How Plants Obtain Minerals from Soil Using this cross-section through a root, we can review the way root hairs take up minerals from the soil. First we enlarge the root hair for better visibility. Roots use ion exchange to detach positive ions from negatively-charged soil particles. Root hairs achieve cation exchange by releasing molecules that react in the soil solution to produce H+ ions, which displace the soil ions. What kind of molecule does the root hair release to start this ion exchange process? Root hairs make carbon-dioxide molecules during respiration. CO2 molecules diffuse into the soil, where they combine with water to release H+ and carbonate ions. The H+ ions bind to soil particles and displace positive mineral ions. Positive ions are called cations, so this process is called cation exchange. Roots also spend ATP to pump H+ ions directly from the root to the soil. Soils hold some minerals more tightly than others, and very heavy rains can wash, or leach, the more loosely-held minerals out of the soil. What types of minerals would you expect to be most depleted by heavy rains? Soil particles tend to be negatively charged, and do not bind negative ions such as nitrate. Positive ions stick to the negative soil particles and resist being leached away. Air pollution can cause acid precipitation, in which raindrops carry H+ ions into the soil. Knowing that the H+ released by roots into the soil helps the root hairs collect minerals, why wouldn't you expect acid precipitation to make soil more fertile? Because running water washes detached cations away. Acid precipitation duplicates the cation exchange effect of roots; H+ ions displace positive mineral ions from negative soil particles. However, the flowing water washes the detached ions away before they can diffuse to the root hair. This makes the soil less fertile.