Question 1

Fulvic acids are a major type of non-humic substances.

Accepted Answer

The answer of Fulvic acids are a major type of...

Question 2

Removing all the straw with the harvest of paddy rice would increase the amount of methane produced by the paddy soil microorganisms.

Accepted Answer

Removing all the straw from the paddy fields actually decreases the amount of methane produced because straw acts as an organic substrate that soil microorganisms decompose anaerobically, producing methane in the process.

Question 3

Non-humic substances formed during organic matter decomposition probably play a more important role than do humic compounds in the mineralization of N and S as well as in the solubilization of iron and other micronutrients.

Accepted Answer

Non-humic substances, such as amino acids and sugars, are more easily mineralized and can provide a source of nutrients for plants. They can also chelate micronutrients and make them more available for plant uptake. While humic compounds can also play a role in nutrient cycling, non-humic substances are generally considered more important.

Question 4

Mineralization of soil organic matter results in the addition of inorganic nutrient ions to the soil solution.

Accepted Answer

Mineralization is the process of breaking down organic matter into inorganic nutrients, which are released into the soil solution. This results in the addition of inorganic nutrient ions to the soil.

Question 5

As decomposition of added organic material proceeds, the C/N ratio in the remaining undecomposed material steadily increases.

Accepted Answer

As organic material decomposes, the C/N ratio decreases as nitrogen is released during decomposition. Therefore, the remaining undecomposed material would have a higher nitrogen content and a lower C/N ratio.

Question 6

When native prairie soils are brought under cultivation, the fraction of soil organic matter which disappears most quickly is the passive fraction.

Accepted Answer

The fraction of soil organic matter that disappears most quickly when native prairie soils are brought under cultivation is the active fraction, not the passive fraction. The active fraction is more labile and responsive to management and environmental changes, decomposing and cycling nutrients more rapidly than the passive fraction, which is more stable and decomposes slowly.

Question 7

Allelopathic chemicals, once added to soil, usually make the affected soil unsuitable for any plant life for periods of years.

Accepted Answer

Allelopathic chemicals can inhibit the growth of certain other plants nearby, but they do not usually make the soil unsuitable for all plant life for years. Many plants and ecosystems have adapted to these chemicals, and their effects can vary widely in duration and intensity.

Question 8

Humus indirectly affects plant growth through its effects on soil physical and chemical properties, but humic substances have not been shown to affect plant growth processes directly.

Accepted Answer

Humic substances have been shown to directly affect plant growth processes, such as root growth, nutrient uptake, and stress tolerance. They can also act as signaling molecules in plant-microbe interactions.

Question 9

The cation capacity of soil humus is two to three times greater than that of an equal volume of high CEC silicate clays.

Accepted Answer

The answer of The cation capacity of soil humus is...

Question 10

Practices that increase the soil organic matter level generally have favorable effects on the chemical and biological properties of soils, but have little or no effect on soil physical properties.

Accepted Answer

Practices that increase soil organic matter levels have been shown to improve soil physical properties such as soil structure, aggregation, and water holding capacity.

Question 11

Aridisols generally contain more organic matter than Mollisols.

Accepted Answer

Aridisols are soils that are dry for most of the year and tend to have low levels of organic matter due to limited plant growth and decomposition. In contrast, Mollisols are soils that are typically found in grasslands and have deep, fertile topsoil with high levels of organic matter. Therefore, Mollisols generally contain more organic matter than Aridisols.

Question 12

The species of microorganisms that are able to sustain themselves by slowly attacking stable soil organic matter are the most active decomposers of freshly added organic materials because of the head start they enjoy over other organisms which were present only in inactive forms prior to the addition.

Accepted Answer

The answer of The species of microorganisms that are able...

Question 13

The soils of the world contain more carbon than all the world's vegetation and the entire atmosphere, combined.

Accepted Answer

According to the Food and Agriculture Organization of the United Nations, soils contain approximately 2,500 billion metric tons of carbon, while the vegetation and atmosphere combined contain only about 560 billion metric tons.

Question 14

40 In the United States, a national goal of maintaining 5% organic matter in all agricultural soils would be a practical and beneficial policy in the long term.

Accepted Answer

Maintaining 5% organic matter in all agricultural soils is not practical due to varying soil types, climates, and agricultural practices across the United States. Different soils have different capacities for organic matter, and a one-size-fits-all goal may not be feasible or beneficial everywhere.

Question 15

Poorly drained soils generally contain more organic matter than well drained soils.

Accepted Answer

Poorly drained soils tend to have more waterlogged conditions which can lead to reduced oxygen availability and slower decomposition of organic matter, resulting in a higher accumulation of organic matter. Well-drained soils allow for better aeration and decomposition, leading to less organic matter.

Question 16

The main contributions of the active fraction of soil organic matter are increased CEC and water holding capacity of the soil.

Accepted Answer

The answer of The main contributions of the active fraction...

Question 17

The priming effect refers to the excretion of growth promoting substance by certain bacteria during decomposition of fresh organic residues.

Accepted Answer

The priming effect refers to the phenomenon where the addition of a small amount of fresh organic matter to soil increases the decomposition rate of native soil organic matter, not the excretion of growth-promoting substances by bacteria.

Question 18

Sandy soils generally are higher in organic matter than fine textured soils.

Accepted Answer

Sandy soils have larger particles and therefore have larger pore spaces, allowing for rapid drainage and less water retention. This makes it more difficult for organic matter to accumulate and remain in the soil. Fine textured soils, such as clay, have smaller particles and more pore spaces, allowing for water and nutrients to be held in the soil, making it easier for organic matter to accumulate. Therefore, fine textured soils generally have higher organic matter content than sandy soils.

Question 19

Microbial metabolism in the guts of termites and in waterlogged soils are two major sources of excess methane that is contributing to the global greenhouse effect.

Accepted Answer

Microbial metabolism in the guts of termites and in waterlogged soils are both known sources of excess methane, and methane is a potent greenhouse gas.

Question 20

Humus is an important nutrient, essential for healthy plants.

Accepted Answer

Humus is not a nutrient itself; it is organic matter that improves soil structure, water retention, and nutrient holding capacity, indirectly supporting plant health.

Quiz 11: Soil Organic Matter Overview