California Crop Fertilization Guidelines

Vegetative growth of N deficient plants is reduced and leaves are small, thin, and stiff. Leaves turn pale green and later yellow. Young shoots, petioles and cluster stems turn pink or red ^{[N4, N17]}.

Nitrogen deficiency slows the growth of shoots, results in shortened internodes, and causes the berries to mature early before they reach their normal size ^{[N11, N18]}. Nitrogen deficiency may first appear in spring ^[N4]. Leaf symptoms appearing after the beginning of ripening are caused by the translocation of N from the leaves to the berries ^[N17].

When N deficiency is accompanied by water stress, the leaf margins may roll slightly upwards, wilt and dry. Sometimes tissue between the veins turns light brown and dies. Under extreme N deficiency, the leaves wilt and drop ^{[N11, N18]}.

Excess N availability is characterized by dark green leaves, reduced fruit set, and delayed maturity ^{[N11, N18]}. High N availability results in high canopy vigor with excessively long shoots. The increased canopy density in turn reduces light interception in the fruit zone, which generally results in reduced concentrations of total soluble solids, anthocyanins, terpenes and total phenols in grapes ^{[N5, N33]}. However, the relationship between N status and berry composition is complex and depends on many other factors as well ^[N5].

High N uptake can also increase the susceptibility of grapevines to diseases, such as Bunch Stem Necrosis in which developing flower clusters are aborted by the vine, causing severe reductions in yield ^{[N18, N33]}.

Nitrogen not taken up by the plants is likely leached with winter rain and irrigation water, contributing to nitrate contamination of groundwater.

Soil analyses are useful for determining the suitability of a site before planting and in establish vineyards to diagnose problems such as extreme pH, high salt concentrations, cation imbalances (Mg:Ca:K), and toxic levels of boron, chloride, or sodium ^{[N23, N32]}. Soil samples should be taken from the part of the soil profile that will be the major root zone. Smart and coworkers ^[N31] compiled rooting depth information from different studies. Their analysis revealed that approximately two thirds of grapevine roots are in the upper two feet of the profile. On average, however, more than 20% of the roots were found below 4 feet. In most cases, soil properties have a greater influence on depth distributions than the type of rootstock ^[N31]. Furrow irrigated vines may also have a deeper and more widespread root system than drip-irrigated vines ^[N2].

As nutrient content changes during the growing season, it is important to collect samples at the proper time. In California, petioles of leaves opposite the flower clusters, sampled at full bloom, are generally used to determine the nutritional status of grapevines in spring, allowing time for adjustments in the fertilization program to prevent nutrient imbalances later in summer and fall. Grapes are in full bloom when approximately two-thirds of the caps have loosened or fallen from the flowers ^[N9].

To obtain a representative sample, 50 to 100 petioles should be collected, with one or two petioles per vine from vines uniformly distributed over the management area ^{[N11, N19]}. Separate petiole samples should be taken from different management units each of which representing a single variety and rootstock ^{[N8, N11, N14]}.

The optimal petiole nitrate-N concentration is between 500 and 1200 ppm (Table 1). Concentrations over 2000 to 2400 ppm can be associated with excessive growth or reduced fruit set, while levels over 3000 ppm can produce direct toxic effects, such as leaf burn ^{[N11, N14]}. The optimal values have been established based on research with ‘Thompson Seedless’ grapevines. However, a study in ‘Chardonnay’ and ‘Cabernet Sauvignon’ vineyards indicated that the values are applicable for other cultivars and rootstocks as well ^[N34]. Table 1: Interpretation of petiole nutrient concentrations at full bloom ^[N11].

Effect of Variety and Rootstock

Grapevines have a low N fertilizer requirement compared to most other crops ^[N23]. On average about 2.9 lbs of N is removed from the vineyard in one ton (2000 lbs) of fresh grapes, with values ranging from 1.8 to 4.1 lbs ^[N21]. Nitrogen should be applied to ensure an adequate N supply during spring development, but available N in late summer should not be high enough to encourage late-season shoot growth, delay maturity, and promote immature canes ^[N24].

