Lecture 17 Nitrogen 4 Fertilizer

What are the major functions of nitrogen (N) in plants?

N is a constituent of amino acids, proteins, nucleic acids, and chlorophyll. Adequate N supports photosynthesis, plant vigor, and dark green foliage.

What are the effects of excessive nitrogen on plants?

Excessive N can stimulate excessive vegetative growth, delay maturity, and increase disease incidence.

Why do nitrogen deficiency symptoms appear first in older plant tissues?

N is mobile within plants, so it can be moved from older tissues to newer growing leaves when N is deficient.

Why does nitrogen deficiency cause chlorosis in older leaves?

N is removed from chloroplasts in older leaves and transferred to new growth, causing older leaves to turn yellow. This yellowing is called chlorosis.

How does severe nitrogen deficiency affect lower leaves?

Lower leaves may turn yellow and die; the death of plant tissue is called necrosis.

What does nitrogen deficiency look like in corn?

Yellowing appears as an inverted V extending down the midrib of the leaf.

What is the typical nitrogen content of healthy plant foliage?

Generally 2–4% N on a dry-weight basis; turfgrass and legumes generally contain 3–6%.

How has global nitrogen fertilizer use changed since 1960?

N fertilizer use has increased much faster than phosphorus (P) and potassium (K) fertilizer use. Use has leveled off or declined in many industrialized countries but continues to increase in China, India, and much of the developing world.

What inverse relationship has been observed between nitrogen fertilizer use and legume seed production in the United States?

As N fertilizer use increases, legume seed production has tended to decrease. The lecture presents this as an inverse relationship.

What do the three numbers in a fertilizer grade, such as 10-10-10, represent?

The percentages of N–P₂O₅–K₂O, respectively. The second number is reported as phosphate (P₂O₅), and the third as potash (K₂O).

Do plants take up N, P, and K in the exact forms shown on fertilizer labels?

No. Fertilizer labels report N as N, phosphorus as P₂O₅, and potassium as K₂O, but plants absorb nutrients in their available chemical forms rather than necessarily as those reported label forms.

What should you check when interpreting a fertilizer recommendation?

Determine whether phosphorus is expressed as elemental P or P₂O₅, and whether potassium is expressed as elemental K or K₂O.

What is the Haber-Bosch process, and why is it important for nitrogen fertilizer production?

It industrially synthesizes ammonia (NH₃) from atmospheric N₂ and hydrogen. Ammonia is the first major product used to manufacture many nitrogen fertilizers.

What is the ultimate source of nitrogen in synthetic nitrogen fertilizers?

Atmospheric nitrogen gas (N₂).

Where is the hydrogen commonly used in nitrogen fertilizer production obtained?

From natural gas, primarily methane.

Why is nitrogen fertilizer production energy intensive?

The industrial production of ammonia and subsequent fertilizer manufacturing require substantial energy.

What are the top three forms of nitrogen fertilizer used in the United States, according to the lecture?

The lecture poses this as a question alongside a chart, but the extracted slide text does not identify the three forms. Check the original chart or instructor's notes for the exact answer.

What is anhydrous ammonia, and what is its fertilizer grade?

Anhydrous ammonia is NH₃ containing no water; its fertilizer grade is 82-0-0.

What are the important properties of anhydrous ammonia?

At room temperature it is a colorless, pungent, suffocating gas. It is transported under pressure as a liquid, has a strong affinity for water, and converts primarily to NH₄⁺ after application to soil.

What are the advantages of anhydrous ammonia fertilizer?

Low cost per unit of N, high N concentration, and low transportation cost per unit of N. It is commonly used for large-scale crop production.

What are the disadvantages of anhydrous ammonia fertilizer?

It can injure seedlings near the injection zone, temporarily raise soil pH near the injection band, and lose N as gas during application. It requires specialized equipment, storage, safety precautions, and potentially higher application costs because injection is slower.

How is anhydrous ammonia normally applied to soil?

