Part II: Chemical Properties of Soil
This fact sheet is the second in a series on soils. The first addressed physical aspects of soils, covering such topics as texture, structure, and organic matter and how these affect soil tilth, moisture retention and drainage. This fact sheet covers some of the basic chemistry of soils and practical implications for soil fertility and nutrient management. We’ll try to unravel the mysteries of “cation exchange capacity” and “buffer pH” without making your eyes glaze over.
A soil test provides information about a soil’s chemical properties. The soil test report indicates the levels of the various nutrient elements in our sample as well as soil pH, buffer pH, cation exchange capacity, base saturation and organic matter. If you are not familiar with these terms, they can sound a bit intimidating. So let’s take some time to learn what they mean and how they relate to practical soil and nutrient management. Note: The UMass basic soil test does not include organic matter; you must ask that it be included.
There are thirteen mineral elements which are essential for plant growth. Six of these are called major or macro elements because the plant uses them in rather large amounts. They are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S). Sometimes Ca, Mg and S are referred to as secondary elements because they are used in somewhat smaller amounts than N, P and K. Seven more are called minor, micro or trace elements. These are every bit as important as major elements, but are used in very small amounts. These elements include iron (Fe), Manganese (Mn), zinc (Zn), boron (B), copper (Cu), molybdenum (Mo) and chlorine (Cl). Nickel (Ni) is accepted by many scientists as the 14th nutrient element derived from soils. We will discuss sources and application of mineral elements in an upcoming fact sheet in this series.
In addition to mineral elements, carbon (C), hydrogen (H) and oxygen (O) are essential elements. Plants take these elements from air and water. We don’t apply fertilizer materials to the soil in order to supply C, H and O, but our soil management practices have an effect on their availability.
Cation Exchange Capacity
Before we talk about “cations”, we should know that “ions” are atoms or groups of atoms (molecules) which have an electrical charge. “Anions” have a negative (-) charge and “cations” (pronounced cat-eye-ons) have a positive (+) charge. Plants take up nutrients from the soil either as cations or anions. Many of the nutrient elements are cations. These include ammonium (NH4+), Ca++, Mg++, K+, Fe+++, Mn++, Zn++, Cu++. Other cations of importance are H+ and Al+++ (aluminum). Cations are attracted to negatively charged surfaces of small clay and organic (humus) particles called colloids. This attraction is called adsorption. Generally, cations are held tightly enough on adsorption sites to restrict their loss through leaching. These cations can move from the adsorption sites on colliods into the soil water solution (and vise versa) where they are available for root uptake and are also subject to leaching (see Figure 1). Cation exchange capacity (CEC) is a measure of the number of adsorption sites in a soil and is an important indicator of the soil’s ability to retain and supply cations for plant use. CEC is reported as milli-equivalents per 100 grams of soil (meq/100 g). A soil with a CEC of one (1), has 600,000,000,000,000,000,000 adsorption sites in 100 grams (about 1/4 lb) of soil. The CEC of agricultural soils ranges from below 5 in sandy soils with little organic matter to over 20 in certain clay soils and those high in organic matter. A soil with a low CEC has little ability to store nutrients and is susceptible to nutrient loss through leaching.
You may recall from the first fact sheet that the soils consist primarily of three sizes of mineral particles; sand, silt and clay. Of these, clay is the only group which makes a significant contribution to CEC. However, there are several types of clays, and they vary considerably in their CEC. Agricultural soils in New England are typically low in clay, and the types of clay we have here have a low CEC. In most of our soils, organic matter is the primary contributor to CEC. This fact is true even of soils with low organic matter. Not only does organic matter improve the physical properties of soil, it also plays a vital role in soil chemistry by increasing CEC.
The cations Ca++, Mg++, K+ and H+ normally account for nearly all cations adsorbed on soil particles, although trace elements that are cations are also present in minute quantities. Ca++, Mg++, and K+ are called bases and H+ and Al+++ are acidic cations that lower soil pH. If all of the adsorbed cations are bases and none are acidic, there would be a 100% base saturation, and the soil pH would be about 7 (neutral) or above. In acid soils there are acid cations present and the per cent base saturation is less than 100. Besides having sufficient quantities of Ca, Mg and K, it is important that they be in balance with each other because an excess of one of these can suppress the uptake of another. As a general rule a Ca:Mg:K ratio of about 20:4:1 is desirable. When expressed as per cent base saturation, desired levels are: Ca 65-80%; Mg 5-15%; and K 2-5%.
Soil pH and Liming
One of the most important aspects of nutrient management is maintaining proper soil pH. Soil pH is a measure of soil acidity . A pH of 7.0 is neutral. A pH below this indicates an acid soil and a pH greater than 7.0 indicates an alkaline soil. Most of our soils are naturally acid and need to be limed periodically to keep the pH in the range of 6.5 to 7.0 which is
optimum for most plants. Soil pH is important because it affects the availability of nutrient elements for plant uptake. As the soil pH falls below pH 6.0, the availability of N, P and K, becomes increasingly restricted. Acid soils usually contain low levels of Ca and Mg, and there may be toxic levels of iron, aluminum and manganese. Under alkaline conditions, the availability of most trace elements is reduced. Phosphorus availability becomes limited above pH 7.5.