Recommended N application rates for raisin production range from 0 to 60 lbs/acre depending on irrigation system, soil type and vigor of the vines (see Table). While the raisin yield averages 9-10 tons/acre, the yield of wine grapes is generally lower, averaging 7 tons/acre in California, with even lower tonnages in most coastal wine grape vineyards. Therefore, the required application rates for wine grapes may be substantially lower than the values reported in the table. However, the values reported in the table for drip irrigated systems correspond to the N rate generally recommended for wine grapes (M. Battany, personal communication). To determine the correct application rate, it is important the take petiole nitrate concentrations as well as vine growth and wine quality into consideration. Suggested N application rates in raisin vineyards ^[N10].

Nitrogen needs to be applied with caution to vines grafted onto the nematode-resistant rootstocks ‘Ramsey’, ‘Freedom,’ or ‘Harmony’ because they are prone to excess vigor ^[N10]. Contact your local farm advisor for more information.

When groundwater is used for irrigation, the nitrate in the water can contribute a significant part of the vineyard's N requirement. These application rates should be corrected for the amount of N contained in the irrigation water ^[N10].

Drip Irrigated Systems

In drip-irrigated vineyards, fertigation is generally the most effective way to apply N ^[N28]. Furrow Irrigated Systems

Under furrow irrigation, fertilizer should be placed near the outer edges of the furrow towards the vine row ^[N11]. Urea and ammonium forms should always be drilled at least 2 inches deep into the soil or immediately incorporated, since they are subject to volatilization losses if left on the surface ^{[N10, N11]}. For young vines and on sandy soils, fertilizer should be applied within 3 feet of the row to ensure root access ^[N11].

Generally, the type of mineral N fertilizer can be chosen based on cost ^[N10]. Urea, ammonium and nitrate fertilizers all provide readily available N. In some situations, however, one type of fertilizer may be preferred.

Urea and ammonium fertilizers, especially ammonium sulfate, acidify soils over time and should be avoided on soils with a pH below 6 ^[N10].

Grapevines fertilized in the fall should have an active, healthy canopy for three to four weeks after application. When uptake time is limited, a nitrate fertilizer, which is mobile and immediately available, may be the best choice ^[N26]. When the canopy is no longer active, the N is not taken up and may be leached below the root zone during the winter.

N uptake is relatively slow between budbreak and bloom but high between bloom and veraison. During this period, about half of the annual N demand is taken up ^{[N13, N27, N30]}.

Due to the ability of grapes to mobilize reserves from permanent structures, time of application has less effect on grapevine performance than it has in annual crops, provided the N is applied during a period of active uptake ^{[N1, N3]}.

Studies in San Joaquin Valley vineyards showed that N is best applied in spring during a period starting after budbreak until fruit set or post-harvest ^{[N12, N27]}. Early applications before or at budbreak are susceptible to leaching from sprinkler frost protection, late rainfall or excessive spring irrigations and should be delayed until after the frost danger period ^[N24]. Nitrogen applied between harvest and leaf senescence refills storage reserves in permanent structures and supports leaf growth the following spring, in regions where the leaf canopy remains active for an extended period after harvest ^[N26]. Postharvest applications need to be made as long as the canopy is healthy and functional to ensure adequate uptake ^[N12]. Nitrogen applications in late fall after leaf fall are inefficient, because N may be leached below the rootzone by winter rains ^[N24].

Furrow Irrigated Systems

Under furrow irrigation, especially in sandy soils, high application rates are best split into two applications in late spring/early summer or one application in spring and the other post-harvest ^[N10].

Drip Irrigated Systems

Under drip irrigation, 2-5 lbs N/acre may be applied weekly in spring ^[N10]. However, when leaching during the growing season is minimized one single application may be as effective as multiple small applications, provided the soil is not very sandy in texture ^[N25].

When P is deficient, shoot and root growth is reduced. Leaves are small and dark green ^[P8]. Under severe P deficiency, younger leaves may turn reddish, whereas red spots near the edges may appear on older leaves ^{[P2, P8]}. While veins remain green, the area between them turns red, appearing as islands or bars of red tissue ^[P7]. Phosphorus deficiency reduces fruit set, resulting in loose and small clusters ^[P2].

Phosphorus deficient grapevine leaves (photo provided by the International Plant Nutrition Institute).

On deep valley soils, P deficiency is generally of little concern and early University of California trials did not find measurable responses in yield or fruit quality to P fertilization ^{[P4, P5]}.

