California Crop Fertilization Guidelines

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Sunflower

	Preplant	Sowing	Early Growth	Flowering	Seed Filling	Maturity
Nitrogen (N)	Soil Test		Leaf Analysis
	Preplant N	Starter N	In-Season N
				Foliar N
Nitrogen (N) Soil Test Leaf Analysis Preplant N Starter N In-Season N Foliar N
Close Sunflower Nitrogen Nutrition Deficiency Symptoms Nitrogen deficient sunflower plants are somewhat smaller and paler green than normal. The leaf area is reduced. Older leaves may become yellow, and may have necrotic margins. Nitrogen deficiency symptoms are difficult to recognize, especially when the deficiency is uniform across the field, and yield reductions have been observed in plants with no evident symptoms [N5]. Thus, it's best to identify deficiencies through tissue analysis. Nitrogen deficient sunflower showing thin stems, pale green coloring and chlorosis of the older leaves. Photo courtesy of IPNI. Excessive Nitrogen Excessive N reduces seed oil concentrations [N8,N13,N30,N37]. Over-fertilized sunflowers also are at more risk of lodging and disease [N8,N25]. If excessive N is present in the early stages, luxuriant early vegetative growth may deplete moisture reserves early and new leaves may shade out older ones, leading to premature senescence during the seed filling period [N3]. Close Soil Nitrate Test The residual soil nitrate level depends on a number of factors related to soil properties, weather and crop management (see Factors Affecting Soil Nitrate-N. Therefore, the test needs to be carried out every year. Soil Sampling Samples can be taken close to planting (pre-plant N test) or in spring before the first in-season N application [N29]. An in-season N test provides a measure of the soil N available to the crop just before the period of greatest N uptake and can be used to determine the need for sidedress N. Measuring N before sidedressing also captures the N which has been mineralized in early spring from crop residues and soil organic matter, and so provides a more accurate estimate of residual N than a pre-plant test [N23]. Soil nitrate should be measured to at least 2 feet regularly, and ideally 4 feet if the soil has not been tested in a while [N11,N25]. Sunflowers have deep taproots that can access water and N from 6 feet deep or more under good conditions [N20,N28], so soil tests may not predict sunflower's nitrogen fertilizer requirement accurately if sampling is too shallow [N30]. Due to the variability of nitrate in the soil, care must be taken to assure that the sample is representative for the field. See Sampling Instructions for more information. Soil Test Interpretation Pounds of potentially available nitrate-N per acre in each foot of soil can be estimated by multiplying parts per million of nitrate-N by 3-4. For example, if the top foot of soil contains 10 ppm nitrate-N and the second foot contains 5 ppm, 45-60 lbs nitrate-N/acre are present in the top two feet of soil. However, not all of this is available to the roots of young seedlings, so even in high-testing soils a small starter application may still be beneficial [N23]. Nitrate in the Irrigation Water 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. For example, with 10 acre-inches of water containing 10 ppm nitrate-N, 22.6 lbs N are applied per acre. Close Leaf Analysis Leaf Sampling For routine monitoring, samples are taken from the entire field. Variable fields should be divided into uniform blocks, which are sampled separately. Samples are taken just before floret emergence. The most recently matured leaf (3rd or 4th from the bud) is sampled. Between 20 and 30 leaves are taken and placed in a clean paper bag. Once tissue samples are collected, they should be dried as quickly as possible, either air- dried out of direct sunlight or in an oven at a temperature of 140-160 °F [N6,N10,N26]. For more information on sampling procedure see Plant Tissue Sampling. Interpretation of Test Results Optimum tissue nutrient ranges in sunflower. Values are for the 3rd-4th leaf below the terminal bud, sampled just before floret emergence [N6]. Nutrient Sufficiency range (%) N 3.4 - 4.0 P 0.25 - 0.35 K 2.5 - 3.2 Leaf N concentrations continue to increase even when yield response has plateaued, so leaf N much above 4% may indicate overfertilization [N1]. However, optimum leaf N may depend on the site and variety [N30]. Therefore, it's most informative to combine tissue analyses with soil analysis, N budgeting and observations of crop performance. Close Preplant N The need for pre-plant N is often minimal, and it may be more efficient to use a small starter N application followed by an in-season sidedress. Irrigated sunflowers have sometimes [N35,N36] but not always [N32] been observed to benefit from in-season N applications. Split applications are likely to be more water efficient than applying the whole rate preplant, as