California Crop Fertilization Guidelines

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Cotton Nitrogen Nutrition

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Deficiency Symptoms

Nitrogen deficiency results in pale green leaves with symptoms first appearing on the older leaves (see picture). Severe deficiency will result in the entire plant showing symptoms. In addition, growth of the plant is stunted and the development of vegetative branches reduced. Bolls may be shed soon after flowering. When N deficiency occurs later in the season, leafs may turn purple, and bolls at late fruiting positions may be shed ^[N31].

A healthy (left) and a nitrogen deficient cotton leaf (right; photo provided by the International Plant Nutrition Institute.

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Excessive Nitrogen

While insufficient N often reduces crop growth and lint yield, excessive N can cause difficulties in defoliation, harvest, and may reduce lint yield ^[N15]. Furthermore, high N application rates may increase aphid populations. In a FREP-funded study carried out in the San Joaquin Valley, cotton aphid infestations were more severe with 200 lbs N/acre than with lower N application rates ^[N11]. Aphid levels were especially high in plots where high N rates and the pyrethroid insecticide Capture (to control lygus bug populations) were applied.

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Soil Nitrate Test

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Soil Nitrate Test

Soil samples for nitrate analysis should be taken near planting time or at least 3 weeks after the last N application ^[N31]. As cotton is a deep rooting crop, samples should be taken to a depth of at least two feet. The distribution of nitrate in the soil profile is very variable. For this reason, the estimation of the potentially available N is improved by taking samples deeper than 2 feet ^[N16]. The Cotton Production Manual recommends taking samples from the top 3 feet of soil ^[N31]. Due to the variability of nitrate in the soil, care must be taken that the sample is representative for the field (see Sampling for Soil Nitrate Determination).

The soil nitrate test provides information on plant available N at the time of sampling. Its level depends on a number of factors (see Factors Affecting Soil Nitrate Concentrations in Spring). Therefore, the test result is field specific and needs to be determined every year.

In a FREP-supported study carried out in the San Joaquin Valley between 1996 and 2000, Hutmacher and collaborators found that the residual nitrate–N level in spring can be fully subtracted from the crop N requirements to determine the amount of fertilizer N that needs to be applied ^[N16].

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Petiole Analysis N

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Petiole Analysis N

Petiole analyses are used to monitor the N status of cotton after the first squares stage. A representative sample of 30 to 40 petioles should be collected at random. Generally, the petiole that is third to fifth from the terminal on the main stem is selected. This petiole should have a fully expanded leaf blade ^{[N2, N24]}. Samples should be sent to the laboratory immediately or dried without delay. For more information on sampling procedure see Plant Tissue Sampling.

Petiole nitrate levels need to be interpreted with care because they not only depend on N availability, but also on variety, growth stage, soil type, weather, and insect damage ^[N10]. The concentration of nitrate in petioles generally declines rapidly after first bloom. Taking petiole samples at first bloom, peak bloom, and at first open boll to monitor the change in petiole nitrate over time may help to overcome the confounding effects of variety and soil type ^[N2].

The following target cotton petiole nitrate levels have been proposed for California and Arizona ^{[N2, N28]}:

Growth stage	Early squaring	First bloom	2 weeks after first bloom	Peak bloom	Open boll	Last Season
	Nitrate-N (ppm)
California, upland cotton [N2]
Should be sufficient		18,000	12,500	7,000	3,500	should be <2,000
May be deficient		12,000	7,500	3,000	1,500	should be <2,000
Arizona, upland cotton [N28]
Desirable level	>18,000	>14,000		>8,000	>4,000
Arizona, Pima cotton [N28]
Desirable level	>10,000	>8,000		>4,000	>2,000

Nitrate concentrations in petioles are slightly higher early in the morning. The values should be adjusted down by 10% when petioles are sampled before 7 a.m., and by 5% when sampled between 7 and 8 a.m. ^[N31].

The nitrate level should fall below 2000 ppm nitrate-N late in the season for best defoliation and harvestability ^[N2]. The results of late-season petiole analyses may be known too late to make corrections for the current crop. However, they can be used to evaluate the fertilization program and make corrections for the following year.