However, in vineyards at higher elevations on marginal, shallow, and acidic soils, P availability may be limiting. Phosphorus deficient grapevines have been found in vineyards located on hillsides of the Coast Range or the Sierra Nevada foothills in northern California ^{[P7, P18]}. Especially soils of the Aiken Series have been found to fix P, resulting in P deficient grapevines ^[P7]. Other soil series with a low pH and P availability include Manzanita, Musick, Pentz, Redding and Sobrante ^[P11]. Soil series information for specific sites can be found with SoilWeb. For more information contact your local farm advisor.

Soil analyses are useful for determining the suitability of a site before planting and in establish vineyards to diagnose problems such as extreme pH, high salt concentrations, cation imbalances (Mg:Ca:K), and toxic levels of boron, chloride, or sodium ^{[P15, P21]}. Soil samples should be taken from the part of the soil profile that will be the major root zone. Smart and coworkers ^[P19] compiled rooting depth information from different studies. Their analysis revealed that approximately two thirds of grapevine roots are in the upper two feet of the profile. On average, however, more than 20% of the roots were found below 4 feet. In most cases, soil properties have a greater influence on depth distributions than the type of rootstock ^[P19]. Furrow irrigated vines may also have a deeper and more widespread root system than drip-irrigated vines ^[P1].

As nutrient contents changes during the season, it is important to collect petiole samples at the proper time. In California, petioles of leaves opposite the flower clusters, sampled at full bloom, are generally used to determine the nutritional status of grapevines. Grapes are in full bloom when approximately two-thirds of the caps have loosened or fallen from the flowers ^[P3].

To obtain a representative sample, 50 to 100 petioles should be collected, with one or two petioles per vine from vines uniformly distributed over the management area ^{[P4, P13]}. Separate petiole samples should be taken from different management units each of which representing a single variety and rootstock ^{[P4, P5]}.

Phosphorus concentrations below 0.1% are considered deficient ^{[P4, P5]} (see Table). Rootstock type and variety affect P accumulation and use efficiency ^{[P9, P10]}. For example, petiole P concentrations of vines on 110R rootstocks were 36-50% higher than those of vines on Freedom, AxR#1 and St. George rootstocks when grown under identical conditions ^[P9]. For this reason, the deficiency threshold of 0.1% should be used loosely ^[P11]. Comparing the present petiole nutrient concentrations with the results of the previous years from the same management unit helps interpreting the results. For more information contact your local farm advisor. Interpretation of petiole nutrient concentrations at full bloom ^[P4].

Based on several studies, the average amount of P in one ton (2000 lbs) of harvested fruits is 0.56 lbs, with values ranging from 0.44 to 0.78 lbs ^[P14]. Therefore, with a yield of 10 tons/acre, about 5.6 lbs P (13 lbs P₂O₅) are removed, while a yield of 7 tons/acre removes 3.9 lbs P (9 lbs P₂O₅). Applying the amount of P removed at harvest ensures that soil P availability is maintained over the years in soils with limited native P availability.

Vines grown on P deficient soils, however, may require larger application rates. For example, yield of grapevines grown in soils with very low P availability (Bray 1 P of 3 ppm or less) and a history of low petiole P concentrations was increased by 0.1-0.4 lb P/vine, which corresponded to 45-180 lbs/acre ^[P18]. Higher application rates did not increase yield further at the same site. For more information contact your local farm advisor.

When a new vineyard is established on P poor soil, P is best applied by shanking dry fertilizer adjacent to the vine row, or by injecting liquid fertilizer through the drip system ^[P11]. In acidic soils that fix P, fertilizer is best applied in a band ^[P11].

As P is immobile in the soil, it does not move downwards into the soil profile. For this reason, the fertilizer should be placed within the wetting zone of the irrigation system so that roots can grow to the fertilizer band ^[P4]. Access to fertilizer left on the soil surface may be reduced because repeated drying may restrict root growth near the soil surface ^[P12]. Surface applied P is also more at risk of being removed with runoff, which can result in contamination of surface water bodies.

Phosphorus should only be applied with the drip system when the magnesium and calcium concentrations in the irrigation water are very low to prevent precipitation of magnesium or calcium phosphates which may clog the emitters. Precipitation can also be prevented by acidifying the irrigation water ^[P11]. Only soluble P fertilizers can be used for fertigation. Fertilizer is normally applied during the middle third of an irrigation event, which allows pre-wetting the soil prior to the application and flushing the drip system after the application ^[P11].

