California Crop Fertilization Guidelines

Soil samples are taken from the top foot of the soil profile, which is the major rooting zone ^[N9]. By the second half of the growth cycle, cauliflower roots can reach to 3-4 feet in the absence of restrictive layers; however, most roots are located in the top foot of the profile ^{[N20, N25]}.

Zones of recently banded fertilizer applications should be avoided so that N availability is not over-estimated ^[N8]. For more information on sampling procedure see Sampling for Soil Nitrate Determination.

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 ^{[N7, N11, N12]}.

For the soil nitrate quick test, 30 mL of a 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. Field 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 ^{[N11, N12]}. More detailed instructions can be found here.

Several studies carried out in commercial cauliflower fields and other cool-season vegetables on the Central Coast found that no fertilizer N is necessary as long as the soil nitrate-N level is above 20 ppm (= 20 mg/kg) ^{[N7, N10]}. A concentration of 20 ppm nitrate-N in the top foot of soil equals approximately 80 lb N/acre. In the absence of leaching, this amount of N could supply a crop for at least ten days to two weeks, even at peak N demand ^[N9]. If the nitrate-N concentration in the soil is below 20 ppm, only enough N to increase the nitrate-N level to 20 ppm is needed (see In-Season N). Contact your local farm advisor for more information.

Recently matured leaves, typically 3-4 nodes down from the growing point, are used for tissue analyses. A minimum of 20 leaves should be collected, each from a different healthy plant of representative vigor. Samples are taken from the entire field. Variable fields should be divided into uniform blocks, which are sampled separately ^[N9. It is important to accurately determine the crop growth stage, since the total N concentration declines as crops develop.

Once tissue samples are collected, they should be dried as quickly as possible ^[N9]. For more information on sampling procedure see Plant Tissue Sampling.

For petiole sap nitrate-N analysis, the entire petiole or midrib is used when plants are young. In more mature plants with large petioles and midribs, the center section can be used ^[N9].

Plant tissue can be analyzed for total nitrogen or nitrate-N concentration. The total N concentration (see Table) of whole leaves indicates the longer-term N availability, while the petiole nitrate-N concentration reflects more the current conditions ^[N9]. Total N concentration is considered a better measure to monitor crop N status; however, this technique has not been widely used in the California vegetable industry ^[N9]. Petiole nitrate-N levels have been found to be relatively insensitive indicators of the current soil N availability, especially when excess N is available ^{[N6, N8, N9, N12, N22]}. Fertilizer N decisions should therefore not be based solely on leaf or petiole analyses ^[N22]. Contact your local farm advisor for more information. Approximate minimum tissue sufficiency level for cauliflower ^[N9].

Under desert conditions in Arizona, critical nitrate-N concentrations in the midribs were 11,000 ppm at the 4-6-leaf stage, 9,000 ppm at the 10-12-leaf stage, 7,000 ppm at folding, 6,000 ppm at early heading, 2,500 ppm when heads reached full size, and 1,500 ppm pre-harvest ^[N5].

The required N rate depends on soil nitrate content. Several studies carried out in commercial fields of cauliflower and other cool-season vegetables on the Central Coast found that when the pre-sidedress soil nitrate-N level is above 20 ppm, no fertilizer N is necessary ^{[N7, N10]}. If the nitrate-N concentration in the soil is below 20 ppm, only enough N to increase soil available nitrate-N to 20 ppm is needed. Approximately 4 lbs N/acre need to be added to increase the soil nitrate level by 1 ppm ^[N12]. As an example, when the soil nitrate-N concentration is 15 ppm, 20 lbs N/acre are needed to increase the level to 20 ppm. Contact your local farm advisor for more information.

Crops produced from late fall through early spring generally require more fertilizer N because the residual soil N content tends to be lower during this period compared to the summer months ^[N9]. When leaching losses are minimized, the seasonal N application rates for cauliflower should not exceed 180-240 lbs/acre for winter and spring production and 150-180 lbs/acre for summer and fall production ^[N9].

