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Field Crop Fertilizer Recommendations for Alaska Vegetables

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Most cool-season vegetables can be successfully grown outdoors in Alaska. Many will benefit from cultural practices designed to lengthen the growing season. These include transplanting greenhouse-grown seedlings rather than seeding directly in the field, growing plants on plastic mulches and using plastic row covers. Most may be successfully produced by direct seeding in bare soil; however, yields may be lower. The recommendations in this publication are general in nature and should apply regardless of the cultural practices used in vegetable production. Warm-season vegetables such as tomatoes and peppers may be reliably produced in much of Alaska only in a greenhouse. These crops are not included in this publication.

Relatively little fertility research has been conducted on vegetables in Alaska. Most previous Alaska research has been conducted on lettuce; the information provided for other vegetable crops is adapted from research and recommendations from other states.

Soil Acidity

The preferred soil pH ranges for vegetable crops are listed in Table 1. These ranges are for mineral soils only. Organic soils (peats and mucks) may be maintained at lower pH levels and are rarely limed. In mineral soils, the soil pH should be adjusted to approximately the middle of the recommended range for best performance. Various liming materials may be used to raise pH; elemental sulfur or aluminum sulfate may be used to reduce it.

Nitrogen source is not considered to be critical for most vegetables, as long as management is suitable for the chosen fertilizer source (for example, urea should be incorporated rather than applied to the soil surface). If organic sources of N are used, release rates must be rapid enough to provide adequate N for the growing plant.

Leguminous crops such as beans and peas can fix atmospheric N if they are inoculated with proper rhizobium bacteria. Inoculum should be applied to legume seeds prior to planting unless the seed has been pre-inoculated. Follow supplier’s recommendations for inoculum rate and handling procedures.

Table 1. Vegetable crop soil pH ranges.

Source: Western Fertilizer Handbook: Horticulture Edition by the Soil Improvement Committee, California Fertilizer Association

Table 2. Recommended nitrogen application rates for vegetables in Alaska

Phosphorus

Phosphorus (P) utilization is affected by soil conditions, including both the past fertilization history of the soil as reflected by the soil test P level and by soil minerology. Generally, soils testing high in P require a lower rate of fertilization than those with a low test level. Also, soils with high capacities for fixing or immobilizing phosphorus may require higher P application rates to overcome this fixing capacity. In non-alkaline soils, P immobilization increases as soil pH drops. Additionally, there are two types of soils in Alaska that have high P fixing capacity: the volcanic ash soils (including the Kachemak, Kashwitna, Naptowne, Rabideaux and Tustumena series) and the alkaline soils of Interior Alaska.

Phosphorus availability is also reduced in cold soils, which are usually found early in the growing season. Therefore, it is important to supply enough P to plants during this critical period. This may be achieved through application of starter fertilizer at the time of planting.

The recommended rates in Table 3 are based on the use of highly soluble P sources such as triple super phosphate, ordinary or single super phosphate, mono- or diammonium phosphates or similar materials. If materials with slowly available P, such as rock phosphates, or some other organic sources are used, application rates will generally have to be adjusted upward. The recommended application rates are for the soil test listed above each recommendation. All recommendations are in pounds of P2O5 per acre. For soil test values between those listed, interpolate from the values in Table 3. For example, a Kenai soil testing 20 ppm P (half-way between the very low and low categories) would require about 175 pounds of P2O5 per acre (half-way between 150 and 200).

Phosphorus fertilizers are usually applied at or before the time of planting. Phosphorus may be broadcast uniformly and incorporated into the soil or may be banded at planting. Fertilizer bands should not be in contact with the vegetable seed or seedling, but should be placed two inches below and two inches to the side of the seed or seedling. If fertilizer is banded, the rate of application may be adjusted slightly downward (decrease by no more than 25 percent).

Potassium

Both potassium chloride and potassium sulfate provide an adequate source of K for vegetables. Potassium chloride is generally less expensive, although potassium sulfate may be preferred for the fertilization of salt-sensitive crops or when salt-sensitive crops are included in a rotation (see Table 4).

Potassium may be applied prior to or at planting. Either band or broadcast applications of K should generally provide satisfactory results, although salt-sensitive crops may be adversely affected by banded fertilizers. High salt levels can burn root tissues, inhibit root growth and slow plant development. Follow the recommended K application rates in Table 5.

