Nitrogen Management in Vineyards

by Pierre Helwi, Ph.D. (June 2019)

Soil mineral composition is one of the key components of viticulture terroir. Indeed, mineral elements such as calcium, iron, potassium, and nitrogen are essential for the synthesis of primary metabolites, as well as the production of many secondary metabolites such as phenolic or aromatic compounds.

Among the minerals that grapevines assimilate from the soil, nitrogen (N) is undoubtedly the one that most strongly influences vine growth and vigor and in consequence, fruit composition. Some authors even consider that the availability of N participates in the terroir effect since it is influenced by the soil. For example, vines generally assimilate more N when they are planted on gravelly soil whereas their N status is lower when planted on clay soil.

N requirements by winegrapes are generally estimated to be between 30 and 70 lbs per acre per year depending on the desired vigor and yield. In Texas, the availability of N naturally occurring in the soil is generally low, often making it a limiting nutrient for vegetative growth. At harvest, fruit contains between 10 and 30 lbs of N per acre resulting in exportation from the vineyard. This phenomenon is strongly correlated with the variety and the vigor and is considered to be the minimum amount that must be supplied each year by the soil and/or by fertilization. However, fertilizer additions to soils must be greater than the amount required because N is mobile in the soil and can be lost by leaching. In contrast, high N availability can lead to excessive grapevine vigor resulting in greater canopy management costs, increase the incidence of fungal diseases, and reduced fruit quality. Therefore, it is essential to maintain a non-limiting, but also a non-excessive level of N to produce a quality crop with an economically acceptable yield. The management of vine N status is therefore crucial in grape production.

NITROGEN IN THE SOIL

In the soil, N is present in two forms, organic and inorganic. The vast majority of N is stored in its organic form, in the form of humus (stable and slowly decomposed) or in the form of fresh organic matter. The inorganic or mineral form of N exists mainly in three forms: elemental (N₂), nitrate (NO₃^–) and ammonium (NH₄⁺). From a plant nutritional point of view, NO₃^– and NH₄⁺ are considered as essential forms. The organic forms of N (humus and fresh organic matter) cannot be assimilated directly by the vine. A mineralization process is essential to make them bioavailable. This step is carried out by the soil microorganisms. It is essential to note that the mineral N is very weakly retained by the soil. In the absence of fertilization, the availability of mineral N for the vines depends on the amount of organic matter stored in the soil and its rate of mineralization. The latter will be more important if the C/N ratio of the organic matter is low, pH close to neutral, the soil is well ventilated and warm and water reserves close to field capacity. Soil N supply is also influenced by irrigation.

Among the many factors influencing vine N status, plant material is predominant. Each variety or even each clone or rootstock has different rates of N assimilation. For example, Cabernet-Sauvignon and Pinot noir are known for their high rate of N assimilation unlike Riesling and Grenache. At the clonal level, Sauvignon blanc clone R3 has a higher assimilation of N compared with Sauvignon CL297.

Cultural practices can also impact N nutrition. It is known that weed cultivation between the rows increases the competition for N between the vine and the weed, thus decreasing the amount of N available for the vine. On the contrary, legumes, often sown as green manure, will enrich the soil with N.

Finally, the climate of the region and the vintage can also influence plant N status since they affect the rate of mineralization of the organic matter.

NITROGEN CYCLE DURING VINE DEVELOPMENT

• Nitrogen reserves

The growth and development of the vine are partly dependent on its mineral reserves, including N, accumulated during the previous season. N reserves are located in the perennial parts of the vine, which are the roots, the trunk, and the shoots and constitute 20 to 40% of the plant annual needs necessary for its growth. At the beginning of the season, N requirements are important to form young organs in the spring. They are mainly taken from reserves formed especially at the end of the previous season before root absorption takes over. Starting at veraison, if N quantities absorbed from the soil remain insufficient, a new phase of remobilization of the reserves will take place.

• Nitrogen absorption

The amount of N taken from the soil varies according to the stage of vine development. Two periods of intense absorption of this nutrient are described: a first period between budbreak and veraison, and a second period after harvest and until leaf fall. The amount of N removed from the soil increases gradually from the 3-5 leaf stage to 50% of the total N content in the pea size stage (first peak of N accumulation). Then, N absorption decreases between veraison and ripeness and increases after the harvest. N quantities absorbed after harvest constitute 30-40% of the total N and will be mainly stored in the trunk.

 

Dynamics of vine N uptake during the season (adapted from Conradie 1980).

 

• Transport and remobilization

N is transported in comparable quantities to all vine organs by the xylem, driving the sap rich in water and nutrients from the soil to the leaves, and the phloem driving the sap rich in carbohydrates.

