California Crop Fertilization Guidelines

Soil samples are taken from the top foot of the soil profile, which is the major rooting zone. Melon roots can penetrate as deeply as 4 feet, but most of the activity is in the top foot ^[N30]. Zones of recently banded fertilizer applications should be avoided so that the N availability is not over-estimated. 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 ^[N9,N13].

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. 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 a nitrate-sensitive colorimetric test strip ^[N12]. More detailed instructions can be found here.

Pounds of nitrate-N potentially available per acre in each foot of soil can be estimated by multiplying parts per million of nitrate-N by 3.5-4. For example, a nitrate-N concentration of 10 ppm in the top foot of the profile corresponds to 35-40 lbs N/acre.

For desert production in Arizona, a pre-plant application of 50 lbs N/acre is recommended for soils which test lower than 10 ppm nitrate-N ^[N8].

When well water is used for irrigation, a considerable amount of N may be applied with the irrigation water. To convert nitrate-N concentration in the water to lbs N/acre, ppm nitrate-N in the water is multiplied by 0.226 and by the number of acre-inches of water applied ^[N3]. For example, with 1 acre-inch of water containing 10 ppm nitrate-N, 2.26 lbs N are applied per acre. More information may be found here.

Samples are taken from the entire field. Variable fields should be divided into uniform blocks, which are sampled separately ^[N11]. It is important to accurately determine the crop growth stage, since the total nutrient concentration declines as crops develop. Tissue N tests are most meaningful if taken several times during the season ^[N21].

Recently matured leaves, typically the 6^th leaf from the growing tip, are used for tissue analyses. A minimum of 20 leaves should be collected, each from a different healthy plant of representative vigor. Once tissue samples are collected, they should be dried or refrigerated as quickly as possible and sent to a lab for analysis ^[N21]. For more information on sampling procedure see Plant Tissue Sampling.

For petiole nitrate-N analysis, the entire petiole is used. Petiole sap nitrate concentrations are affected by environmental conditions, so it's recommended to collect samples at around the same time each day, after 8 am but before 2 pm ^[N25].

Fresh sap nitrate may also be tested on-farm, using the Cardy nitrate meter. While not as accurate as lab testing, it can give approximate values. Instructions and additional information may be found here.

Sufficiency ranges for melon tissue analyses (whole leaf, 6^th leaf from the growing tip) ^[N4].

Melon requires little N in the early phase of growth, and N fertilizers applied prior to planting are at risk of leaching, causing yield reductions and groundwater pollution ^[N1,N8]. To prevent N stress during early growth, a small quantity of N is often banded with P preplant or at planting, and the remainder applied in-season ^[N14].

For cantaloupe production in Arizona, it's recommended that when a soil test is below 10 ppm, 50 lbs N/acre can be broadcast prior to listing and shaping the beds ^[N8].

To avoid the risk of salt injury, the combined rate of banded N+K₂O should not exceed 50 lbs/acre ^[N15]. Twin bands, 4-6 inches below and to the side of the seed, have been shown to be effective and help reduce the risk of salt injury ^{[N15,N18,N29]}.

Melons don't appear to be affected by the source of N ^[N20] and choice may be made based on price and convenience. In general, seedling uptake of N and P tend to be higher when both are included in the starter band ^[N16].

Research on the effects of slow-release products applied prior to planting generally does not suggest any benefits over conventional N sources. Melons are a fast-growing crop, and early trials normally found slow-release products were unable to release sufficient N in time to support maximum cantaloupe growth ^{[N7,N23,N31,N32]}. However, little research has compared preplant applications of modern slow-release products with modern fertigation techniques.

More information on common fertilizer types can be found at the International Plant Nutrition Institute website.

To lessen the risk of loss by leaching, preplant N applications should be made as close as possible to the time of planting and after any pre-irrigation.

Annual N application to melons can be based on expected uptake and the N available from non-fertilizer sources. Melons have a dense root system and are efficient at using soil N, so N rates should be adjusted for soil and irrigation water nitrate (see Soil N Test).

Estimated cantaloupe N uptake based on expected yield.

The numbers in the table are based on studies from California, Arizona and Spain that found cantaloupe contained about 3-5 lbs N in each ton of harvested fruit, and that fruits accounted for on average 50-60% of the plant's total N ^{[N5,N10,N27,N28]}.

