A Basic Soil Chemistry Review

By Justin Keay

[This article was first published in The Gateway Gardener Winter 2019 issue. It is part of a 4-part series of articles on how to test soil, get the sample analyzed, understand the report and use it to apply the correct type and amount of fertilizer.]

Before you take on the task of analyzing a soil test, it may be helpful to review some basic chemistry.

There are 17 elements plants require to complete their life cycle. Carbon, hydrogen and oxygen are three elements that plants can obtain from water and air and never need to be added as fertilizer. Of the remaining 14 other essential elements, 3 are known as primary macronutrients, 3 are known as secondary macronutrients, and the remaining 8 are known as micronutrients. The primary macronutrients are needed in the greatest supply and consist of nitrogen, phosphorous and potassium. The secondary macronutrients that may need to be added in smaller amounts are calcium, magnesium and sulfur. Micronutrients are needed only in very small or trace amounts and consist of the following elements; boron, chlorine, copper, iron, manganese, molybdenum, nickel and zinc.

Why isn’t Nitrogen Included in Soil Tests

One of the things that may surprise you about your soil test results, is that nitrogen is not tested, which seems strange considering it is the element needed in the greatest supply. The reason for this omission is that testing for nitrogen in the soil is very challenging because nitrogen exists in forms easily moved downward in the soil profile, or lost to the atmosphere. Nitrogen testing is possible and sometimes used by large-scale commercial farmers, but the test results are only valid for a short amount of time due to the mobility and volatility of nitrogen in the soil. One form of nitrogen that is easy to estimate is the nitrogen found in soil organic matter. Organic matter is anything that was once a living organism or part of a living organism. This including leaves, grass clippings, earthworms and their castings, insects, bacteria, fungi and more. Organic matter slowly releases nitrogen, and the percentage of organic matter in your soil is used to help calculate the amount of nitrogen you actually need to apply. Every 1% of organic matter will release approximately 20 pounds of nitrogen per acre per year and therefor organic matter in your soil gives you a credit of nitrogen.

Soil Acidity (pH)

A graph showing soil nutrient availability at certain pH levels.

This is a common chart showing soil nutrient availability at certain pH levels. The blue column represents the best zone for all nutrients.

The pH value represents the acidity or alkalinity of your soil and runs on a scale from 0 to 14, with 7 being neutral. Anything lower than 7 is acidic (as are most of Missouri’s soils) and anything higher than 7 is considered alkaline. A single point difference on the pH scale might not seem like much, but every point actually represents a difference of a factor of 10. Therefore, a pH of 5 is 10 times more acidic than a pH of 6. pH is hugely important because certain plants require very different pH values. Additionally, pH has a great impact on nutrient availability in the soil. For example, phosphorous may be abundant in a certain soil but if the soil has a very low pH the phosphorous will not be present in a chemical form that the plants can uptake and utilize.

With that basic chemistry lesson, you may now wish to read about how to interpret a Soil Test, or one of the other related articles below:

How Why and When to Test Your Soil

How to Interpret a Soil Test

How to Choose and Apply An Appropriate Fertilizer

Justin Keay is the MU Extension Field Specialist in Horticulture for St. Louis City/County and St. Charles County in Missouri.