Do I need to continue fertilizing my tomato plants after they’re out of the pots and in the ground?
We have a whole webpage dedicated to growing tomatoes. Check it out: https://www.skagitmg.org/home/tomato-info/. Here is an excerpt from the WSU Fact Sheet FS145E “Growing Tomatoes in Home Gardens” that can be found there.
Performing a soil test is the best method for determining soil pH, as well as a plant’s potential nutrient needs and nutrient overloads during the growing season. Tomatoes grow best in slightly acidic soils, those with a pH between 6.0 and 6.5. West of the Cascades, soil pH is often natu-rally in this range or lower. Where pH is below 6.0, add the manufacturer’s recommended quantity of lime (calcium carbonate) to adjust the pH to the ideal range. East of the Cascades, soil pH is often above the ideal range; pH can be lowered using the manufacturer’s recommended amount of elemental sulfur. Typically, tomatoes require 1.5 to 2 ounces of nitrogen per 10 linear feet of planting bed and roughly equal amounts of phosphorus and potassium. Prior to planting, submit garden soil samples to a soil-testing laboratory for nutrient-level testing. (See Further Reading section for information on laboratories serving the Pacific Northwest.) Based on test results, chose a fertilizer that meets the nitrogen, phosphorus, and potassium requirements of your site in order to give your tomato plants the best possible care. For more information on how to calculate the amount of fertilizer to add to your tomato-growing area, see the publication Soil Fertility in Organic Systems: A Guide for Gardeners and Small Acreage Farmers (Collins et al. 2013) cited in the Further Reading section. Mixing fertilizer and other soil amendments into the soil immediately before planting will help get your tomatoes off to a good start. Early plant needs can be met by tilling in the recommended quantity of lime or sulfur and one-half the annual nitrogen requirement for your planting area.
As the tomato plant grows, resist providing too much nitrogen. Excess nitrogen stimulates leaf growth, decreasing fruit production. Use a slow-release fertilizer in arid regions, where frequent watering is necessary.
Judy says she fertilizers her tomatoes once a month with 10-20-20 Miracle Grow.