Honeydews likely require less nutrients per unit yield than cantaloupes. An early study in Riverside and more recent studies with hybrid varieties in the Central Valley and in Brazil all measured lower concentrations of N and K in honeydew fruit than that of cantaloupe, and somewhat lower P concentrations ^[N6,N10,N28]. The Riverside study found crenshaw melons to have similar nutrient uptake to honeydews ^[N28].

Several early California and Arizona trials observed that cantaloupe generally did not respond to applications greater than 120 lbs N/acre ^{[N18,N20,N22,N23,N24]}. While these studies used older varieties and furrow irrigation, a more recent trial in the Central Valley suggests that melon nutrient requirements per unit expected yield do not differ notably with irrigation type or between open-pollinated and hybrid varieties ^[N10]. Typical California application rates range between 80-150 lbs N/acre annually ^[N14].

Somewhat higher rates are typically used for desert cantaloupes than for those grown in the Central Valley ^[N14]. Higher rates may be helpful to desert production because these fields do not use N as efficiently since they are normally on lighter soils and are furrow irrigated, and also to facilitate earlier harvests (Tim Hartz, personal communication). However, no recent field studies have been performed to confirm optimal N rates for desert cantaloupes.

In-season N may be sidedressed, or applied with irrigation water in drip-irrigated fields ^[N14].

To quickly address emerging in-season deficiencies, nitrate or urea N-sources are recommended. They are more mobile in the soil than ammonium, and are immediately available to plant roots. However, the source is less important if plants are not deficient since ammonium is quickly transformed to nitrate in moist, aerated soils. Anhydrous or aqua ammonia may cause ammonia toxicity and should be used with caution. The risk is greatest on sandy soils ^[N8].

More information on common fertilizer types can be found at the website of the International Plant Nutrition Institute.

In-season N may be side-dressed at thinning, or applied with irrigation water any time during the season ^[N14]. Melons take up N at a steady rate from early bloom until maturity ^[N26], and may even benefit from late in-season fertilization ^[N24]. Based on field experiments in Spain with Santa Claus melons, which are closely related to cantaloupes and honeydews, Cabello and colleagues ^[N2] suggest that drip-irrigated melons can receive constant daily doses of N from early bloom until 2-3 weeks before harvest. No irrigations should be made in the last 7-10 days before harvest, as this can reduce fruit quality ^[N14].

Samples for P and K should be taken in late fall or early spring, when no fertilizer has recently been applied ^[P3]. A total of 20-30 cores should be taken from random locations within the field. If P has been previously banded in the field, twice as many cores should be taken to account for the additional variability ^[P3]. Soil samples are taken from the top foot of the soil profile, which is the major rooting zone. Melon roots can penetrate as deeply as 4 feet, but almost all of the activity is in the top foot ^[P19]. For more information on sampling procedure see Soil Test Sampling instructions.

In California, the available soil P is generally determined on sodium bicarbonate extracts (Olsen-P test). This test is appropriate for soils with a pH between 5.5 and 8.5 containing less than 3% of organic matter ^[P14]. For information about the appropriate P test when your soil is outside of this range, contact your local farm advisor.

Soil test critical ranges for cantaloupe ^[P14,P18].

A crop response is likely when the soil has an Olsen P value of less than 8 ppm. Between 8-15 ppm a response is possible, especially for cool season crops in the early spring. Over 15 ppm, a response is unlikely in warm weather plantings, although cool-weather plantings may still benefit from a small starter application ^[P14,P18].

Samples are taken from the entire field. Variable fields should be divided into uniform blocks, which are sampled separately ^[P5]. It is important to accurately determine the crop growth stage, since the total nutrient concentration declines as crops develop. Tissue tests are most meaningful if taken several times during the season ^[P10].

Recently matured leaves, typically the 6^th leaf from the growing tip, are used for tissue analyses. A minimum of 20 leaves should be collected, each from a different healthy plant of representative vigor. The petioles may also be analyzed. Petiole sap concentrations are affected by environmental conditions, so it's recommended to collect samples for petiole analysis at around the same time each day. Once tissue samples are collected, they should be dried or refrigerated as quickly as possible and sent to a lab for analysis ^[P10]. For more information on sampling procedure see Plant Tissue Sampling.

Sufficiency ranges for melons (petiole or whole leaf, 6^th leaf from the growing tip) ^[P1].