high early N availability may lead to overly luxuriant early growth and reduced water use efficiency [N3]. Heavy preplant applications are therefore especially risky when moisture may become limiting, as early depletion of soil water reserves can lead to premature leaf senescence and a shorter grain-filling period [N3]. In addition, when the entire N rate is applied pre-plant, there is more risk that early irrigations may leach it below the root zone of the young crop, inducing early season deficiencies. Mode of Application If ammonium or urea fertilizers are broadcast preplant they should be incorporated into the soil, or washed into the soil with irrigation water to avoid loss by volatilization. If the surface soil is likely to dry out during the season, deep incorporation or placement is better than surface placement, as shallowly incorporated fertilizer N may become unavailable as the soil dries [N30]. Fertilizer Type In greenhouse studies it's been observed that sunflowers grown with nitrate were larger and had a higher seed weight and oil concentration than those grown with urea [N17,N22]. However, because urea and ammonium are rapidly transformed to nitrate in warm and moist soils, the source is unlikely to make a difference in the field [N17]. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute. Close Starter Nitrogen Sunflower N uptake is very low until the 8-leaf stage; however, any N deficiency during this period strongly affects subsequent crop growth rate and yield [N7,N36,N37]. Therefore, a small starter application which provides the young seedling with sufficient N for early growth is likely to be sufficient, and the rest may be sidedressed at layby. See In-season N: Seasonal N Requirement for more information. Application Rate and Type For irrigated California sunflowers, it's recommended to apply about 13 lbs N/acre at planting as starter [N25] (see Table). A low-N, high-P blend is an effective starter fertilizer, as simultaneous addition of ammonium and P can increase uptake of both nutrients on low-fertility soils [N4,N38]. Commonly used types and rates of starter fertilizer materials in California. Target N application rate is 13 lbs N/acre. Starter fertilizer Analysis Form Application rate P2O5 (lbs/acre) K2O (lbs/acre) 8-24-6 starter 8-24-6 Liquid 15 gal/acre 40 10 Ammonium polyphosphate 10-34-0 Liquid 11.4 gal/acre 45 0 Monoammonium phosphate 11-54-0 Solid 120 lbs/acre 65 0 Liquid fertilizer rates calculated using a density of 11.0 lbs/gal for 8-24-6 and 11.6 lbs/gal for ammonium polyphosphate. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute. Mode of Application Starter N is most efficiently injected [N40]. Fertilizer should not be placed in direct contact with the seed (Rachael Long, personal communication). Bands should be at least 2 inches below and to the side of the seed [N25,N41]. Close In-Season N While it is important that adequate N is available during early growth, sunflower plants take up less than 5% of their N before the 8-leaf stage [N36,N37]. However, uptake after this point is very rapid, with 60-100% of the crop's total N uptake occurring by seed fill [N316,N37]. Applying moderate levels of N before or at planting and sidedressing the rest at layby is therefore an efficient strategy [N25]. Seasonal N Requirement Estimated nitrogen uptake of a typical sunflower crop Yield Nitrogen uptake (lbs N/acre) (lbs/acre) Seeds Residue Total 1000 25-30 20-40 45-70 1500 37.5-45 30-60 68-105 2000 50-60 40-80 90-140 2500 63-75 50-100 113-175 Values in the table are based on studies from around the world, which typically observe that between one and two-thirds of the total N is found in the seed, and that seeds contain about 2.5-3.0% N [N2,N4,N9,N14,N16,N27,N30,N33,N34,N36,N37]. However, concentrations ranging from 1.5% to 4.0% have been observed [N12,N42]. Values in the table are in line with the rule of thumb from the Great Plains that for every 100 lbs expected yield, 5-7 lbs N should be available in the top 2 feet of soil (fertilizer + residual soil N) [N11,N24,N41]. Yield-based estimates may be increased to account for male pollinator sunflowers, since they are not harvested for seed. However, sunflower can also access soil N from deeper in the soil profile than is practical to sample [N29], meaning soil nitrate levels in the top 2 feet of the profile may underestimate the total amount of residual N available to sunflowers [N11]. Conditions that may cause significant nitrate leaching below 2 feet include: heavy irrigation or fertilizer on the previous crop, a wet winter, and a preceding crop that yielded poorly [N29]. Application Rate In-season N application rates can be calculated by subtracting the residual soil N and the N applied at planting as starter from the estimated total N requirement. See Seasonal N Requirement for more information. Mode of Application Sidedress N may be injected in a drip line or banded [N31,N39]. Fertilizer Type Many types of N fertilizers are available. Type of fertilizer can be based on price and application method. