Nutrients and the Soil
Early experiments on mineral uptake were performed by Hoagland, Stout and Amon in 1923. They showed that minerals were taken up from the soil primarily in ionic form. The rate of uptake of different ions by roots varied and one ion influenced the uptake of other ions. As soil is the medium for the storage and exchange of mineral ions, its properties, ion exchange capacity, pH and the presence of different cations and anions affect the availability of ions to the plant. In other words, the presence of a certain mineral ions in abundance in the soil cannot ensure its availability to the plant because ions may adhere to clay or precipitate out of the solution as insoluble salts. The soil with high water holding capacity generally has high mineral holding capacity as well.
The fine particles of clay and humus possess a relatively large surface to volume ratio and are negatively charged. Hence, they have higher ion-binding capacity than the soil composed of coarse particles. Figure shows the colloidal clay crystals (micelles) with innumerable negative surface charges. The cations are loosely bound to negative charges by ionic bond and are capable of exchanging rapidly and reversibly with those in the soil solution. H+ ions have greater affinity for charged soil particles than ca2+, Mg2+or K+ ions. Therefore, these cations are released in soil water by H+ ions and made available for uptake by roots. The acidity of soil also increases due to respiration because CO2 released reacts with soil water to form carbonic acid.