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Onion Nitrogen Nutrition
Deficiency Symptoms
Nitrogen-deficient onion plants are stunted, and mature early. Foliage is uniformly light green and becomes increasingly yellow as the deficiency becomes stronger. The symptoms are first seen in the older leaves and progress towards the younger leaves at the center. Leaves tend to be small, thin and erect. The leaf tips start to die back, and whole leaves may be lost [N1]. In some varieties, bolting incidence is increased [N8].
Excessive Nitrogen
Large N applications at planting may reduce stands and overall yields [N33,N34]. Excess N can delay maturity and cause bulbs to be soft and more susceptible to field and storage rots [N1,N8,N35]. Total seed yield may be reduced, since high N levels can reduce the number of seed stalks [N1].
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Soil Nitrate Test
Soil Nitrate Test
Onion N needs are best determined by a combination of soil and tissue analysis [N26,N29].
Soil samples for nitrate analysis should be taken before the first N application of the season, either before planting or before the first in-season N application. Nitrate is easily leached below the root zone with excess irrigation water, so the test may overestimate available N if a heavy irrigation or rainfall occurs.
Preplant sample taken from the top six to eight inches of soil [N5,N12,N26]. Onion roots can grow to a depth of 2-3 feet in the absence of restrictive layers [N2,N30]. However, most of the roots are located in the top 7 inches [N10]. In addition, if planted in cold weather root growth may be slow for the first half of the season [N28]. Zones of recently banded fertilizer applications should be avoided so that N availability is not over-estimated. For permanent drip systems, samples should be taken from within the wetting zone. Before planting or during early growth, samples may be taken within the seed row, as long as starter fertilizer was not placed directly below the seed [N26].
For more information on sampling procedure see Sampling for Soil Nitrate Determination.
For spring-planted onions where high-N crop residues like alfalfa have been decomposing over winter, or if ammonium- or urea-based fertilizers were used recently and the weather has been cold and dry, a significant amount of ammonium may be present in the soil. Analyzing these samples for ammonium as well as nitrate can give a more accurate estimate of total residual N [N26].
Soil samples can be sent to a laboratory or extracted and analyzed on the farm. The soil nitrate quick test with colorimetric test strips is highly correlated with the standard laboratory technique and is a reliable estimate of current soil N status. Although the quick test is less accurate than a standard laboratory analysis, its accuracy is generally sufficient for routine on-farm use when done correctly. With the quick test, soil nitrate can be determined in a timely manner in order to make N fertilization decisions [N14].
For the soil nitrate quick test, 30 mL of a calcium chloride (0.01 M) or an aluminum sulfate (0.025 M) solution are measured into a clean 2-oz bottle or centrifuge tube with a mark at 40 mL. A homogenous sample of sieved, moist soil is added until the extractant reaches the mark. The tube is then capped and vigorously shaken for about 1 minute to disperse all soil aggregates. The soil particles are allowed to settle until clear supernatant forms. The nitrate concentration in the supernatant can then be measured with nitrate- sensitive colorimetric test papers [N13,N14]. More detailed instructions can be found here.
Every ppm of soil test N is equivalent to 2-4 lbs N/acre, depending on soil bulk density. That is, a test value of 20 ppm N would mean that about 40-80 lbs N/acre may be subtracted from the total N rate. The lower value is for intensively tilled soils with a high soil organic matter content.
Research with cool-season vegetables on the central coast has established that if soil nitrate-N in the top foot of soil exceeds 20 ppm, a fertilizer application may be skipped or postponed [N11]. This threshold is also valid for onions in the Pacific Northwest [N5]. While it has not been tested for onions in California, unless leaching occurs this amount of N would be sufficient to support growth for at least 2 weeks even at peak uptake rates [N3,N36]. See Soil Applied N for more information.