Secondary and Micronutrients

Deficiencies of secondary nutrients (calcium (Ca), magnesium (Mg) and sulfur (S)) are uncommon. They are most likely to occur on well-drained soils. Adequate S will almost always be provided if sulfate salts of potassium or nitrogen are used. Calcium and Mg levels usually will be adequate if the soil pH is maintained in the proper range. If secondary nutrients are found to be lacking, they may be broadcast or band applied before or at the time of planting according to Table 6. If the soil pH is too low, Ca can be supplied by applying calcitic lime, or Ca and Mg can be provided with application of dolomitic lime.

Micronutrient nutrition usually is not a problem in vegetable production. However, copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn) may become deficient if the soil pH is high (pH of 7.0 or above). In this case the best remedy is to lower the soil pH by addition of elemental sulfur. If required, the micronutrients may be added at the rates suggested in Table 7 with the following exceptions. Beets, cauliflower and celery have very high boron (B) requirements; add one pound/acre every year when growing these crops. If B deficiency is suspected, soil and plant analyses are recommended. Also, molybdenum (Mo) deficiencies are fairly common in cole crops (broccoli, cauliflower, cabbage) and may be indicated by elongation of the leaf base. Apply Mo according to Table 7. Please note that over-application of micronutrients can result in plant damage.

Table 3. Recommended phosphorus application rates for vegetables¹

¹ From Michaelson & Ping, 1989.

² Mehlich 3 extraction.

³ When soil phosphorus tests are at the very high level and above, it is generally recommended that a small amount of phosphorus (about 50 lb P2O5/a) be applied as a starter fertilizer to provide adequate nutrition in cool soils.

Table 4. Salt-sensitivity of vegetable crops

Source: Diagnosis and Improvement of Saline and Alkali Soils. 1954. USDA Agriculture Handbook Number 60.

Tissue Analysis

Problems suspected to be caused by lack of nutrients often can be confirmed by plant tissue analysis. Plants of various ages differ in nutrient content; different plant parts also contain varying levels of plant nutrients. Therefore, it is critical that the plant structure collected is one for which standard values are known. In small plants, the whole above-ground portion of the plant is usually sampled. In older plants, the most common method of sampling is to collect the youngest fully mature (grown to its full size) leaf or to take the petioles (leaf stems) associated with those leaves. For those plants requiring leaf sampling, the petiole is usually not included. Petioles are often used for sampling soluble nutrients (nitrate, phosphate and potassium) because this is the conducting tissue where nutrients travel from the stem to the leaf and may provide a more sensitive test for these nutrients than leaf analysis. The recommended plant part for sampling various vegetable crops is given in Table 8.

If a field contains both healthy and unhealthy plants, leaf samples can be collected from both the healthy and unhealthy plants, making sure that the same plant part is taken in both cases. The healthy plant can then be used as the standard value to compare the unhealthy plant against.

Plant tissue samples should be taken from plants representative of the sampling area. Dead or damaged plants, those with insect or disease problems, those at the end of rows or in edge rows or plants that differ significantly from those in the rest of the planting should not be sampled. Plants that have been recently sprayed with foliar fertilizers should be avoided. It is important that at least the recommended number of plants are sampled to ensure that a representative sample is obtained. If the recommended sample size is 25 mature leaves, all leaves should be taken from separate plants. In addition, the sampled plants should be randomly selected from a field, not concentrated in one area.

Try to sample clean leaves. Leaves should be washed only if they are to analyzed for iron or aluminum. Washing should be done quickly in a mild (2%) detergent solution if required. Fresh tissue samples must be dried rapidly at 150 to 175 degrees F until all water is removed. Drying at higher temperatures may destroy plant tissues; drying at lower temperatures will not stop biological activity. Tissue samples will dry best in open containers, cloth bags, or opened paper bags. Samples should be dried immediately following sampling. If this is not possible, samples may be refrigerated for short periods of time prior to drying.

Nutritional status of the sampled plants can be evaluated by comparison with appropriate nutrient sufficiency ranges (Tables 9 and 10). It will often be impossible to correct nutritional problems in the same growing season the samples were collected. However, nutritional information from tissue analyses should be used to adjust fertility practices in subsequent years.

