The Significance of Soil Colloids in Plant Growth

Basics

• Positively charged nutrient cations, seeking a neutral electrical state, adhere or cling to the negatively charged surface of colloids. Plants require both the nutrients adsorbed to the surface of colloids and the air and water in the spaces surrounding them.

  The surface area of large, loosely packed colloids is smaller than small, densely packed colloids. Very coarse sand has 90 colloids or particles per gram; coarse sand has 720; medium sand has 5,700; fine sand has 46,000; very fine sand has 722,000; silt has 5,780,000 and clay has 90,300,000,000. One lb. of coarse sand has 1,000 times less surface area than 1 lb. of clay. The Biogrow company estimates that a layer of clay 6 inches deep covering 1 acre has a surface area that equals 110,000 square miles, the size of Nevada.

Soil Fertility

• The fertility of soil is measured by its cation exchange capacity (CEC), or the ability of cations to adhere to the surface of colloids. Fertile soils have high CEC; poor soils have low CEC. Calcium, magnesium potassium and sodium are among cations commonly found in soil. Rain and heat can leach these nutrients from tropical and subtropical soils, increasing the acidity of the soil and decreasing its fertility.

Soil Properties

• Since positive cations are attracted to the negative surface of colloids, the greater the surface area of colloids in a soil, the more the soil will stick together. Sand, which has fewer, larger colloids, is loose and easy to cultivate. Clay, which has numerous, small colloids, is dense and more difficult to cultivate.

Water and Air

• Water that bears nutrients that plants need for growth forms a thin layer on soil particles. The larger the surface area of colloids in the soil, the more water the soil will hold. Since clay has numerous small colloids with a large total surface area, it will hold more water than sand, which has fewer, larger colloids with a smaller total surface area. Heavy clay soils with compacted colloids can block out water and air that plants need.

Micro-organisms

• Micro-organisms that add to the fertility of soil and are necessary for vigorous plant growth grow and form colonies on soil particles. Soils with a high surface area on the accumulated colloids have more space on which these organisms can grow, hence soil that is dense with small colloids is more fertile than loose, sandy soil with large colloids.