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute. Time of Application Nitrogen should be sidedressed before the 8- leaf stage, so that the N will be available during the period of rapid uptake. Unless N is applied through a drip line sidedressing should be done before the sunflower roots extend too far into the row middles, to avoid root injury [N31].. In California, this is normally at layby, 4-6 weeks after planting [N25]. Sunflower N uptake is high through flowering and under adequate moisture some N uptake may continue well into seed-filling [N36]. It's possible that in N-deficient sunflowers fertilization during seed-filling can slow the rate of leaf senescence and increase yield [N2,N7]. However, N should not be applied after flowering if sunflowers are not N-deficient, as it's unlikely to increase yields and may reduce seed oil content [N2,N18]. Close Foliar Nitrogen Foliar N applications to sunflower have not been well researched and are not commonly practiced. Sunflower leaves appear to be able to take up N foliarly. In a greenhouse experiment, Goswami and Srivastava [N15] measured higher rates of photosynthesis in sunflower leaves that had been sprayed with a 0.5% urea solution than in an unsprayed control. The effect was most pronounced when the leaves were sprayed three times (30, 60 and 90 days after sowing), and only had a long-term effect on the upper leaves. In addition, greenhouse and lysimeter studies have observed that foliar sprays of potassium nitrate increased leaf nitrate and soluble protein levels in sunflowers grown on saline soil, and improved nutrient uptake and water regulation in sunflowers grown under drought conditions [N19,N21]. These results suggest that foliar N sprays may be useful for quickly relieving N deficiencies in the short term, especially under stress conditions and when combined with foliar K. Such quick N boosts may possibly delay leaf senescence and helping to extend the seed-filling period [N7]. However, foliar N sprays are not likely to be an economically sound technique for field grown California sunflowers.
	Preplant	Sowing	Early Growth	Flowering	Seed Filling	Maturity
Phosphorus (P2O5)	Soil Test		Leaf Analysis
	Preplant / Starter P		In-season P
Phosphorus (P2O5) Soil Test Leaf Analysis Preplant / Starter P In-season P
Close Sunflower P Nutrition Deficiency Symptoms Phosphorus deficient sunflowers are usually stunted and have thin stems. Older leaves may turn yellow and senesce early. With very severe deficiencies lower leaves may develop water-soaked areas that later become circular necrotic spots. Phosphorus deficiency has been observed to delay flowering, and to decrease seed oil concentration [P1,P4,P20]. In practice sunflower P deficiencies are very difficult to diagnose visually, as mildly affected plants may not have evident symptoms and the symptoms of severe deficiencies look similar to those produced by some fungal diseases [P5]. Soil and plant tissue analysis are the most reliable diagnostic methods. Close Soil Analysis Soil Sampling Samples for P and K should be taken in late fall or early spring, when no fertilizer has recently been applied [P12]. 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 [P12]. Samples are generally taken from the top foot of the profile where the root density of sunflower is highest. However, sunflowers have deep taproots that may extend deeper than 6 feet in the absence of restricting soil layers [P16,P23]. See Soil Test Sampling for further sampling instructions. In California, the available soil P is generally determined by extracting the soil samples with a bicarbonate solution (Olsen-P test). The Olsen-P test works well in soils with pH 6.5 and above. Consult your local farm advisor for appropriate soil testing when soil pH is below 6.5. Soil Test Interpretation Published critical values below which sunflower is likely to respond to P fertilization range generally from 10 to 20 ppm bicarbonate extractable P [P5,P18,P19,P20,P25]. However, in some studies, sunflowers responded to P applications at bicarbonate-extractable P levels of over 25 ppm, while in other studies no response was observed even at soil test levels as low as 6 ppm [P5,P10,P18]. This variability is partly due to the associations sunflower roots form with mycorrhizal fungi, which help it exploit native soil P. In soils that have been fallow a long time or that have recently been fumigated, sunflower may respond to fertilizer at a higher level of extractable P [P5]. In addition, sunflowers may access nutrients below the soil layer tested, so soil test values do not always match the nutrient availability of a site. To make accurate P fertilization decisions, therefore, soil test values are best combined