Table 5. Recommended potassium application rates for vegetables

1 Mehlich 3 extraction.

Table 6. Recommended secondary nutrient application rates and sources for vegetables

1 Elemental S should never be banded. As a broadcast treatment, 1,000 lb of elemental S/a generally will reduce soil pH between 1 to 2 units over time.

Table 7. Recommended micronutrient application rates and sources for vegetables

References

Information in this document is derived from original research and from the following publications.

Carling, D.E., G.J. Michaelson, C.L. Ping and G.A. Mitchell. 1987. The Effects of Nitrogen Fertilization Rates on Head Lettuce Yields. Â鶹¹ÙÍø Fairbanks, Agricultural and Forestry Experiment Station Â鶹¹ÙÍø Progress Report 3.

Carling, D.E., G.J. Michaelson and C.L. Ping. 1988. The Effects of Nitrogen Fertilization Rates on Yields of Transplanted and Direct-Seeded Head Lettuce. Â鶹¹ÙÍø Fairbanks, Agricultural and Forestry Experiment Station Â鶹¹ÙÍø Progress Report 6.

Dow, A.I. 1980. Critical Nutrient Ranges in Northwest Crops. Western Regional Extension Publication No. 43.

Geraldson, C.M. and K.B. Tyler. 1990. "Plant Analysis as an Aid to Fertilizing Vegetables." In Soil Testing and Plant Analysis., R.L. Westerman, ed., Soil Science Society of America, Madison, WI.

Garrison, S.A. 1987. Commercial Vegetable Production Recommendations. Rutgers, The State University of New Jersey. Rutgers Cooperative Extension Service Publication 8001C.

Jones, J.B. Jr., B. Wolf and H.A. Mills. 1991. Plant Analysis Handbook. Athens, GA : Micro-Macro Publishing, Inc.

Kelling, K.A., P.E. Fixen, E.E. Schulte, E.A. Liegal and C.R. Simson. 1976. Soil Test Recommendations for Field, Vegetable and Fruit Crops. University of Wisconsin Cooperative Extension Service Publication A2809.

Michaelson, G.J. and C.L. Ping. 1989. Interpretation of the Phosphorus Soil Test for Alaska Agricultural Soils. Â鶹¹ÙÍø Fairbanks, Agricultural and Forestry Experiment Station Circular 66.

Sanchez, C.A., H.W. Burdine and V.L. Guzman. 1989. "Soil Testing and Plant Analysis as Guides for the Fertilization of Celery on Histosols." Soil and Crop Science Society of Florida Proc. 49:69–72.

Sanchez, C.A., G.H. Snyder and H.W. Burdine. 1991. "DRIS Evaluation of the Nutritional Status of Crisphead Lettuce." HortScience, 23:274–276.

Table 8. Recommended plant part and stage of growth for selected vegetables

1 YML: youngest mature (fully expanded) leaf. 2 Whole tops are the entire above-ground portion of plants.

Table 9. Nutrient sufficiency ranges for selected vegetables¹

¹ Standard nutrient levels are for plant parts and growth stages specified in Table 8.

Table 10. Sufficiency levels for nitrate, phosphate and potassium in petioles and leaf midribs of vegetables

¹ YML: youngest mature (fully expanded) leaf.

Western Fertilizer Handbook: Horticulture Edition. 1990. Soil Improvement Committee, California Fertilizer Association. Danville, IL: Interstate Publishers, Inc.

Turner, D.O., A.R. Halvorson and M.L. Jarmin. 1974. Fertilizing Guide: Cabbage, Broccoli, Cauliflower and Brussels Sprouts for Western Washington.
Washington State University Cooperative Extension Service Pub. FG-47.

Turner, D.O., W.C. Anderson, A.R. Halvorson and M.L. Jarmin. 1979. Fertilizing Guide: Carrots for Western Washington. Washington State University Cooperative Extension Service Publication FG-51.

United States Salinity Laboratory Staff. 1954. Diagnosis and Improvement of Saline and Alkali Soils. USDA Agriculture Handbook Number 60.

Walworth, J.L., D.E. Carling and G.J. Michaelson. 1992. "Nitrogen Sources and Rates for Direct-Seeded and Transplanted Head Lettuce." HortScience, 27: 228–230.

Mingchu Zhang, Professor, Agriculture and Forestry Experiment Station. Originally prepared by James Walworth, former Associate Professor of Soil Science, AFES

Revised April 2023