think of your farm’s soil as the storeroom for your plant nutrients.

Nutrients, such as calcium and magnesium, may be supplied to plants solely from reserves held in the soil. Others like phosphorus and nitrogen are added regularly to soils as fertiliser to be withdrawn as needed by crops.

“It’s a complicated concept, but critical to understanding your soil’s fertility. We’ve tried to simplify the concept to help you understand why measuring CEC as part of your soil testing and analysis is important and useful.”

All nutrients are either positively or negatively charged particles/ions. Positive particles are called cations and negative charged particles are called anions.

Soil cations essential for plant growth include calcium (Ca++), ammonium (NH4+), magnesium (Mg++), and potassium (K+). Three additional soil cations that are not essential plant elements but affect soil pH include sodium (Na+), aluminium (Al2+), and hydrogen (H+). The calcium soil cation is the most important and dominant cation affecting the uptake of nutrients and consequently plant growth. Calcium not only acts as a bonding agent in the aggregation of soil particles to help bind organic and inorganic substances, but it is also responsible for contributing to the building of cell walls in plants. Calcium deficiency can lead to improper cell wall formation in new tissue which may show up as distorted new growth on plants.

Additionally there are other cations, equally important for plant health but less abundant called minor and micronutrients. These micronutrient cations include iron (Fe); manganese (Mn); zinc (Zn); copper (Cu) and cobalt (Co).

Given the importance of cations, in particular calcium cations, the ability of your soil to store or hold them is important to soil fertility optimisation. The relative ability of soils to store positively charged nutrients essential for plant growth is referred to as cation exchange capacity (CEC).

CEC is an inherent soil characteristic that is difficult to alter significantly but can have a significant effect on the fertility management of your soil, and therefore your crop growth. A high CEC soil can hold more positive nutrients and deliver them to the plant when it’s needed.

Soil cations (i.e. essential nutrients) are consumed in the production of crops and stock and must be replenished to avoid reduced yield output and quality. Given the critical importance of CEC in determining your soil’s health and ability to sustain your crop, BioAg includes CEC analysis in its soil testing and analysis program.

Slobodan Vujovic BioAg Area Manager

Soils are composed of sand, silt, clay, and organic matter/humus. Clay and organic matter ions/particles have a net negative charge and are therefore anions. Cations are held by negatively charged particles of clay and humus called colloids (see diagram). They act as a storeroom of nutrients for plant roots.

As plant roots take up cations, other cations in the soil water replace them on the colloid. If there is a concentration of one particular cation in the soil water, those cations will force other cations off the colloid and take their place, potentially causing toxicity or poor plant growth.

Soils containing clay and/or organic matter have a higher CEC while sandy soils have a low CEC. In low CEC soils many of the cations that are present may be in the water around the soil particles, and not actually held by the particles. These cations are very susceptible to being leached or drained away in the soil water. Low CEC soils are typically low in calcium.

Maintaining the amount and ratio of available cations (as measured through soil test base saturation) is key to highperforming crops or pastures and is a key measure in BioAg’s soil testing and analysis program. Lower CEC sandy soils are frequently acidic and respond well to lower rates of lime and/or dolomite. BioAg’s approach is to balance cations as part of the nutrient input recommendation. The BioAgPhos® solid fertiliser range and biostimulant options are proven to build organic matter and provide long term nutrient availability, especially important in low CEC soils where nutrients not bound to clay colloids are more susceptible to leaching and lockup.

Soil & Seed® biostimulant, in particular, is a source of humus and soil carbon, thereby providing short-term improvement in CEC in sandy soils, and greatly reducing the amount of nutrients lost through leaching.