with tissue P concentrations and P budgets (see Starter Application Rate). Both N and P are less available when soils are cold, especially when seedling roots are still small [P7,P17]. Since California sunflowers are often planted early, a small starter application may still be beneficial, even when soil tests show adequate P levels. Close Leaf Analysis It's most efficient applying all P and K fertilizer at planting. Therefore, tissue analysis is more useful for assessing the current program and adjusting P and K rates for subsequent crops than for correcting current-season deficiencies. Sampling Procedure For routine monitoring, samples are taken from the entire field. Variable fields should be divided into uniform blocks, which are sampled separately. Samples are taken just before floret emergence. The most recently matured leaf (3rd or 4th from the bud) is sampled. Between 20 and 30 leaves are taken and placed in a clean paper bag. Once tissue samples are collected, they should be dried as quickly as possible, either air-dried out of direct sunlight or in an oven at a temperature of 140-160°F [P8,P11,P21]. For more information on sampling procedure see Plant Tissue Sampling. Interpretation of Test Results Optimum tissue nutrient ranges in sunflower. Values are for the 3rd-4th leaf below the terminal bud, sampled just before floret emergence [P7]. Nutrient Sufficiency range (%) N 3.4 - 4.0 P 0.25 - 0.35 K 2.5 - 3.2 These values are in line with results from a greenhouse study by Abbadi and Gerendás [P2], who found that 0.3-0.4% P in the youngest mature leaf blade was associated with maximum seed and oil yield. Concentrations of less than 0.2% almost always indicate deficiency [P6]. Close Preplant / Starter P Both N and P are less available when soils are cold, especially when seedling roots are still small [P7,P17]. Since California sunflowers are often planted early, a small starter application may still be beneficial, even when soil tests show adequate P levels. Application Rate Sunflower yield response to P fertilizer is relatively rare [P4,P9]. To maintain adequate P levels in the soil in the long term, the P removed with the harvested seed may be replaced. Sunflower seeds contain about 0.4-0.8% P, so about 9-18 lbs P2O5/acre are removed with 1000 lbs of harvested seeds [P14,P24]. A low-N, high-P starter fertilizer applied at normal rates will more than replace the P removed by a typical 1500-2000 lb/acre crop. See Starter N for more information Higher rates may be beneficial in low-P soils which fix P, either because they are acid or calcareous. Field studies in South Africa and Pakistan on P-fixing soils with low organic matter content found that yields were maximized with applications of 90 lbs P2O5/acre (40 lbs P/acre) [P3,P4]. In both these studies the P fertilizer was broadcast as single- or triple-superphosphate prior to planting. Lower rates could likely be used if P is banded [P28]. Sunflower uses native soil P efficiently in part because it forms associations with mycorrhizal fungi. In P-deficient soils that have been fallow a long time or that have recently been fumigated, sunflowers may respond to higher P rates [P15]. Mode of Application Phosphorus is not very mobile in the soil, so P fertilizer needs to be incorporated. Banded or seed-placed P is taken up more efficiently than broadcast P [P27,28]. If the starter fertilizer contains only P, any amount of fertilizer may be applied with the seed. If the material used contains N or K,starter fertilizer should be banded 2 inches below and to the side of the planting row [P28]. Fertilizer Type Low-N, high-P fertilizers are applied at planting to many crops, as the presence of N, especially ammonium, helps improve P uptake for many crop species [P13]. A greenhouse study in Germany observed that sunflower had a greater root density and took up more P when it was applied in combination with ammonium. [P27]. Many liquid and solid P fertilizers are available, and under normal conditions are equally effective [P28]. In California, dry monoammonium phosphate (11-54-0) or liquid blends such as 8-24-6 or ammonium polyphosphate (10-34-0) are common [P20]. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute. Time of Application All P should be applied at planting. Phosphorus fertilizer does not have to be applied every year. Close In-Season Phosphorus Adequate P availability is especially important early in the season, as sufficient P is important for seedling root growth [P22]. All P is therefore normally applied at planting. There is very little information about the best way to apply in-season P to address emerging deficiencies. Roots will be fewer and less active in dry soil [P23,P26] and P fertilizer cannot be washed deeply into the soil profile. Therefore, in-season P applications are only likely to be taken up if the fertilizer is placed deeply into moist soil.