These application rates are in line with studies from Europe, where optimum yields were produced with a total N availability ranging from 200 to 240 lbs N. The available N included fertilizer N plus pre-plant nitrate-N in the top two feet of the soil profile ^{[N4, N19, N29]}.

Studies carried out in Arizona found that marketable yields ranging from 200 to 260 cwt/acre were achieved with 300-360 lbs N/acre ^{[N23, N26, N27]}. However, in one of the studies with cauliflower produced with subsurface drip, the residual soil nitrate N in the top three feet of the profile after the harvest of the cauliflower generally was between 135 and 180 lbs/acre. This amount does not include N leached during the cropping season and N in residues remaining in the field ^{[N26, N27]}.

In addition to residual soil nitrate, nitrate in the irrigation water should be accounted for to reduce the amount of N fertilizer applied. Nitrogen mineralization from soil organic matter and crop residues also add available soil N. Net N mineralization is generally higher after broccoli or cauliflower than after lettuce ^[N9] (see Factors Affecting Soil Nitrate Concentrations in Spring). These factors are taken into account by CropManage, a web-based irrigation and N management software tool developed by Cahn and coworkers ^[N2]. CropManage can be accessed href="https://cropmanage.ucanr.edu/Home/">here.

Subsurface band

Everaarts and de Moel ^[N4] reported that placement of fertilizer in a band 2 inches to the side of the row and 2 inches below the soil surface tended to slightly increase cauliflower yield compared to a broadcast application. However, in Oregon, Hemphill and Hart ^[N14] found that band applied in-season N resulted in significantly lower yield and mean head size than when the N was broadcast.

Fertigation

In drip-irrigated fields, N can be applied through the drip system ^[N17].

Surface band

A surface band application of an ammonium nitrate solution between rosette stage and harvest serves as contact herbicide against broadleaf weeds and as a N source ^[N28]. The waxy cuticle that cauliflower and other cole crops develop once they have at least three true leaves prevents damage to the crop, unless the plants are very wet ^[N28]. A shielded spray should be used to avoid spraying the growing point or emerging new leaves of the broccoli plant ^[N28].

Agamalian ^[N1] found that topical applications of 50 to 60 gallons per acre of undiluted 20% ammonium nitrate were effective in killing weeds. Ammonium thiosulfate was also effective in killing weeds, while the results with urea/sulfuric acid solutions were variable ^[N1]. The amount of N in these applications is equivalent to 105-125 lbs N/acre. Therefore, such high topical applications should only be used when the amount of N applied is needed by the plants at the time of application. The N applied must be integrated into the N fertilization program.

Foliar N

Little information on the effect of foliar N application is available.

While N uptake is minimal during early growth, the N uptake rate is high during the second half of the season, and may exceed 10 lbs/acre per day and N uptake rates generally remain high until harvest ^{[N16, N21]}.

Early in the season, N uptake is low and the plant’s demand is covered by residual soil N and starter N or pre-plant N. Once the plants reach the 4-6 leaves stage, N uptake increases quickly, and sufficient N needs to be available ^[N3]. N is only applied when the pre-sidedress soil nitrate-N concentration is below 20 ppm (see above).

Under greenhouse conditions the type of mineral N applied (nitrate, ammonium or urea) has been found to have little effect on cauliflower N uptake ^[N18]. Furthermore, once applied urea is generally hydrolyzed quickly to ammonium and ammonium is generally quickly converted to nitrate by soil microorganisms, making nitrate the major form of N available to plants, except when soil temperatures are low.

A considerable quantity of N is left in the field with leaves and stems. In a study in high-yielding fields on the Central Coast, Smith and coworkers ^[N25] found the stems and leaves that are left in the field as residues contained 175-230 lbs N/acre, which corresponds to 60-80% of the total N in the aboveground biomass. The aboveground biomass is very N rich, having an N concentration of 4% or more, and is easily decomposed. This leads to high net soil N mineralization rates and soil N availability after cauliflower ^[N9]. A large proportion of the mineralized N will be available to the next crop, provided it is not leached below the root zone.

