Question 1

On the average,agricultural soils in the US are currently being depleted of their nitrogen supply.

Accepted Answer

Agricultural soils in the US are typically managed with practices that replenish nitrogen, such as crop rotation and the application of fertilizers, to maintain soil fertility.

Question 2

Forests in decline because of acid deposition can often be brought back to high productivity with just a few applications of nitrogen fertilizer.

Accepted Answer

Acid deposition can cause a complex set of problems in forests, including nutrient imbalances and the leaching of essential minerals from the soil, not just nitrogen deficiency. Simply adding nitrogen fertilizer may not address these broader issues and could potentially exacerbate certain problems, such as soil acidification or nutrient imbalances. Effective recovery strategies often require a more comprehensive approach, including liming to neutralize soil acidity and the addition of other missing nutrients based on soil and foliage analyses.

Question 3

Highly weathered soils low in available sulfur in their A horizons often have adequate supplies of available sulfur in deeper,clay-rich horizons.

Accepted Answer

This statement is true. Due to the leaching of sulfur from the topsoil, the deeper clay-rich horizons may accumulate sulfur and have adequate supplies of available sulfur.

Question 4

Because of their huge numbers and direct access to soil organic matter,free living microorganisms such as Azotobacter typically fix more nitrogen per hectare of land than do nitrogen-fixing bacteria living in plant roots.

Accepted Answer

Free-living microorganisms like Azotobacter generally fix less nitrogen per hectare of land compared to nitrogen-fixing bacteria living in symbiosis with plant roots, such as those in the Rhizobium genus with legumes, because the symbiotic relationships allow for more efficient nitrogen fixation due to the direct energy supply from the plant to the bacteria.

Question 5

Although slow release fertilizers take longer to have an effect,they are more economical than regular nitrogen fertilizers and so are favored by foresters and farmers.

Accepted Answer

Slow release fertilizers are generally more expensive upfront than regular nitrogen fertilizers, but they are favored for their efficiency and long-term cost savings due to reduced application frequency and minimized nutrient loss.

Question 6

Ammonia volatilization is a process by which much needed nitrogen is added to the pool of plant-available nitrogen in soils.

Accepted Answer

Ammonia volatilization is a process where nitrogen is lost from the soil as ammonia gas, reducing the amount of plant-available nitrogen.

Question 7

Applying heavy dressings of N fertilizer will tend to produce sugary-sweet fruits and vegetables.

Accepted Answer

Applying heavy dressings of nitrogen fertilizer can actually decrease the sweetness of fruits and vegetables by diluting their natural sugars. Over-fertilization can also lead to other negative effects, such as reduced yield and increased susceptibility to diseases. It is important to apply fertilizer in appropriate amounts and timing based on the specific needs of the crop and soil conditions.

Question 8

The terms "ammonium fixation" and "biological nitrogen fixation" refer to the same soil process.

Accepted Answer

Ammonium fixation refers to the process where ammonium ions (NH4+) in the soil are immobilized and become unavailable for plant uptake. Biological nitrogen fixation, on the other hand, is the process by which certain bacteria convert atmospheric nitrogen (N2) into a form (such as ammonia or nitrate) that can be used by plants. These are different processes with different outcomes for plant nutrient availability.

Question 9

Ammonium fixation occurs to the greatest extent in the O and A horizons.

Accepted Answer

Ammonium fixation occurs to the greatest extent in the B horizon, where clay particles are more abundant and can trap ammonium ions more effectively.

Question 10

Leguminous forest trees,such as Locust,enrich the soil in nitrogen,to the benefit of non-nitrogen fixing plants growing nearby.

Accepted Answer

Leguminous trees have root nodules that contain symbiotic bacteria that fix atmospheric nitrogen and convert it into a form that plants can use. This nitrogen is then released into the soil, enriching it and benefiting other nearby plants that do not have this ability. Locust is a leguminous tree, so this statement is true.

Question 11

About half the nitrogen in the A horizon of most soils becomes available to plants over the course of a whole year in a warm,humid climate.

Accepted Answer

The rate at which nitrogen becomes available to plants depends on various factors, including soil type, temperature, moisture, and microbial activity. Typically, only a small fraction of nitrogen in the soil becomes available to plants in a year.

Question 12

Ammonia losses from the shallow,algae-filled water of a non-tidal wetland are likely to be much greater at night than during the day.

Accepted Answer

Ammonia losses are typically greater during the day because photosynthetic activity by algae and other aquatic plants increases pH levels, converting ammonium (NH4+) to ammonia gas (NH3), which is then lost to the atmosphere more easily.

Question 13

Nitrification requires the presence of a source of readily available carbon,such as manure.

Accepted Answer

Nitrification is the biological process by which ammonia is first converted to nitrite, then to nitrate by aerobic bacteria in soil or water. While carbon can aid in the growth and activity of these bacteria, it is not a requirement for the process itself.

Question 14

Pesticides and other chemicals added to soils would be more likely to slow down nitrification than mineralization.

Accepted Answer

Pesticides and other chemicals can affect the microorganisms involved in nitrification, thus slowing down the process, while they may have a lesser impact on mineralization.

Question 15

In most temperate,humid regions,the period of greatest potential for nitrate leaching is during the summer when nitrogen fertilizer is applied to crops.

Accepted Answer

In most temperate, humid regions, the period of greatest potential for nitrate leaching is typically during late fall to early spring when crops are not actively growing and taking up nutrients, and when excess rainfall or snowmelt can leach nitrates into the groundwater.

Question 16

Unlike Rhizobium bacteria,Bradyrhizobium bacteria are effective at fixing N,regardless of the type of plants present.

Accepted Answer

Bradyrhizobium bacteria are more specialized in forming symbiotic relationships with specific plants and are not universally effective at fixing nitrogen for all plant types.

Question 17

The natural microbial oxidation of nitrogen released by organic matter mineralization is a major source of soil acidity.

Accepted Answer

During the microbial oxidation of nitrogen released by organic matter mineralization, nitrification produces nitric acid which contributes to soil acidity.

Question 18

Nitrogen taken up by plants growing in a compost-treated soil is indistinguishable from the nitrogen taken up by plants growing in a fertilizer-treated soil.

Accepted Answer

Compost contains organic matter that has decomposed to release nutrients, including nitrogen, that plants can use. This nitrogen uptake by plants from compost is chemically identical to the nitrogen uptake from fertilizer. Therefore, plants growing in a compost-treated soil and a fertilizer-treated soil take up indistinguishable nitrogen.

Question 19

Most nitrogen leaching to groundwater is in the form of ammonium.

Accepted Answer

Most nitrogen leaching to groundwater is in the form of nitrate, not ammonium. Nitrate is a highly soluble form of nitrogen and can easily travel through soil and reach groundwater. Ammonium, on the other hand, is less mobile in soil and is often converted to nitrate by soil microorganisms before it reaches groundwater.

Question 20

Nitric acid deposited on forest soils with "acid rain" can increase the leaching of nutrient cations.

Accepted Answer

Nitric acid can increase the acidity of the soil, which can cause the leaching of essential nutrient cations such as calcium, magnesium, and potassium out of the soil. This can lead to nutrient deficiencies in plants and negatively impact the ecosystem.

Quiz 13: Nitrogen and Sulfur Economy of Soils