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Tissue Analysis
The tallest leaf is sampled. Samples from about 20-30 onion plants should be taken randomly throughout the field, avoiding field edges, sand streaks and other non-representative areas. Unusual or poorly-performing parts of a field should be sampled separately. Leaves should be washed with distilled water, packed loosely in a paper bag and mailed by overnight delivery to a lab as soon as possible. More instructions for plant tissue sampling are available here.
Test values are most informative if several samples are taken per season, and if samples are taken at the same time of day and at similar soil moisture contents [N1,N26].
Total N concentration critical values for the tallest onion leaf [N18].
| |
N (% dry weight) |
| Sampling date |
Deficient |
Sufficient |
| Pre-bulbing |
3.0 |
4.0 |
| Bulbing |
2.5 |
3.0 |
| Post-bulbing |
2.0 |
2.5 |
Onion tissue N varies a lot between years, locations and varieties [N31]. For this reason, it’s best for each grower to fine-tune these values by keeping good records of yield as well as soil and tissue N concentrations over time [N31]. For more information on values used in your area, contact your local farm advisor.
Roots may be a more reliable indicator of fertility status than leaves [N1,N26].
Root samples are taken at early bulbing (bulbs 0.5-1.5 inch diameter). A FREP-funded study with long-day onions in San Benito County found that nitrate-N of fresh root sap measured with a handheld nitrate meter (Cardy Meter) had a good correlation with lab-analyzed dry tissue nitrate-N.
Nitrate-N analysis on fresh onion root sap using the Cardy meter [N21,N26]:
- Collect root tissue from about 20-30 representative onion plants located throughout the field.
- Cut roots from bulbs.
- Wash soil off in distilled water and blot dry with a paper towel.
- If samples are being analyzed in the field:
- Mix roots well.
- Squeeze sap for about 1-2 plant's worth of randomly selected roots into a clean cup.
- Pour a small amount of sap from the cup onto Cardy meter sensor for analysis.
If samples are being sent to a lab, washed roots should be packed loosely in a paper bag and shipped by overnight delivery to the lab. They should reach the lab within 24 hours of sampling.
The FREP study’s findings suggest fresh sap from long-day onions should range between 350-500 ppm nitrate-N at early bulbing [N24]. For dried onion roots, nitrate-N levels between 5,000- 7,000 ppm nitrate-N are considered sufficient [N7].
More information on the nitrate quick tests can be found here.
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Preplant N
California studies on intermediate- and long-day onions in Antelope Valley and Salinas suggest that onions take up less than 20% of their total N requirement in the first half of the growing season [N3,N36]. For this reason, it is recommended that no more than 1/3 of the total N required be available at planting [N23,N29]. Available N includes residual soil nitrate-N and pre-plant fertilizer N. See Soil-Applied N for more information.
Nitrogen may be banded as granular or liquid fertilizer at or directly prior to planting, or broadcast and incorporated as granular fertilizer. Banding at least some as starter N is likely more efficient than broadcasting the whole application, especially at low N rates and if P is included in the starter band [N25]. However, banding is more difficult in high density plantings (e.g. 4 seedlines on 40 inch beds and 10 seedlines on 80 inch beds) which makes broadcast applications a necessity in many cases. Broadcast fertilizer should be incorporated to reduce the risk of volatilization.
Placement with the seed should be avoided. A general recommendation is that bands should be made at least 2 inches below and to the side of the onion row [N6]. However, studies from Oregon with a controlled-release fertilizer [N16] and the UK with ammonium phosphate [N25] observed that placing a starter band one or two inches directly below the seed, rather than to the side, improved early seedling growth [N25] and increased the proportion of ‘colossal’ size onions [N16]. It is safer to place the band further from the seed in dry, sandy soils, with high application rates or when fertilizers with a high salt index or a high ammonium concentration are applied.
Topical applications of acid based fertilizers help reduce crusting of the soil surface and improve stand establishment. At typical application rates of these materials, 10 - 20 lbs of N is added which can serve to supply the needs of the young, developing crop. These materials also provide useable amounts of P (Richard Smith, personal communication).
