Soil Chemistry


If you garden, then you know vegetables are grown virtually everywhere.  However, you also know that the soil those vegetables are grown in has a huge impact.  Soil can simply be defined as the portion of the Earth’s surface where plant growth occurs.  Even though many people call it dirt, there is a lot more to soil than most people realize.

Soils are dynamic systems subject to changing temperature and moisture regimes.  Despite the dynamic nature of soils, insights about soil reactions and processes can be gained by consideration of soils at equilibrium.

The chemistry of soil water or sediment water systems is the study and quantitative description of simultaneous reactions, many of which are at virtual equilibrium.


One of the most important features of soil is pH.  The pH of soil can be modified by adding different chemicals.  Soil pH indicates how acid or alkaline the soil is.  The pH scale ranges from 0 to 14.  The pH of a soil is crucial because crops grow best in a narrow pH range which can vary somewhat among crops.  Depending on the crop you are growing the pH levels can vary, but will typically fall somewhere between 5.5 and 7.5.

There are two reasons that a pH of 5.5-7.5 is the range you will find most quality soil.  It allows sufficient microorganisms to break down organic matter. It also determines the solubility and hence availability, of various ions.


Each year the soil undergoes a series of cycles in which materials are added and then taken away.  Organic matter and nutrients, in various forms, are constantly being added to the soil.  Nutrients are the minerals required by plants to survive.  It is very important that plants receive all the required nutrients.  There are a total of seventeen mineral elements required for plant growth.  Each is required in different amounts.

The most important nutrients are called macronutrients. Nutrients which are essential, but only needed in small quantities are called micronutrients.  Carbon, hydrogen, and oxygen are the big three macronutrients. These are obtained in almost unlimited amounts from the atmosphere and from the water around the plant.  The other mineral nutrients should be available for plant uptake from the soil.  They are called essential because the absence of any one of these will cause the plant to grow poorly or develop disease.

Organic Matter

Many pleasant drives in the country have been affected when the passengers of a car are hit with an unpleasant, but familiar odor.  Many people complain about the smell without questioning why it exists.  There is actually a very good reason for this practice.  Although they are often given more unpleasant names, these animal wastes are known as organic fertilizers.  These fertilizers have a very high content of organic matter.  Organic matter is simply dead decaying matter that originated from a living source.  It prevents nutrients from being lost from the soil by binding these nutrients.  Therefore, the best soil for crop production will have a very high organic content.  Most organic fertilizer livestock waste from cattle, pigs, poultry and other livestock.  Compost is also an organic fertilizer.  Compost can be made up of grass clippings, table scraps, ashes, seaweed, and many other types of food products.  Organic fertilizers contain high levels of nitrogen and moderate levels of phosphorus and potassium.  The nutrient content of organic fertilizer can vary according to the source that produced it.  The process of spreading organic fertilizers gives farmers the opportunity to rid themselves of accumulated livestock waste.  It also provides farmers with a free source of fertilizer which is sometimes sufficient to meet the needs of the desired crop.  Organic fertilizers are also less harmful to the environment though many contain significant levels of heavy metals.  This may be one of the first recycling practices that ever developed.