You are going to your local garden shop to buy fertilizer, and you may see many bags of fertilizer with a bunch of numbers on them. 10-10-10, 10-30-20, 20-20-20. What do those mean, and which one do you want?
Bags of fertilizer are labelled according to the percentage, by weight, of the major macronutrients that a plant needs for healthy growth. They are always listed in the same order, and are known as the NPK numbers. The N stands for nitrogen, the P for phosphorous, and the K for potassium.
The higher the number, the more concentrated the nutrient is in the bag. In a bag of 20-20-20 fertilizer there is 20% nitrogen, 20% phosphorous, and 20% potassium. (The remaining 40% of the bag is micronutrients and filler to allow for the appropriate application of the fertilizer.) In the bag of 10-10-10, there is 10% of each nutrient and 70% filler. (So the 20-20-20 bag will be more expensive.)
A 5 pound bag of 10-10-10, then, would contain .5 pounds of nitrogen. (10% of 5 pounds.) So if you have had your soil tested or are following the guidelines from a book or your local garden center for how much nitrogen to apply per 100 square feet, you’ll know how many bags to purchase to get the job done.
So which numbers do you want? That depends on what you are fertilizing. Nitrogen (the N or first number) is important for the growth of the leaves. So when you are fertilizing leafy plants like lettuce, spinach, … or grass, you might want a bag with a high first number. If you are fertilizing something grown more for the fruits or flowers, like tomatoes or… well, flowers, then you might opt for the higher middle number, the P for phosphorous, which is valuable for roots, fruits and flowers. The third number, the K or potassium number, is important for general health, insect and disease resistance.
If you really don’t know which is best, opt for a balanced fertilizer like a 10-10-10 which provides all three nutrients in equal amounts.
Bags of organic fertilizers will carry these same numbers, but will usually be lower numbers, like 4-3-3. This, of course, means that the amount of available nutrients is lower in those products. Organic gardeners believe that these products work better over a longer period of time than chemical fertilizers which may work more quickly but are then carried off by rain or watering into the ground water. Requests for organic alternatives are becoming more common in our area as time goes on and the concern for the environmant becomes more mainstream. But, in the end, it is a personal preference which one you use.