C R O P S E R I E S

SOIL
Fertilizing Sunflowers no. 0.543
1
by J.J. Mortvedt, D.L. Johnson and R.L. Croissant

Adequate soil fertility is one of the requirements for profitable sunflower

production. Nitrogen (N) is the most yield-limiting nutrient, unless there are high
residual NO3-N levels in the soil. Phosphorus (P) is the next most limiting
Quick Facts... nutrient. Levels of available potassium (K), sulfur (S), and micronutrients
generally are sufficient for sunflower production in Colorado soils.
Nitrogen normally is the most
limiting nutrient for sunflower Soil Tests
production. The value of a soil test to predict nutrient availability during the growing
season directly relates to how well the sample collected represents the area
Apply nitrogen fertilizers at rates sampled. Take surface samples to a 1-foot depth. To improve the accuracy of N
based on expected crop yields recommendations, take subsoil samples to a depth of 2 to 3 feet to determine
minus credits for residual soil available NO3-N. If high residual NO3-N levels are suspected, take soil samples
to 4 feet because sunflower roots will remove NO3-N from this depth. A good
nitrates and nitrogen
sample is a composite of 15 to 20 soil cores taken from an area uniform in soil
mineralized from soil organic
type. Sample separately areas with major differences in soil properties or
matter, manure and previous management practices.
legume crops. Thoroughly air dry all soil samples within 12 hours after sampling by
spreading the soil on any clean surface where the soil will not be contaminated.
Apply phosphate fertilizers at Do not oven-dry the soil because this can change soil test results. Place the air-
rates based on soil test results. dried soil in a clean sample container for shipment to the soil test laboratory.
Submit a carefully completed information form with the soil sample.
Most Colorado soils contain This form provides information so fertilizer suggestions can be tailored to your
sufficient available potassium, specific situation. Take soil samples for NO3-N analysis every year for optimum
sulfur, and micronutrients for N fertilization of crops. Soil analyses for availability of the other nutrients, pH,
sunflower production. and organic matter content every three to four years may be sufficient.
More detailed explanations of the importance of taking proper soil
samples are found in fact sheets 0.500, Soil Sampling, 0.501, Soil Testing, and
0.502, Soil Test Explanation. These fact sheets are available at your Colorado
State University Cooperative Extension county office or from the Cooperative
Extension Resource Center, 115 General Services Building, Fort Collins, CO
80523; (970) 491-6198.
The Colorado State University Soil, Water, and Plant Testing Laboratory
is located at Room A319, Natural and Environmental Sciences Building,
Colorado State University, Fort Collins, CO 80523; (970) 491-5061.

