Nutrient Management

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Analytical Research Laboratory

Extension Soil Testing Laboratory

Soil and Water Science Department

Manure Testing Laboratory



Soil Textural Triangle

Use a textural triangle to determine textural classes of soil if given the percent of two of the soil separates

Figure 1 shows a textural triangle. The bottom of the triangle is labeled sand, the left side silt, and the right side clay. Each side is divided 100 segments. If we know the percentage of sand, silt, and clay in the soil, we can determine its texture. For example, if a soil is 40% sand, 30% silt, and 30% clay, the texture is clay loam.

Note: To determine the texture, lines from the sides must be extended in the correct direction. The triangle is equilateral i.e., all angles are 60 degrees. Proceed as follows:

clay extend line horizontal from the % clay
i.e., parallel with side labeled sand

silt extend line downward from % silt at 60 degrees
i.e., parallel with side labeled clay

sand extend line upward from % sand at 120 degrees
i.e., parallel with side labeled silt


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Mineral versus Organic Soil Material

Mineral soil material is defined by exclusion from either of these two requirements that define organic soils:

1. Organic soil has more than 20 percent organic carbon (about 35 percent humus) if it is never saturated with water more than a few days.

2. Organic soil has water saturation periods (unless drained by people) and has:

a. At least 12 percent organic carbon (about 21 percent humus) if the soil has no clay, or
b. has at least 18 percent organic carbon (about 31 percent humus) if the soil has 60 percent or more clay, or
c. has an intermediate, proportional amount of organic carbon for intermediate amounts of clay.

Soils with less organic carbon than these amounts are called mineral soils.

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Chelates complex organic molecules used to maintain the plant availability of micronutrients.

Chelates prevent the micronutrient cation from interacting with the soil and forming insoluble compounds with soil constituents.

Effectiveness of the chelate is related to the stability of the chelate at different pH levels. Chelates should be chosen dependent on the soil pH. For example:

EDTA chelates Fe at a pH < 6.3

DTPA chelates Fe at a pH < 7.5

EDDHA chelates Fe over a pH range of 4 9

If pH = 8.0 then only EDDHA will be effective. If pH = 6.2 then all will be effective.

For soil applications much lower rates of chelated micronutrients are needed to supply the plant than inorganic sources. This is because the availability is higher. In some cases the application of the non chelated micronutrient is so rapid that it is ineffective as a nutrient source.

Chelates are generally more expensive than inorganic sources. Applications should be as near the point of crop demand as possible.

Inorganic sources and chelates are equally effective for foliar applications.

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The pH Defined

Soil pH is the negative logarithm of the active hydrogen ion (H+) concentration in solution. When water (HOH) ionizes to H+ and OH- (a neutral solution), both H+ and OH- are in concentrations of 10-7 mole* per liter. The [ ] = concentration.

HOH = H+ + OH-

---------- = 1 X 10-14 , [H+] = [OH-] = 1 x 10-7


Thus, the negative logarithm of [H+] is 7, or PH 7. When the H+ concentration is greater (more acidic), such as 10-4 mole per liter, the pH is lower (e.g., pH 4). In basic solution the OH concentration exceeds the H+ concentration. The product of the H+ and OH- concentrations equals 10-14 mole per liter. When H+ is 10-5, OH- is 10-9, for example. (The weight and volume units in chemistry are always expressed in the centimeter- gram-second system, and conversions to the U.S. units are not applicable here.)

"A mole is one molecular weight of ion or of the molecule.



FIGURE 5-8 The entire pH scale ranges from 0 to 14, but soils under field conditions vary between pH 3.5 and 10.0. Few soils have pHs outside this range. in general, most plants are best suited to a pH of 5.5 on organic soils and a pH of 6.5 on mineral soils, (Source: Adapted from Winston A. Way, "The Whys and Hows of Liming," University of Vermont Brieflet 997, 1968.)

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