Soil is an ecosystem, far more complex than other ecosystems of the planet. Climatic or environmental factors, seasons of the year, geologic factors, and hydrologic factors determine the physicochemical and biological properties of the soils. Plant nutrients, some 16 in number, come from soil, water, and atmosphere. Alkaline soils reduce the availability of Zn, Mn, and B. Acidic soils increase the solubility of Mn, Al, and Fe which, in excess, are toxic for the plants. Availability of an ion is influenced—enhanced or suppressed—by the presence of other ion(s). The sum of equivalents of cations in a soil solution is equal to the sum of soluble anions. The passive transport of an ion is down the electrochemical gradient but usually against the chemical concentration gradient for that ion. Intercellular transport of ions in plant roots takes place through plasmodesmata, the living connectors between the cells, called the symplast pathway. Occurring through a concentration gradient, the ions passively diffuse through the cells of the cortex and endodermis to be eventually delivered into a xylem vessel or tracheids. The movement of the ions from cell to cell across their plasma membrane actively and passively through the cortex and epidermis cells is called the transmembrane pathway. Further, the movement of ions through cell walls and intercellular spaces between the cells in the cortex is called the apoplast pathway. The rate of root uptake of the ions increases with an increase in ion concentration. Nutrient uptake by plants is influenced by a root:shoot ratio and root:weight ratio, root diameter and root hairs, root density and distribution, rhizosphere, and microbial populations in the soil. Nitrogen fixation in the soil is influenced by environmental factors, such as CO2, C:N ratios, weather factors (heat, drought, soil moisture), soil pH, mineral nutrients, and mycorrhizas.