B. Procedurs 1. Place 5 g of a sandy soil in a tes tube. Add 10 ml of 002 N BaCl

ID: 288384 • Letter: B

Question

B. Procedurs 1. Place 5 g of a sandy soil in a tes tube. Add 10 ml of 002 N BaClh to the test tube and shake one minute 2. Filter and collect 1 ml of extract. 3. Repeat steps (I) and (2) witdh a clay soil. 4. Add 1 ml of 0. 02 N BaCl to a third test tube as a check. 5. To each test tube add one drop of saturated K,Cr20, solution and shake. The Clay moot hecipta amount of yellow BaCrOs precipitate indicates the amount of Ba in the filtrate. (NOTE: all of the solutions will be equally yellow, compare the amounts of precipitate in each) C. Results CEC 1. Which soil has the higher cation exchange capacity? Explain? 2. If the sandy soil and the clay soil were equally acid in terms of pH, would both require the same amount of lime? Explain. 3. A cmol, is equal to the amount of electrical charge associated with I cgram of H'or additionally, the cgrams of any other ion that will react with or displace 1 cgram of H The cmol, weight of an ion is equal to the ioaic weight in cgram divided by the valence of the ion. Using a periodical table, complete the following table: Cation gnokwL H* Ca" Mg K' Cation gnokwt NH so2 Al (mg/cmole) (mg/emol.)

Explanation / Answer

CEC, an abbreviation for Cation Exchange Capacity, refers to the amount of negative charges available on the surface of soil particles.

It gives an indication of the potential of the soil to hold plant nutrients, by estimating the capacity of the soil to retain cations, which are positively-charged substances.

Therefore, the CEC of the soil directly affects the amount and frequency of fertilizer application.

Clay soil particles and organic matter carry a negative charge on their surfaces. Cations are attracted to the negatively-charged particles by electrostatic forces. The net charge of the soil is, therefore, zero.

Soils with high CEC typically have a high clay and organic matter content. These soils are considered to be more fertile, as they can hold more plant nutrients. Sandy soils typically have a lower CEC and require more frequent fertilizer applications.

Typical Cation Exchange Capacities of soil components and soil types

Material

CEC (meq/100g)

Clays

Kaolinite

Illite

Montmorillonite

3-15

15-40

80-100

Soil Texture

Sand

Loamy Sand to Sandy Loam

Loam

Clay Loam

Clay

1-5

5-10

5-15

15-30

>30

Soil nutrients exist as positively charged or negatively charged ions when dissolved. The positively charged ions are known as cations and the negatively charged ions are known as anions. The nutrients which exist as cations are calcium (Ca2+), Magnesium Mg2+, ammonium (NH4+), potassium (K+), hydrogen (H+), sodium (Na+) aluminium (Al3+), iron (Fe2+), manganese (Mn2+), zinc (Zn2+) and copper (Cu2+). Some of the nutrients which exist as anions are chloride (Cl-), nitrate (No3-), sulphate (S04)-2 phosphate (H2PO4- and HPO4–), borate (BO3-), and molybdate (MoO4–).

Cation exchange capacity (CEC) is a soil chemical property. It is the ability of the soil to hold or store cations. When soil particles are negatively charged they attract and hold on to cations (positively charged ions) stopping them from being leached down the soil profile. The cations held by the soil particles are called exchangeable cations. On the other hand, negatively charged soil particles repel anions (negatively charged ions). The implication of this is that negatively charged nutrients such as nitrates, sulphate and chlorides are vulnerable to leaching down the soil profile.

In fact, there are two types of CEC measured in laboratories: CEC at neutral conditions and CEC at the actual soil pH. The values resulting from these two different measurements may vary greatly. The CEC measured at the actual soil pH is referred to as "Effective CEC" and abbreviated as CEC Inaccuracies in the measurement occur when soil pH is greater than 7.5 or when lime was recently applied. The resulting CEC is overestimated under such conditions.

Clay soils need a much greater amount of lime to shift the pH than sandy soils.
The pH scale measures whether substances are acid or alkaline.
pH1 very strong acid that can burn you, e.g. battery acid
pH6 weakly acid, e.g. soda water
pH7 neutral, neither acid nor alkaline, e.g. water
pH8 weakly alkaline, e.g. soap
pH14 very strong alkali that can also burn you.
Acids have a sharp raw taste, e.g. unripe oranges or bush limes.
Alkalis have a slippery feel, e.g. soap, saliva.
Plants can absorb plant nutrients best when pH is 6 to 7.
In soils formed from coral rock, the pH will be too high.
In swampy land, the pH will be too low.
Experiment
To lower the pH add rotten compost.
To raise the pH add lime.
In this lesson show how to use a "Soil pH Test Kit".
You will need: Coral soil or coral sand, well drained dark soil, swamp soil.
Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline.
Soils that are not well-drained are too acid.
Soils made from coral rocks are too alkaline.
You can test the soils using a colour test.
If the colour of soil in the test turns yellow orange, the soil is too acid.
If colour if soil in the test turns blue purple, the soil is too alkaline.
If colour if soil in the test turns dark green, the soil is not too acid nor too alkaline.
Collect just enough soil from just under the surface of the soil to cover your little finger nail, and place on a white plate.
Shake two drops of the indicator on the soil and mix to a paste with the stick.
Sprinkle some special white power on the paste.
Wait a few minutes then match the colour of the powder with the colour chart.
Do this for swampy soil, coral soil, dark well drained soil.
Acids have a sharp sour taste and can dissolve substances, e.g. in a car battery.
Alkalis have a slippery feel and can dissolve substances, e.g. soap.
Plants cannot absorb plant nutrients from the soil if the soil is too acid or too alkaline.
Soils in swampy ground are too acid for most plants.
Soils made from coral sand are too alkaline for most plants.
Make soil less acid by adding burnt shells hammered to a powder, and by draining the soil.
Make soil less alkaline by adding rotten plants from a compost heap.
Good soil is dark in colour from the rotten plants and is well drained.
To make sure that the soil is not too acid and not too alkaline the agric.