Common P fertilizers are monoammonium phosphate, and single or triple superphosphate ^{[P11, P22]}. The N in monoammonium phosphate may increase P uptake ^[P11]. Rock phosphate is also used as a P source, but has poor solubility and should only be used on acidic soils.

Phosphorus is immobile in soil and is not readily leached below the rooting zone. Therefore, P fertilizer can be applied during the dormant season. However, when ammonium phosphate is used, the ammonium may be nitrified to nitrate, which is easily leached. Ammonium phosphate should be applied close to the time the vines can take it up.

Repeated small applications of P every 2-3 years to vineyards grown on soils with low P availability may be necessary to maintain adequate vine P status ^[P18].

Early in the season, K deficiency results in light colored leaves with some necrotic spots along the edge of the leaf blades. Leaf margins dry and may either roll upwards or downwards. Leaf blades become distorted and ruffled. The symptoms are more pronounced under water stress ^[K11].

Late symptoms include yellowish to dark brown surface of older leaves at the base of the shoots exposed to sunlight ^{[K2, K11]}. The leaf margins turn upwards. The discoloration first appears between the veins, and spreads over the entire leaf area when K deficiency continues. Potassium can be translocated from the leaves to the ripening berries. For this reason, leaf symptoms are more severe on heavily cropped vines ^[K11].

Potassium deficiency may decrease frost resistance and increases susceptibility to powdery mildew ^[K11].

Potassium deficient grapevine leaves (photo provided by the International Plant Nutrition Institute).

Many soils located on the east side of the San Joaquin Valley, including the Lodi Wine Grape District, tend to fix K ^[K18]. These soils are formed from Sierra Nevadan alluvium and are weakly to moderately weathered ^{[K22, K12]}. A map showing the K fixation potential of soils located between Stockton and Sacramento can be found here. Soils with low K availability are also found in other regions. For more information contact your local farm advisor.

Within a vineyard, K deficiency can occur in small areas where the K-rich topsoil was removed during land leveling ^[K19].

Berries are a strong sink for K, containing 60% or more of the K found in the entire aboveground biomass at harvest. Excess K levels in grape berries may impact wine quality by decreasing free tartaric acid, which in turn results in a higher pH of grape juice, must and wine ^[K16].

Furthermore, a field study on San Joaquin sandy loam revealed that excess K application rates may decrease water infiltration, especially when irrigation water with a low soluble salts content (low EC) is used ^[K19].

Soil analyses are useful for determining the suitability of a site before planting and in establish vineyards to diagnose problems such as extreme pH, high salt concentrations, cation imbalances (Mg:Ca:K), and toxic levels of boron, chloride, or sodium ^{[K21, K27]}. Soil samples should be taken from the part of the soil profile that will be the major root zone. Smart and coworkers ^[K26] compiled rooting depth information from different studies. Their analysis revealed that approximately two thirds of grapevine roots are in the upper two feet of the profile. On average, however, more than 20% of the roots were found below 4 feet. In most cases, soil properties have a greater influence on depth distributions than the type of rootstock ^[K26]. Furrow irrigated vines may also have a deeper and more widespread root system than drip-irrigated vines ^[K1].

As nutrient contents changes during the growing season, it is important to collect petiole samples at the proper time. In California, petioles of leaves opposite the flower clusters, sampled at full bloom, are generally used to determine the nutritional status of grapevines. Grapes are in full bloom when approximately two-thirds of the caps have loosened or fallen from the flowers ^[K5].

To obtain a representative sample, 50 to 100 petioles should be collected, with one or two petioles per vine from vines uniformly distributed over the management area ^{[K6, K14]}. Separate petiole samples should be taken from different management units each of which representing a single variety and rootstock ^{[K6, K9]}.

Petiole K concentrations of 1.5% or higher are considered adequate (see Table). Interpretation of petiole nutrient concentrations at full bloom ^[K6].

Vines with petiole K concentration in the questionable range should be resampled 6 to 8 weeks after bloom. Potassium levels below 0.5% in the petioles of the most recently matured leaves suggest K deficiency, while concentrations above 0.8% can be considered safe ^[K6].