Banding starter fertilizers that contain both N and P improves both N and P uptake efficiency and early seedling growth, even in soils with sufficient P fertility [N25].
Ammonia, which is generated by ammonium and urea fertilizers, is toxic to onions. This limits the amount that can safely be applied. Two experiments with processing onions near Tulelake observed that when rates higher than 60 lbs N/acre as urea were broadcast and incorporated at planting (in addition to about 64 lbs N/acre banded as a liquid fertilizer), yield [N33] and stand [N34] were reduced.
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Soil Applied N
Split N applications are necessary for good N use efficiency. Onions grow very slowly in the first half of the season and take up little N, but grow rapidly in the second half, taking up N at a steady rate until harvest [N2,N36]. Onions have a shallow and slow-growing root system, so excess early season N is easily leached.
Onion N fertilization should depend on the N which the crop is expected to take up. In a study carried out in different commercial fields in the Antelope Valley, harvested bulbs of fresh market intermediate-day and long-day onions contained between 110 and 140 lbs N/acre. An additional 50-70 lbs N/acre was in the tops, and the total N required for the crop was about 180-190 lbs N/acre. In a study near Salinas with long-day processing onions, about 140 lbs N/acre were in the harvested bulbs, and about 180 lbs N/acre in the total crop [N36]. Assuming efficient irrigation, around 250-300 lbs N should be available to the crop from all sources [N19], including residual soil nitrate, nitrate in the irrigation water and fertilizer. With inefficient irrigation systems, or if significant in-season rainfall occurs, higher rates may be necessary.
Fertilizer application rates should be reduced based on soil and irrigation water nitrate tests (See Soil Test N). Additional N also may become available during the growing season from soil organic matter, crop residues and organic amendments. The N mineralized during the season will be much greater in soils with high organic matter content. For information on how to estimate non-fertilizer N contributions see Site Specific Adjustments.
Poor stands will require less N. Research from Idaho suggests that for each 10% stand reduction below 90%, 15-20 lbs N/acre should be subtracted from the total N requirement [N6].
For drip irrigated onions in California, the UC onion production manual advises that applications of up to 250 lbs N/acre are normally sufficient to maximize yields [N23].
Rate trials at the UC West Side Research station in Fresno County on drip-irrigated fresh market onions found that maximum yields of 50-70 tons (1000-1400 cwt) per acre could be achieved with rates of 150-200 lbs N/acre [N17]. In the same trial, furrow-irrigated processing onions reached maximum yields of 15-20 tons (300-400 cwt) per acre with 100-150 lbs N/acre. In these studies, dry matter contents were 8% and 20%, respectively.
In general, drip and sprinkler irrigated systems deliver N more efficiently than furrow-irrigated systems, reducing the N requirement. However, more consistent irrigation may increase yields, thereby increasing N needs, so N requirements may or may not be lower in drip systems [N20,N22].
In-season applications should supply the N required by onions that is not provided by residual soil nitrate, nitrate in the irrigation water, or pre-plant or starter fertilizer.
Uptake studies in commercial fields in the Antelope Valley and Salinas suggest that direct-seeded onions take up less than 20% of their total N requirement in the first half of the season, until about the 3-leaf stage, or just before bulb initiation. After this point, uptake increases steadily at average rates of about 1.5-3.5 lbs N/acre/day [N3,N36]. To ensure that sufficient N is available in this period, 65-80% of the total rate should be applied in-season [N23,N29]. Individual applications should not exceed 100 lbs N/acre [N26].
Residual soil nitrate and nitrate in the irrigation water are taken into account by CropManage, a web-based irrigation and N management decision support tool developed by UC Cooperative Extension. The program integrates CIMIS reference ET data and field specific soil, plant and management information to calculate crop water needs and estimates fertilizer N needs on a field-by-field basis. While CropManage is not yet calibrated for onions, a version is being developed and will be available shortly.