Starting bloom, total N concentration of the shoots increases gradually to reach its maximum level at the end of the vegetative growth. This increase is due to a translocation of this element from the leaves to the shoots and the perennial parts. N content in the aerial parts of the plant (limbs, petioles and berries) reaches its maximum in the early stages of development, concordant with the phase where N uptake peaks, then decreases over the season with minimum values at harvest or in the fall.

• Dynamics and location of nitrogen in the berry

In the berry, a significant allocation of N begins after flowering thus causing a drop in the content of this macro-element in the perennial parts of the plant despite the continuous supply from the roots. Thus, cluster development and N reserve formation seem to be two antagonistic processes. The translocation of N in the grape berry increases gradually during ripening. N is accumulated mainly two weeks before the pea-size stage and then after mid-veraison. During these two periods, the berry accumulates more than 50% of the N present in the plant. At the end of ripeness stage, a large amount of N is transported from the roots to berries.

SYMPTOMS OF NITROGEN DEFICIENCY

• Overall yellowing of lower and older leaves. As deficiency worsens, younger leaves will show symptoms.
• Slow growth, short internode and small leaves.
• Reduction of yield.

Symptoms of N deficiency (Photo courtesy of Fran Pontasch; Texas A&M AgriLife Extension Service).

NITROGEN FERTILIZATION

N fertilization stimulates N metabolism and consequently protein synthesis. Two periods of fertilization are recommended depending on the stage of development and must be reasoned according to indicators of soil and vine N status. The first period is between budbreak and fruit-set, when N uptake and plant needs are at maximum. The second period is the post-harvest stage when reserves for the following season growth are constituted. Small quantities via drip irrigation (5 to 15 lbs N/acre) are recommended at this second period. If N application is made close to leaf fall, uptake will be low and N will be leached through the soil profile. Large applications of N should be avoided in late season to ensure that shoots properly harden off and acclimate for winter.

In vineyards deficient in N, a foliar N supply between flowering and veraison generally increases must available nitrogen (YAN), essential for a better alcoholic fermentation.

Application prior to budbreak is of no value since N uptake is low and the vine is still using N from reserves. In addition, N should not be applied close to flowering as it may result in bunch shatter. Applications around veraison and during ripening should also be avoided as this could promote excess growth and decrease berry quality.

Fertilization is generally accompanied by an increase in leaf area coupled with an increase of chlorophyll synthesis and development of storage tissues such as the trunk, the shoots and the roots.

APPLICATION METHODS

Direct soil surface application – dry N fertilizers are applied directly to the soil surface either by banding in the weed-free strip under the trellis where vine roots are concentrated or by broadcasting over the vineyard floor in clean cultivated vineyards. The disadvantage of such applications is the dependence on rainfall for incorporation and availability. Some growers mix N with their herbicide sprays to enhance herbicide activity and as a source of nutrition.

Fertigation – more efficient technique since N is applied in small amounts at regular intervals rather than large rate all at once as done by direct soil surface application. On sandy soils, multiple applications of small amounts are recommended to reduce the risk of salt injury and to compensate for potentially greater losses of N via leaching.

Foliar application – although having limited benefits, small applications of N in the form of urea may be used to correct visual symptoms of deficiency. With foliar applications, a risk of leaf burn can occur mainly when sprays are done between bloom and harvest.

NITROGEN SOURCES

Dry N for soil surface application – Examples: Urea (46% N), Calcium nitrate (15% N), Potassium nitrate (13% N), Ammonium sulfate (21% N), Monoammonium phosphate (11% N) and Diammonium phosphate (18% N). The N percentage given for fertilizers indicates the percent by weight of N (i.e. 100 lbs of urea (46% N) contains 46 lbs of N).

Liquid N for soil application – Examples: Aqua ammonia (20%), Calcium ammonium nitrate (32%), Urea ammonium nitrate (32%), Urea (23%) and Ammonium nitrate (20%).

If using ammonium form of N, the fertilizer must be incorporated into the soil by tillage or rainfall, otherwise, valuable N may be lost through volatilization.

Complete fertilizers such as 10-10-10 could also be considered.

Manure and other organic matter – slow and long term release of N.

Cover crops – a winter legume such as vetch or clover can be a good N sources and will extend N availability.