	Preplant	Sowing	Early Growth	Flowering	Seed Filling	Maturity
Potassium (K2O)	Soil Test		Leaf Analysis
	Preplant / Starter K		In-season K	Foliar K
Potassium (K2O) Soil Test Leaf Analysis Preplant / Starter K In-season K Foliar K
Close Sunflower Potassium Nutrition Deficiency Symptoms Potassium deficiency symptoms in sunflower first appear as yellowing on the lower leaves. Yellowing and tissue death are most severe between the major veins and along the margins, especially near the tip. In some cases, the leaves become narrower near the tips, and exhibit upward or downward cupping. The base of the leaf normally remains green for a long time [K5]. Plants may be subject to lodging [K1,K24]. Furthermore, seed oil content is reduced in K-deficient sunflowers [K1,K4]. The K deficient leaf is smaller and has patches of dry tissues between the veins and near the tip (photo taken by Patricia Lazicki). Potassium has an important role in regulating sunflower water use. The effects of K deficiency are more severe when sunflowers are under moisture stress, and the effects of drought are stronger under low-K conditions, especially in older plants [K16]. Sunflower K deficiency is most likely to occur on sandy soils [K15]. Close Soil Analysis Soil Sampling Samples for P and K should be taken in late fall or early spring, when no fertilizer has recently been applied [K10]. 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 [K10]. Samples are generally taken from the top foot of the profile where the root density of sunflower is highest. However, sunflowers have deep taproots that may extend deeper than 6 feet in the absence of restricting soil layers [K13,K19]. See Soil Test Sampling for further sampling instructions. Soil Test Interpretation A soil test value of 150 ppm extractable K indicates sufficient K for sunflower production [K9]. Close Leaf Analysis It's most efficient applying all P and K fertilizer at planting. Therefore, tissue analysis is more useful for assessing the current program and adjusting P and K rates for subsequent crops than for correcting current-season deficiencies. Leaf Sampling For routine monitoring, samples are taken from the entire field. Variable fields should be divided into uniform blocks, which are sampled separately. Samples are taken just before floret emergence. The most recently matured leaf (3rd or 4th from the bud) is sampled. Between 20 and 30 leaves are taken and placed in a clean paper bag. Once tissue samples are collected, they should be dried as quickly as possible, either air-dried out of direct sunlight or in an oven at a temperature of 140-160 °F [K6,K8,K17]. For more information on sampling procedure see Plant Tissue Sampling. Interpretation of Test Results Optimum tissue nutrient ranges in sunflower. Values are for the 3rd-4th leaf below the terminal bud, sampled just before floret emergence [K6]. Nutrient Sufficiency range (%) N 3.4 - 4.0 P 0.25 - 0.35 K 2.5 - 3.2 The values in the table are slightly hither than the results of a greenhouse study by Spear et al. [K22], who observed maximum sunflower growth when the youngest fully expanded leaf had a concentration of at least 2.4% K. Excess K applications increased leaf K without increasing sunflower growth or yields. Close Preplant / Starter K Application Rate When soils have adequate K, fertilization rates may be based on the K removed with harvested seeds in order to maintain soil K levels in the long term. Only about 5-15 lbs K2O are removed with 1000 lbs of seed [K7,K11,K20,K21]. However, sunflower K uptake is much greater, as 80-90% is in the stover [K18,K20,K21]. If stover is also removed, much more K will be needed to replace what was lost. Higher K applications may be necessary in soils with K-test levels below 150 ppm. In the northern Great Plains, an annual application of 100 lbs/acre potassium chloride (60 lbs K2O/acre) is recommended for low-K soils [K9]. Mode of Application Preplant K may be broadcast and incorporated, or banded. If K is included in the starter fertilizer, bands should be made 2 inches below and 2 inches to the side of the seed. Fertilizer Type A number of granular and liquid K fertilizers are available, and are equally effective. Choice therefore may be based on price and convenience [K24]. Fact sheets of the most common fertilizers can be found on the web site of the International Plant Nutrition Institute. Time of Application Since K is relatively immobile in most soils, K fertilizer may be applied at or any time prior to planting and does not have to be applied every year. Close In-Season Potassium About 90% of sunflower K is taken up before flowering [K20]. Therefore, all K is normally applied at or before planting. There is very little information about the best way to apply in-season K to address emerging deficiencies. Roots will be fewer and less active in dry soil [K19,K23] and unless the soil is very sandy, K fertilizer cannot be washed deeply into the soil profile. Therefore, in-season K applications will only be effective if the fertilizer is placed in contact with moist soil. Close Foliar Potassium Adequate K is important to a sunflower's ability to regulate water [K16]. However, K uptake is limited in dry and saline soils. In a lysimeter tank study in Pakistan, Hussain and coworkers [K12] observed improved yield, drought tolerance, and nutrient uptake in sunflowers that were sprayed with a solution that contained 0.5% N as urea, 0.5% K as potassium sulfate, and a surfactant. The plants were sprayed once during the vegetative stage and once during the reproductive stage (35 and 60 days after sowing, respectively). Similarly, in two greenhouse studies, sunflowers which received foliar K were less affected by irrigations with saline water than unsprayed sunflowers, although sufficient soil K was provided [K2,K14]. Akram and coworkers [K3] observed that potassium sulfate (1.25% K; sprayed 25 and 32 days after emergence) was more effective at increasing growth and seed yield under saline conditions than the same rate as potassium chloride (KCl). Potassium nitrate (KNO3) had an intermediate effect. These results suggest that foliar K sprays may be useful for improving sunflower's resilience to drought and salinity stresses, even when soil K is not limiting. However, no information is available on whether foliar K sprays are an economically sound technique for field grown California sunflowers.