It is injected about 10 cm below the soil surface through a slit made by a coulter or knife. The slit must be closed immediately to prevent NH₃ loss.

Under what soil conditions can anhydrous ammonia application be difficult?

Stony soils, dry soils, poorly structured soils, and soils where the injection slit cannot be sealed.

What can happen when anhydrous ammonia is injected into soil?

Limited lateral movement can create alternating zones of high and low N availability. High NH₃ concentrations may temporarily suppress microorganisms and damage roots.

How does soil pH change after anhydrous ammonia injection?

The pH near the injection zone may initially rise to approximately 9–10, then later become acidic as nitrification occurs.

What is the chemical formula and fertilizer grade of urea?

Urea has the formula CO(NH₂)₂ and a fertilizer grade of 46-0-0.

What are the major advantages of urea fertilizer?

It has a high N concentration, relatively low cost, is easy to handle, transport, and apply, and can be blended with other dry fertilizers.

Why is urea considered the most concentrated commonly available solid nitrogen fertilizer?

It contains approximately 46% nitrogen by weight.

How does urea behave immediately after application to soil?

Urea initially exists as a highly water-soluble, uncharged molecule. Before conversion, it can move with water through the soil.

What is urease, and what does it do to urea?

Urease is an enzyme that converts urea into ammonium-related products, producing ammonium and ammonia. Under favorable conditions, this conversion generally occurs within approximately 1–3 days.

What happens to urea after it is converted to ammonium in soil?

NH₄⁺ can be retained by the soil's cation-exchange complex instead of remaining as the original uncharged urea molecule.

Why can surface-applied urea lose nitrogen through ammonia volatilization?

Urea transformation can create conditions that favor NH₃ formation near the soil surface. If the ammonia escapes into the atmosphere before the fertilizer moves into the soil, nitrogen is lost.

What are the disadvantages of urea fertilizer?

It can lose N through ammonia volatilization when surface-applied and can injure germinating seeds and young seedlings.

How can ammonia volatilization from surface-applied urea be reduced?

Incorporate urea into the soil, irrigate it into the soil, apply it shortly before adequate rainfall, or use a urease inhibitor when appropriate.

Why should high rates of urea-N not be placed directly in contact with seeds?

Urea can injure germinating seeds and young seedlings.

What is the long-term acidifying effect of urea in soil?

Urea can contribute to soil acidification after its nitrogen is converted to ammonium and then nitrified.

What is the chemical formula and approximate fertilizer grade of ammonium nitrate?

Ammonium nitrate is NH₄NO₃ and has a fertilizer grade of approximately 34-0-0.

What forms of nitrogen does ammonium nitrate supply?

It supplies both NH₄⁺ and NO₃⁻, which are readily available or can quickly become available to plants.

What are the advantages of ammonium nitrate fertilizer?

It is easy to apply, produces a rapid crop response, supplies both ammonium and nitrate, and has lower volatilization potential than surface-applied urea.

What are the disadvantages of ammonium nitrate fertilizer?

It has a relatively high salt index, can injure seeds and seedlings at excessive rates near seeds, may burn foliage when applied directly to leaves, has a lower N concentration than urea, and can be explosive under certain conditions.

Why is ammonium nitrate subject to special storage and transportation regulations?

It can be used in explosive materials, so its storage, transportation, and availability are highly regulated.

What is urea-ammonium nitrate (UAN) solution?

A liquid mixture of urea and ammonium nitrate dissolved in water.

What are common fertilizer grades of UAN solution?

28-0-0, 30-0-0, and 32-0-0.

What three forms of nitrogen are present in UAN solution?

Urea-N, NH₄-N, and NO₃-N.

What are the advantages of UAN solution?

It is easy to store, transport, and apply as a liquid; can be applied uniformly; can be combined with some other liquid products; and works with several application systems.

How can combining UAN with herbicides benefit field operations?

It can reduce the number of trips over a field when the products are compatible.

What are the disadvantages of UAN solution?