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Preplant N

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Preplant N

High preplant N applications have generally no beneficial effect on yield compared to well-timed sidedress applications, but may result in lower yields ^{[N6, N34, N35]}. This can be explained by the fact that only a small proportion, generally less than 15-25% of the total N is taken up by the cotton plant during the first two months after planting ^{[N1, N9]}. Therefore, preplant N remains in the root zone for a relatively long time before being taken up by the plants. This increases the risk of N leaching below the root zone, especially in furrow irrigated systems. Applying moderate levels of N before or at planting and sidedressing the rest minimizes the risk of N losses.

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Starter Nitrogen

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Starter Nitrogen

Starter fertilizers are an effective way of providing sufficient N and phosphorus to young plants. Starter fertilizer strengthen the root system and encourage early rapid growth necessary to compete with weeds ^[N7]. However, these beneficial effects do not always translate into higher yields ^{[N3, N13, N29]}. Starter fertilizer are most beneficial when cool weather slows down germination and early plant growth or in soils that fix P or K ^{[N3, N21]}. Under conditions, where starter fertilizer has a positive effect, a yield increase of up to 10% may be achieved ^{[N13, N32]}.

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Application Rate

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Mode of Application

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Fertilizer Type

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Sidedress N

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Application Rate

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Mode of Application

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Fertilizer Type

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Time of Application

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Foliar Nitrogen

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Foliar Nitrogen

Foliar fertilizers are best used to supplement soil-applied N in order to correct for N deficiencies during boll development ^{[N22, N25]}. While N deficient plants respond to foliar N ^{[N22, N33]}, yield of plants with an adequate N supply is generally not increased. Therefore, foliar N is applied when petiole levels or crop development indicates that the need for N exceeds the plant’s capacity to take up N from the soil and when it is too late in the season to side-dress N ^[N22]. However, foliar N should be applied before deficiency symptoms develop ^[N22].

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Application Rate

Up to three applications with rates of 6-10 lbs N/acre per application have been shown to be effective ^[N18].

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Fertilizer Type

Urea is the most popular form of N used as foliar fertilizer in cotton ^{[N10, N22]}. Ammonium toxicity can be avoided by buffering the solution to a pH of 6.5 to 7 ^[N18].

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Time of Application

Foliar applications are generally made between peak and late bloom. During this period, soil-applied N would promote excessive late-season vegetative growth ^{[N19, N20]}. Absorption rates are highest and the risk of leaf burn lowest when foliar N is applied in the late afternoon or early morning. Water stress may reduce N absorption ^{[N10, N18, N22]}. Therefore, the crop should be well watered.

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Cotton Phosphorus Nutrition

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Deficiency Symptoms

Phosphorus deficiency results in stunted growth and impacts the ability to take up nutrients. Deficient plants have dark green leaves and, in some cases, the leaves and stems turn purple. As phosphorus is mobile in plants, P deficiency symptoms first appear on older leaves. Leaves of P deficient plants senesce often prematurely. In addition, flowering, fruiting and maturity are delayed as a result of P deficiency ^{[P5, P6]}.

Phosphorus deficiency symptoms on cotton leaves (photo provided by the International Plant Nutrition Institute.

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Excessive Phosphorus

Excessive P application may reduce Zn uptake, inducing Zn deficiency ^[P5].

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Soil Analysis

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Soil Sampling

Soil samples should be taken in spring from the first or second foot of soil (see Soil Test Sampling Instructions). In California, plant available P is generally determined by extracting soil samples with sodium bicarbonate (Olsen-P). Olsen-P concentrations of less than 3-5 ppm are considered low and a yield response to fertilizer P is likely ^{[P17, P21, P25]}. In contrast, when the concentration is higher than 8 ppm, a yield response is unlikely (see Table). Interpretation of soil test values.

Soil P test level	Olsen-P (ppm)		P application rate
	California [P17, P21, P25]	Arizona [P19]	(lbs P/acre)	(lbs P2O5/acre)
Low	< 3-5	< 5	18 - 40	40 - 90
Intermediate	5 - 8	5 - 8	9 - 22	20 - 50
High	> 8	> 8	0	0

Soil Test Based Application Rate

When the soil P test level is high, a small starter application may be considered when the growth conditions are suboptimal (e.g. cool weather conditions).

