Good soil fertility management is one key ingredient for successful production of melons in Arizona. Inadequate nutrients can lead to loss of plant vigor, susceptibility to pests and, of course, lower marketable yields. On the other hand, excessive applications of nutrients can lead to unnecessary production costs and, in the case of nitrogen, pollution of groundwater supplies. Experience, soil testing, plant tissue testing, and a knowledge of irrigation water quality are important tools for proper fertility management for melons.

Nitrogen (N) and phosphorus (P) are the nutrients that have the greatest influence on melon yields in Arizona. Without fertilization, Arizona soils can rarely supply sufficient N or P for high-yielding crops. However, most Arizona soils will supply adequate potassium (K) for melon yields. Under some conditions, K deficiencies may occur. Such conditions include melons after alfalfa, on very sandy soils, or early-planted fall melons. Deficiencies of other plant nutrients are very rare for melons in Arizona. A well fertilized cantaloupe crop yielding 20-30 tons/acre will take up about 90 lb N/acre, 25 lb P/acre, and 150 lb K/acre. A watermelon crop yielding 30-40 tons/acre will take up about 100 lb N/acre, 30 lb P/acre, and 160 lb K/acre.

Because adequate N fertility is critical for good melon yields, it is important to account for all sources of N available to the crop. These sources include nitrate (NO3-N) in the soil at the beginning of the season and NO3-N added in irrigation water. Soil NO3-N is measured in preplant soil samples. Nitrate added in irrigation water can be determined by the formula: ppm NO3-N in irrigation water * 2.7 = lbs. NO3-N/acre foot. For example, irrigation water containing 10 ppm NO3-N will supply 27 lbs N per acre foot of water applied.

Soil samples collected before planting can give important information for early-season fertility decisions. Soil samples should be collected randomly to a 1 foot depth throughout "uniform" areas. The areas should be relatively uniform with respect to soil texture and crop and soil management history. Within each uniform area, a minimum of 20 samples should be collected, then mixed together to form one sample. The samples can be analyzed for a wide range of values, but a recommended minimum set of measurements is: nitrate-N, available P and K, and pH, soluble salts and exchangeable sodium percentage. Refer to Table 1 for the recommended preplant fertilizer rates for melons.

Nitrogen is easily lost from soils because of nitrate leaching and volatilization of gases. Therefore, it is recommended that N be split into two or more applications during the season. These applications should be guided by feedback from leaf petiole samples collected at various growth stages, beginning at the 3-4 leaf stage. The leaf sampled should be the youngest mature leaf. After runners develop, this is the 4th or 5th leaf from the growing tip for cantaloupe, and the 5th or 6th leaf from the growing tip for watermelon. Collect petioles from a minimum of 15-20 average plants within uniform areas. Cool or dry the petioles immediately and submit them to a laboratory for analysis. Refer to Figures 1 and 2 and Tables 2 and 3 for interpretation of cantaloupe and watermelon petiole NO3-N analysis.

Any N fertilizer source is usually suitable for use on melons. However, if petiole NO3-N is in the "warning" or "deficient" ranges, apply nitrate or urea sources. Also, use care when applying ammonium-N sources at high rates or in sandy soils, because ammonia toxicity can result.

Figure 1 Interpretation of cantaloupe petiole NO3-N concentrations at different growth stages.

Figure 2 Interpretation of watermelon petiole NO3-N concentrations at different growth stages.

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Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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