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Tomatoes 2008
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Fertilization Vegetable gardening has become so popular in recent years that about half the nation’s homeowners now grow some vegetables. In particular, New Mexico has experienced a tremendous upsurge in vegetable gardening. Success in gardening depends much upon soil fertility. If soil lacks fertility, gardeners must add plant nutrients in the form of fertilizer. This guide will help gardeners select and use fertilizers to obtain high yields of good quality vegetables. SOIL TESTS Having the soil tested is an excellent way to know the fertility level of soil and ho much fertilizer to add. In every New Mexico county, a county Extension agent can provide information about how to have soil tested. MAJOR PLANT NUTRIENTS The major plant nutrients are nitrogen (N), phosphorus (P), and potassium (K). New Mexico soils are commonly deficient in nitrogen and phosphorus, but potassium is usually adequate. Commercial fertilizer labels usually show, in percentages, how much actual nitrogen (N), phosphorus as available phosphoric acid (P2O5), and potassium as potassium oxide (K2O) they contain. Fertilizer recommendations are often given in terms of N, P, and K. To convert a recommendation for P into the P2O5 form, which is most common, multiple the P value by 2.29. Similarly, multiple the K value by 1.2 to obtain the recommended amount with K2O (although this is rarely needed in New Mexico). Thus, a 20-20-20 (N-P-K) recommendation would mean the fertilizer contained 20 units of nitrogen, 45.8 units of phosphorus, and 24 units of potassium. AMOUNTS FOR SMALL AREAS Fertilizer recommendations are often made in terms of pounds per acre. However, most gardeners plant areas much smaller than an acre. Table 1 shows the nitrogen and phosphorus requirements of some common garden vegetables for 1,000 ft 2. Table 2 shows the requirements for the same vegetables for 10 ft of row. For easy measuring, use the following equivalents, which are accurate with most granular fertilizers.
1 pint = 1 lb Table 1. Nutrient requirements of some common garden vegetables (lb per 1,000 sq ft) Table 2. Nutrient requirements of some common garden vegetables, given in oz per 10 ft row.* MATCHING NUTRIENT NEEDS AND FERTILIZERS Commercial fertilizer comes in many different formulations that supply different amount of nutrients. Therefore, calculations are necessary to determine how much product is required to supply needed nutrients. Table 3 shows some fertilizer formulations and the proportion of fertilizer needed for one unit of nitrogen (N) or phosphoric acid (P2O5). Use Table 3 to calculate how much fertilizer to supply a certain nutrient requirement (see examples below). Table 3. Nutrient content of selected commercially available fertilizers and proportion of each needed for one unit of nitrogen (N) or phosphoric acid (P2O5 ). Example 1: A gardener has planted chile. Table 1 lists the nitrogen requirement for chile as 1.8 lb per 1,000 sq ft. The gardener plans to use urea to fertilize. According to Table 3, 2.17 is the proportion needed for one unit of N. Therefore, multiple 1.8 lb per 1,000 sq ft by 2.17. 1.8 x 2.17 = 3.91 The gardener will need to apply 3.91 lb of urea per 1,000 sq ft for chile.
Example 2: For the same chile crop, the gardener needs to know how much P2O5 to apply. Table 1 recommends applying 1 lb of P2O5 per 1,000 sq ft. The gardener is going to use triple superphosphate (44% P2O5 ) to fertilize. According to Table 3, 2.27 is the proportion needed for one unit of P2O5 Therefore multiply 1 lb per 1,000 sq ft by 2.27.