VINE NITROGEN STATUS AND ITS EFFECT ON VINE GROWTH AND GRAPE COMPOSITION

The effect of N nutrition on vine growth and berry composition was explicitly described by Bell and Henschke in 2005. In their review, they mentioned that N deficiency, defined by low vigor and a YAN less than 100 mg/L, leads to a decrease in growth and yield as well as yellowing of the leaves. The presence in the must of low concentrations of YAN causes delays or difficulties of fermentation and results in the presence of hydrogen sulfide and/or undesirable sulfur compounds in the wine. This deficiency can be caused by an insufficient availability of mineral N in the soil, but also by a strong water stress which prevents the assimilation of N by the root system. A YAN value close to 200 mg/L is recommended to ensure a smooth running of the fermentation.

In contrast, an excessive N supply causes increased yield, vigor, photosynthetic activity and susceptibility to Botrytis cinerea. Among other things, the excessive vigor alters canopy microclimate, thus resulting in a decrease of temperature and radiation received by the various organs (leaves, buds and clusters). Radiation decrease reduces the concentration of anthocyanins, terpenes and total phenols in the berry. Moreover, the low temperature inside the canopy causes high titratable acidity and a low pH due to an increase in malic acid content. A reduction in temperature can also causes a slower maturation resulting in insufficient phenolic maturity at harvest.

NITROGEN FERTILIZATION IN TEXAS

In Texas, the majority of vineyard soils, particularly sandy soils, are naturally low in organic matter. In consequence, N fertilization seems to be primordial for optimal vine growth and fruit quality. As a mobile nutrient and due to its high leaching rate on these types of soils, N application should be made more frequently adding small doses at each application. A split application between the 3-5 inch leaf stage and fruit-set is recommended with quantities depending on tissue testing and overall vine visual observations. As mentioned previously, attention should be made to the planted variety and the scion/rootstock combination as some cultivars has a tendency to better assimilate N and has the potential of excessive vigor that influences vine health and grape quality.

 

 

Pierre Helwi, Ph.D. | Assistant Professor and Extension Viticulture Specialist
Texas A&M AgriLife Extension Service | 1102 E. Drew Street, Lubbock, Texas 79403
Phone: (806) 723-8447
E-mail: pierre.helwi@ag.tamu.edu

 

by Pierre Helwi (October 2017)

Cover crops are an important component of sustainable viticulture systems as they have a major and direct impact on the health of vines and the surrounding ecosystem. Growing a cover crop minimizes the use of chemicals which may negatively affect the environment and reduce the physical impact of frequently running heavy equipment on vineyard soil. This article discusses the benefits and drawbacks of using cover crops in sustainable viticulture and includes guidelines for sound practices.

A cover crop can be defined as any vegetation grown in vineyard middles and occasionally under vines without being harvested. Cover crops may be planted annually fall and spring or maintained perennially. The implemented technical aspects of this approach are delicate and must be well considered in order to benefit from the positive effects of the planted crop.

Benefits of cover crops

Improve soil structure: vegetation roots bind soil particles together, ameliorating soil structure and water infiltration. In additions, the mechanical action of cover crop roots loosen the soil up to 60 inches of depth, reduce its compaction and improve the penetration of water and air.

Improve mineral fertility: besides increasing soil nitrogen, the decomposition of the cover crop increases soil cation exchange capacity therefore the ability of a soil to hold and exchange nutrients allowing their restitution to the vine in an assimilable form. Cover crops limit also mineral leaching by rain by storing them during the winter time. In addition, legumes contribute to enrich the soil with nitrogen by symbiotic fixation of the atmospheric form.

Improve soil biological activity and organic matter content: cover crops stimulate rapidly and intensely the biological activity of the soil during their growth and especially after decomposition. The quantities of formed humus (organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms) allow to maintain the organic matter content of the soil.

Protect against erosion and run-off:  cover crop protects the soil surface from raindrop impact that dislodges soil aggregates, enabling them to move with water run-off.

Limit weed germination and growth.

Provide habitat for beneficial insects and predators: some cover crops attract beneficial insects and arthropods which can contribute to control harmful insects and mites.

Suppress some populations of nematodes: an anti-nematode action is sometimes described for some cover crops. This action, due to compounds released during the decomposition of the plant, concerns only Root-knot and Pratylenchus nematodes responsible for direct damages.

Influence grapevine growth: the presence of vegetative cover influences grapevine growth by competing for water and nutrients or by providing additional nitrogen for vine development.

Provide firm footing for cultural operations and are aesthetically pleasing.

The use of cover crops may also have some drawbacks. The presence of a cover crop may increase water use, frost hazard, and the competition with vines for soil moisture and nutrients. Pest problems may also result from the presence of cover crop mainly when it is not kept in a reasonable height, in addition to a possible increase in costs and management.

Which species can be used as cover crop?

