When should I fertilize my crops?


I am an avid backyard gardener and am familiar with the big three nutrients, Nitrogen, Potassium and Phosphorous, the differences between full, partial shade, etc. I also know the importance soil testing and proper pH. What I don’t know is when to fertilize? I generally buy Neptune’s Harvest and Espoma products as well as use my own compost and a neighbor’s pig manure.

Is there a schedule of some sort to make it easier to plan general fertilizing? For instance, we have some established blueberry bushes and some new ones coming in this weekend. After planting, when do I fertilize those during the season? Tomatoes? Lettuce? Perennial flower bulbs? Apple trees? Is there an easy reference guide or is it a matter of soil testing and adjusting? 


Abi Griffith, Horticulture Community Education Assistant

Great question!  You are definitely ahead of the game by getting your soil tested and making adjustments. That is really going to usually be the answer to most of these questions, and if you are aware of what is going on in your soil and have been consistently amending according to the results, you should be in great shape overall!  This fact sheet – Garden Soil Management from the Soil Lab, reiterates this and cautions against, just “always adding more!” There isn’t an easy one all in one schedule, but you could sort of create your own using a garden planner, etc. I’m going to walk through some of the specific things you refer to.

Blueberry bushes: This fact sheet Growing Highbush Blueberries has a fertilization chart, depending on the age of your bushes. It suggests fertilizing 3 to 4 weeks after planting and has suggestions for more established bushes

Apple trees: Would want to base off of a soil test and doesn’t generally need a lot of fertilizer, but it would usually be applied in early spring if the test indicates – look at Fruit Trees – Fertilization for a more nuanced take on this. In general, you want to avoid fertilizing apples and other woody plants in the late summer/early fall because that can cause a flush of tender growth that will be more susceptible to winter damage.

Tomatoes: Again, if you have been testing your soil and making adjustments, that is a great start, but since tomatoes are a heavy feeders, they generally benefit from at least one application of fertilizer or more during the growing season (e.g. when fruits are small and just setting and perhaps again at harvest). Avoid excessive N (nitrogen) application which can cause excessive foliage.

Lettuce: should be fine if your soil is good

Perennial flower bulbs: this is a good reference –  Bulbs and More – University of Illinois and they suggest “If bulbs are going to be maintained in a planting bed more than one year, it is important to supply additional fertilizer. Spring flowering bulbs should have mixed into the soil in the fall five tablespoons of 10-10-10 soluble fertilizer (or equivalent bulb fertilizer) plus two cups of bonemeal per ten square foot area. As soon as the shoots break through the ground in the spring, repeat the above soluble fertilizer application. Do not fertilize spring flowering bulbs after they have started flowering. This tends to encourage the development of bulb rot and sometimes shortens the life of the flowers.”

Since you mentioned that you use your neighbors pig manure, I just wanted to refer you to this as a reference – Guidelines for Using Manure on Vegetable Gardens as it suggests it not be used in vegetable gardens.