| Soils, Crops and Fertilizer Use |
|Chapter 10: Fertilizer guidelines for specific crops|
Most vegetables are very low in calories but have a high nutrient density in terms of vitamins and minerals. The dark-green leafy vegetables like kangkong, bok choy, amaranth, and collards are excellent sources of vitamin A (as carotene), Vitamin C, B vitamins, calcium, iron, and potassium. (However, amaranth, spinach, Swiss chard, and beet greens contain oxalates which bind up much of their iron and calcium; they can be partially deactivated by cooking). Dark-green leafy vegetables also provide significant protein.
The deep-yellow and orange vegetables like cantaloupe, carrots, and Hubbard squash are excellent sources of vitamin A, vitamin C, and potassium. For example, one large carrot contains twice the adult daily requirement of vitamin A. Aside from preventing vitamin A deficiency which leads to blindness and death from infections, carotene is now known to play an important role in preventing several type" of cancer.
The Asian Vegetable Research and Development Center in Taiwan is the international research center most involved with tropical and subtropical vegetable production. The AVRDC has developed a number of heat-tolerant varieties of cool-season vegetables like cauliflower and also work" on disease resistance and general production practices. ( Bee Appendix G for the address.)
General Fertilizer Needs of Vegetables
Since most Third World small farmers grow vegetables on small plots, this is an ideal situation for using organic fertilizers (see Chapter 8), and responses are excellent. However, in cases where organic fertilizers are in short supply, chemical fertilizer can be used if resources permit and will usually be very cost-effective on well--managed plots.
NPK Needs: The kind and amount of fertilizer needed varies a lot with the soil, the vegetable, and other key factors covered in Chapter 9. Table 10-4 gives a range of NPK rates from a number of research and extension sources worldwide.
Susceptibility of Vegetables to Secondary Nutrient Deficiencies
Cabbage, eggplant, pepper, tomato, cucumber, watermelon
Asparagus, onions, and the Crucifer family (cabbage, collard, broccoli, turnips, bok choy, kale, cauliflower)
Response of Vegetables to Micronutrients When Soil Levels are Low.
CHEMICAL FERTILIZER APPLICATION GUIDELINES FOR VEGETABLES
The band method of application is very suitable for direct-seeded vegetables like turnips, radish, mustard, bok choy, leaf lettuce, spinach, Chinese cabbage, and okra. The halfcircle method works well with cucurbits (cucumber, squash, etc.) and transplants. Apply all the P and K along with 1/3-1/2 of the N at planting; sidedress the remaining N in one or more applications, depending on time to maturity and harvest method. For example, leafy greens, like leaf lettuce, spinach, Swiss chard, and bok choy, can be harvested either all at once or picked a few leaves at a time over a month or two (new leaves keep emerging from the center). In the latter case, 2 or 3 sidedressings can be made at 3-week intervals.
Tomatoes, peppers, eggplant, cabbage, collard, broccoli, cauliflower, head lettuce, and onions usually do better if first started out in a nursery seedbed, seedbox, or small containers and then transplanted to the field 3-6 weeks later.
Nursery Seedbed or Seedbox: In most cases, manure or compost will supply enough nutrients for the nursery stage. (See Chapter 4 for how to prepare a nursery seedbox mix.) However, there are 3 cases where chemical fertilizer might be needed:
• If the soil has been heat sterilized before planting, there may not be enough beneficial bacteria left to convert the organic N in the manure or compost into available N. However, fresh manure has a good amount of available N.
• If the manure or compost is of poor quality due to poor storage and exposure.
• If plants become N deficient while in the nursery. Watering is often high enough to cause lots of leaching.
If NPK fertilizer is needed, broadcast the equivalent of 60-80 grams/sq. meter (600-800 kg/ha) of 10-20-10 or 10-30-10 and mix it thoroughly into the top 10-15 cm of soil. Don't exceed 60-80 kg/ha of N or plants may become overly succulent and more prone to damping-off fungus disease. Fertilizers with a 1:2:1 or 1:3:1 ratio work best since they allow you to apply sufficient broadcast P without exceeding N or K rates.
NOTE: Disregard the amount of NPK applied in the nursery when calculating NPK rates needed' fro. transplanting onward.
N Deficiency in the Nursery Seedbed: If the plants turn yellow from N deficiency, dissolve a straight N fertilizer in water and apply it over the bed at the rate of 30 kg/ha of N which equals:
15 grams ammonium sulfate (21-0-0) per sq. meter
10 grams ammonium nitrate (33-0-0) per sq. meter
7-8 grams urea (45-0-0) per sq. meter.
Water plants with plain water afterwards to wash off any fertilizer solution from the leaves. If plants are being "hardened" in preparation for field setting (usually done the last 7-10 days before setting), N fertilizer will be counterproductive.
Using a Starter Solution for Transplants: Pouring a liquid starter fertilizer solution around the base of the plants after setting them will help get things off to a good start. Manure tea (see Chapter 8) or chemical fertilizer can be used for this. If using chemical fertilizer, here are the guidelines:
• Since P is the nutrient most involved in stimulating root regeneration and development, choose a formula with a good ratio of P in it such as 12-24-12, 18-46-0, or 10-30-10. Some N is helpful too, since it helps in the uptake of N.
• Except for a few like 18-46-0, most granular NPK fertilizers dissolve poorly in water. Grinding or mashing is helpful and will improve solubility.
• Dosage: Mix up 8-15 cc of fertilizer per liter of water and apply about 1 cup (240 cc) around the base of each transplant after setting.
• The starter solution only supplies enough nutrients for the first week or so of growth; additional organic or chemical fertilizer will be needed.
• NOTE: As in the case of fertilizer applied to a nursery seedbed, this starter fertilizer application is not counted when calculating overall NPK totals.
Applying Solid Fertilizer at Transplant Time: Use an NPK fertilizer that supplies 1/3-1/2 of the total N and all the P needed. If leaching is likely to be high, only about 1/3-1/2 of the K should be applied. The half-circle method works very well for transplants and should be made about 7.5-10 co (about 4 fingers-width) out from the stem and 5-10 cm deep.
How to Sidedress Nitrogen: Review the sidedressing guidelines in Chapter 9 before proceeding. Here are some more specific suggestions for vegetables:
• Long-duration crops like indeterminate tomatoes, eggplant, and peppers may require 3-4 or more sidedressings at 3-4 week intervals.
• Medium duration crops like broccoli and cauliflower will normally need 1-2 at 3-4 week intervals.
• Apply about 30 kg/ha of actual N per sidedressing as a rough figure. It's more accurate to subtract the at-transplanting dosage from the total N and then divide the result by the number of sidedressings needed.
• Apply the N in a band or half circle about 20 cm out from the plant; cover it lightly with soil. (This can be done by weeding with a hoe following the application.)