Rootstock type (see Table) and variety affect K accumulation and use efficiency ^[K15]. Comparing the present petiole nutrient concentrations with the results of the previous years from the same management unit helps interpreting the results. For more information contact your local farm advisor. Petiole K concentrations at bloom as affected by rootstock. The study was carried out in ‘Chardonnay’ and ‘Zinfandel’ vineyards located in Sacramento and Amador Counties ^[K15].

Based on several studies, the average amount of K in one ton (2000 lbs) of harvested fruits is 4.9 lbs, with values ranging from 3.2 to 7.4 lbs ^[K17]. Therefore, with a yield of 10 tons/acre, approximately 50 lbs K/acre (60 lbs K₂O/acre) are removed from the field, while a yield of 7 tons/acre removes some 35 lbs K/acre (42 lbs K₂O/acre). Applying the amount of K removed at harvest ensures that soil K availability is maintained over the years.

Vines grown on K fixing soils, however, may require much higher application rates. In a study with ‘Thompson Seedless’ grapes grown in a moderately to severely K deficient vineyard with a low K fixation potential under drip irrigation, the application of 0.22 and 0.44 lbs K/vine was effective over a two-year period, while applying 0.9 to 1.8 lbs K/vine (520 vines/acre) corrected K deficiency for at least 3 years ^[K7]. In a severely K deficient vineyard, deficiency symptoms during the first two years after K fertilization could only be reduced compared to the unfertilized control with a single application of 2.2 lb K/vine, but not with annual applications of 0.44 lb K/vine. The positive effect of the single heavy application lasted four years ^{[K3, K13]}. For more information contact your local farm advisor.

Furrow Irrigated Systems

Potassium is best applied to the bottom of a 6-8 inch deep furrow located at a distance of 18-24 inches from the vine on each side of the row ^{[K6, K19]}. However, in K deficient sandy soils, 'Thompson Seedless' grapes were found to respond more rapidly to surface applied K compared to K applied in a furrow. This may have been due to the fact that during the first year after surface application, most K is confined to the top foot of the profile where the root density is high ^[K3].

Drip Irrigated Systems

In drip irrigated systems, K can be fertigated. Because the nutrients are placed into moist soil with active roots, K use efficiency is generally increased when K fertilizer is fertigated as compared to a band application in a furrow close to the vine ^[K20].

Different K fertilizers can be used in vineyards (see Table), with potassium sulfate (K₂SO₄) being most popular ^[K20]. Differences between in-season applications of K in the form of K₂SO₄ and potassium chloride (KCl) to drip-irrigated ‘Thompson Seedless’ grapes at annual applications of up to 0.44 lbs K/vine were minimal ^[K7]. However, drainage problems or high soil chloride concentrations may prevent the use of KCl in some areas ^[K7]. In general, chloride fertilizers should be avoided in areas with salinity concerns. In another study carried out in a K deficient vineyard in Fresno County, K₂SO₄, KCl and potassium nitrate (KNO₃) were equally effective in correcting foliar symptoms of K deficiency. The application of 2.2 lbs K/vine in the form of KCl in January increased foliar chloride concentrations, but no symptoms of excess chloride were detected ^[K13].

When potassium nitrate is used, the N needs to be considered when determining application rates ^[K20]. Potassium carbonate has an alkaline soil reaction and is often used in acidic soils to increase soil pH. Potassium thiosulfate, on the other hand, acidifies the soil and is used in alkaline soils ^[K20]. K fertilizers ^{[K12, K20]}.

Potassium demand is highest between bloom and veraison. During this period roughly 60% of the annual K demand is taken up ^{[K8, K23]}. Between veraison and harvest, only about 10% of the annual K demand is taken up. This period is characterized by redistribution of K from vegetative organs to the clusters. About 15% of the annual K demand may be covered with postharvest uptake ^{[K8, K28]}. Furrow Irrigated Systems

Dry material is most commonly applied during dormancy either in fall or early spring ^{[K6, K12]}. Drip Irrigated Systems

In vineyards with drip irrigation, K can be fertigated during the growing season ^[K7], with applications being made before veraison ^[K20]. A single application of K in late April to drip-irrigated grapevines was equally effective as eight weekly split-applications between late April and mid-June in a ‘Thompson Seedless’ vineyard ^[K7]. However, it may be more practical to apply 10-15 lbs K₂O/acre on a weekly basis than apply a large amount with one single application ^[K20].