A collaboration between

Acknowledgments

Acknowledgments

Funding provided by:

Guidelines and Webpage Design:

Daniel Geisseler, Ph.D.; UCCE Specialist in Nutrient Management; Department of Land, Air and Water Resources, University of California, Davis

Patricia Lazicki, Department of Land, Air and Water Resources, University of California, Davis

Reviewers:

XXXXXX; Farm Advisor

Support:

Moradi A. Barzin, Ph.D.; Senior Environmental Scientist FREP, California Department of Food and Agriculture

Amadou Ba, Ph.D.; Branch Chief Feed, Fertilizer, and Livestock Drugs Regulatory Services, California Department of Food and Agriculture

Amrith Gunasekara, Ph.D.; Science Advisor to the Secretary; California Department of Food and Agriculture

Last Update: January, 2018

Additional Information:

Crop Nitrogen Uptake and Partitioning

California Fertilization Guidelines

Principles of NPK Management

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Preplant

Sowing

Vegetative
Growth

Flowering

Fruit/Seed
Growth

Maturation

Residual soil nitrate and irrigation water nitrate contribute to a crop's N supply. Soil samples can be sent to the lab for nitrate analysis or analyzed on-farm. A full discussion of how to use soil nitrate testing to support nutrient management in annual crops may be found here.

Soil nitrate analysis can be combined with records of yields, fertilizer application rates and tissue analyses as part of a nutrient budget approach. Over time these values can be used to develop site- and variety-specific fertilizer rates and tissue N thresholds.

Soil Sampling

Soil Nitrate Quick Test

Interpretation

Irrigation Water Analysis

Tissue Analysis

Sampling Procedure

Diagnosing Suspected Deficiencies

Interpretation of Results

Preplant N

Seasonal N Requirements

Application Rate and Timing

Usually it is more efficient to split N applications than to apply the whole rate prior to planting (see In-Season N). This is because most annual crops have a phase of slow early growth when they take up little N, which is followed by a period of rapid vegetative growth and N uptake. For many crops, a small starter application at planting is sufficient to support early growth (See Starter N). High rates risk being leached during the early season. Side-dressing most or part of the N rate just before the rapid growth phase helps ensures that enough N will be present to meet plant demand ^[N37]. Information on the seasonal uptake curve for major California crops can be found here, and for a number of minor crops here.

If in-season or sidedress applications are not possible, preplant N should be applied as close to sowing/transplanting as possible. To avoid nitrate leaching, nitrate-based fertilizers should be applied after any pre-irrigation, especially if surface irrigation is used.

General shape of the N uptake curve for annual crops.

Crops that have a short season or are planted in warm weather (especially from transplants) have a shorter early lag phase and may benefit from a greater proportion of N applied pre-plant than crops with a long season, or crops planted in cool weather ^[N34]. More information on site-specific adjustments can be found here.

Mode of Application

Fertilizer Type

When properly managed, the fertilizer type does not affect the quality or yield of most crops ^{[N29,N47,N55,N67]}. Fertilizer choice depends on the weather (current and forecast), and fertilizer costs. Urea is inexpensive, high analysis and nitrifies less quickly than ammonium fertilizers. However, ammonium and especially urea-based fertilizers are prone to volatilization and should only be applied if they can be incorporated or if a rain or irrigation event of at least 1/2 inch is expected within a few days after application ^[N33,N49].

In shallow or coarse soils, nitrate-based fertilizers are not recommended because of their leaching potential ^[N47].

Fact sheets of the most common fertilizers can be found on the website of the International Plant Nutrition Institute.

Slow-release materials

Nitrate is more mobile in the soil than ammonium, and is more at risk of leaching. Urease and nitrification inhibitors and controlled release fertilizers (CRFs) aim to reduce nitrate leaching by slowing the rate at which nitrate-N is released to the soil ^[N57]. They are most likely to be effective when there's significant in-season rainfall, excess irrigation, or when in-season applications aren't practical ^[N23], and with shallow-rooted crops ^[N3,N57]. However, for products where the release rate depends on temperature, insufficient N may be released to support peak uptake if the weather is too cold ^[N13,N47]. Research in California vegetable crops suggests that under normal conditions slow-release materials are not consistently better than split in-season applications of conventional materials, which is less expensive ^[N23].