Soil samples are taken from the top foot of the soil profile, which is the major rooting zone ^[P4]. By the second half of the growth cycle, cauliflower roots can reach to 3-4 feet in the absence of restrictive layers; however, most roots are located in the top foot of the profile ^{[P10, P13]}.

Zones of recently banded fertilizer applications should be avoided so that P availability is not over-estimated ^[P3]. For more information on sampling procedure see Soil Test Sampling.

Bicarbonate extractable P levels (Olsen-P) above 50 ppm are adequate for cauliflower growth and P fertilization is generally not necessary (see Table) ^[P7]. Cauliflower grown in soils with less than 50 ppm plant available P may benefit from P fertilizer (see Pre-Plant P). Contact your local farm advisor for more information. Soil test interpretation for cool season vegetables ^{[P7, P12]}.

The values in the table are in line with results from an Australian study, where 95% of maximum yield were reached when soil test values exceeded 40 ppm Olsen-P ^[P8].

Recently matured leaves, typically 3-4 nodes down from the growing point, are used for tissue analyses. A minimum of 20 leaves should be collected, each from a different healthy plant of representative vigor. Samples are taken from the entire field. Variable fields should be divided into uniform blocks, which are sampled separately ^[P4]. It is important to accurately determine the crop growth stage, since the total nutrient concentration declines as crops develop.

Once tissue samples are collected, they should be dried as quickly as possible ^[P4]. For more information on sampling procedure see Plant Tissue Sampling.

Tissue P analyses are more valuable as a planning tool for succeeding crops than as a management tool for the current crop because P is generally applied pre-plant. Adequate whole-leaf P and K concentrations in the most recently matured leaves ^[P6].

At the buttoning stage, the Western Fertilizer Handbook recommends a slightly higher sufficiency range of 0.5-0.7% PO₄-P ^[P1]. These values are in line with a study in Australia, where the maximum yield was reached with leaf P concentrations exceeding 0.47% at buttoning ^[P8].

The sufficiency range in the midrib of the most recently matured leaf ranges from 3,500 to 5000 ppm PO₄-P. This range remains stable between the 6-8 leaf stage and pre-harvest ^[P1].

Phosphorus application rates should be based on soil test results. Bicarbonate extractable P levels above 50 ppm are adequate for cauliflower growth and a yield response is not likely. However, to promote early growth, a small starter application may be beneficial (see Starter P). When the soil P test level is below 50 ppm, P fertilization may increase yield. Applying the amount removed with the harvested curds ensures that P availability remains at the same level over the years. When P availability is very low, higher application rates may be required.

In a study carried out in different commercial fields on the Central Coast, total P uptake reached 35-45 lbs/acre ^[P13]. However, only about 40% of the P taken up is removed from the field with the harvested curds ^[P11]. Therefore, to replace the P removed from the field with one crop, 30-40 lbs P₂O₅/acre (14-18 lbs P/acre) are required.

For the southern desert and Central Valley, where soil P test levels are generally lower than on the Central Coast, application rates exceeding the P removed at harvest may be needed ^[P7]. In eastern Canada, in soils with low to medium P availability, the highest cauliflower yields (180 cwt/acre) were produced with 100 of 200 lbs P₂O₅/acre. However, yield differences due to application rate were small ^[P2]. Over the long term, such high rates are likely to increase soil test values. Regular soil and leaf analyses indicate whether the P fertilization program is adequate or should be modified to maintain optimal P availability. For more information, contact your local farm advisor.

In general, band applications are more effective than broadcast applications. Bands should be located 2-3 inches to the side of the seeds or plants and 3-4 inches deep ^[P5]. However, in a study on soils low in available P, McPharlin and coworkers ^[P8] did not find a significant yield difference between broadcast and banded P applications.

Broadcast P fertilizer needs to be disked into the soil because P is immobile in the soil. Because the surface soil periodically dries out, the root density near the soil surface is low ^[P9].

Pre-plant P fertilizers are generally best applied close to the time of planting since P may interact with soil minerals and become less available over time ^[P9]. The further away soil pH is from neutral, the stronger P interacts with soil minerals.