In-season N may be fertigated or sidedressed. In soils that are susceptible to leaching, fertigation is the most efficient means for supplying in-season N. Drip and sprinkler irrigation are used in wide bed production systems (80 inch wide beds). Surface and sub-surface drip systems appear to be equally effective [N20]. Nitrogen may also be injected through the sprinkler system. However, for fresh market onions drip is preferred to sprinkler once bulbing has started, as sprinkler irrigation is associated with disease problems such as bacterial soft rot and sour skin rot [N15,N17,N27].
In furrow-irrigated systems, sidedressed N may be knifed into the shoulder of the beds [N26]. Fertilizer may also be water-run in furrow irrigated systems, but this can be inefficient due to uneven distribution and potential losses with water that leaves the field.
Onions are sensitive to ammonia. High rates of urea- and ammonium-based fertilizers may be risky, especially when temperatures are low. However, in warm, aerated soils urea and ammonium are quickly converted to nitrate. Ammonium nitrate, ammonium sulfate and urea ammonium nitrate solutions are commonly used for in-season applications. Aqua ammonia and anhydrous ammonium should be avoided, especially for sprinkler-irrigated onions [N6,N29].
Controlled-release products aim to reduce N leaching losses by slowing the availability of fertilizer N. Results from experiments in other states suggest that some products can improve N use efficiency and increase the proportion of large onions [N4,N9,N16]. However, not much research has been done in California. A drip-irrigated fertilizer trial at the West Side Research station in Fresno County found that at a low N rate (150 lbs N/acre), onions grown with 3 applications of the slow-release product NFusion® yielded more than onions grown with 7 applications of UN 32. At higher N rates, however, the UN 32 treatment yielded better [N17]. Tests in the Tulelake area suggest that in that climate onions fertilized with slow-release N at planting do not yield as well as those fertilized with split applications of urea ammonium nitrate solutions, likely because not enough N is released in time to support peak crop uptake (Rob Wilson, personal communication).
More information on N fertilizers may be found at the website of the IPNI.
Some varieties may take up N steadily until harvest, while in others uptake plateaus near maturity [N3,N26]. If drip irrigation is used, in-season N may be fertigated throughout the season, starting at bulb initiation, when rapid N uptake begins [N6]. Guidelines from the Pacific Northwest recommend that no more than 100 lbs N/acre be used in a single application [N26], and 30 lbs N/acre may be a safer threshold in high organic matter soils (Rob Wilson, personal communication). Late-season N fertilization should be avoided, especially for processing onions, as excess N at this stage can delay maturity and lead to quality and storage problems [N1,N29]. Contact your local farm advisor for information about the recommended cutoff time in your area.
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Foliar Nitrogen
The erect structure and waxy surface of onion leaves make foliar fertilization difficult [N1]. Little N may be safely applied foliarly in a single application, and the onion plant only absorbs a small fraction of that [N32], making its use uneconomical.
Organic foliar sprays
Two recent California trials tested the effectiveness of supplementing a traditional fertilizer program with organic foliar sprays such as humic acid, fish emulsion or seaweed-based products [N17,N34]. In both of these studies, no significant yield or quality differences were observed between fertilized onions which did and did not receive foliar sprays.
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Onion Phosphorus Nutrition
Deficiency Symptoms
Onions are most likely to be affected by P deficiency in early growth, when their root systems are not yet developed and cool soil temperatures limit P movement to the root surfaces [P8].
The first symptom of P deficiency in onion plants is a dull, dark green leaf color. As the deficiency progresses, leaf tips wilt and die, but without the yellowing seen in N and K deficiencies. Tissue death proceeds towards the base of the leaves, and finally the dead tissues turn black. Deficient plants grow slowly, and show delayed maturity and a high percentage of thick necks at harvest [P1].
Excessive Phosphorus
Very high rates of P fertilizer may encourage growth of the tops at the expense of the bulbs, resulting in reduced proportion of large onions compared with more moderate rates [P6,P17].