References

Nitrogen

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36. Steer, B.T., Hocking, P.J., 1983. Leaf and floret production in sunflower (Helianthus annuus L.) as affected by nitrogen supply. Annals of Botany 52, 267-277.
37. Steer, B.T., Coaldrake, P.D., Pearson, C.J., Canty, C.P., 1986. Effects of nitrogen supply and population density on plant development and yield components of irrigated sunflower (Helianthus annuus L.). Field Crops Research 13, 99-115.
38. Strasser, B., Werner, W., 1995. Einfluss von Form und Plazierung der N- und P-Düngung auf Wurzelentwicklung, Phosphat-Aufnahme und Wachstum von Sonnenblumen (Helianthus annuus) und Zuckerhirsen (Sorghum bicolor). Journal of Agronomy & Crop Science 175, 157-165.
39. Tahir, M., Khaliq, A., Waseem, M., Ali, A., Tanveer, A., Iqbal, A., Atta Ullah Mohsin, H., Khan, M.A., 2011. Influence of different nitrogen application methods and sowing techniques on sunflower hybrid. Legume Research 34, 90-97.
40. Vigil, M.F., Benjamin, J., Schepers, J., 2001. Yield response and fertilizer nitrogen recovery by dryland sunflowers in a no-till rotation. National Sunflower Association Research Forum Paper.
41. Vigil, M.F., Hergert, G.W., Mengel, D.B., 2009. Nutrient management. In: Meyer, R., (Ed.). High Plains Sunflower Production Handbook. Kansas State University Agricultural Experiment Station and Cooperative Extension Service MF-2384.
42. Wan, P.J., Baker, G.W., Clark, S.P., Matlock, S.W.,1979. Characteristics of sunflower seed and meal. Cereal Chemistry 56, 352-355.

Phosphorus

1. Abbadi, J., Gerendás, J., 2011. Effects of phosphorus supply on growth, yield, and yield components of safflower and sunflower. Journal of Plant Nutrition 34, 1769-1787.
2. Abbadi, J., Gerendás, J., 2015. Phosphorus use efficiency of safflower (Carthamus tinctorius L.) and sunflower (Helianthus annuus L.). Journal of Plant Nutrition 38, 1121-1142.
3. Ammanullah, Khan, M.W., 2010. Interactive effects of potassium and phosphorus on phenology and grain yield of sunflower in northwest Pakistan. Pedosphere 20, 674-680.
4. Blamey, F.P.C., Chapman, J., 1981. Protein, oil, and energy yields of sunflower as affected by N and P fertilization. Agronomy Journal 73, 583-587.
5. Blamey, F.P.C., Edwards, D.G., Asher, C.J., 1987. Nutritional disorders of sunflower. Department of Agriculture, University of Queensland, Australia.
6. Blamey, F.P.C., Zollinger, R.K., Schneiter, A.A., 1997. Sunflower production and culture. In: Schneiter, A.A., (Ed.). Sunflower Technology and Production. American Society of Agronomy, Madison, WI. pp. 595-670.
7. Bryla, D.R., 2011. Application of the '4R' nutrient stewardship concept to horticultural crops: getting nutrients in the 'right' place. HortTechnology 21, 674-682.
8. California Plant Health Association, 2002. Western Fertilizer Handbook 9^th edition. Interstate Publishers, Inc.
9. Franzen, D., 2016. Fertilizing sunflower. North Dakota State University Extension Service SF713 (Revised).
10. Franzen, D.W., Schultz, E.C., Graham, C., Endres, G., Sharma, L.K., 2015. Nitrogen and phosphorus recalibration for sunflower.
11. Flynn, R., Ball, S.T., Baker, R.D., 1999. Sampling for plant tissue analysis. New Mexico State University Cooperative Extension Service, Guide A-123.
12. Geisseler, D., Miyao, G., 2016. Soil testing for P and K has value in nutrient management for annual crops. California Agriculture 70(3), 152-159.
