Garden Topics:  Fertilizer

Garden Elements

Taking The Mystery Out of Fertilizer

With so many fertilizers on the market, it can be difficult to decide which is best for your plants. There are differences in the amount and release rate of nutrients in each fertilizer.  

When selecting a fertilizer, the first question to answer is, “What analysis do I need?” The analysis is actually the three large numbers you see on every fertilizer label such as 10-20-10 or 10-10-10 or 18-46-0. These numbers represent the percentage (by weight) of the three major nutrients required for healthy plant growth, always in the same order: nitrogen-phosphorus-potassium (N-P-K). Each of these nutrients affects plant growth differently, and the formulation you select should depend on your specific gardening needs.

1. Nitrogen:

The first number is the percentage of nitrogen in the bag. So a bag of 24-8-4 has 24 percent total nitrogen. Nitrogen provides plants with the ability to produce more chlorophyll, which in turn allows plants to grow quickly. With each additional nitrogen application, plants will grow taller and develop a darker green color, but at the expense of flower and fruit growth if too much is used. So if you want a dark green lawn, use a lawn fertilizer that’s high in nitrogen.  Under nitrogen deficiency, the older mature leaves gradually change from their normal characteristic green appearance to a much paler green. As the deficiency progresses these older leaves become uniformly yellow (chlorotic). Leaves approach a yellowish white color under extreme deficiency. The young leaves at the top of the plant maintain a green but paler color and tend to become smaller in size. Branching is reduced in nitrogen deficient plants resulting in short, spindly plants. The yellowing in nitrogen deficiency is uniform over the entire leaf including the veins

2. Phosphorous:

The second number in the analysis is the percentage of phosphorus in the mix. For example, a bag of 24-8-4 would contain 8 percent phosphorus. Phosphorous aids in root and stem development and increases seed production, flowering ability and bloom size. The fertilizer industry sometimes markets high phosphorus fertilizer as “Bloom Booster.” A major visual symptom of phosphorus deficiency is that the plants are dwarfed or stunted. Phosphorus deficient plants develop very slowly in relation to other plants growing under similar environmental conditions, but without phosphorus deficiency. Some species such as tomato, lettuce, corn and the brassicas (cole crops) develop a distinct purpling of the stem, petiole and the under sides of the leaves. High-phosphorous fertilizer should be used when plants are being established in your garden, when sowing a new lawn or planting a new tree.  Because phosphorus can become fixed to soil particles, it is important to place it close to the roots.  

3. Potassium (Potash)

The third number represents the percentage of potassium found in the product. A bag of 24-8-4 has 4 percent potassium in the mix. Potassium is necessary for strong roots and stems as well as deep flower color. Symptoms of potash deficiency are weak stems and yellowing or browning leaf tips and edges. The onset of potassium deficiency is generally characterized by a marginal chlorosis (yellowing), progressing into a dry leathery tan scorch on recently matured leaves. This is followed by increasing interveinal scorching. As the deficiency progresses, most of the interveinal area becomes necrotic, the veins remain green and the leaves tend to curl and crinkle

4. Calcium 

There are really four major nutrients that are most used by plants: nitrogen, phosphorus, potassium, and calcium. The first three nutrients previously discussed, are found in most mixed fertilizers. Calcium which effects soil acidity is  found in the form of limestone or gypsum. Calcium-deficient leaves show necrosis around the base of the leaves. Classic symptoms of calcium deficiency include blossom-end rot of tomato tip burn of lettuce, blackheart of celery and death of the growing regions in many plants. All these symptoms show soft dead necrotic tissue at rapidly growing areas. You may note that deficient leafs tend to cup downward.  

5. Chemical or Organic

a. Chemical fertilizers are less expensive than organic fertilizers and are  faster acting, but since they are water soluble they are  more subject to loss by leaching with heavy rainfall or irrigation than organic or natural fertilizers. This is because they  consist of water-soluble nutrients. If you are using a synthetic (chemical)  fertilizer, you should supplement it with some type of organic matter such as compost or manure, to insure the availability of the many minor nutrients.  

b. Organic fertilizers are made from natural ingredients and often have lower concentrations of the three major nutrients, so you will often need to use larger amounts. Organic fertilizers differ from chemicals, in that, they feed your plants while building the soil's structure. Soils with lots of organic material, remain loose and airy, are better able to hold moisture and nutrients, foster growth of soil organisms, including earthworms, and promote healthier root development.  Other Advantages include slow nutrient release, and a much lower risk of over-fertilization. Organics like Espoma Garden Tone, also contain many other minor nutrients that feed both the plant and the soil.   There is no one size fits all fertilizer. Fertilizer choice depends on the type of plant being grown and the soil it is being grown in. Some fertilizers claim to be better for spe­cific plant types, such as roses, hollies, camel­lias, tomatoes or azaleas, and they are. These fertilizers work well, we recommend them, but, they may be a bit  more expensive per pound of nutrient. Some of the compounds used in these plant-specific fertilizers (such Holly Tone) have an acidic reaction that is beneficial for acid-loving plants.  

Other Important Information

1. An experienced gardener may recognize a plant’s need for fertilizer. (For example, plants that are deficient in nitrogen may start turning light green or yellow, and purple foliage (on an otherwise green plant) is a telltale sign of phosphorus deficiency),  but the only true way to determine how much fertilizer a crop needs is to conduct a soil test. Having a soil test done before you start adding amendments will tell you what you actually need. If your soil pH (soil acidity) is too high or too low, your plants will not be able to access some nutrients, even if they are present in the soil. For a soil test kit, call Penn State Co-Op Extension at 412-473-2540.

2 Always follow the label instructions. Many gardeners have the false impression that the more they fertilize, the better their plants will grow. But fertilizing does not always result in improved plant growth. As is often the case, too much of a good thing may result in something bad. If a chemical fertilizer is applied too heavily, it may cause plant tissue to burn, and even result in plant death. Read product labels carefully and follow directions to avoid toxicity problems.  

3. You can think of fertilizer as a supple­ment, like a vitamin. Plants produce their own food using water, carbon dioxide, nutri­ents from soil and fertilizer, and energy from the sun. It is a combination of appropriate conditions that produces a healthy plant. If a plant appears healthy and is producing at a normal rate of growth, you may not need to fertilize.  

A few last tips

  • Liquid plant foods are good for accurate applications and quick plant uptake for house and container plants.
  • Slow release fertilizers such as SR4 Slow Release, feed plants over an extended period of 3-4 months. This type of fertilizer can be applied without the risk of burning.
  • Limestone (calcium) neutralizes the acid level in soil. A soil test will tell you how much to use. Side dressing adds fertilizer to plants during the growing period. Apply the fertilizer on top of the soil at least six inches away from the base of the plant.  
  • Base feeding is especially good for shrubs and roses. Apply fertilizer on top of the soil at least six inches from the base of the plant and extend to approximately twelve inches beyond the branch tips. Scratch the fertilizer into the soil without disturbing the roots.



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