Spectrum Agronomic Library

Knowledge is key to using your analytic results to their fullest. The Spectrum Agronomic Library provides you with useful information that will help you to better understand the complex science of agronomy. Our agronomists will be continually adding original and reprinted articles, so check the library regularly for new information.

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The Nutrient Uptake Process

Calculating Salt Index

Dr. John J. Mortvedt

Salt content is one of the most critical characteristics of fertilizers that should be considered when fertilizers are applied, especially with seed-row or “in furrow” placement.

Summary: Salt index (SI) of a fertilizer is a measure of the salt concentration that fertilizer induces in the soil solution. SI does not predict the exact amount of a fertilizer material or formulation that could produce crop injury on a particular soil, but it does allow comparisons of fluid formulations regarding their potential salt effects. As we all know, placement of some formulations in or near the seed may decrease seed germination or result in seedling injury.

Fluid fertilizers containing potassium phosphate as the source of K have lower SI values than those containing KCI. When applied near the seed, fertilizers with lower SI values generally cause fewer problems in seed germination or seedling injury. SI of any fluid formulation can be calculated using the SI values of the most common fertilizer sources. Dealers or growers then can select those formulations with lower SI values that best fit their needs.

Banding of nutrients has received much attention over the years. Usually, the fertilizer is placed at a depth greater than that of the seed to allow root interception of the fertilizer band as roots grow outward and downward in the soil.

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Sulfur (S)

Cation Exchange Capacity (CEC)

CEC, as reported by nearly all soil testing laboratories, is a calculated value that is an estimate of the soils ability to attract, retain, and exchange cation elements. It is reported in millequivalents per 100 grams of soil (meq/100g).

In order for a plant to absorb nutrients, the nutrients must be dissolved. When nutrients are dissolved, they are in a form called “ions”. This simply means that they have electrical charges. As an example table salt is sodium chloride (NaCl), when it dissolves it becomes two ions; one of sodium (Na+) and one of chloride (Cl-). The small + and - signs with the Na and the Cl indicate the type of electrical charges associated with these ions. In this example, the sodium has a plus charge and is called a “cation”. The chloride has a negative charge is called an “anion”. Since, in soil chemistry “opposites attract” and “likes repel”, nutrients in the ionic form can be attracted to any opposite charges present in soil.

Soil is made up of many components. A significant percentage of most soil is clay. Organic matter, while a small percentage of most soil is also important for several reasons. Both of these soil fractions have a large number of negative charges on their surface, thus they attract cation elements and contribute to a higher CEC. At the same time, they also repel anion nutrients (“like” charges).

Some important elements with a positive electrical charge in their plant-available form include potassium (K+), ammonium (NH4+), magnesium ( Mg++), calcium (Ca++), zinc (Zn+), manganese (Mn++), iron (Fe++), copper (Cu+) and hydrogen (H+). While hydrogen is not a nutrient, it affects the degree of acidity (pH) of the soil, so it is also important. Some other nutrients have a negative electrical charge in their plant-available form. These are called anions and include nitrate (NO3-), phosphate (H2PO4- and HPO4), sulfate (SO4-), borate (BO3-), and molybdate (MoO4). Phosphates are unique among the negatively charged anions, in that they are not mobile in the soil. This is because they are highly reactive, and nearly all of them will combine with other elements or compounds in the soil, other than clay and organic matter. The resulting compounds are not soluble, thus they precipitate out of soil solution. In this state, they are unavailable to plants, and form the phosphorus “reserve” in the soil.

Larger CEC values indicate that a soil has a greater capacity to hold cations. Therefore, it requires higher rates of fertilizer or lime to change a high CEC soil. When a high CEC soil has good test levels, it offers a large nutrient reserve. However, when it is poor, it can take a large amount of fertilizer or lime to correct that soil test. A high CEC soil requires a higher soil cation level, or soil test, to provide adequate crop nutrition. Low CEC soils hold fewer nutrients, and will likely be subject to leaching of mobile “anion” nutrients. These soils may benefit from split applications of several nutrients. The particular CEC of a soil is neither good nor bad, but knowing it is a valuable management tool.

The following, is data on how CEC is calculated at Spectrum Analytic.

Milli-equivalents (Meq.) of Selected Cations and Their Equivalent ppm
Cation Atomic Weight Valence Milli-equivalents Equivalent
ppm Lbs/acre
H+ 1 1 1 10 20
Ca++ 40 2 20 200 400
Mg++ 24 2 12 120 240
K+ 39 1 39 390 780
NH4+ 18 1 18 180 360
Al+++ 27 3 9 90 180
Zn++ 65 2 32.5 325 650
Mn++ 55 2 27.5 275 550
Fe++ 56 2 28 280 560
Cu++ 64 2 32 320 640
Na+ 23 1 23 230 460

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library/start.txt · Last modified: 2010/03/31 11:41 by wayland