California Crop Fertilization Guidelines

In young safflower or with mild deficiencies, N-deficient plants are uniformly light green to pale yellow. In more severe deficiencies 'firing' occurs, as the older leaves become darker yellow to orange, starting from the leaf tips, and finally turn brown and die ^[N14]. Nitrogen-deficient safflower has fewer heads per plant ^[N6].

Excess N on dry-farmed safflower can decrease yields by encouraging excessive vegetative growth at the expense of flower and seed formation ^[N11].

Samples should be taken close to planting. Safflower is one of the deepest-rooted annual crops. For deep, well-drained soils, water and nutrient uptake from depths of 9 feet and below has been measured ^[N1,N8]. In a study on a deep clay loam soil near Five Points, pre-irrigated safflower took up less than 20% of its N from the top foot of soil ^[N1]. Sampling to three feet or below will therefore improve the accuracy of a soil nitrate test. 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.

Pounds of potentially available nitrate-N per acre in each foot of soil can be estimated by multiplying parts per million (ppm) 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, 30-40 and 15-20 lbs nitrate-N/acre are present in the top and second foot of the profile, respectively. See Preplant N for for information on adjusting application rates.

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.

To analyze safflower in the pre-bloom stage, only the middle section of the stem is sampled. At the first bud stage, whole recently matured mid-stem leaves are selected. For a representative sample, about 40 stems or leaves should be sampled. Samples should be placed in a clean paper bag and dried at a temperature of 140-160 °F ^[N2,N15].

Non-uniform fields are best divided into uniform areas which are sampled separately. Atypical areas in a field should be avoided (if small) or sampled separately. For more information on sampling procedure see Plant Tissue Sampling.

Optimal nutrient range at pre-bloom and first bud stages of safflower ^[N2].

These thresholds may not be consistent across varieties and production conditions. Therefore, growers should combine tissue analyses with soil analysis, N budgeting and observations of crop performance.

Nitrogen application rates can be based on expected yield (see table). The values in the table are based on a study with irrigated safflower in Yolo County, where on average 20-30 lbs N were removed for every 1000 lbs of seed, with an additional 20-30 lbs N taken up in the residues (Cavero, unpublished data).

N requirements of safflower, based on yield expectation.

These values are in line with the common rule of thumb that safflower requires 50 lbs of N for every 1000 lbs expected yield ^[N17].

Safflower is a deep-rooted crop that is efficient at taking up soil residual N throughout the soil profile ^[N1]. The N application rate should therefore be adjusted for soil and irrigation water nitrate (see Soil Nitrate Test). This is especially important if safflower is grown as a dryland crop after alfalfa or a heavily fertilized vegetable crop like tomatoes, to avoid yield reductions from excess N ^[N11,N20]. In addition, even best management practices result in some nutrient movement below the root zone of most crops, where it may be impractical to take soil samples. If growers suspect that their fields have accumulated nitrate below their sampling depth, then fertilizer N application should be carried out at the lower end of suggested ranges (see table).

Manured or high organic matter soils and residues from leguminous crops like alfalfa or beans may also release significant amounts of N during the season. More information calculating N credits from these sources can be found here.

Rate trials in the Sacramento Valley in the 1960s found that yield was normally maximized with applications of around 100-150 lbs N/acre where moisture was not limiting and expected yield ranged between 2000-3000 lbs/acre. For non-irrigated safflower in dry or shallow soils, around 20-60 lbs N/acre was more appropriate ^[N11,N20].

Safflower sometimes has a lower yield potential after rice, even if other nutrients are sufficient ^[N11]. Trials in Glenn and Colusa counties in the 1960s found that when safflower followed rice, yield response plateaued at 60 lbs N/acre.

Safflower only takes up N which is located in the moist root zone. Therefore, N fertilizer should be placed at least 4 inches deep in the soil, either by incorporation or deep injection, as aqua ammonia. Broadcast N fertilizer needs to be disked in and washed into the soil with at least 1 inch of rain, or with sufficient irrigation water ^[N20].