13. Grunes, D.L., 1959. Effect of nitrogen on the availability of soil and fertilizer phosphorus to plants. Advances in Agronomy 11, 369-396.
14. Heard, J., Park, R., 2008. Nutrient uptake and partitioning by sunflowers in Manitoba. In Proceedings of the 2008 Manitoba Agronomist Conference.
15. Hibberd, D.E., Want, P.S., Hunter, M.N., Standley, J., Moody, P.W., Blight, G.W., 1991. Marginal responses over six years by sorghum and sunflower to broadcast and banded phosphorus on a low P Vertisol, and changes in extractable soil phosphorus. Australian Journal of Experimental Agriculture 31, 99-106.
16. Jaafar, M.N., Stone, L.R., Goodrum, D.E., 1993. Rooting depth and dry matter development of sunflower. Agronomy Journal 85, 281-286.
17. Lazicki, P.A., Geisseler, D., 2017. Soil nitrate testing supports nitrogen management in irrigated annual crops . California Agriculture, DOI: 10.3733/ca.2016a0027.
18. Lewis, D.C., Potter, T.D., Weckert, S.E., 1991. The effect of nitrogen, phosphorus and potassium fertilizer applications on the seed yield of sunflower (Helianthus annuus L.) grown on sandy soils and the prediction of phosphorus and potassium responses by soil tests. Fertilizer Research 28, 185-190.
19. Lilleboe, D., 2011. Fertilization of sunflower. Special supplement to "The Sunflower" magazine. 2/2011.
20. Long, R., Mathesius, K., Vaccaro, B., Symmes, E., Bali, K., Gulya, T., (in preparation). Sunflower Hybrid Seed Production in California. UC ANR Publication.
21. Lorenz, O.A., Tyler, K.B., 1976. Plant tissue analysis of vegetable crops. In: Reisenauer, H.M., (Ed.). Soil and Plant-Tissue Testing in California. University of California, Division of Agricultural Sciences, Bulletin 1879. pp. 24-29.
22. Lopez-Bucio, J., Cruz-Ramirez, A., Herrera- Estrella, L., 2003. The role of nutrient availability in regulating root architecture. Current Opinion in Plant Biology 6, 280-287.
23. Merrill, S.D., Tanaka, D.L., Hanson, J.D., 2002. Root length growth of eight crop species in Haplustoll soils. Soil Science Society of America Journal 66, 913-923.
24. Narem, R.A., 1982. Nitrogen fertility requirements, dry matter production and nutrient uptake of sunflower. M.S. thesis. South Dakota State University, Brookings.
25. Shaver, T.M., Hergert, G.W., Binford, G.D., Blumenthal, J.M., 2014. Sunflower. In: Shaver, T.M., (Ed.). Nutrient Management for Agronomic Crops in Nebraska. University of Nebraska Extension Circular 155, 127-130.
26. Stone, L.R., Goodrum, D.E., Schlegel, A.J., Jaafar, M.N., Khan, A.H., 2002. Water depletion depth of grain sorghum and sunflower in the central High Plains. Agronomy Journal 94, 936-943.
27. Strasser, B., Werner, W., 1995. Einfluss von Form und Plazierung der N- und P-Düngung auf Wurzelentwicklung, Phosphat-Aufnahme und Wachstum von Sonnenblumen (Helianthus annuus) und Zuckerhirsen (Sorghum bicolor). Journal of Agronomy & Crop Science 175, 157-165.
28. Vigil, M.F., Hergert, G.W., Mengel, D.B., 2009. Nutrient management. In: Meyer, R., (Ed.). High Plains Sunflower Production Handbook. Kansas State University Agricultural Experiment Station and Cooperative Extension Service MF-2384.

Potassium

1. Abbadi, J., Gerendás, J., Sattelmacher, B., 2008. Effects of potassium supply on growth and yield of safflower as compared to sunflower. Journal of Plant Nutrition and Soil Science 2008, 272-280.
2. Akram, M.S., Ashraf, M., Akram N.A., 2009a. Effectiveness of potassium sulfate in mitigating salt-induced adverse effects on different physio- biochemical attributes in sunflower (Helianthus annuus L.). Flora 204, 471-483.
3. Akram, M.S., Ashraf, M., Shabhaz, M., Akram N.A., 2009b. Growth and photosynthesis of salt- stressed sunflower (Helianthus annuus) plants as affected by foliar-applied different potassium salts. Journal of Plant Nutrition and Soil Science 172, 884-893.
