Some fertilizer should be mixed into garden soil before you seed or transplant annual vegetables and flowers. When you buy fertilizer, look at the package label to find out how much nitrogen (N), phosphorus (P) and potassium (K) it contains. The percent of N-P-K is listed in this order on every fertilizer package. Ammonium phosphate fertilizer 16-20-0 contains 16% nitrogen, 20% phosphorus and 0% potassium.
A starter, or pre-plant fertilizer, should contain more phosphorus than nitrogen. Products that have up 2 or 3 times more phosphorus than nitrogen are good pre-plant fertilizers. The amount of potassium should be the same or less than the amount of nitrogen. This will supply enough phosphorus and potassium during the growing season for annual crops. If a soluble pre-plant fertilizer is used, some additional nitrogen will need to be applied during the growing season to maintain good plant growth.
A soluble chemical fertilizer with an N-P-K ratio of 1:2:1 would be a good pre-plant fertilizer. Examples of products with this ratio include fertilizers labeled 4-8-4 or 5-10-5 and 10-20-10. Don’t worry if a specific fertilizer does not have an exact ratio of 1:2:1. Any fertilizer will work well if the amount of phosphorus is about double the amount of nitrogen and the amount of potassium is about the same as nitrogen.
The nutrients in soluble chemical fertilizers are in a form that plant roots can immediately absorb. Apply the fertilizer just before planting at the rate given on the product label. Scatter fertilizer evenly over the area to be planted and mix it into the top 6-10 inches of soil. Organic fertilizers must break down or decompose to release the nutrients they contain before roots can absorb them. It’s best to apply organic fertilizers 2 to 4 weeks before planting so they can start to decompose. Mix the fertilizer into the soil 6-10 inches deep, then water thoroughly. If poultry manure is used, apply it at the rate of 20 pounds per 100 square feet of garden soil. If steer manure is used, apply it at the rate of 50 pounds per 100 square feet of garden soil. Manures should be mixed into the soil about a month before planting and the soil should be irrigated several times to leach out harmful salts.
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, August 2012