It is corrosive to copper, zinc, and some alloys; part of its nitrogen is in urea form and can volatilize; excessive rates near seeds can cause salt injury; foliar application can burn leaves; it has lower N concentration than dry urea or anhydrous ammonia; and transporting its water increases transportation costs per unit of N.

What are slow-release nitrogen fertilizers?

Fertilizers that release plant-available N gradually rather than supplying it all rapidly in soluble forms.

Where are slow-release nitrogen fertilizers commonly used?

Turfgrass, greenhouse production, and nursery production.

What are the potential advantages of slow-release nitrogen fertilizers?

Reduced nitrate leaching, more gradual N supply, better synchronization between N release and crop demand, fewer applications, and lower risk of salt injury at an equivalent total N rate.

What are examples of slowly soluble nitrogen fertilizers?

Urea-formaldehyde, methylene urea, and isobutylidene diurea (IBDU).

What are examples of coated or controlled-release nitrogen fertilizers?

Sulfur-coated urea, polymer-coated urea, and resin-coated fertilizers.

What are examples of natural organic slow-release nitrogen sources?

Composted poultry manure, composted cattle manure, other composted organic materials, and commercial organic fertilizer products.

What are urease inhibitors?

Products that slow the conversion of urea into ammonium and ammonia.

How do urease inhibitors help reduce nitrogen loss?

They slow urea conversion, providing more time for rainfall or irrigation to move surface-applied urea into the soil and reducing ammonia volatilization.

What is a common urease inhibitor, and what is one product name associated with it?

NBPT; it is sold in some products under the name Agrotain.

When are urease inhibitors particularly useful?

With surface-applied urea in pasture or hay fields, no-till fields, and situations where incorporation is not possible.

What is a disadvantage of fertilizers containing inhibitors?

They generally cost more than untreated fertilizers.

What are nitrification inhibitors?

Products that slow the conversion of NH₄⁺ into NO₃⁻.

How do nitrification inhibitors slow nitrification?

They interfere with nitrification, including through direct toxicity to Nitrosomonas nitrifying bacteria.

What nitrogen losses may nitrification inhibitors reduce?

Nitrate leaching and denitrification losses.

What are examples of nitrification inhibitors?

Nitrapyrin, sold as N-Serve, and dicyandiamide (DCD).

Which fertilizers are nitrification inhibitors commonly used with?

Anhydrous ammonia and other ammonium-producing fertilizers.

What is the main difference between a urease inhibitor and a nitrification inhibitor?

A urease inhibitor slows urea conversion into ammonium and ammonia, helping reduce NH₃ volatilization. A nitrification inhibitor slows the conversion of NH₄⁺ into NO₃⁻, helping reduce nitrate leaching and denitrification.

A farmer surface-applies urea to a no-till field and no rainfall is expected. Which inhibitor may help?

A urease inhibitor, because it slows urea conversion and provides more time for rainfall or irrigation to move urea into the soil.

An ammonium fertilizer is applied several weeks before rapid crop N uptake. Which inhibitor may help reduce nitrogen loss?

A nitrification inhibitor, because it slows conversion of NH₄⁺ to NO₃⁻, reducing the amount of mobile nitrate produced before the crop can use the nitrogen.

A farmer specifically wants to slow the conversion of NH₄⁺ to NO₃⁻. Which inhibitor should be used?

A nitrification inhibitor.

A farmer wants more time for rainfall to move surface-applied urea into the soil. Which inhibitor should be used?

A urease inhibitor.

Ammonium nitrate is applied to a waterlogged field. Would a urease or nitrification inhibitor directly address the main problem?

Neither directly addresses the main problem in the lecture's scenario. Waterlogging promotes denitrification; improving drainage and managing fertilizer timing are more directly relevant.

Urea is incorporated immediately after application. Would a urease or nitrification inhibitor directly address the main problem?

Neither inhibitor is specifically indicated by the lecture's scenario, because incorporation helps move urea into the soil and reduces the risk of surface ammonia volatilization.