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Soil Analysis
Soil samples are generally taken from the top 12 inches of the soil profile [P7,P13,P22]. While onion roots can reach up to 2-3 feet in the absence of restrictive layers [P3,P25], growth is slow for the first half of the season [P23] and about 90% of the roots are located in the top 7 inches [P12]. Zones of recently banded fertilizer applications should be avoided so that P availability is not over-estimated. For permanent drip systems, samples should be taken from within the wetting zone. Before planting or during early growth, samples may be taken within seed row as long as P hasn’t been banded directly beneath it [P22].
For more information on sampling procedure see Soil Test Sampling.
Critical soil analysis thresholds for onion yield response in California. Adapted from [P15].
| Yield response |
Concentration (ppm) |
| |
Olsen P |
K |
| Likely |
< 8 |
< 80 |
| Unlikely |
> 12 |
> 100 |
Thresholds are similar for green onion production [P19].
Because onions are often planted in cold soils and have slow-growing root systems, some starter P is often beneficial even in relatively high testing soils. A rate trial in the UK found that starter P was useful at soil Olsen P concentrations up to 80 ppm [P20]. See Starter P: Application Rate for more details.
When soil P is marginal or low, onions depend on beneficial mycorrhizal fungi to exploit soil P. For this reason, more P fertilizer may be required in low-P soils that have recently been fumigated, as fumigation reduces mycorrhizal populations [P8]. A higher critical soil P threshold may also be appropriate for fumigated soils.
Sampling on a grid (variable rate sampling) allows the grower to map areas of high and low fertility and adjust fertilizer rates accordingly, avoiding over- and under-fertilization in a large, variable field. A 2009 study on six commercial onion fields in Antelope Valley documented the costs, fertilizer savings and yield increases associated with grid sampling at different sampling densities. Average fertilizer savings were about $7.50/acre and analysis costs were about $2/acre. The study concluded that 1 sample for every 6 acres was sufficient to represent most of the variability in the fields included [P5].
For more information, consult your local farm advisor.
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Leaf Analysis
The tallest leaf is sampled. Samples from about 20-30 onion plants should be taken randomly throughout the field, avoiding field edges, sand streaks and other non-representative areas. Unusual or poorly-performing parts of a field should be sampled separately. Leaves should be washed with distilled water, packed loosely in a paper bag and mailed by overnight delivery to a lab as soon as possible. More instructions for plant tissue sampling are available here.
Test values are most informative if several samples are taken per season, and if samples are taken at the same time of day and at similar soil moisture contents [P1,P22].
Tissue P should remain between 0.1 to 0.2% of the dry tissue weight, which corresponds to 1000-2000 ppm PO4-P [P15]. The Western Fertilizer Handbook recommends a higher sufficiency range of 0.2-0.4% P [P10].
Since all P is normally applied prior to or at planting, tissue samples are most useful for guiding next year’s P application rate decisions.
Roots may also be sampled for P and K concentrations, but adequate ranges for root P and K have not been established in California. Research in the Pacific Northwest suggests that values between 1500 and 3500 ppm PO4-P (0.15-0.35%) in the onion roots are associated with high yields [P22]. See Tissue N: Root Nitrate-N sampling for root sampling instructions.
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Preplant P
Onion root systems are shallow and sparsely branched [P25], meaning they are very inefficient at capturing immobile soil nutrients such as P. High concentrations of accessible P are important in the early season when the roots are small [P2]. For this reason, all P is usually applied at or prior to planting.
In non-deficient soils P application can be based on removal with harvest. In a study carried out in different commercial fields in Antelope Valley, P removed with harvested bulbs of fresh market intermediate-day and long-day onions ranged from 61-98 lbs P2O5 (26-43 lbs P/acre) [P4]. A study with long-day processing onions near Salinas found that about 60 lbs P2O5/acre (26 lbs P/acre) were removed with the bulbs [P26]. However, because onions are not very efficient at using fertilizer P, they may respond to much higher rates, especially in deficient soils. When a soil test Olsen P is <10 ppm, the University of California onion production manuals suggest application rates of up to 200 lbs P2O5/acre [P18,P24].
