Soils Exam Week 7

Which of the following is the H+ concentration of a solution with pH 5.0?

0.000,01 moles per liter

Which of the following is the H+ concentration of a solution with pH 7.0?

0.000.000.1 moles per liter

The amount of lime which must be applied to an acidic soil is determined by:

1. Exchangeable acidity in the soil 2. All of the these, (these terms are synonymous) THIS IS THE ANSWER! 3. Soil buffering capacity 4. Potential acidity in the soil 5. Reserve acidity in the soil

List and discuss several liming practices.

1. Lime according to soil test: You cannot tell if a soil needs lime by just looking at it, nor can you determine LR from a pH measurement alone. About 67% of the East Texas soil samples run through the SFA Lab need lime, but about 33% don't need lime. If you don't need lime, liming is an un-necessary expense. 2. Use Mg containing lime on low Mg soils: About 40% of the soils in East Texas are low in Mg. If a Mg containing lime can be obtained at reasonable cost, it is the best way to correct low Mg soil conditions. 3. Apply several months in advance of planting: Lime is not very soluble and the neutralization of soil acidity is a slow process. Applying lime 6 to 12 months in advance of planting a crop that is sensitive to acidic pH gives the lime time to act. 4. Mix or incorporate lime into soil: For the reasons mentioned in #3, it is desirable to mix lime into the soil to speed up the neutralization reaction and also to affect more of the root zone. Thus, apply lime before tillage operations not after them. In permanent sod crops or no till systems, go ahead a apply the lime to the soil surface where it can at least affect the surface 1 to 3 inches. 5. Lime is effective for 3 to 5 years: In most cropping systems, lime is effective for several years. Applying an entire recommended lime application rate (sometimes 3 tons/acre) at one time and mixing it into the soil is generally thought to be better than multiple applications. However, due to the high cost of lime in some areas, multiple applications over 2 or 3 years is acceptable 6. Do not over apply lime: Not only is over application an un-necessary cost of production, it can also cause Micronutrient deficiencies. Any time is lime time: Many East Texas fertilizer dealers like to apply lime in the late summer or early fall because these are "slow" times in the fertilizer business, because some fields are to wet to support spreader trucks at other times, and the more acid sensitive small grains and legumes are planted in the fall. As a result many people think fall is the only time you can apply lime. Any time it it convenient in the farming operation can be a time to apply lime.

The base saturation of an Ultisol is:

< 35%

The base saturation of an Alfisol is:

>35%

Using the drawing shown below explain the factors that determine the amount of acidifier or lime needed to lower or raise the pH of a soil. Be sure to relate the illustration to soil CEC. (caution, in answering this question we are not looking for differences in the quality of lime or soil acidifier or preference of plants. We want to know what factors determine why some soils need more lime or acidifier than others)

Active acidity is the H ions in soil solution. In the illustration it is represented by the small glass tube that tells you how full (or empty) the coffee pot is. Reserve acidity is another term used to refer to exchangeable or residual acidity. This is the H and Al ions held on exchange sites and shown as the large coffee reservoir in the illustration. The quantity of acidity held on the cation exchange sites is very very large (the coffee pot) compared to the acidity in soil solution (the glass site tube). When we add a small amount of lime to an acid soil it neutralizes the acidity in soil solution but that is quickly replaced from the exchangeable acidity. Thus we say the soil is buffered. The mechanism works the same way when we try to acidify a soil. In the case of neutral & alkaline soils we must neutralize the bases (exchangeable bases) held on the cation exchange sites. Soils with higher CEC (soils with more clay, coffee pot "a") are going to require more acidifier than sandy soils with low CEC (coffee pot "d").

The illustration below show tolerance and sensitivity to what chemical element.

Al

Which of the following are most sensitive to acidic soils or show reduced growth on acidic soils?

Alfalfa, Clovers, Legumes

What two soil orders have similar morphology and are often distinguished by being above or below 35% base saturation?

Alfisols & Utisols

Which of the following fertilizer materials is the most acid forming?

Ammonical fertilizers

Which of the following soil orders will most likely have an alkaline pH?

Aridisol

Which of the following are acid loving plants?

Azalea, camelia, & rhododendron

The portion of the soils cation exchange capacity that is occupied by Na, K, Ca, and Mg is called:

Base saturation

A chemical solution that resists change in pH is called what?

Buffer solution

Which of the following is not a liming material?

CaSO4

As soil pH increases, the quantity of H+ in soil solution will:

Decrease

As pH increases from 5 to 7 what happens to the H+ concentration in solution?

Decreases

If the Ca, Mg, K, & Na found on the cation exchange complex are called "exchangeable bases", then what do you call exchangeable H+ and Al+++ ?