When the Olsen-P test indicates moderate P availability, the main goal of P fertilization is to maintain the current level of P availability by replacing the P removed from the field with seeds and lint, which ranges from 9 to 22 lbs P/acre (20-50 lbs P₂O₅/acre) for high-yielding cotton ^{[P7, P10, P22]}. These amounts are in line with several studies, where application rates ranging from 20 to 60 lbs P₂O₅/acre were reported ^{[P3, P4, P11]}.

The P contents in the aboveground biomass ranges from 18 to 40 lbs P/acre (40-90 lbs P₂O₅/acre) ^{[P7, P10, P22]}. This amount of P may be applied when the P soil test indicates low P availability. By applying more P than is removed at harvest, the soil P test level should increase over the years, reaching the intermediate level. Contact your local farm advisor for more information.

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Petiole Analysis

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Petiole Analysis

If P deficiency is suspected early in the season, the entire aboveground biomass of young plants can be sampled. Deficiency symptoms occur when tissue phosphate-P levels are below 400 ppm ^[P15].

Later in the season, the petiole from the youngest fully expanded leaf on the main-stem is usually sampled. This is the third to fifth leaf from the terminal and the most active ^{[P1, P20]}. A representative sample of 30 to 40 petioles should be collected at random ^[P1]. Petiole P levels can be determined on the same sample taken for nitrate or potassium analysis ^[P14]. For more information on sampling procedure see Plant Tissue Sampling. Phosphorus concentrations (in ppm phosphate-P) in petioles ^[P1].

Growth stage	First bloom	Peak bloom	First open boll	Late season
Low	1,500	1,200	1,000	800
High	2,000	1,500	1,200	1,000

The phosphate-P content in the petioles should be maintained above 1000 ppm during the main fruiting period ^[P1]. At the time petiole analyses are done, it may no longer be possible to correct P availability. Therefore, petiole P analyses are mainly used to assess the P fertilization management for adjustments in the following years. High petiole P concentrations do not necessarily indicate abundant P availability, as other factors, such as acute N deficiency can result in an accumulation of P in the petiole ^[P1]. Petiole P concentrations should therefore be interpreted together with other nutrients.

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Preplant P

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The optimum time to apply P to cotton is at planting ^[P12]. Phosphorus may be applied preplant when the soil P test level is intermediate to high and the goal of fertilization is to maintain the current level of P availability (for more details, see Soil Test).

Band applications of P are generally more effective than broadcast applications. This is especially true in soils with an alkaline (>7.5) or acidic pH (<5.5). Phosphorus availability highly depends on soil pH. In acidic soils, P may be fixed by aluminum and iron oxide minerals, while fixation by calcium is increased at high pH. Fixation is less of a problem in neutral soils or acidic soils with a high sand content. Under these conditions, a broadcast application is possible ^[P13].

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Starter Phosphorus

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Starter Phosphorus

Starter fertilizers are an effective way of providing early nutrients. Starter fertilizer may strengthen the root system and encourage early rapid growth to compete with weeds ^{[P8, P18]}. However, these advantages do not always translate into higher yields ^{[P3, P11, P23, P26]}. Starter fertilizer may be most beneficial in years where cool weather after planting slows down germination and early plant growth or in soils that fix P ^{[P3, P16]}. In contrast, in soils with very high soil test P levels, starter P application may not have an effect on cotton yield and growth ^[P4].

Under conditions, where starter fertilizer has a positive effect, a yield increase of up to 10% may realistically be expected ^{[P11, P25]}.

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Application Rate

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Mode of Application

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Fertilizer Type

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Sidedress Phosphorus

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The optimal time to apply P to cotton is at planting. However, when deficiency symptoms indicate suboptimal P supply, a sidedress application should be attempted. Johnson Hake and coauthors recommend shanking in ammonium phosphate as close to the active root zone as possible at rates of 50 to 75 lbs of P₂O₅/acre ^[P14].

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Foliar Phosphorus

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When P availability (from soil or fertilizer) is adequate, foliar P applications generally have no positive effect on cotton lint yield. In a soil at the Westside Field Station which was not considered deficient in either P or K, Zelinski and coworkers found that foliar application of these two nutrients had no effect on lint yield ^[P26]. This is in line with Bednarz and coworkers who found that foliar fertilization with N and P (applied as urea phosphate or ammonium polyphosphate) had little effect on yield and plant P concentrations of cotton grown on soils that were P fertilized based on soil test recommendations ^[P2].