4. Bailey, L.D., Soper, R.J., 1985. Potassium nutrition of rape, flax, sunflower and safflower. In: Munson, R.D., (Ed.) Potassium in Agriculture. American Society of Agronomy, Crop Science Society of America and Soil Science Society of America, Madison, Wisconsin, pp. 765-798.
5. Blamey, F.P.C., Edwards, D.G., Asher, C.J., 1987. Nutritional disorders of sunflower. Department of Agriculture, University of Queensland, Australia.
6. California Plant Health Association, 2002. Western Fertilizer Handbook 9^th edition. Interstate Publishers, Inc.
7. Deibert, E.J., Utter, R.A., 1989. Sunflower growth and nutrient uptake: Response to tillage system, hybrid maturity and weed control method. Soil Science Society of America Journal 53, 133-138.
8. Flynn, R., Ball, S.T., Baker, R.D., 1999. Sampling for plant tissue analysis. New Mexico State University Cooperative Extension Service, Guide A-123.
9. Franzen, D., 2016. Fertilizing sunflower. North Dakota State University Extension Service SF713 (Revised).
10. Geisseler, D., Miyao, G., 2016. Soil testing for P and K has value in nutrient management for annual crops. California Agriculture 70(3), 152-159.
11. Gholamhoseini, M., Ghalavand, A., Dolatabadian, A., Jamshidi, E., Khodaei-Joghan, A., 2013. Effects of arbuscular mycorrhizal inoculation on growth, yield, nutrient uptake and irrigation water productivity of sunflowers grown under drought stress. Agricultural Water Management 117, 106-114.
12. Hussain, R.A., Ahmad, R., Mawaz, F., Ashraf, M.Y., Waraich, E.A., 2016. Foliar NK application mitigates drought effects in sunflower (Helianthus annuus L.). Acta Physiologiae Plantarum 38:83.
13. Jaafar, M.N., Stone, L.R., Goodrum, D.E., 1993. Rooting depth and dry matter development of sunflower. Agronomy Journal 85, 281-286.
14. Jabeen, N., Ahmad, R., 2011. Foliar application of potassium nitrate affects the growth and nitrate reductase activity in sunflower and safflower leaves under salinity. Notulae Botanicae Horti Agrobotanici Cluj-Napoca 39, 172-178.
15. Lewis, D.C., Potter, T.D., Weckert, S.E., 1991. The effect of nitrogen, phosphorus and potassium fertilizer applications on the seed yield of sunflower (Helianthus annuus L.) grown on sandy soils and the prediction of phosphorus and potassium responses by soil tests. Fertilizer Research 28, 185-190.
16. Lindhauer, M.G., 1985. Influence of K nutrition and drought on water relations and growth of sunflower (Helianthus annuus L.). Zeitschrift fur Pflanzenernährung und Bodenkunde 148, 654-669.
17. Lorenz, O.A., Tyler, K.B., 1976. Plant tissue analysis of vegetable crops. In: Reisenauer, H.M., (Ed.). Soil and Plant-Tissue Testing in California. University of California, Division of Agricultural Sciences, Bulletin 1879. pp. 24-29.
18. Mathers, A.C., Stewart, B.A., 1982. Sunflower nutrient uptake, growth, and yield as affected by nitrogen or manure, and plant population. Agronomy Journal 74, 911-915.
19. Merrill, S.D., Tanaka, D.L., Hanson, J.D., 2002. Root length growth of eight crop species in Haplustoll soils. Soil Science Society of America Journal 66, 913-923.
20. Narem, R.A., 1982. Nitrogen fertility requirements, dry matter production and nutrient uptake of sunflower. M.S. thesis. South Dakota State University, Brookings.
21. Robinson, R.G., 1973. Elemental composition and response to nitrogen of sunflower and corn. Agronomy Journal 65, 318-320.
22. Spear, S.N., Asher, C.J., Edwards, D.G., 1978. Response of cassava, sunflower, and maize to potassium concentration in solution I. Growth and plant potassium concentration. Field Crops Research 1, 347-361.
23. Stone, L.R., Goodrum, D.E., Schlegel, A.J., Jaafar, M.N., Khan, A.H., 2002. Water depletion depth of grain sorghum and sunflower in the central High Plains. Agronomy Journal 94, 936-943.
24. Vigil, M.F., Hergert, G.W., Mengel, D.B., 2009. Nutrient management. In: Meyer, R., (Ed.). High Plains Sunflower Production Handbook. Kansas State University Agricultural Experiment Station and Cooperative Extension Service MF-2384.