Nitrogen Suggestions
Base nitrogen rates for sunflowers on the expected yields for each field.
Nearly all sunflower crops require some N fertilizer, unless there is a substantial
N carryover. As a rule, a 1,500 pounds per acre sunflower yield will require the
 Colorado State University
Cooperative Extension. 3/96. same amount of N fertility as a 30 bushels per acre wheat crop (about 75 pounds
www.colostate.edu/Depts/CoopExt of nitrogen per acre).
Table 1: Suggested nitrogen rates for dryland Give credit for the level of residual NO3-N in the
sunflowers (expected yield, 1,500 lb/A). root zone. Other credits for N include the N expected to
ppm NO3-N Soil organic matter content, % become available during the season from mineralization
in soil* 0 - 1.0 1.1 - 2.0 >2.0 of soil organic matter, manure, and previous legume
----Fertilizer rate, lb N/A---- crop residues, as well as NO3-N in irrigation water.
0-6 75 55 35 Subtract these credits from the total crop needs to
7 - 12 55 35 15
determine the suggested N fertilizer rate. When a soil
13 - 18 35 15 0
19 - 24 15 0 0 test for organic matter is not available, assume a level of
25 - 30 0 0 0 1.5 percent organic matter for eastern Colorado soils.
*Concentration of NO3-N in the surface 0 - 1 foot soil depth. Dryland Sunflowers
NOTE: Adjust N rate by 6 lb/A for each cwt difference in
expected yield. Suggested N rates are lower for dryland
sunflowers because of lower plant populations and less
Table 2: Suggested nitrogen rates for irrigated than optimum soil moisture conditions. Table 1 suggests
sunflowers (expected yield, 2,400 lb/A). N rates at an expected yield of 1,500 pounds per acre.
ppm NO3-N Soil organic matter content, % Fertilizer rates decrease with increasing levels of NO3-N
in soil* 0 - 1.0 1.1 - 2.0 >2.0 in the top foot of soil and increasing soil organic matter
----Fertilizer rate, lb N/A---- content. Suggested N rates in this table do not account
0-6 130 110 100 for other N credits. Subtract these credits from the N
7 - 12 110 95 85 rates in Table 1 to determine the N rate for the field.
13 - 18 95 80 70
19 - 24 80 60 50 Irrigated Sunflowers
25 - 30 60 45 35 Table 2 suggests N rates for irrigated sunflowers
*Concentration of NO3-N in the surface 0 - 1 foot soil depth. at an expected yield of 2,400 pounds per acre. Fertilizer
NOTE: Adjust N rate by 6 lb/A for each cwt difference in
N rates decrease with increasing levels of NO3-N in the
expected yield.
top foot of soil and increasing soil organic matter
content. Suggested N rates in this table do not account
Table 3: Suggested phosphorus rates for dryland for other N credits. Subtract these credits from the N
and irrigated sunflowers. rates in Table 2 to determine the N rate for the field.
ppm P in soil Relative Fertilizer rate,
AB-DTPA NaHCO3 level lb P2O5/A Methods and Timing of N Applications
0-3 0-6 low 40 Apply nitrogen fertilizer preplant or sidedressed.
4-7 7 - 14 medium 20 Some N may be applied in combination with starter
>7 > 14 high 0
fertilizers, but the rate should be less than 10 pounds of
NOTE: Suggested P rates are for band application. N per acre. Most efficient use can be obtained by
applying N just ahead of planting. However, apply all of
Table 4: Suggested potassium rates for dryland and
irrigated sunflowers. the fertilizer before heading (bud stage) to maximize
yields and N use efficiency. All sources of N fertilizers
ppm K in soil Relative Fertilizer rate, lb K2O/A
AB-DTPA or NH4OAc level Dryland Irrigated are equally effective per unit of N if properly applied.
0 - 60 low 30 50
Base choice of N fertilizer on availability, equipment
61 - 120 medium 0 30 available and cost per unit of N.
> 120 high 0 0 Sunflower roots grow quickly into the soil
between the rows. Sidedress N fertilizers early in the
growing season to avoid root pruning.
For irrigated sunflowers, it is convenient to apply N fertilizers with high-
efficiency irrigation systems. This method allows for split applications that
improve N use efficiency.
Use in-season soil or plant analysis to determine the nutrient status of the
growing crop. If the N status of the crop is low or growing conditions appear to
be above average, apply additional N with the next irrigation.

Phosphorus Suggestions
Crop responses to applied P are most likely on soils with low or medium
levels of extractable P. The main tests for extractable P in Colorado soils are the
AB-DTPA and sodium bicarbonate (NaHCO3) tests. Suggested P fertilizer rates
(Table 3) are for banded applications related to soil test levels.
 Placement of P fertilizers in the root zone is important because P is not
mobile in soil. Band application at planting (starter fertilizer) is the most efficient
placement method for P, and suggested rates for broadcast application are about
double those for band application.
Incorporate broadcast applications of P fertilizers into the soil prior to
planting. Sunflower seed is sensitive to salts in fertilizers. Use popup placement
(directly with the seed) with caution because seedling emergence may decrease in
dry soil, especially at rates supplying more than 10 pounds of nitrogen per acre.
Monoammonium phosphate (MAP, 11-52-0), diammonium phosphate (DAP, 18-
46-0), and ammonium polyphosphate (10-34-0) are equally effective per unit of
P if properly applied. Base your choice of product on availability, equipment
available, and cost per unit of P.

Potassium Suggestions
Most Colorado soils are relatively high in extractable K, and few
crop responses to K fertilizers have been reported. Suggested K rates related to
soil test values (AB-DTPA or NH4OAc) are given in Table 4. The main K
fertilizer is KCl (potash). Broadcast application incorporated into the soil prior to
planting is the usual method.

Other Nutrients
Most Colorado soils contain adequate levels of available S, and soil tests
for available S are not routinely performed. Irrigation water from most surface
waters and some wells often contains appreciable SO4-S, so irrigated soils
usually are adequately supplied with S. Some deep well waters are low in SO4-S,
so analyze water samples if soils are low in organic matter content and you
suspect S deficiency.
There have been no confirmed deficiencies of boron (B), copper (Cu),
iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn) in sunflowers in
Colorado.

1
J.J. Mortvedt, Colorado State University
Cooperative Extension soils specialist; D.L. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in
Johnson, Cooperative Extension agronomist cooperation with the U.S. Department of Agriculture, Milan A. Rewerts, Director of Cooperative
and associate professor; and R.L. Croissant, Extension, Colorado State University, Fort Collins, Colorado. Cooperative Extension programs are
Cooperative Extension crops specialist and available to all without discrimination. No endorsement of products mentioned is intended nor is
professor (retired); soil and crop sciences. criticism implied of products not mentioned.