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Cotton Potassium Nutrition

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Deficiency Symptoms

Potassium deficiency symptoms generally become visible after flowering and intensify during the boll set period ^[K25]. Symptoms include a yellowish white mottling of the foliage and yellow spots appearing between the veins of the leaves, with the leaf tips and margins being first affected (see picture). When the centers of the spots die, they turn into brown specks. Leaf tips curl downwards. Under severe K deficiency, the leaves turn reddish brown, dry and drop prematurely ^[K6]. Potassium deficiency may also affect boll development and fiber quality ^[K5].

Potassium deficiency symptoms on cotton leaves (photo provided by the International Plant Nutrition Institute.

Potassium is a mobile element in the plant. Deficiency symptoms therefore should appear first on older leaves ^[K25]. In the field, however, symptoms are often seen on younger leaves ^[K24]. Symptoms of potassium deficiency have been widely observed for cotton grown on soils in the San Joaquin valley derived from Sierra Nevada alluvium. These soils tend to fix K ^[K20] (see K Fixation in the San Joaquin Valley).

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Soil Analysis

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Soil samples are taken in spring from the first or second foot of soil (see Soil Test Sampling Instructions). A commonly used method to determine the quantity of K available is the ammonium acetate extraction. If the test value is below 120 ppm K in the first foot of the soil, or below 80 ppm K in the second foot of soil, a response to K fertilization is likely ^[K25]. The following application rates have been recommended for a sampling depth of one foot ^[K18]:

	Soil Test Exchangeable K (ppm)
	< 80 or K fixation	80 - 110	110 - 120	> 120
Fertilizer recommendation (lbs K2O/acre)	400	200	100	more

For soils that fix K, the extraction with ammonium acetate is not a reliable measure for available K ^[K20] (see K Fixation in the San Joaquin Valley). A K fixation measurement should be considered for soils located within the area identified as potentially K fixing when the exchangeable K values are between 50 and 200 ppm. Soils from this area with exchangeable K test levels below that range are very likely to fix K, while soils with higher values generally do not fix K ^[K20].

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Petiole Analysis

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Petiole analyses can be used to determine the K status of the crop (see Table). The petiole from the youngest fully expanded leaf on the main-stem is usually sampled. This is the third to fifth leaf from the terminal and the most active ^{[K2, K21]}. A representative sample of 30 to 40 petioles should be collected at random ^[K2]. For more information on sampling procedure see Plant Tissue Sampling.

The K concentration in the petioles decreases almost linearly after bloom by about 1% every 16 days ^{[K2, K25]}. Petiole K concentrations also depend on the variety and may vary from one year to another ^[K16]. Therefore, the levels shown in the table below are only general guidelines and knowledge about the fields and experience with the cotton variety grown are important for the interpretation of petiole K levels for a particular location and soil.

Growth stage	Cotton Petiole K (%)
	Very low	Low	Medium	High	Very high
	Variety Maxxa [K18]
First flower	< 2.5	2.5 - 3.0	3.0 - 3.5	3.5 - 4.0	> 4.0
2 weeks after first flower	< 1.8	1.8 - 2.4	2.4 - 2.8	2.8 - 3.2	> 3.2
10 days after cutout	< 1.0	1.0 - 1.4	1.4 - 1.8	1.8 - 2.4	> 2.4
	Upland cotton [K2]
First flower		4.0		5.5
Peak bloom		3.0		4.0
First open boll		2.0		3.0
Late season		1.0		2.0

The levels reported as “low” by Bassett and MacKenzie are identical to the sufficiency levels reported by Oosterhuis who based the recommendation on several studies ^{[K2, K19]}.

Results from late-season petiole analyses may be known too late to make corrections for the current crop. However, they are helpful for evaluating the current fertilization management and to make adjustments for the following year ^{[K16, K23]}.

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Preplant / Starter K

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Application Rate

The amount of K removed at harvest with lint and seeds ranges from 35 to 60 lbs K/acre (40-70 lbs K₂O/acre) for high-yielding cotton ^{[K9, K10, K22]}. The K in the aboveground biomass is considerably larger, ranging from 160 to 220 lbs K/acre (190-270 lbs K₂O/acre) ^{[K7, K9, K10]}.