Exchangeable Acidity

All soils in East Texas should be limed. (T or F)

False

All soils in dry regions need to be limed. (T or F)

False

All soils should be limed. (T or F)

False

Fungi are much more sensitive to strong acid conditions than are bacteria or actinomycetes. (T or F)

False

Iron and manganese are commonly deficient in very acid soils. (T or F)

False

Lime should only be applied after tillage. (T or F)

False

Lime should only be applied in the Fall. (T or F)

False

The amount of limestone required to increase the pH from 5 to 7 would be higher for an Oxisol than for an Alfisol. (T or F)

False

Most of the lime spread in Texas south of Interstate 20 comes from what town.

Georgetown

The term pH comes from the French words pouvoir hydrogene. What do these French words mean?

Hydrogen Power

As soil pH decreases, the quantity of H+ in soil solution will:

Increase

Adding too much lime to soil can:

Induce micro-nutrient deficiencies and cost too much.

As pH decreases from 10 to 8, what happens to the OH- concentration in solution?

It decreases

As the base saturation of soil increases, what happens to soil pH?

It generally increases

How do most soil testing laboratories determine lime requirement (LR)?

LR can be determined in several ways. All of these methods involve some way or measuring or estimating the amount of exchangeable acidity. The amount of exchangeable acidity is sometimes called the soil "buffering capacity." One way to do this is to measure the soils CEC and then calculate lime rate. Soil texture can be used to estimate CEC and at a given pH, say 5.0, the more clay there is in a soil, the higer the LR. Most laboratories use what is called a buffer pH method. The SFA Soil Testing Lab uses the Adam-Evans buffer method. This method measures the amount of reserve acidity in the soil.

The major reason for the soil being acidic is:

Leeching of bases from the soil where rainfall exceeds 35 inches per year.

Lime should be incorporated (mixed) into soil because:

Lime is not very soluble and mixing with the soil gets the CaCO3 in contact with the soil colloids where the acidity is.

Which two factors determine lime quality:

Lime neutralizing value and particle size

Describe the two factors that determine lime quality and explain what ECCE is.

Lime quality is determined by the chemical purity and particle size. Chemical purity of lime is really how much acid the lime will neutralize. This test is carried out in the laboratory using a titration procedure. Liming materials are compared to pure CaCO3 which is called neutralizing value (NV). Most calcitic ag. lime has impurities in it and thus have neutralizing values less than 100%, although some materials can have over 100% . The other lime quality factor is particle size with small particles being more effective than large particles. A fineness efficiency is determined for the lime material based on the particle size distribution of the lime. Large particles have low efficiency and small particles (<60 mesh) have 100% efficiency. The NV and fineness efficiency are multiplied together to get Effective Calcium Carbonate Equivalent (ECCE). Most ag. limes sold in Texas have ECCE values ranging from about 60% to about 100%.

Re-word the following statement to make it a true statement: All soils in East Texas need to be limed.

Many of the soils in East Texas need to be limed but not all of them. Data from the SFASU Soil Plant and Water Analysis indicates that about 66% of the soil samples from the East Texas region need to be limed but abut 34% do not need lime.

If you want to quickly (in just a few days) increase the pH of a soil from 5.5 to 7.0, which of the following would you use?

NOT calcitic limestone

Which of the following is the H+ concentration of a solution with pH 8.0?

NOT: 0.000,000,1 moles per liter

What is the major reason for reduced plant growth on alkaline soils?

NOT: All of these

Liming a soil adds what cation to the exchange complex ?

NOT: H+

Which is the major reason for reduced plant growth on acidic soils?

NOT: all of these

If a soil is low in Mg, what type of lime should be applied?

NOT: all of these materials contain Mg.

Poor plant growth in a well-drained irrigated soil in an arid region (pH=8.0) is most likely due to ________.

NOT: manganese toxicity

An acid soil is known to contain toxic quantities of a certain plant nutrient. Which of these essential elements is it most likely to be?

NOT: nitrogen

If a soil has pH of 5.2 and acid loving plants (azalea, camellia, blueberry) are to be planted, should the soil be limed?

NOT: yes

Chemical quality of lime is indicated by which of the following?

Neutralizing Value

Which of the following is the most significant of the secondary causes of soil acidity?

Nitrification of ammonium N in fertilizers

Which type of lime particles are most efficient in neutralizing soil acidity?

Particles <60 mesh

Which type of particles are larger?

Particles >60 mesh

Soil A & B have a pH of 5.0. Soil A has a CEC of 25 cmole(+)/kg and Soil B has a CEC of 5 cmole(+)/kg. Which soil will have the larger quantity of reserve acidity?

Soil A

Soil A and B have pH of 4.9. Soil A has CEC of 2.5 c mole (+)/kg and Soil B has a CEC of 5 c mole(+)/kg. Which soil will have a higher lime requirement?

Soil B

Explain how soil pH affects plant growth. Be sure to distinguish between the effects of acidic soils and alkaline soils.

Soil pH measures the hydrogen ions in soil solution. The direct effects of these hydrogen ions on plants is small. Most of the effects of soil pH on plants are indirect effect. Acidic pH's (pH's in the 5's and high 4's) reduce the activity of soil bacteria but increase the activity of soil fungi. Below pH's of about 5.2, Al and Mn begin to become toxic to plants, Al toxicity being the dominant problem on acidic soils. Also acidic soils have reduced availability of N, P, K, Ca, Mg, S, and Mo. At alkaline pH's plant growth may be reduced because of reduced availability of micro-nutrients especially Fe, Mn, Zn, & B.