Lower pH soils are desirable for “acid-loving” plants such as blueberries and rhododendrons. Potatoes are often thought of as “acid loving” plants. They are not. They are acid tolerant and will grow reasonably well at soil pH levels down to about 5.0, but will grow better at pH 6.0 to 6.5. The only good reason for growing potatoes on acid soil is to manage potato scab. This works because the organism that causes common potato scab becomes inactive when the soil pH is below 5.3. Scab resistant varieties grow well at pH 6.0 – 6.5.
Besides raising soil pH, lime is the most economical source of Ca and Mg for crop nutrition. It is important to select liming materials based on Ca and Mg content with the aim of achieving desired base saturation ratios. If the Mg level is low, a lime high in Mg (dolomite) should be used. Lime high in calcium (calcite) is preferable if Mg is high and Ca is low. The average agricultural limestone is about 5% Mg and 35-40% Ca by weight. It is appropriate if Ca and Mg are both needed.
The neutralizing power of lime is determined by its calcium carbonate equivalence. Recommendations are based on an assumed calcium carbonate equivalence of 100%. If your lime is lower than this you will need to apply more than the recommended amount, and if it is higher, as in the with some dolomites, you will need less. To determine the amount of lime to apply, divide the recommended amount by the per cent calcium carbonate equivalence of your lime and multiply by 100. For example, if the lime recommendation is 2 tons per acre and the lime you are going to apply has a calcium carbonate equivalence of 72% you should apply at the rate of 2.7 tons per acre according to the following calculation:
recommended amount X 100
calcium carbonate equivalence
2 tons/A X 100 = 2.7 T/A
By law the calcium carbonate equivalence of the lime you are purchasing must appear on the bag, or on the delivery slip for bulk lime.
The speed with which lime reacts in the soil is dependent on particle size and distribution in the soil. To determine fineness, lime particles are passed through sieves of various mesh sizes. A 10 mesh sieve has 10 openings per linear inch, or 100 openings per square inch (10 X 10) and a 100 mesh sieve has 10,000 openings per square inch (100 X 100). Lime particles that pass through a 100 mesh sieve are fine and react rapidly–within a few weeks. Coarser material in the 20 to 30 mesh range will react over a longer period such as one to two years or more. Agricultural ground limestone contains both coarse and fine particles. About half of a typical ground limestone consists of particles fine enough to react within a few weeks or months, but to be certain you should obtain a physical analysis from your supplier. Super fine or pulverized lime is sometimes used for a quick fix because all of the particles are fine enough to react rapidly. Hydrated lime, “quick lime” and pelleted lime react very quickly, but their effects are short-lived and the materials are expensive.
Wood ashes can also be used to raise soil pH. The calcium carbonate equivalence of wood ashes varies considerably, typically ranging from 30 to 50%. They are chemically similar to quick lime and supply K as well as Ca and Mg. CAUTION: Do not over-apply wood ashes. I have seen numerous cases where wood ashes were spread in a concentrated area causing soil pH to become extremely high.
Lime will react most rapidly if it is thoroughly incorporated to achieve intimate mixing with soil particles. Mixing is best accomplished when lime is applied to a fairly dry soil and disked (preferably twice) or rotary tilled into the soil. When spread on a damp soil, lime tends to cake up and doesn’t mix well. A moldboard plow has little mixing action and can bury lime too deep for good results in the first year. If soil pH is monitored yearly and lime is applied before the soil becomes very acid, a rapid reaction is not needed and you can be less particular about incorporation or the use of very fine lime.
In addition to soil pH, many soil tests provide a reading called buffer pH (sometimes called lime index). Soil pH is a measure of hydrogen ion (H+) concentration in the soil solution. This is called active acidity. It is an indicator of current soil conditions. When lime is added to a soil, active acidity is neutralized by chemical reactions of H+ with carbonates, bicarbonates or hydroxides generated from limestone. However, H+ is also attached to soil colloids and is released into the soil solution to replace those neutralized by the lime. These adsorbed H+ are called reserve acidity. To effectively raise the soil pH we must neutralize both active and reserve acidity. At a given soil pH, soils with a high CEC will have a higher reserve acidity than those with a low CEC. Buffer pH is a measure of reserve acidity and is used by the soil testing laboratory to calculate lime requirement. Low buffer pH readings indicate high amounts of reserve acidity and therefore high amounts of lime will be recommended. Buffer pH is used only for calculation of the lime requirement and should not be confused with soil pH which is the value of importance to growers.
We will concentrate on soil organic matter in the next fact sheet.