Many types of plants can be used as cover crops. Legumes and grasses including cereals are the most extensively used, but there is increasing interest in brassicas (such as rape, mustard, and forage radish) and continued interest in others, such as buckwheat.

Families of cover crop are classified according to their ability to provide carbon (“slow” or “fast”) and nitrogen (N). “Slow” carbon sources correspond to materials rich in cellulose and lignin such as cereals, “rapid” carbon sources are associated with grasses and brassicas with easily degradable sugars, and legumes are the N providers. In order to ensure that microorganisms can properly degrade the organic matter without depriving N, it is desirable to use a cover crop with a balanced formulation between slow carbon, fast carbon and N sources. Legume-grass mixtures complement each other in their soil improving functions. This blend offers the benefit of both tap and fibrous root systems and supplies the vines with moderate N. Monocultures may be preferred where the species has a history of proven performance. Single-species plantings should usually be rotated to reduce the potential for buildup of insects or pathogens. For each specific crop, ask the seed supplier about seedbed cultivation, as well as moisture and fertilizer requirements.

The table below adapted from “Principles of Cover Cropping for Arid and Semi-arid Farming Systems, NM State University” lists some of the species that can be used:

Winter annual

Name	Family	Characteristics	Seeding Rate (lb/ac)
Annual grasses (wheat, barley, oats, annual ryegrass, cereal rye, triticale)	Grass	Cold-tolerant, high lime tolerance, low drought and generally low salinity tolerance, moderate moisture use	Wheat, barley, oats, triticale: 60-120 Annual ryegrass: 15-30
Austrian winter pea	Legume	Moderately cold and drought tolerant, moisture efficient	60-80
Brassicas (mustards, turnips, forage radish)		Tap-rooted, moderate to high drought tolerance	Mustard: 5-12 Turnip: 4-7 Radish: 8-12
Hairy vetch	Legume	Cold tolerant, moderate tolerance to drought and soil lime; low salinity tolerance	15-20

Winter annual cover crops are most often planted in vineyards because they grow during the dormant season and spring when rainfall is often most abundant, thereby aiding in erosion control and are not in competition with the vines for water and nutrients. They are sown in the fall and are mowed and disked in the spring or killed with an herbicide. Summer annual cover crops are usually planted in the spring and they are ready to mow or till in in about a month assuming adequate rainfall.

Summer annual

Name	Family	Characteristics	Seeding Rate (lb/ac)
Buckweat	Grass	Cold sensitive, moderate drought tolerance	50-60, drilled
Cowpea	Legume	Drought tolerant	50-100
Foxtail millet	Grass	Cold sensitive, drought tolerant	15-20
Lablab	Legume	Vining and spreading legume	50-60
Pearl millet	Grass	Cold sensitive, drought tolerant	15-20
Sesbania	Legume	Fast and vigorous growth	30-40
Sorghum-Sundangrass	Grass	Cold sensitive, drought tolerant	15-40

Perennial cover crops are generally sown in the fall, but some can be planted in early spring. They usually do not require replanting for several years. Perennial species are most commonly used in vineyards planted on fertile sites where vines are seriously out of vegetative balance but are also utilized in less fertile sites in order to maintain soil structure in the aisles and provide firm footing for viticulture operations.

Name	Family	Characteristics	Seeding Rate (lb/ac)
Alfalfa	Legume	Cold tolerant, drought tolerant	15-18
Red clover	Legume	Cold tolerant, moderate tolerance to soil lime, low drought and salinity tolerance	20-28

Legumes

Legumes provide N to the soil with the aid of symbiotic bacteria. A legume plant produces a tap root that does not penetrate well into compacted soil layers, so they are less useful for loosening soils and improving water penetration than cereals. A legume green manure cover crop can provide all of the N required under ideal circumstances after 2 seasons of careful management. The N contribution could be reduced by planting alternate row middles, combining legumes and cereals in the cover crop mixture, or reducing the width of the cover crop band. Legume cover crops should be used with caution in excessively vigorous vineyards and high rainfall areas of the state. Legume seed must be inoculated with the appropriate strain of N-fixing rhizobium bacteria prior to planting.

Grasses

Grasses do not fix N but may be useful as a trap crop to take up soil N and release it more slowly upon decomposition in the soil. Grasses have numerous small, fibrous and fine roots that are more likely to grow into compacted layers.