Phosphorus application rates should be based on soil test results. When soil P is adequate, P applications can be based on removal with the harvested fruit to maintain soil fertility in the long term.

Approximate amount of P removed with harvested cantaloupe

Values in the table are based on values from studies performed in California and Arizona which found that cantaloupes removed between half a pound and 1.5 lbs P ₂O₅ for each ton of harvested fruit ^[P4,P16,P17]. Honeydews likely remove less P per unit yield ^[P2,P4,P17].

Higher rates are typically used for desert cantaloupes than for those grown in the Central Valley ^[P6]. Higher P rates may be helpful to facilitate earlier harvests ^[P8]. However, no recent field studies have been performed to confirm optimal P rates for desert cantaloupes.

Higher rates may also be required for low testing soils and for melons planted in cool weather, when soil P is less available to plants. The University of California Small Farms fertilization guide suggests an application of 120 lbs P₂O₅/acre for low-testing soils, and 100 lbs P₂O₅/acre for early cool-weather plantings on soils with adequate P ^[P18]. 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.

The whole P rate is normally banded at planting ^[P6]. Early studies in Fresno County found that banding P fertilizer was about five times more efficient than broadcasting ^[P8]. Their results suggest the exact distance from the seed is not too important, but about 4-6 inches below and to the side is likely best ^[P8]. Banding on both sides of the seed reduces the risk of salt injury if N and K are included in the starter band ^[P7,P18].

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.

If applied pre-plant, P fertilizers are generally best applied close to the time of planting since P may interact with soil minerals, becoming less available over time ^[P13]. The further away soil pH is from neutral, the more strongly P interacts with soil minerals.

Samples for P and K should be taken in late fall or early spring, when no fertilizer has recently been applied ^[K4]. A total of 20-30 cores should be taken from random locations within the field. If P has been previously banded in the field, twice as many cores should be taken to account for the additional variability ^[K4]. Soil samples are taken from the top foot of the soil profile, which is the major rooting zone. Melon roots can penetrate as deeply as 4 feet, but almost all of the activity is in the top foot ^[K18]. For more information on sampling procedure see Soil Test Sampling.

In California, the available soil K is generally determined by the ammonium acetate extraction method.

Soil test critical ranges for cantaloupe ^[K13,K16]

Samples are taken from the entire field. Variable fields should be divided into uniform blocks, which are sampled separately ^[K6]. It is important to accurately determine the crop growth stage, since the total nutrient concentration declines as crops develop. Tissue tests are most meaningful if taken several times during the season ^[K12].

The youngest full size leaf, usually the sixth leaf from the growing tip, is selected. Either the petiole or the whole leaf may be taken. Petiole sap concentrations are affected by environmental conditions, so it's recommended to collect samples for petiole analysis.at around the same time each day. Between 20-40 samples should be collected. Once tissue samples are collected, they should be dried or refrigerated as quickly as possible and sent to a lab for analysis ^[K12]. For more information on sampling procedure see Plant Tissue Sampling.

Sufficiency ranges for melons (petiole or whole leaf, 6^th leaf from the growing tip) ^[K1]

The K application rate should be based on soil test results. Melon grown in soils with greater than 100 ppm of ammonium acetate exchangeable K is unlikely to respond to K fertilization. When the soil K test level contains adequate K, the K removed with the harvested crop can be replaced to maintain soil fertility in the long term (see table).

Approximate amount of K removed with harvested cantaloupe.

Values in the table assume that 5-6 lbs K₂O are removed with every ton of harvested cantaloupe ^{[K2,K5,K14,K15]}. Honeydews likely remove less K per unit yield ^[K3,K5,K15].

In fields with low soil test values the application rate may be increased. A higher application rate may also be necessary in very sandy soils where K may be leached.

Potassium fertilizers are most commonly applied pre-plant. Pre-plant applications can be broadcast and incorporated into the soil or banded 4-6 inches from the plant, on both sides of the row ^[K17]. Banding is more likely to be effective on calcareous, heavy, or K-fixing soils. To avoid the risk of salt injury, the combined rate of banded N+K₂O should not exceed 50 lbs/acre ^[K8].

In drip irrigated fields, K can also be applied by fertigation during the growing season ^[K17].

For fertigation, KCl or potassium thiosulfate (KTS) are generally preferred over K₂SO₄ due to their higher solubility.

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