When soil P is marginal or low, onions depend heavily on beneficial mycorrhizal fungi to exploit soil P. For this reason, more P fertilizer may be required in low-P soils that have recently been fumigated, as fumigation reduces mycorrhizal populations [P8].
All P that is not included in the starter band may be applied preplant. See Starter P.
Banding is usually more efficient than broadcasting, as high P concentrations relatively close to the seed ensure that the small root systems can take it up [P20]. However, in soils that have recently been fumigated research from the Pacific Northwest suggests that broadcasting could be more effective [P9]. Banding is more difficult in high density plantings (e.g. 4 seedlines on 40 inch beds and 10 seedlines on 80 inch beds) which makes broadcast applications a necessity in many cases.
Phosphorus is not mobile in the soil and will not leach, so the timing of preplant P application is not important. Fertilizers containing N should be applied as close to planting time as possible, as starter applications, to avoid N leaching.
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Starter Phosphorus
When rates are low, the entire P rate can be banded as starter fertilizer. The University of Minnesota recommends that not more than 70 lbs P2O5/acre be included in the starter band [P11]. In California, it’s recommended that when Olsen P is greater than 30 ppm, no more than 50 lbs/acre P2O5 should be applied as starter P [P18].
Many anticrustants contain usable amounts of P. Any P applied as an anticrustant should be taken into account when calculating P application rates.
A general recommendation is that bands should be made at least 2 inches below and to the side of the onion row [P8]. However, studies from Oregon with a controlled-release product [P14], Texas with triple superphosphate, [P16] and the UK with ammonium phosphate [P21] have reported that placing a starter band one or two inches directly below the seed, rather than to the side, can result in better yields [P16], early seedling growth [P21], reduced days to maturity [P16], and an increased proportion of ‘colossal’ size onions [P14]. The Texas study found that for a low P rate (20 lbs P/acre), placement with the seed gave the highest yields [P16]. However, the fertilizer used in this study, triple superphosphate, did not include ammonium. Ammonium phosphates should never be placed with the seed [P22].
Onions are also able to use P contained in acid fertilizers applied topically as anticrustants.
Banding starter fertilizers that contain both N and P can improve both N and P uptake efficiency and early seedling growth, even in soils with sufficient P fertility [P21]. Onions are sensitive to both salt and ammonia toxicity. Ammonium phosphates with a high salt index, such as diammonium phosphate (DAP), should be used with caution, and placed further from the seed.
A number of granular and liquid P fertilizers are available. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute.
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Soil Applied Phosphorus
Onions need a high concentration of soil P during early growth [P2]; therefore, all P is normally applied at or just prior to planting.
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Onion Potassium Nutrition
Deficiency Symptoms
Leaves of K deficient onion plants are dark green. Starting with the oldest leaves, the tips wilt and the leaves droop. Wilting leaves have first a satiny and then a wrinkly appearance. As the deficiency progresses leaves yellow and then die, as with severe N deficiency [K1].
While onions are susceptible to K deficiency, most California soils contain sufficient K for onion production [K1,K18]. Deficiencies are most likely to be observed on sandy soils.
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Soil Analysis
Soil samples are taken from the top 12 inches of the soil profile [K5,K8,K16]. While onion roots can reach up to 2-3 feet in the absence of restrictive layers [K2,K19], growth is slow for the first half of the season [K17] and about 90% of the roots are located in the top 7 inches [K11]. Zones of recently banded fertilizer applications should be avoided so that P availability is not over-estimated. For permanent drip systems, samples should be taken from within the wetting zone. Before planting or during early growth, samples may be taken within seed row as long as fertilizer hasn’t been banded directly beneath it [K16].
