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Soil & Fertility


Soil

Deep sandy to loamy soil with a pH of 6.5 to 7.5 is ideal for early potatoes. Heavier soils while more productive, should be used for main season production. Most soils in Utah are suitable for potatoes provided they are well drained and fertile. Practice good crop rotations remembering that potatoes are related to eggplant, peppers and tomatoes and most insects and diseases are common to all solanaceous vegetables. Note: plant residue from these related crops serves as a host for plant diseases and insects that may infect or infest the next crop. Plant residues from the prior crop should be completely buried at the bottom of the furrow to facilitate decomposition.

When green-manure or cover crops are part of the production system, incorporate these early and allow sufficient breakdown time of crop residues so they do not tie-up nutrients or interfere with planting. Some residues (straw, corn stalks, grassy sod, or grain stubble) require additional nitrogen to enhance breakdown. Manures are also beneficial in providing extra nutrients and maintain organic matter. Prior to planting, be sure to incorporate green manures, cover crops, residues, composts and manures.

Potato fields should be well-tilled to create a smooth, firm seed bed for uniform planting and emergence. Several weeks before planting, prepare the field to create a loose, moist seedbed which ensures good soil-seed piece contact. Over-working the seedbed encourages soil crusting and compaction.

Fertility

Prior to planting, have the soil tested to determine nutrient needs and deficiencies (Table 8.3). Soil sampling approaches, forms, test details, and interpretation can be accessed through the Utah State University Analytical Laboratories (www.usual.usu.edu). Organic growers find it is a good idea to incorporate composted organic matter before planting to sustain soil fertility. An initial application of 5 tons per acre of high quality compost of known nutrient analysis may be helpful. This can be broadcast over the whole field or banded and incorporated into the individual rows.

A common practice is to add ¼ to ⅓ of the required nitrogen fertilizer and all the phosphorous and potassium prior to planting (See Table 8.3). In soils with high P and K levels, broadcast applications are acceptable, then work the fertilizer into the soil during normal field preparations. Banding is a good method to ensure the fertilizer is near the plant and makes sense where wide row spacings are common. Fertilizer bands should be 3 inches beside and 3 inches below the seed to minimize salt injury.

Nitrogen (N) – Nitrogen fertilizer is most efficiently used in split applications. Apply up to 50 lbs. N/acre prior to planting with an additional 100-150 lbs. N/acre applied in two or three applications. The first side dressing occurs when stolons start to form, the next around flowering, and the last during early bulking. Use the smaller amount if the site has added manure, compost, or when potatoes are grown after a legume crop (beans, alfalfa, etc.). Nitrogen is particularly suited for application by sprinkler or drip irrigation. In these systems, it is common to apply 20-30 lbs. N/A every 10-14 days. Nitrogen management can be greatly improved through tissue testing.

Table 8.3. Phosphorus (P2O5) and Potassium (K2O) based on soil test results.

Phosphorus Test Results Lb/acre Potassium Test Results Lb/acre
0-14 100-150 0-74 100-150
15-29 70-100 75-149 50-100
30-45 40-70 150-199 25-50
46-60+ 0-30 200+ 0-25

Use the higher amount when soil test values are in the lower part of the range