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.

Soil samples are taken from the top foot of the soil profile, which is the major rooting zone ^[K4]. By the second half of the growth cycle, cauliflower roots can reach to 3-4 feet in the absence of restrictive layers; however, most roots are located in the top foot of the profile ^{[K9, K13]}.

Zones of recently banded fertilizer applications should be avoided so that K availability is not over-estimated ^[K3]. For more information on sampling procedure see Soil Test Sampling.

To determine plant available K, soil samples are most often extracted with an ammonium acetate solution ^{[K8, K10]}.

While a response to K fertilization is unlikely when soil test K levels exceed 150 ppm; cauliflower may respond to fertilization when the available K level is 100-150 ppm. A response is likely when the available K concentration is below 100 ppm ^[K12]. Contact your local farm advisor for more information. Soil test interpretation for cool season vegetables ^{[K8, K12]}.

In addition to the exchangeable K content, competition with other cations may affect K uptake by plants. In soils where K makes up less than 2% of exchangeable cations, K uptake may be restricted ^[K12].

Recently matured leaves, typically 3-4 nodes down from the growing point, are used for tissue analyses. A minimum of 20 leaves should be collected, each from a different healthy plant of representative vigor. Samples are taken from the entire field. Variable fields should be divided into uniform blocks, which are sampled separately ^[K4]. It is important to accurately determine the crop growth stage, since the total nutrient concentration declines as crops develop.

Once tissue samples are collected, they should be dried as quickly as possible ^[K4]. For more information on sampling procedure see Plant Tissue Sampling.

Adequate whole-leaf P and K concentrations in the most recently matured leaves ^[K6].

At the buttoning stage, the Western Fertilizer Handbook recommends a slightly higher sufficiency range of 2.6-4.2% K ^[K1].

The sufficiency range in the midrib of the most recently matured leaf ranges from 4.0 to 6.0% K. This range remains stable between the 6-8 leaf stage and preharvest ^[K1].

Potassium is typically taken up in higher quantities than any other nutrient which emphasizes the need for careful attention to supplying this nutrient to cauliflower.

The K application rate should be based on soil test results. Cauliflower grown in soils with greater than 150 ppm of ammonium acetate exchangeable K is unlikely to respond to K fertilization and K fertilization is generally not beneficial. When the soil K test level is below 150 ppm, the K removed with the harvested crop should be replaced ^[K8]. In a study carried out in different commercial fields on the Central Coast, total K uptake was 240-300 lbs/acre ^[K13]. However, only about 30% of the K taken up is removed from the field with the harvested curds ^[K11]. Therefore, to replace the K removed from the field with one crop, 70-90 lbs K₂O/acre (85-110 lbs K/acre) are required.

In fields with very low soil test values, the application rate may be increased, especially when leaf analyses indicate that K supply is insufficient. A higher application rate may also be necessary in very sandy soils where K leaching may increase the amount of K exported from the field. However, in eastern Canada, cauliflower grown in soils with low ammonium acetate extractable K levels ranging from 52 to 81 ppm, K fertilization had little effect on yield ^[K2]. In this trial, the highest yields, averaging 190 cwt/acre, were produced with 100 lbs K₂O/acre.

In fields with sprinkler or furrow irrigation, K fertilizers are most commonly applied pre-plant. Pre-plant applications can be broadcast and incorporated into the soil or banded. Due to salt effects, the total of N plus K₂O in the band should not exceed 90 lbs/acre ^[K5]. If more K is needed, the remainder should be broadcast and incorporated into the seedbed prior to planting ^[K5].

In drip irrigated fields, K can also be applied by fertigation during the growing season. Cauliflower takes up most of the K during the second half of its growth cycle ^[K11], which is the time when the K supply needs to be adequate.

Potassium chloride (KCl), also known as muriate of potash, and potassium sulfate (sulfate of potash; K₂SO₄) are the most commonly used fertilizers. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute.

For fertigation, KCl is generally preferred over K₂SO₄ due to its higher solubility ^[K7].