Retaining Nutrients in Your Soil
(Conservation Currents, Northern Virginia Soil and Water Conservation District, Fall 2006)
Anyone who has put a few drops of plant food into a watering can knows that dissolved nutrients such as nitrogen, phosphorous, potassium, and iron are essential for plant growth, but many people do not know that a nutrient must be dissolved in order to facilitate growth. Roots are the conduit through which all nutrients from the soil enter into a plant, but roots take up only one thing: water. Because of this, nutrients must be dissolved before they can be used by the plant.
Yet, dissolved nutrients are also a pollution threat. Stormwater can run off the surface or percolate through highly fertilized soils to contribute dissolved nutrients to our streams and groundwater. This abundance of nutrients disrupts the natural ecology of streams and creates well water that is unsafe to drink. Luckily, as long as fertilizer is not over-applied, soil can handle the influx of nutrients thanks to some unique chemical characteristics of clay and organic matter.
Clay and organic matter are both known as colloids; tiny particles in the soil that carry a small electric charge. Both come about their charge in very different ways. Clay is a crystal, made of repeating sheets of aluminum and silicon oxides. Aluminum and silicon are both cations – positively charged atoms – and both carry a fairly strong charge. Oxygen, on the other hand, combines with the aluminum and silicon to make the clay oxides and is an anion – a negatively charged atom. The positive charges of the aluminum and silicon and the negatively charged oxygens cancel each other out. However, during the lifespan of a particle of clay, dissolved cations in the soil water will migrate into the clay’s crystal structure and replace some of the aluminum and silicon. These cations, such as iron and calcium, do not carry as strong a charge as aluminum or silicon. Thus over time, as more and more substitutions are made, clay develops an increasingly negative charge.
Organic matter does not have an ordered crystal structure like clay. Rather, it is a chaotic and variable mess of carbon chains with hydrogen, oxygen and other atoms bonded to its tips. Oxygen, as stated before, is an anion. Hydrogen is a very weak cation. As the environment in the soil changes due to fluctuations in pH, moisture and other variables, the hydrogen cations leave and return to the bonding sites on the tips of the carbon chains. This two-way migration causes small negative and positive charges to appear and disappear on the surface of organic matter, kind of like twinkling lights on a Christmas tree. However, like clay, the total sum of the organic matter’s charges is usually negative.
So clay and organic matter tend to be negatively charged, but how does this relate to nutrients? Well, most nutrients are cations and one of the most basic rules of physics is that opposite charges attract. Positively charged nutrients will be attracted to the negatively charged clay and organic matter. This attraction is strong enough to pull many of the nutrients out of the water and bond them to the negatively charged surfaces, but not strong enough to make the bond permanent. This means that soil can hold onto nutrients strongly enough to prevent them from leaching into surface and groundwater, but will also release the nutrients slowly enough that plants will not lack them.
The ability of a soil to hold onto nutrients is measured by its cation exchange capacity, or CEC. Soils with an abundance of clay or organic matter will have a very high CEC, while soil with lots of sand or little organic matter will have a low CEC.
To increase the ability of your soil to retain nutrients, the best thing to do is add organic matter such as compost. Organic matter has a higher cation exchange capacity than clay, and adding compost to your soil will have the added benefit of improving structure and aeration, reducing density, and increasing nutrient content. Adding clay also will increase a soil’s nutrient retention, but the secondary effects are almost all negative – such as decreased water infiltration. So if you want a soil that can fight pollution and nourish your plants, look no further than compost.