The soil environment sets challenges for water and mineral ion acquisition. The soil is a heterogeneous substrate consisting of: Liquid phase: soil solution containing dissolved mineral ions. The nutrients required for plant growth occur in the soil as ions, charged particles. Anions are negatively charged particles, while cations are positively charged. Anions usually dissolve in soil water, because they interact with water molecules via hydrogen bonding (the h+ ion is a cation and hence anions are attracted to it). As solutes, anions are available to plants for absorption, but are easily washed out of the soil by rain - leaching. Cations dissolve in soil water but are not as immediately available as anions. Cations interact with the negative charges on two types of soil particles: organic matter rich in organic acids, and the surfaces of clay. Soil ph can also influence the availability of essential elements. Soils can be acidic (low ph) or alkaline (high ph).