Many types of N fertilizer may be used. Aqua ammonia is a common choice for injection and urea for broadcast applications ^[N20]. When used in alkaline soils, urea fertilizers can generate ammonia, which is toxic to germinating safflower seeds and seedlings. Therefore, broadcast applications of urea>50 lbs N/acre should be avoided ^[N6].

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

Even though safflower is efficient at recovering soil residual nitrate, N fertilizer should be applied in spring at or shortly before planting, rather than the previous fall ^[N6]. Fall-applied N is at risk of being lost over winter to the atmosphere through denitrification on heavy soils or leached below the seedling root zone on lighter soils ^[N13,N20].

Nitrate is very mobile in the soil and is easily leached. Therefore, nitrate-based fertilizers should be applied after any pre-irrigation, especially if surface irrigation is used.

Sidedress N may be knifed in or broadcast. If ammonium fertilizer or urea is broadcast, it should be moved into the soil with irrigation water to avoid ammonia volatilization losses and to ensure root access ^[N20]. If a drip system is used, in-season N may be fertigated.

Most of the roots and N uptake in established safflower may be below the top foot of soil ^[N1,N16]. Therefore urea- or nitrate-based fertilizers, which move more readily into deeper soil depths with irrigation water, may be more effective than ammonium-based fertilizers. Unlike during germination and early growth, urea doesn't appear to negatively affect established safflower ^[N3,N6].

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

If N is sidedressed, it should be done at or before stem elongation ^[N19]. A greenhouse study in Australia found that safflower yield was most strongly affected by the N supply between the time of stem elongation and the first visible buds ^[N19]. Nitrogen applications after bud visibility had less effect on yield, and tended to decrease seed oil content. Little to no N uptake occurs between full bloom and maturity ^{[N3,N5,N10,N12]}.

Samples for P and K should be taken in late fall or early spring, when no fertilizer has recently been applied ^[P5]. In deep, well-drained soil safflower's active rooting depth can be 9 feet or deeper ^[P2,P7]. Deep sampling (to at least three feet) may therefore be useful in fields where safflower is being grown for the first time ^[P15]. However, since P and K tend to concentrate at the surface and are not very mobile in the soil, the top foot is likely sufficient for routine sampling (Konrad Mathesius, personal communication).

A total of 20-30 cores should be taken from random locations within the field and mixed well. If P has been previously banded in the field, twice as many cores should be taken to account for the additional variability ^[P5]. See Soil Test Sampling for 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.

In soils with an Olsen-P value of 5 ppm or less, a response to P fertilization may be expected ^[P17].

In soils coming out of rice, P is less available to subsequent crop. Safflower responses to P have been observed in rice soils with Olsen P values as high as 11 ppm ^[P9,P12]. If soils have been under rice for several years, in general it can be assumed that safflower will respond to starter P ^[P12].

Because safflower plants can also access nutrients below the soil layer tested, soil test values do not always match the nutrient availability of a site. To make accurate P fertilization decisions, soil test values are best combined with tissue P concentrations and P budgeting (see Preplant Application Rate).

To analyze safflower in the pre-bloom stage, only the middle section of the stem is sampled. At the first bud stage, whole recently matured mid-stem leaves are selected. For a representative sample, about 40 stems or leaves should be sampled. Samples should be placed in a clean paper bag and dried at a temperature of 140-160 °F ^[P3,P10].

Non-uniform fields are best divided into uniform areas which are sampled separately. Atypical areas in a field should be avoided (if small) or sampled separately. For more information on sampling procedure see Plant Tissue Sampling.

Optimal nutrient range at pre-bloom and first bud stages of safflower ^[P3].

The range in the table is slightly below the critical threshold of 0.6% P for young mature leaves identified in a greenhouse study by Abbadi and Gerendás ^[P1].

