During seedbed preparation, any nutrient deficiencies should be corrected with fertilizer. Regardless of whether you use an organic or chemical (granular) fertilizer, the application should be based on a soil test report. Contact your local county or state Cooperative Extension office for information on soil testing.
With the exception of nitrogen, all the recommended nutrients should be applied during bed preparation. Only a portion (30 to 40 percent) of the crop's total nitrogen requirement should be present in the soil at planting because too much nitrogen applied prior to planting can result in delayed fruiting. For most vegetable crops, this portion is equivalent to 40 to 50 pounds of actual nitrogen per acre, enough to get your garden off to a good start. If a nitrogen deficiency is reported, consult your soil test report for a pre-plant nitrogen fertilizer recommendation.
Typically, additional nitrogen is supplied by one or more fertilizer applications over the life of the crop. Fertilizer should be applied to the soil surface around plants (topdressed) at critical stages of crop growth. For a few vegetable crops, Table 3 lists topdressing times based on stage of growth.
Although topdressing addresses the problem of overfertilization, it does have limitations, the most obvious being its incompatibility with plastic mulch. The plastic film acts as a barrier to the fertilizer. Second, the practice of topdressing is not calibrated to the nitrogen requirement (growth curve) of plants. Figure 2 illustrates the relationship of a crop's nitrogen requirement to its stage of development or growth curve. Early in the life of a crop, plants are small and devoid of fruit; their requirement for nitrogen is limited. As plants grow, their demand for and ability to use nitrogen increase, peaking during reproduction (fruiting).
Matching fertilizer application to a crop's growth curve is a far better way to fertilize and is best accomplished by using your drip irrigation system to deliver small, frequent doses of soluble fertilizer to the plant's root system, a process referred to as fertigation. Under plastic, drip fertigation permits precise nutrient placement with less waste.
The one disadvantage of drip fertigation is the need for multiple applications of fertilizer. Ideally, crops should be fertilized no less than once a week, a routine that has yielded excellent results at the Noble Foundation. Plan on designating one day of the week as "feeding" day.
A fertigation schedule for 16 popular vegetable crops is given in Table 4 and was developed by researchers at the University of Florida for the commercial vegetable industry. We have modified it for garden use.
Note this about the schedule: it was designed to be used as a guide only. The fertilizer rates have been developed for crops grown on plastic-mulched raised beds composed of sandy soils low in organic matter. If a soil test-based pre-plant fertilizer application has been made or if the soil test indicates sufficient levels of all plant nutrients, the first two scheduled nitrogen applications may be omitted. Ammonium nitrate (34-0-0) is the most readily available form of soluble nitrogen. In the absence of a pre-plant fertilizer application, use a soluble form of a complete fertilizer such as Miracle-Gro 18-18-21, Rapid-Gro or Peter's.
When using this schedule on soils amended with large quantities of organic materials, expect to lower the scheduled rate according to (1) nitrogen content of the material (compost is an excellent source of nitrogen, while peat moss is a poor one); (2) stage of decomposition (nitrogen is more readily available from a well-decomposed source of organic matter, such as compost, than from raw animal manure or lawn clippings); and (3) soil temperature (as soil temperature increases, the rate of decomposition increases, thereby increasing the availability of nitrogen).
In certain situations in which organic soil amendments are used, little if any additional nitrogen will be required. Normally, several trials are needed to fine-tune this schedule to a particular practice. Regular soil testing, in combination with careful observation and note taking, will help you control nutrient management.