Based on soil test results, the following K application rates have been recommended ^[K18]:

	Soil Test Exchangeable K (ppm)
	< 80 or K fixation	80 - 110	110 - 120	> 120
Fertilizer recommendation (lbs K2O/acre)	400	200	100	more

Mode of Application

Potassium fertilizer is most often broadcast before discing and bedding up or banded on the shoulder of the bed to a depth of 6 to 8 inches ^[K17]. As the cotton root system in the surface soil is weak, it is important to incorporate the fertilizer ^[K12]. In soils with a high K fixation potential, a band application, 6 to 8 inches deep, may be more efficient than a broadcast application, as the contact with soil minerals is reduced ^[K13].

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Fertilizer Type

Both, muriate of potash (KCl; potassium chloride) and sulfate of potash (K₂SO₄; potassium sulfate) can be applied to cotton. Muriate of potash is most often used, as it is cheaper and cotton is tolerant to chlorine ^[K25].

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Time of Application

Potassium is most often added preplant. In sandy soils with a low cation exchange capacity, multiple annual applications may be required, as K can be leached. Split applications may also be beneficial in soils that tend to fix K.

Cotton tends to be more sensitive to low K availability than most other major field crops due to its relatively low root density ^[K19]. Therefore, when cotton is grown in rotation with other crops, and K is not applied on an annual basis, it is recommended to apply K before cotton.

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Starter Potassium

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Starter Potassium

Potassium is not commonly applied as a starter fertilizer. However, applying starter K may be beneficial in soils that fix K.

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Application Rate

The amount of K that con be applied as a starter is limited, as high K application rates may have negative effects on seedling development because of increased salt concentrations in the soil near the application band. According to the Western Fertilizer Handbook, when starter fertilizer is applied at a distance of more than 2 inches from the seed, the amount of N plus K₂O that can be applied safely is 20 and 40 lbs/acre in sandy and fine-textured soil, respectively ^[K4]. Contact your local farm advisor for more information.

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Mode of Application

The most common placement for starter fertilizer is 2 inches to the side and 2 inches below the seed drill ^{[K3, K11, K14]}. Starter fertilizer should not be placed in direct contact with the seed in the furrow due to salt effects.

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Fertilizer Type

Liquid fertilizers that contain N and P, such as 10-34-0 (ammonium polyphosphate) are most commonly used as starter fertilizers ^{[K3, K8, K11]}. In K fixing soils, e.g. on the east side of the San Joaquin Valley, starter fertilizers containing K may be beneficial. Contact your local farm advisor for more information.

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Sidedress Potassium

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Sidedress Potassium

Potassium is most often applied preplant. In sandy soils with a low cation exchange capacity, however, multiple annual applications may be required, to minimize leaching losses. Split applications may also be beneficial in soils that tend to fix K (see Soil K Test).

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Application Rate

The sidedress application rate equals the K fertilizer recommendation, which is based on the soil test result, minus the quantities applied preplant and as starter.

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Mode of Application

The fertilizer should be placed below the bottom of the irrigation furrow, as the mobility of K in the soil is limited and plants have restricted access to K in dry surface soil ^[K15].

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Fertilizer Type

Both muriate of potash (KCl) and sulfate of potash (K₂SO₄) can be used for cotton. Muriate of potash is most often used, as it is cheaper and cotton is tolerant to chlorine ^[K25].

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Time of Application

The period of high daily uptake rates begins after first bloom ^[K1]. Therefore, the last split application should be done before first bloom ^[K15], so that the K is available when the plants need it.

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Foliar Potassium

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Deficiencies occurring after first bloom can be addressed with foliar applications. Late in the season, foliar applications are generally more effective than water-run applications. This is especially true when the K supply of the plant is limited by a poor root system due to diseases, root damage or soil conditions restricting rooting depth. In soils that fix K, foliar applications may also be beneficial ^[K17,K25].

If the petiole test indicates suboptimal K supply, two foliar applications of 10 lbs K₂O/acre at 7 and 14 days after first flower are recommended ^[K17]. The recommended source of K for foliar application is potassium nitrate, however, potassium sulphate and potassium thiosulfate seem to be similarly effective ^[K19].