Exam 12: Soil Organic Matter

Soils with thick horizons consisting mainly (> 30% by weight) of organic matter and that form in topographic depressions or low-lying places usually belong to the order ______.

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.

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.

Suppose leaves containing 1000 kg of carbon fell to the forest floor in a humid temperate environment.After a year or so, how many kg of that carbon would you expect to remain in the soil as microbial biomass and humus?

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

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

To avoid a nitrate depression period when it is added to a garden soil, about how many kg of nitrogen must be added to a load of grass clippings that contains 1000 kg of C and 25 kg of N?

Well aerated soils enriched with organic amendments can help reduce the methane load in the atmosphere.

Which of the following is a greenhouse gas contributed to the atmosphere by soils?

Green plant residues such as grass clippings or a green manure crop generally have a water content of about ____% by weight.

Non-humic substances consist of ____.

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

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

The active fraction of soil organic matter largely accounts for which of the benefits of adding organic matter to soils?

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

Given this Table of plant residue analyses, choose the "highest quality" residue in terms of suitability to support an active community of soil organisms and rapid nutrient cycling. C/N ratio 15 15 15 45 45 Polyphenol content, \% 0.5 6.0 6.0 0.5 0.5 Lignin content, \% 10 40 10 40 10

Which of the following added to a soil would be likely to result in the greatest level of soil organic matter after several years of decomposition?

Humus is an important nutrient, essential for healthy plants.

Soil organic matter consists of ______.

Humic substances consist of _____.