For more information on sampling procedure see Soil Test Sampling.
Critical soil analysis thresholds for onion yield response in California. Adapted from [K12].
| Yield response |
Concentration (ppm) |
| |
Olsen P |
K |
| Likely |
< 8 |
< 80 |
| Unlikely |
> 12 |
> 100 |
Thresholds are similar for green onion production [K14].
Sampling on a grid (variable rate sampling) allows the grower to map areas of high and low fertility and adjust fertilizer rates accordingly, avoiding over- and under-fertilization in a large, variable field. A 2009 study on six commercial onion fields in Antelope Valley documented the costs, fertilizer savings and yield increases associated with grid sampling at different sampling densities. Average fertilizer savings were about $7.50/acre and analysis costs were about $2/acre. They concluded that 1 sample for every 6 acres was sufficient to represent most of the variability in the fields they studied [K4].
For more information, consult your local farm advisor.
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Leaf Analysis
The tallest leaf is sampled. Samples from about 20-30 onion plants should be taken randomly throughout the field, avoiding field edges, sand streaks and other non-representative areas. Unusual or poorly-performing parts of a field should be sampled separately. Leaves should be washed with distilled water, packed loosely in a paper bag and mailed overnight delivery to a lab as soon as possible. More instructions for plant tissue sampling are available here.
Test values are most informative if several samples are taken per season, and if samples are taken at the same time of day and at similar soil moisture contents [K1,K16].
Critical tissue K levels for the tallest onion leaf blade [K12].
| |
K (% dry weight) |
| Sampling date |
Deficient |
Sufficient |
| Pre-bulbing |
3.0 |
4.0 |
| Bulbing |
2.5 |
4.0 |
| Post-bulbing |
2.0 |
3.0 |
At early bulbing, the Western Fertilizer Handbook recommends a slightly higher sufficiency range of 3-4% K [K7].
Roots may also be sampled for P and K concentrations, but adequate ranges for root P and K concentrations have not been established in California. Research in the Pacific Northwest suggests that values between 3-5% ppm K in onion roots are associated with high yields [K16]. See Tissue N: Root nitrate-N sampling for root sampling instructions.
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Soil Applied Potassium
Onions don’t respond to K fertilizer in soils with high K levels. However, for long-term productivity K that is removed at harvest should be replaced. In a study carried out in different commercial fields in Antelope Valley, K removed with harvested bulbs of fresh market intermediate-day and long-day onions ranged from about 170-225 lbs K2O (150-190 lbs K/acre) [K3]. A study on commercial fields near Salinas with long-day processing onions found that they removed about 115 lbs K2O/acre (95 lbs K/acre). However, in this trial K was probably limiting [K20].
In soils testing less than 100 ppm extractable K, the University of California onion guides recommend applying up to 150 lbs K2O/acre to ensure good K supply [K13].
Potassium fertilizer is usually broadcast and incorporated during seed bed preparation [K6]. However, in a low-K soil, including some K in the starter band at planting may be beneficial [K15]. Since onions are very sensitive to salinity, no more than 30 lbs K2O/acre should be included in the starter band [K10]. If more K is needed, the remainder should be broadcast and incorporated into the seedbed prior to planting.
In drip irrigated fields, K can also be applied by fertigation during the growing season. Onion takes up most of the K during the second half of its growth cycle [K3], which is the time when the K supply needs to be adequate. However, no research has been done in California to test whether in-season fertigation is more effective than applying the whole rate pre-plant. Benefits are most likely to be observed on sandy soils [K16].
It may be safest to wait until the 4-leaf stage to apply in-season K, since onions are especially salt-sensitive in the seedling stage [K9].
Many K fertilizers are available. Onions are salt sensitive, so care should be taken when using high salt-index fertilizers like potassium chloride (muriate of potash, KCl). Potassium sulfate has a lower salt index and is an alternative to potassium chloride where elevating salt loads in the soil is a concern.
Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute.
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