Module 3

Question 1

Chemical formula of phosphate in soil solution?

Answer 1

H2PO4-

Question 2

Chemical formula of Sulphate in soil solution?

Answer 2

SO4-2

Question 3

Chemical formula of Nitrate in soil solution ?

Answer 3

NO3-

Question 4

Chemical formula of Sodium in soil solution?

Answer 4

Na+

Question 5

Chemical formula of Magnesium in soil solution?

Answer 5

Mg2+

Question 6

Chemical formula of Potassium in soil solution?

Answer 6

K+

Question 7

3 types of ion mechanisms?

Answer 7

Root interception
Mass Flow
Diffusion

Question 8

What mechanism does phosphorus adopt to reach the plant root?

Answer 8

Diffusion

Question 9

How does phosphorus behave in soil?

Answer 9

As phosphate isn’t very mobile (is adsorbed by soil), fertiliser placement is critical but the potential for nutrient loss is small. Phosphorus doesn’t move more than 6mm to get to a root. Phosphorus is only lost if the soil itself is lost through erosion. Phosphorus accumulates in the soil.

Question 10

How does nitrogen behave in soil?

Answer 10

As nutrient mobility increases, fertiliser placement becomes less critical, but the potential for nutrient loss becomes greater. Nitrogen placement relative to the plant isn’t very critical for uptake, but the potential for loss of nitrogen once it is applied to the soil is generally very high, and little available nitrogen will accumulate in the soil.

Question 11

how do micronutrients behave in soil?

Answer 11

The behaviour of most micronutrients is similar to that of phosphorus. The other nutrients fall somewhere between these two extremes in mobility behaviour (phosphorus and nitrogen)

Question 12

proportion of nutrient uptake by every mechanism

Answer 12

nitrogen - 99% mass flow
phosphorus - 90% diffusion
potassium - 78% diffusion
calcium - 72% mass flow
magnesium - 87% mass flow
sulphur - 95% mass flow
all consist of some root interception

Question 13

what is pH buffering?

Answer 13

When a solution resists pH change of soil when acidic or alkaline materials are added.

Question 14

what happens when lime is added to an acidic soil?

Answer 14

CaCO3 is a basic salt - it dissolves in water to form a strong base (Ca(OH)2) and weak acid (H2CO3), so the net effect is a basic solution.
For every mole of CaCO3 that reacts with water, 2 moles of OH- ions are generated. These two moles of OH- ions neutralise excess H+ in soil solution. H+ then moves into soil solution from the soil surface, buffering the change in pH.

Question 15

explain “Point of Zero Net Charge”?

Answer 15

PZNC is the pH at which the amounts of negative and positive charges on the soil particle surface are the equal resulting in zero net charge.

Question 16

PNZC related to the dissociation of H+/OH- ions?

Answer 16

When the pH is above PZNC, the soil is net negatively charged, resulting in more cation (H+) retention; when the pH < PZNC the soil is net positively charged, resulting in more anion (OH-) retention.

Question 17

definition of Cation Exchange Capacity?

Answer 17

This is the number of cation adsorption sites ( or negative charge) per unit mass of soil

Question 18

what causes a soil to have a high CEC value?

Answer 18

high CEC is when cations are held by the negatively charged clay and organic matter particles in the soil through electrostatic forces. (negative soil particles attract the positive cations). High CEC is caused by high negative charge.

Question 19

what is the benefit of high CEC?

Answer 19

high CEC means more plant available cations. It represents the total amount of exchangeable cations that the soil can adsorb. The cations used by plants in the largest amounts are calcium, magnesium, and potassium.

Question 20

what is flocculation?

Answer 20

the process in which particles disperse in a solution contact and adhere each another, forming clusters, flocks, flakes, or clumps of a larger size.

Question 21

what is a diffuse layer?

Answer 21

the DL is the specific distance from the clay surface that the concentration of the bulk soil solution is equal, the anionic and cationic charges are balanced in solution.

Question 22

Below the PZNC?

Answer 22

the surface bears a positive charge and can attract anions

Question 23

Above the PZNC?

Answer 23

the surface bears a negative charge and can attract cations

Question 24

increase of K+ from effluent…

Answer 24

K+ over time can displace adsorbed Ca2+ and Mg2+, releasing these cations into soil solution and thereby increasing their risk of leaching. Over time the Diffuse Layer may increase leading to soil deflocculation

Question 25

Increase in H+ from increases in acidity

Answer 25

H+ over time can displace adsorbed Ca2+ and Mg2+, releasing these cations into soil solution and thereby increasing their risk of leaching. Over time the Diffuse Layer increases leading to soil deflocculation