Starter N

Starter Nitrogen

Starter fertilizer, as opposed to preplant, refers to a small amount of fertilizer applied at planting, near or sometimes with the plant row. It is especially useful in cold, wet soils and for crops with small root systems.

Application Rate

Mode of Application

Fertilizer Type

Soil Applied N

Application Rate and Timing

In-season application rates should be adjusted to account for the nitrate present in the soil and irrigation water (See Soil N Test).

As a general rule, crops will use N most efficiently when available N matches the crop demand. The cumulative N uptake curve of annual crops from planting to maturity generally follows an S-shaped (sigmoid) curve (see Figure). Little N is taken up during germination and seedling development. Nitrogen uptake is high and reaches a maximum during vegetative growth as the root system expands and leaves and stems develop ^[N15]. For maximum effectiveness, sidedress N and N fertigation should be timed to maintain an adequate N supply during this period of high N demand. If only one side-dress application is practical, the best time is normally just before rapid vegetative growth begins ^[N26].

Generalized N uptake curve.

Information on the seasonal uptake curves for major California crops can be found here. An N-rate calculator for several minor California crops with information on the seasonal uptake pattern is available here.

If fertilizer is knifed in, it should be done early enough that the roots are not pruned ^[N26]. Late applications may be risky for crops where the mature product is harvested, as they are sometimes linked with quality reduction. This seems to especially be true where the harvested part grows belowground ^{[N30,N36,N45,N62,N65]}. However, late applications can increase protein in wheat ^[N40].

Mode of Application

Appropriate application of in-season N varies with cropping system and fertilizer type. Some major forms are outlined below.

Fertigation

In fields with pressurized irrigation systems, N is often most efficiently applied with irrigation water. This allows synchronization of N additions with plant demand and, in the case of drip systems, allows the nutrient to be directly placed in the root zone.

Care should be taken that nutrients neither accumulate at the soil surface nor are leached past the root zone. Therefore, it's generally recommended that fertilizer is injected towards the middle of an irrigation, and terminated far enough from the end that it can be flushed from the system and washed into soil ^[N26].

In some crops, topdressing N through a sprinkler system can lead to disease issues ^[N46]. In addition, high rates may cause leaf burn in some crops ^[N47].

Water-run fertilizer in furrow irrigated systems can be inefficient due to uneven distribution and potential losses with tailwater^[N59].

Broadcast (topdress)

Topdress applications are common in small grains, turfgrass, and pastures. Both liquid and solid fertilizers can be used ^[N26].

When N fertilizer is broadcast, it needs to be applied 1-2 days before an irrigation or rainfall event to move the N into the soil ^[N33,N68]. Irrigating the field is especially important when urea or UAN are applied, because the hydrolysis of urea by urease increases soil pH, which can result in high ammonia volatilization losses when the material is left on the surface ^[N5].

Broadcast N may be immobilized by high-carbon residues decomposing on the soil surface, such as occur in conservation tillage systems ^[N26].

Banding/injection

Liquid or solid fertilizers can be knifed in, injected, or dribbled on the soil surface and washed in with irrigation water. This tends to be the most efficient method for furrow-irrigated crops ^[N59].

The band should be far enough away from the plant row that it does not damage the growing roots, either through root pruning or toxicity from salts or ammonia in the fertilizer ^{[N11,N26,N56]}. If fertilizer is knifed in, it should be done early enough that it does not prune the roots ^[N26].

Fertilizer Type

Foliar N

Foliar Nitrogen

Foliar N applications normally don't have any value for non-deficient annual plants. They cannot replace soil applications, since only very dilute solutions can safely be applied without risking leaf burn, so it is generally not possible to apply enough to meet plant demand ^[N12,N26].

Foliar N may be useful when:

Quick, temporary relief from a deficiency is needed-i.e., a late-season deficiency develops but too much late-season soil N would have undesirable effects ^[N50]
Deficiencies occur in fields where a soil application is not practical ^[N48]
The crop's ability to take up N from the soil is limited (i.e. impaired root function, cold or waterlogged soil) ^{[N18,N31,N50]}
A late-season protein boost is needed for wheat ^[N12]

Application Rate

Fertilizer Type

Application Timing

Phosphorus (P₂O₅)