Most gardeners know that vegetable crops need fertilizer to produce well, but sometimes the questions of what kind, when and how much can cause confusion.
The nutrients your leafy greens need are available in commercial fertilizers such as 5-10-10 or 10-10-10, and in organic fertilizers like bonemeal, bloodmeal and dehydrated manures. The numbers 5-10-10 or 10-10-10 refer to the percentages of nitrogen (N), phosphorus (P) and potassium (K) in the bag of fertilizer. They're always listed in that order, too: N-P-K.
Nitrogen is essential to all growing vegetation for healthy, dark green leaves. Phosphorus helps plants grow strong roots and potassium or potash conditions the whole plant, helping it to bear fruit and resist disease. A balanced diet is important for plants, but remember that you really want to encourage quick, steady leaf growth. Nitrogen is the key here. It gives salad crops their dark green color and encourages stems and leaves to grow.
Of course, plants need more than just the three major plant nutrients to grow normally. Secondary plant nutrients, such as magnesium, and some minor elements, such as zinc and iron, are also important, but usually needed only in small quantities. Most soils contain these minor elements, but mixing compost or other organic matter, such as composted manure, into the soil helps to insure their presence.
It's best to apply a complete chemical fertilizer shortly before planting time because you want the nitrogen it contains to be readily available to your young plants.
How to Apply
To apply fertilizer, spread 10 pounds of composted manure or two to three pounds of 5-10-10 or other complete fertilizer per 100 square feet. (If the fertilizer you use has a higher percentage of nitrogen - in other words, if the first number is greater than 5 - use correspondingly less fertilizer. For example, 10-10-10 contains twice as much nitrogen as 5-10-10, so you would use half as much.)
Toss the fertilizer over an area as evenly as possible. You don't want to get a lot of the commercial fertilizer or dried manure in one place. Always mix the fertilizer into the top two to three inches of soil before planting. Seeds are sensitive and can get burned by any fertilizer that touches them, so spreading it evenly and mixing it into the soil prevents any trouble.