Question: I have two types of fertilizer that don't have any kind of pesticide mixed in. One is for lawns and the other is for tomatoes. The lawn fertilizer is a 32-10-20 and the tomato fertilizer is 15-30-15. Can I combine them to apply in my garden? Does that give me a 47-40-35 fertilizer? I want to get the benefits of both fertilizers, but I only want to fertilize the lawn and garden once.
Answer: You can mix the fertilizers, assuming they are both made from similar materials. They need to be spread or sprayed together. However, you won't get the benefits you are assuming.
The numbers on a fertilizer package are always listed in the same order. Nitrogen, phosphorus and potassium are the three major nutrients that plants use that don't get to the plant from the air or water as oxygen, carbon and hydrogen do. All of the other nutrients such as iron, boron or copper are used in such tiny amounts that they are called minor nutrients and they get to plants from the soil, water and possibly through fertilizer.
The numbers on the package are the percent of active ingredient of each element. Add the numbers up and the rest of amount necessary to get to 100 percent is the inactive ingredients, such as filler or materials used to create even fertilizer spreading — like sand or ground up corncobs. The active ingredient numbers can't add up to more than 100 percent as they do in your example of the combined fertilizers. Each active ingredient in one of the fertilizer packages is also an inactive ingredient to the other fertilizer. When you add equal amounts of the two fertilizer percentages you then must also divide by two. In your case, an equal amount of the 32-10-20 added to the 15-30-15 becomes a final fertilizer of 23.5-20-17.5.
When you divide all the major nutrient percentages by the lowest number (for example: 32-10-20 divided by 10 results in 3.2-1-2) you get the fertilizer's ratio. The ratio is useful for comparing fertilizers. The actual numbers are less important than the ratio.
The three major ingredients are used by plants in different ways. They are mixed into the fertilizer in different ratios depending on what aspect of plant growth they are trying to promote. Nitrogen is used in making proteins and chlorophyll and tends to promote the growth of leaves and stems. Use it in the vegetable garden to promote leaf growth on such things as lettuce or cabbage. Phosphorus is the middle number and it is used throughout the plant, but it is especially useful in promoting new flowers, which on vegetable crops means more fruit. Potassium is used throughout the plant, but does help promote flowers and roots.
A lawn fertilizer should typically have a ratio of around 4-1-2 or 4-1-3 or 3-1-2. Your 32-10-20 fertilizer has a ratio of 3.2-1-2 and would be good on a lawn. A ratio of 1-2-1 or 1-3-2 would be good for roses, azaleas, lilacs, tomatoes, peppers and other flowering or fruiting plants. Your 15-30-15 fertilizer has a ratio of 1-2-1 and would be good for the flower or vegetable garden.
A ratio of 1-1-1 is called a balanced fertilizer and is a good general fertilizer for any plant when you don't need to promote flowers or leaves. Your combined fertilizer is a 1.34-1.14-1 fertilizer. It would be good for hedges, trees or most perennials at any time.
You mention there are no pesticides in either package. That is good because if there is a weed killer in the lawn fertilizer, it will kill any garden plants it is used on.
Email questions to Jeff Rugg at [emailprotected] To find out more about Jeff Rugg and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.