If two liming materials are sold at the same price per ton. One lime has an ECCE of 60% and the other has an ECCE of 90%, which would be the best buy?

The material with the ECCE of 90%.

Using terms like soil pH, CEC, active acidity and exchangeable acidity, describe the factors that determine the amount of lime needed on a soil.

The pH of a soil determines whether or not the soil needs to be limed. The term active acidity is the same as soil pH. Only soils with a pH below about 6.0 or 6.2 are limed for the production of most crops. Soil pH does not indicate the amount of lime to apply, if lime is needed. To determine the amount of lime needed requires some measure of the amount of acidic cations held on the cation exchange complex. The term exchangeable acidity is a measure of the quantity of acidity held on the cation exchange sites. The higher the CEC (cation exchange capacity) at a given pH the larger the amount of reserve or exchangeable acidity and the more lime is needed raise soil pH. Generally as CEC increases the amount of lime needed to raise the pH of an acid soil increases. Or as the CEC increases, the amount of acidity needed to lower pH (soil acidification) increases. Because clay textured soils have higher CEC, they usually have higher lime requirements.

Using terms like CEC, base saturation, and exchangeable acidity, describe the factors that determine soil pH and how pH can be changed.

The pH of a soil is determined by the exchangeable cations that occupy the exchange site on soil colloids. The exchange sites can be occupied by exchangeable bases (Na+, K+, Ca++, Mg++) or exchangeable acidity (H+ & Al+++). Soils with low base saturation (20 to 30%) have acidic pH's. As percent base saturation increases, exchangeable acidity decreases and pH goes up. As you approach pH of 7.0, base saturation will be between 80 and 100%. Alkaline soils (soil pH >7.3) will usually have over 100% base saturation. The CEC (cation exchange capacity) of a soil determines the quantity of exchangeable bases and exchangeable acidity in the soil. The higher the CEC the more resistant the soil is to pH change or the more buffered the soil is. Generally as CEC increases the amount of lime needed to rasies the pH of an acid soil increases. Or as the CEC increases, the amount of acidity needed to lower pH increases. Because clay textured soils have higher CEC, they usually have higher lime requirements.

The illustration below show tolerance and sensitivity to what chemical element. How does this relate to soil pH?

The set of photos on the left show tops and roots of wheat plants grown at pH 4.4. The variety on the left is tolerant of Al while the variety on the right is not. The reduction of growth in the tops and roots of the Al sensitive variety is quite obvious. The right hand set of photos are at a higher pH (pH = 5.7) where the is little or no soluble Al in soil solution. Thus there is no difference in the two varieties.

The pH of a soil tells you if you need to apply lime but does not tell you how much lime to apply. Which of the following are needed to determine the lime requirement.

The soil texture from which exchangeable acidity can be estimated. ***All of these (correct answer) Some measure of the potential or reserve acidity. Some measure of the soil's cation exchange capacity.

Which soil texture is more likely to have an acidic pH?

There is really no relationship between soil texture and soil pH.

Active soil acidity is a measure of the H+ ion activity in the soil solution.

True

If you wanted to quickly increase the pH of a soil to 7.5 to combat a cabbage disease you would prefer hydrated lime rather than calcium carbonate. (T or F)

True

It will be easier to change the pH of a sandy textured soil than to change the pH of a clay textured soil. (T or F)

True

Lime should only be applied before tillage. (T or F)

True

Residual soil acidity is that acidity in the soil associated with cation exchange sites. (T or F)

True

Soil reaction indicates the degree of acidity or alkalinity in a soil. (T or F)

True

Soils in a region with >40 inches of average precipitation are more likely to need liming than soils with <30 inches of average precipitation. (T or F)

True

Some nitrogen-containing fertilizers are significant sources of soil acidity in cultivated soils. (T or F)

True

The cation exchange capacity of humus is mostly pH-dependent. (T or F)

True

The higher the cation exchange capacity of a soil, the greater is its buffer capacity. (T or F)

True

The pH of a soil tells you if you need to apply lime but does not tell you how much lime to apply. (T or F)

True

Which of the following soil orders will most likely have an acidic pH?

Ultisol

Among the soils of which order would you most likely find a soil with a high buffering capacity?

Vertisol

You are growing soybeans and soil pH is 5.1, should lime be applied?

Yes

The pH of a soil tells you if you need to apply lime and also tells you how much lime to apply.

false

The amount of lime needed to raise the pH of the soil to some target pH and to maintain it for 3 to 5 years is:

lime requirement

Under which of the following conditions would you favor a finely ground dolomitic limestone as your choice of a liming material?

need to provide magnesium in addition to calcium

Based on your understanding of soil pH and soil genesis, what soil pH would you expect to find in an Oxisol?

pH < 6