Cover crop management

Prior to seeding, the soil must be sufficiently cultivated to allow for good germination. Many growers begin by shallow ripping using a shallow tiller. The soil is then moistened and disked twice (about a week apart), leveled, and seeded. Seeding will ideally be from mid-September to mid-October for most cover crops, when soils are warm and rainfall is likely. Seeding after mid-October in many areas of the state becomes risky due to cooling soil temperatures, slow germination, and early frost. No-till drilling method is highly recommended for seeding cover crops because, besides conserving soil texture, it offers a uniform seed placement and an excellent seed-to-soil contact, which leads to a high cover crop establishment rate. After seeding, the seedbed should be firmed to lightly pack the soil. Irrigation after seeding helps ensure successful germination and establishment.

Cover crops should be fertilized and soil amendments should be added on the basis of soil test results. Grasses and brassicas may require the addition of N for adequate growth. When planting legume-grass mixes, avoid or limit N fertilizers, which stimulate grasses to the point that the will shade out the legumes. Many growers use compost, which in most cases will adequately provide what the cover crops need.

The presence of the cover crop increases the risk of damage by spring frost. Often the cover crop is mowed in early spring for frost protection and then allowed to resume growth and go to seed. After the seed matures, the cover crop is mowed and left on the soil surface or incorporated into the soil using a shallow tiller.

The following factors should be considered when choosing whether or not to incorporate the cover crop:

• It may allow for rapid N release and availability for the current season.
• Maximum N release occurs about 3 weeks after incorporation, assuming that the soil remains moist.
• For perennial cover crops, several mowings might be required to keep the foliage from growing excessively tall.

Conclusion

Vineyard floor management is an important component of sustainable winegrowing systems. If a cover crop is to be utilized, choices in cover cropping should be site-specific. Growers must consider their style of farming, yield and quality objectives, and any other criteria that they consider important.

By Fran Pontasch (July 29, 2018)

The demand of carrying a grape crop to harvest is stressful for grapevines, even when rainfall and high temperatures are normal. This season is placing extraordinary demands on our grapevines. Demands for water vary by soil type, vine age, competition from weeds, and temperature.

Water loss from evaporation and transpiration must be replenished so the vines can continue to function. In drought conditions, the older, basal leaves dry out and brown, leaving a diminished number of functioning leaves, and fruit quality can be affected if the berries do not receive their essential requirements. In addition, berry pH tends to elevate and berries ripen faster in extremely hot, dry weather. Drought stress causes variation within the vineyard, thus a larger berry or cluster sample is needed to represent an upcoming harvest. Conversely, excess water intake will slow the accumulation of sugar. Combined with an extended period of cloud cover, the ripening process tends to stall.

Some of the Visual Signs and Effects of Drought Stress

• Leaf margins become scorched
• Basal leaves become chlorotic and drop. Tendrils wilt and brown.
• Fruit ripening delay or stalls. Berries shrivel
• Periderm formation starts early and can be incomplete

The root system is dynamic. When water is limited, roots die back. The root tips regenerate as water availability increases. Irrigate carefully to replenish lost water without overwatering.

Irrigation need can be determined by how much water is used and lost in the vineyard due to evaporation and transpiration (evapotranspiration). The Texas ET website reports evapotranspiration estimates by area at http://texaset.tamu.edu/

Below are the calculations for irrigation need using current data from http://texaset.tamu.edu/, adapted from an example provided by Dr. Justin Scheiner, Texas A&M Agrilife Extension Viticulture Specialist .

Location: 12′ x 6′ vineyard – College Station/Bryan

Vines: Mature, at harvest

Date: July 20, 2018 through July 26, 2018

1 acre inch = 27,154 gallons

 

1) Determine the reference ET (ETo) for a given irrigation period and adjust using proper crop coefficient. Determine the proportion (%) of ET you want to replace

 ETo = 2.07 inches

Crop coefficient = 0.7

ET replacement = 80%

2.07 inches ETo x 0.7 crop coefficient x 0.80 ET replacement = 1.159 inches

1.159 inches x 27,154 gallons per acre inch = 31,471.486 gallons

2) Determine irrigation system output

 Vines per acre = 43,560 square feet per acre / 12’ row x 6’ vine spacing = 605

Emitters per vine = 2

Emitter output = 1 gallon per hour (gph)

Drip irrigation efficiency = 90% (approx. 10-15% is lost to evaporation)

605 vines x 2 emitters per vine x 1 gallon per hour / 0.90 irrigation system efficiency = 1,344 gallons per hour total outpu

3) Determine irrigation system run time to replenish the water lost during the last 7 days in BCS

31,471.486 gallons to apply / 1,344 gallons per hour output = 23.41 hours of run time

Particular sites’ water demands vary by soil depth, soil type, canopy, vine age, etc, so keep in mind that the calculation is an estimate. Vines need a lot of water now, and most systems are not capable of delivering the determined amount of water for the determined run time … do the best you can, grapevines are pretty tough.