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Gardening

Don't get burned: Better to underfertilize plants

By MARY COLLISTER
Published June 24, 2005

I had a neighbor ask me about fertilizer recently. He is a serious hobby gardener and excels at propagating the plant material in his yard. He always has more tomatoes than his family can eat, which benefits me, as he is very generous. As much as he gardens, he says he still has trouble with fertilizer. He maintains that nine times out of ten he burns his plants when he fertilizes. If a seasoned gardener has trouble like this, I figure there are more of you out there struggling to feed your plants.

Understanding fertilizer starts with understanding the numbers listed on the label. There are always three numbers, each separated with a hyphen, such as 10-10-10 or 20-10-5. These numbers indicate the fertilizer analysis or percentage by weight of nitrogen (N), phosphorus (P) and potassium (K), in that order.

Therefore a 50-pound bag of fertilizer labeled 20-10-5 would contain 20 percent nitrogen (10 pounds), 10 percent available phosphates (5 pounds), and 5 percent soluble potash (2.5 pounds of potassium). This product would be considered a "complete" fertilizer, since all three nutrients are present.

An "incomplete" fertilizer would have a label like 0-0-60 or 46-0-0, since it would only have one of the three major nutrients present. Another example of an incomplete fertilizer would be 0-20-20, since one of the three nutrients is missing.

Fertilizers also have ratios, which indicate the relative amounts of nutrients to each other. For example, a 10-10-10 fertilizer is a 1-1-1 ratio, and a 20-10-5 fertilizer is a 4-2-1 ratio. Ratios can be helpful when looking for the right mix for a certain type of plant or situation. For example, vegetable gardens often call for a 1-2-1 ratio, which would translate into a 5-10-5 or 10-20-10 fertilizer. Many trees like a 2-1-1 ratio, which would be a fertilizer product such as 10-5-5 or 20-10-10.

Lawns prefer a 3-1-2 ratio fertilizer, so a fertilizer product with 30-10-20 on the label would be a good ratio match. The higher nitrogen leads to quicker and greener growth, and of course in turn, more frequent mowing! If you are after blooms, look for a 1-2-2 ratio, such as 5-10-10. This concentrates the growth away from the foliage and into flower production.

High analysis fertilizers (those with larger numbers on the label) would be applied at a lower rate to yield the same results. In other words, 5 pounds of a 20-20-20 fertilizer would yield the same amount of actual nutrients as 10 pounds of a 10-10-10 fertilizer. So, a fertilizer with high numbers makes it easier to over-apply and "burn" your plants.

It helps to understand what each of the three major nutrients does.

An application of high nitrogen fertilizer should be used during periods of vegetative growth, when the plant requires a higher rate of nitrogen phosphates or potash.

Phosphorus is important for root growth and blooming in plants and is the main ingredient in starter fertilizers as well as liquid fertilizer bloom boosters. It also is important for young plants when rapid root growth is necessary.

Potassium helps plants resist disease and aids in winter hardiness. "K" is the symbol for "kalium" or potash, and is commonly used to represent potassium. Most winterizer fertilizers used on lawns in late fall are high in potassium, since it promotes winter hardiness in turf. Potassium is fairly mobile and readily leaches out of the soil profile. Potassium fertilizers have a high salt index and should be used with caution since they can burn plant foliage.

Ah, here's that burn issue again; back to my neighbor's problem.

Fertilizer burn is created when the flow of diffusion in the root is reversed. Huh? There is a membrane in the root of plants that allows water and dissolved nutrients to move from the soil, across the membrane and into the root. If that direction of flow is reversed we get tip burn or fertilizer burn. This can be caused either by over fertilization or an excessive amount of dissolved salts in your water (Hmm, maybe too much potassium?) Once burned, there isn't much you can do. Just keep the plant moist and hope it recuperates. Taken to the extreme, fertilizer burn can kill your plants.

For very young plants, such as seedlings, use a water-soluble fertilizer at less than half strength. Better to fertilize often with a weaker mixture than use a strong formulation. Many of the water soluble types call for one tablespoon per gallon of water, so if you want it half strength add just one half of a tablespoon to a gallon of water. You can than use a watering can or a mister to apply.

It's always important to read and follow the directions on the label. For granular lawn fertilizer, the label will usually explain how many square feet the bag will cover. It also gives the settings for different types of fertilizer spreaders. Once again always better to err on the side of caution.

Time-release fertilizers are less likely to burn, but do initially cost more. They also last longer and therefore require less frequent applications. Potting soil can be purchased with the fertilizer included. This is great for container gardening. I have found this works very well and certainly takes the guess work out of trying to decide when and how much fertilizer to add. You do pay for the convenience, of course.

Then, of course, you have the choice between chemical and organic fertilizers. There may not be a topic that polarizes gardeners quicker than the organic versus inorganic (chemical) debate. Suffice to say here, that you need to do your homework and may decide that organic has a place in your garden. I use a combination in my yard, but it seems logical to me that the fewer chemicals we can use, the better.

Hope this enlightens a bit when it comes to fertilizer. Don't over-think the problem, but do read and follow the label instructions. When applied correctly, fertilizers lead to healthier plants and a more beautiful, bountiful garden.

Mary Collister of Valrico writes about how to garden successfully in Florida's climate and offers problem-solving tips for your home garden. Mail questions to: Mary Collister, Brandon Times, 426 W Brandon Blvd. Brandon, FL 33511.

[Last modified June 23, 2005, 01:08:26]