How Do Roots Pull Nutrition Out of the Soil?

Not every carrot has the same nutritional value. The secret to highly nutritious plants is, well, in the soil.

by Grace Hensley
PHOTO: KateLeigh/iStock/Thinkstock

I hefted two identical bunches of carrots at the grocery store the other day, both firm and bright orange with a tousled top of vibrant green leaves; however, one was labeled organic and the other conventionally grown. Scientists can’t seem to agree about the nutritional value of these two different types of carrots. Some claim organically grown produce is more nutritious than conventionally grown produce, while others insist there is no difference. I grow my own carrots in soil heavily amended with decomposed chicken manure, but how do I really know what’s inside my food, and how did it get there?

The first place to start when trying to answer this question is to perform a soil test on your garden; balanced soil yields healthy crops. Fall is the best time to take soil samples and send them to your county’s Conservation District Soil Testing Laboratory. Follow their recommendations to improve your soil so that plants can make the most of the nutrients you add.

Two Types of Roots

You probably remember from high-school biology class how plants breathe in carbon dioxide from the air through tiny holes in their leaves called stomata, and then convert the air into sugars using the energy from the sun. Plants draw water and dissolved nutrients up from the soil through their roots, and control where they go throughout the plant by opening and closing those tiny stomata. The sticky nature of water molecules moves nutrients throughout the plant, but plants cannot live on air and water alone. They need a balanced diet of elements drawn up from the soil through their roots.

Some root systems, like the carrot, have a single thick taproot, while others, like grasses and sunflowers, have a fibrous network. When you dig into the soil, you can easily see the side roots on the carrot, but you’ll need an electron microscope to see the tiny root hairs, only one cell thick, branching off and increasing the surface area of the roots. The root hairs, living only a couple of days up to a few weeks, use calcium like ankle weights to help them grow down!

Roots can grow shallow or deep, depending on the soil type and amount of water available while the plant is growing. I’m always reminding my kids to stay on the garden paths; compacted soils from heavy tractors or careless feet will restrict those roots from reaching deeply into the soil, and I’ll have to water more frequently. The same goes if I over-feed my plants—there would be no need for those roots to grow deep to search for the next meal, and I’d just be wasting money as fertilizers wash downstream.

As the growing tips of the root move through the soil, the cells are elongating and dividing. The carbohydrates shed from the root tips attract fungi in the soil forming a mycorrhizal partnership. Tiny root hairs reach out into the soil and intertwine with the fungi that are capable of dissolving minerals from rocks and decaying plants. The thread-like fungal mycelia are much smaller and absorb nutrients better than the root hairs and can reach out further in the soil to draw dissolved minerals closer to the plant’s roots. These fungi only work in specific pH ranges that are favorable for biological activity. This is another reason why checking your soil test report and balancing your soil pH is so important.

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From Soil to Root

Most of that carrot you’re eating—nearly 96 percent of its weight—consists of the basic elements, hydrogen, oxygen and carbon, which the plant gets from carbon dioxide dissolved in water and taken up through the roots. But the remaining part, a mere 4 percent, consists of essential macronutrients (N, P, K, Ca, Mg and S) and micronutrients (B, Cl, Cu, Fe, Mn, Zn, Mo, and Ni) which are essential to plant growth and nutrition. These macronutrients and micronutrients are also essential to our own health.


Nitrogen is the structural component of amino acids in enzymes, which do everything in the cell, and is an essential component of chlorophyll; without it photosynthesis won’t occur. If plant leaves turn yellow, suspect nitrogen deficiency. Fortunately, bacteria in the soil are great at breaking down ammonium and nitrate, providing nitrogen for the plant. The nitrogen-fixing nodules on the roots of your beans and peas are full of rhizobia bacteria, making the nitrogen bioavailable, which can then easily pass into a plant’s roots.


Phosphorus is required for plant energy—think of it like a power-smoothie. Without it, our carrot growth is stunted and will either not grow tall or fail to bloom and set seed. The best source of phosphorus is from decaying organic matter and manures, but it binds tightly to soils and is hard for plants to utilize it. Fortunately, the mycorrhizal fungi produce acids and enzymes to dissolve it, providing plants with the necessary phosphorus.


Potassium is an element that helps regulate water balance in leaves. Potassium binds to clay soils, but easily dissolves in water and is taken up through the roots. Plants low in potassium tend to wilt and form dead spots on the leaves.

Because a plant accumulates nutrients and has a higher concentration in its roots than in the soil, simple diffusion of these molecules from the soil into the cells of the plant may not be sufficient. Special active-transport proteins use energy to move phosphorus, as well as nitrogen, sulfur and other trace minerals into plant cells. You can easily diagnose a sulfur deficiency, typical in plants grown in sandy soils, when new and young leaves turn yellow.

This fall my soil test came back with a whopping 17 percent organic matter from all of that great composted chicken manure. With a balanced pH and plenty of raw material in my soil, the fungi and bacteria are working together to break down the nutrients and bring them to the roots of all of my crops. I’m looking forward to a productive growing season next spring!

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