Soil with a pH above 7.4 is normally referred to as alkaline. This soil property can occur naturally, due to the presence of alkali salts. Although some plants do prefer slightly basic soil (including cabbage family vegetables and buffalograss), most plants prefer a mildly acidic soil (pH between 6.0 and 6.8), and high pH levels can cause a problem.

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In alkali lakes (a type of salt lake), evaporation concentrates the naturally occurring alkali salts, often forming a crust of mildly basic salt across a large area.

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The terms 'base' and 'alkali' are often used interchangeably, since most common bases are alkalis. It is common to speak of 'measuring the alkalinity of soil' when what is actually meant is the measurement of the pH (base property). Similarly, bases which are not alkalis, such as ammonia, are sometimes erroneously referred to as alkaline.

In chemistry, an alkali (from Arabic: al-qaly 'the calcined ashes') is a specific type of base, formed as a carbonate, hydroxide or other ionic salt of an alkali metal or alkali earth metal element. The word alkali or the adjective alkaline are frequently used to refer to all bases, since most common bases are alkalis, although strictly speaking this is inaccurate. The original production source of alkaline substances, were ashes used in conjunction with animal fat to produce soap, a process known as saponification.

Alkalis are all Arrhenius bases and share many properties with other chemicals in this group (Arrhenius bases form hydroxide ions when dissolved in water). Common properties of alkaline solutions include: