Sep

Starter Fertilizer with Organic Amendments: Needed or Not?

Nitrogen is a critical nutrient for plant growth and is often the most limiting nutrient in agricultural systems. Organic fertilizers such as manure, compost, and other organic amendments are valuable sources of nitrogen. However, unlike synthetic fertilizers that provide readily available nitrogen, the nitrogen in organic fertilizers is released gradually. Understanding when and how much nitrogen becomes available after applying organic fertilizers is essential for effective nutrient management and maximizing crop yield. This article discusses the dynamics of nitrogen availability from organic sources, the factors influencing release rates, and practical strategies for integrating organic fertilizers into fertilization programs.

Forms of Nitrogen in Organic Fertilizers

Organic fertilizers like manure and compost contain nitrogen in different forms, each with a distinct timeline for availability:

Ammonium nitrogen (NH₄⁺): A portion of nitrogen in fresh manure is present in the ammonium form, which is immediately available for plant uptake upon application. Ammonium nitrogen can be lost quickly through ammonia volatilization if manure or compost is not incorporated into the soil shortly after application. Therefore, rapid incorporation is crucial to retain this nitrogen and make it available to plants.

Organic Nitrogen (Org-N): The majority of nitrogen in organic fertilizers is in complex organic forms (proteins, amino acids, urea, etc.). This organic nitrogen is not immediately available to plants and must be mineralized by soil microorganisms. It is first mineralized into ammonium and then converted to nitrate (NO₃^–) through nitrification. The mineralization process depends on environmental factors such as temperature, moisture, and microbial activity. and varies significantly between manure, compost, and other organic materials.

The fraction of ammonium can range from 10% to 50% of the total nitrogen, depending on the type of organic fertilizer:

Poultry Manure: Contains about 30-50% of total nitrogen as ammonium nitrogen.
Swine Manure: Typically has 20-35% of total nitrogen as ammonium nitrogen.
Dairy Cattle Manure: Contains about 10-20% of total nitrogen as ammonium nitrogen.
Compost: Due to the composting process, which stabilizes organic matter, compost generally has less than 5-10% of total nitrogen in the ammonium form.

Nitrogen Availability Timeline After Application

The release of nitrogen from organic fertilizers occurs in stages, influenced by the type of organic material, environmental conditions, and management practices. Here is a breakdown of nitrogen availability over time after applying manure, compost, or other organic amendments:

Immediate to Two Weeks After Application

Manure:

Ammonium nitrogen availability: In the first 1-2 weeks after manure application, ammonium nitrogen is the primary form of available nitrogen. Because ammonium is immediately plant-available, crops can access this nitrogen almost right away. However, without rapid soil incorporation, ammonium can be lost to volatilization.
Organic nitrogen mineralization: During the first two weeks, the mineralization of organic nitrogen is minimal. Typically, only about 1-5% of organic nitrogen begins to mineralize in this period as microbial activity starts breaking down the more easily decomposable organic fractions

Compost:

Ammonium nitrogen: Compost, having undergone a decomposition process, contains much lower ammonium nitrogen levels than fresh manure. As a result, only a small amount of nitrogen is available immediately.

Organic nitrogen mineralization: The mineralization process in compost is slower due to its more stabilized organic matter. Therefore, in the first two weeks, only about 1-3% of organic nitrogen may mineralize.

Other organic amendments (e.g., cover crop residues, green manures):

Availability varies widely, depending on the C:N ratio and the degree of decomposition, nitrogen availability can vary. High C:N ratio materials (e.g., straw) may immobilize nitrogen rather than release it, whereas low C:N ratio materials (e.g., green manures) can begin mineralizing quickly.

Two to Eight Weeks After Application

Manure:

Continued organic nitrogen mineralization: From 2 to 8 weeks after application, there is a noticeable increase in organic nitrogen mineralization as microbial activity ramps up. This period is crucial for releasing plant-available nitrogen, especially for crops entering early vegetative growth stages.
Rate of mineralization: In favorable conditions (warm temperatures, adequate moisture, and neutral pH), approximately 10-20% of the organic nitrogen in manure may mineralize during this period.

Compost:

Slow, gradual release: Compost continues to release nitrogen, but at a slower rate than fresh manure. During this period, about 5-10% of the organic nitrogen in compost may mineralize, providing a steady but moderate supply of nitrogen to crops.
Lower risk of losses: Due to the lower ammonium content and more stabilized organic matter, compost is less prone to nitrogen losses through volatilization or leaching.

Other organic amendments have variable mineralization rates: Green manures or fresh cover crop residues with low C:N ratios may have 10-15% of their organic nitrogen mineralize in this period. In contrast, high C:N ratio materials may continue to immobilize nitrogen.

Two to Six Months After Application

Manure:

Continued Mineralization: The mineralization process continues, with 30-50% of the organic nitrogen potentially becoming available in the first growing season. This gradual release provides a steady supply of nitrogen throughout the crop growth period.
Decreasing rate of release: The mineralization rate slows down over time as the more labile (easily decomposable) organic matter fractions are depleted.

Compost:

Extended nitrogen release: Compost provides a slow and sustained release of nitrogen over several months. Over this period, 15-25% of the total nitrogen in compost may become available. This makes compost an excellent source for maintaining long-term soil fertility.
Residual effects: The residual nitrogen from compost continues to mineralize over a longer timeframe (1-2 years), contributing to the nitrogen pool for subsequent crops.

Other Organic Amendments: Ongoing mineralization or immobilization, depending on the material’s composition, nitrogen may continue to mineralize or, in the case of high-carbon materials, remain immobilized for an extended period.

Green manure vs. Compost

Aspect	Green Manure	Compost
Definition	Crops grown to be incorporated while green to boost soil fertility.	Decomposed organic matter from plant and animal waste.
N Content	High; releases quickly after incorporation (low C:N ratio).	Moderate; releases slowly over time (higher C:N ratio).
C:N Ratio	Low (10:1 to 20:1), fast breakdown.	Higher (15:1 to 30:1), slower breakdown.
Decomposition	Fast (2-8 weeks).	Slow; can last months to years.
Benefits	Quick N release, soil structure, weed control.	Long-term soil health, organic matter, moisture retention.
Application	Grown in place, incorporated before seed set.	Made off-site, applied on the surface or incorporated.
Nutrient Timeline	Quick N availability after use.	Slow, steady nutrient release over time.
Microbial Activity	Rapid increase with fresh organic matter.	Gradual support for diverse microbes.
Use Cases	Short-term N boost, soil fertility.	Long-term organic matter and nutrient cycling.
Environmental Impact	Less synthetic fertilizer, better soil health.	Recycles waste, improves soil carbon.

Practical Considerations for Managing Nitrogen Availability

Use of starter fertilizers: For crops with high early nitrogen demand (e.g., corn, wheat), applying a starter fertilizer with readily available nitrogen (such as urea or ammonium nitrate) can complement organic fertilizers and ensure adequate nitrogen supply in the early stages.
Timing and incorporation: Apply manure or compost close to the time of planting and incorporate it into the soil to reduce nitrogen losses and enhance availability.
Split applications: Consider split applications of organic fertilizers or combining them with mineral fertilizers to provide a consistent nitrogen supply throughout the growing season.
Regular soil testing: Monitoring soil nitrogen levels and organic matter can help adjust fertilization rates based on crop needs and prevent under- or over-application.