Apply 2.27 lb of triple superphosphate per 1,000 sq ft for the chile. Too much nitrogen can burn plants. If you are applying a mixed fertilizer, apply amounts according to the nitrogen requirement to avoid applying too much nitrogen. For example, you have planted tomatoes. The N requirement (according to Table 1) is 3.7 lb per 1,000 sq ft. You have decided to use 18-12-6 fertilizer; according to Table 3, the proportion needed for one unit of N is 5.6. Multiply the fertilizer recommendation (3.7) by the proportion needed (5.7). 3.7 x 5.7 = 21.09 You will need to apply about 21 lbs of 18-12-6 fertilizer per 1,000 sq ft for the tomatoes. What if you had applied the fertilizer according to the P2O5 recommendation? According to Table 1, the P2O5 recommendation for tomatoes is 3.4 lb per 1,000 sq ft. For 18-12-6 fertilizer, the proportion needed for one unit of P2O5 is 8.3. Multiply the recommendation by the proportion needed: 3.4 x 8.3 = 28.22 lb per 1,000 sq ft. If you had figured the amount to apply by the P2O5 recommendation, you would have applied about 28 lbs of the fertilizer, which would have provided too much nitrogen and risked burning the tomatoes. For vegetable gardening, do not use fertilizers that have herbicides or pesticides mixed in them, as plants are easily burned when these are applied to the garden. Some gardeners may want to use organic fertilizers. Table 4 shows the nitrogen and phosphorus content of selected organic materials and the proportion of each material needed for one unit of nitrogen or one unit of phosphoric acid. Table 4. Nutrient content of selected organic materials and the proportion of each needed to supply one unit of nitrogen (N) or phosphoric acid (P2O5 ). WHEN TO APPLY FERTILIZERS Phosphorus becomes available to plants gradually, so it can be applied all at one time before planting. Nitrogen applications, however, should be split. Provide nitrogen in two or three applications, which allows plants to feed more evenly and decreases the possibility of salt burn. As a general rule, apply no more than 1 lb of nitrogen at one time for 1,000 sq ft of garden space. When the total nitrogen requirement is more than 1 lb, supply it in two or more applications. Apply about one-third of the required nitrogen before planting, apply one-third when plants reach four to five inches tall, and apply the remaining one-third about a month later. This system of split applications is also good for applying fertilizers. PLANTING VEGETABLES BY GROUP Home vegetable gardeners tend to have a potpourri of vegetables in a small area, so fertilizing different crops with differing amounts can be tedious. However, if the vegetables are grouped according to their fertilizer needs, then only two or three different fertilizer amounts have to be applied. Table 5 shows vegetable groups and the amount of fertilizer per 1,000 sq ft of any single crop or combination of crops within the group. Use this table to help you group vegetables according to their nutrient requirements. Table 5. Vegetables with similar nutrient requirements, grouped by pounds of nitrogen (N) and phosphorus (P2O5 ) needed per 1,000 sq ft. FERTILIZING SMALL GARDEN AREAS If the garden area is small, a compromise fertilizer application can be made. The application will not provide the correct amount of nutrients for each crop, but it should provide enough nutrients for adequate growth and development. For most gardens and most vegetables, 3 lb of nitrogen and 2 lb of phosphoric acid per 1,000 sq ft is suitable. Because applied phosphorus is released slowly in most New Mexico soils, the amount applied in the compromise formulation may seem high, but it should satisfy the nutrient need. Determining Amounts of Fertilizer for Small Areas A home gardener knows that a good fertilization program will result in optimum yields from vegetable crops. A complete fertilizer is one that includes nitrogen, phosphorus and potassium, and it should supply the plants’ nutrition requirements from planting to harvest. Unfortunately, the recommended rates of commercial fertilizers are often given on the per-acre basis. Using the following tables, you can easily convert per-acre rates into amount for small areas. Use the table to convert per-acre amounts of fertilizer into rates for 10 feet of row at different row spacings. When individual plants are established and the recommended fertilizer rate has been determined on the per-acre basis, use the table to find the amount of fertilizer needed per plant. The accuracy of the soil test is a reflection of the sample taken. Be sure the sample is representative of the area to be treated. Sample the soil from 10 random areas in the garden up to a depth of 6 inches. Avoid sampling unusual areas such as those near gravel roads, manure or compost spots, brush piles or under eaves. Place the samples in a plastic bucket or similar container and thoroughly mix the soil. Place about a pint of the mixed soil in the container provided by your local county agent, fill out the necessary form and send it to the indicated address. |
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Go to nmmastergardeners.org/ to find these and other articles in a pdf. format that you may use to print out a book with much of this material from that web site. Those articles, however, may not have been modified since they were originally printed in 2001.
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