GARDENING : When Plants Are Hungry, They Show It

If starved plants could squeal like hungry pigs, we would pay more attention to fertilizer needs. Plants cannot squeal, but they can show their hunger. A deficiency of any needed nutrient causes a characteristic visual symptom in a plant.

Plant nutrient deficiencies can stunt plants, cause abnormal growth or cause leaves to yellow and die. You have to know what to look for to pinpoint a nutritional problem.

Look closely. Do your plant’s leaves look unhealthy because they are dying beginning at their tips, along their margins or between their veins? What color are the leaves? Note whether symptoms appear first near the base of the stem, on older parts of the plant, or near young, growing tips.

The following nutrients are those most likely to become deficient in your plants: nitrogen, phosphorus, potassium, calcium, magnesium and iron. Here are characteristic deficiency symptoms for each:

Nitrogen: The oldest leaves are affected first, turning pale green and then yellow. Yellowing usually begins at the tips of the leaves, but in corn the midrib of the leaf is the first part to yellow.

Phosphorus: Here again, the oldest leaves are the first to be affected. The leaves take on a reddish or purplish cast.

Potassium: The oldest leaves die beginning at their tips and proceeding, in contrast to nitrogen deficiency, along their margins.

Calcium: When plants become deficient in calcium, the tips of the plant no longer can grow. Calcium deficiency also causes the bottoms of tomato fruits to turn black, a disorder called “blossom end rot.”

Magnesium: Older leaves are affected first. These leaves turn yellow in between the veins on the surface of the leaf.

Iron: Iron deficiency symptoms are similar to magnesium deficiency symptoms, except that the yellowing between the veins occurs first on younger leaves. When severe, the young leaves may turn almost completely white.

Before you run outside and drench your plants with fertilizer, consider what else might influence plant nutrition, besides how much of a given nutrient is in the soil. An excess of one nutrient can cause a deficiency of another.

For example, if you load up the soil with too much phosphorus, plants become deficient in magnesium. Also, plants cannot absorb nutrients from the soil unless the soil pH is in the correct range--about 6.5 for most garden plants.

Water can influence plant nutrition. On hot, dry days, tomato plants may not be able to absorb calcium from the soil at a rate quick enough to reach the ends of the rapidly developing fruits. The result: blossom end rot.

Insects, disease, wind and seasonal change may have taken their toll on plants, so other problems can be mistaken for nutrient deficiencies. Some plant viruses cause leaves to become mottled yellow and green. Natural aging also causes color changes in leaves--maple leaves turn fiery orange at the end of the season because they are losing nitrogen, as they are supposed to.

If you have determined that the sickly color of a plant is due to a nutrient deficiency, what do you do?

Purchase a fertilizer containing only, or at least a high percentage of, the deficient nutrient. To tide annual plants over until the end of the season and correct deficiencies in houseplants, use a quick-acting fertilizer mixed with water. Most organic fertilizers are slow-acting, so a chemical fertilizer is better for this quick effect.

Handle perennial plants differently from annuals. Fertilization, especially with nitrogen, could stimulate new growth at a time of year when such plants should be slowing down as preparation for cold weather.

So unless trees, shrubs and other perennials are on the verge of death, leave them alone for now. Fertilize them after the weather has turned too cool for growth to resume, or just before their spring spurt of growth.

By the time leaf symptoms become evident, plant growth has suffered to some degree. Make a note of fertility problems this year so that you can avert them next year, before they occur.