When soil test P is adequate, fertilizer application can be based on the P removed with the harvest to maintain P availability at an optimal level in the long term. However, the economically optimal P application rate may be lower. Safflower removes about 12 lbs P₂O₅ for every 1,000 lbs of seed ^[P3,P14].

Safflower P deficiency is normally corrected by banding 40-60 lbs P₂O₅/acre (17-26 lbs P/acre) at planting ^[P9]. If P is broadcasted, rates should be doubled ^[P18].

Phosphorus fertilizer should be placed near the seed ^[P9]. Broadcast applications are much less efficient, especially for low P rates ^[P18].

On soils coming out of rice even very high rates of broadcast and incorporated P fertilizer do not correct deficiencies ^[P12].

Starter P can be drilled below or with the seed ^[P16]. Trials after rice in Glenn and Colusa counties found that banding 100 lbs/acre ammonium phosphate (11-48-0 or 10-50-0) 1 inch below the seed was safe and effective ^[P12]. If starter P is drilled with the seed, a low-N, high-P fertilizer should be used to prevent seed injury ^[P16]. If more than 15-20 lbs N and/or K₂O will be included, the starter fertilizer should be banded at least two inches away from the seed ^[P4].

The presence of ammonium in a starter fertilizer can increase P uptake efficiency ^[P6]. Trials in Glenn and Colusa Counties on soils that had been under rice for 15 years found that banding ammonium phosphates resulted in more P uptake and higher yields than much larger applications of triple superphosphate, although sufficient N had already been injected as aqua ammonia ^[P12].

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

Generally it's recommended to apply all P at planting ^[P9]. In soils with adequate available P, phosphorus fertilizer does not have to be applied every year.

Samples for P and K should be taken in late fall or early spring, when no fertilizer has recently been applied ^[K5]. In deep, well-drained soil safflower's active rooting depth can be 9 feet or deeper ^[K2,K7]. Deep sampling (to at least three feet) may therefore be useful in fields where safflower is being grown for the first time ^[K16]. However, since P and K tend to concentrate at the surface and are not very mobile in the soil, the top foot is likely sufficient for routine sampling (Konrad Mathesius, personal communication).

A total of 20-30 cores should be taken from random locations within the field and mixed well. If P has been previously banded in the field, twice as many cores should be taken to account for the additional variability ^[K5]. See Soil Test Sampling for sampling instructions.

Safflower grown in soils with an extractable K level>100 ppm is not likely to respond to K fertilizer ^[K17]. For soils that fix K, the extraction with ammonium acetate is not a reliable measure for available K ^[K14] (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 ^[K14].

To analyze safflower in the pre-bloom stage, only the middle section of the stem is sampled. At the first bud stage, whole recently matured mid-stem leaves are selected. For a representative sample, about 40 stems or leaves should be sampled. Samples should be placed in a clean paper bag and dried at a temperature of 140-160 °F ^[K3,K12].

Non-uniform fields are best divided into uniform areas which are sampled separately. Atypical areas in a field should be avoided (if small) or sampled separately. For more information on sampling procedure see Plant Tissue Sampling.

Optimal nutrient range at pre-bloom and first bud stages of safflower ^[K3].

Safflower is not likely to respond to K fertilization on soils where K is adequate ^[K1]. However, in order to maintain adequate soil K availability over the long term, applications to non-deficient soils can be made based on the K expected to be removed with the harvest. About 80 lbs of K₂O are removed with one ton of harvested seed ^[K4,K15]. However, the economically optimal K application rate may be lower. Safflower is efficient at using soil K even when soil K availability is very low ^[K7]. An application rate of 70-90 lbs K₂O/acre has been found to be sufficient to correct K deficiencies observed in California ^[K11].

Most K should be broadcast and incorporated prior to planting. A small amount may also be applied as starter. See Starter K.

Many types of N fertilizer may be used. Fact sheets of the most common fertilizers can be found on the website of the International Plant Nutrition Institute.