How to Use Fertilizers in Urban Farming

In most situations, adding raw organic matter, compost, and manures to your urban soil will be enough to grow healthy flower and vegetable gardens, trees, shrubs, and lawns. But sometimes, you have to give your plants an added boost. That’s where commercial fertilizers come in.

Commercial fertilizers apply the nutrients that plants need directly to the plants. Some are complete, meaning they contain relatively equal quantities of nitrogen, phosphorous, and potassium. Others have higher levels of one or more of these nutrients to offset a deficiency in your soil. You can determine when and how much fertilizer to apply based on your soil test.

The “big three” in commercial fertilizers

Often you see three numbers on your fertilizer bag or container indicating the levels of nitrogen, phosphorous, and potassium in the fertilizer. These nutrients are called the “big three” or macronutrients because plants need them the most.

  • Nitrogen (N): This nutrient plays a key role in building proteins and chlorophyll (the pigment responsible for photosynthesis) in plants. It helps keep plant leaves green and healthy. Nitrogen leaches out of the soil quickly, so many urban gardeners have to add it on a regular basis to have a steady supply all season long.

  • Phosphorous (P): Phosphorus helps promote good root growth and bulb formation. It’s also important in fruit and seed formation and increased disease resistance. Phosphorous accumulates in soils and doesn’t move about in the soil easily, so you may need to add it only occasionally.

    Phosphorous isn’t readily available to plants if soils are cold or if the pH is below 5 or above 7. So before you add a fertilizer high in phosphorous, check these conditions to see whether you really need to add it.

  • Potassium (K): This nutrient promotes plant vigor, fruit and vegetable flavor, disease resistance, and general cellular functioning in plants. Like phosphorous, potassium doesn’t move easily in the soil, so you may have to add it only occasionally.

The best fertilizers are ones that contain these three nutrients in varying amounts (2-12-4, for example), otherwise known as complete fertilizers. Balanced fertilizers contain the same amount of the three nutrients. Single-element fertilizers contain only one of these three nutrients. There are also fertilizers that feature a certain nutrient over another that should be used if a soil test indicates a deficiency of that nutrient.

Plants need other nutrients, such as magnesium, calcium, and sulfur, in smaller quantities. These are called secondary nutrients. Plants also need minerals or micronutrients, such as boron, copper, and iron, in even smaller quantities.

In most cases, if you have a nutrient problem in your soil, the deficiency is in one of the three macronutrients. If you’re regularly adding organic matter, you’re probably adding enough of the micronutrients and minerals to keep the soil healthy.

Wet versus dry fertilizers

Fertilizer products come in two forms: liquid or granular.

Liquid fertilizers

Liquid fertilizers give plants a quick boost of nutrients because plant roots and leaves readily absorb them. When you apply liquid fertilizers to leaves, the process is called foliar feeding. Liquid fertilizers are a great way to remedy a nutrient deficiency quickly or add highly soluble nutrients to a plant. Some examples of liquid fertilizers include

  • Fish emulsion: Fish emulsion is made from the by-products of the fish industry. It can be smelly, but it has a good balance of nutrients (5-2-2) that help young seedlings, in particular, grow strong. It often comes as a concentrate that’s diluted in water.

  • Worm tea: You can make your own worm tea or buy a liquid worm food already made for you. Worm tea offers many of the advantages of fish emulsion, but it doesn’t have the bad smell.

  • Manure tea: You can use the same process you use for making worm tea to make manure tea, or you can buy it already made. Either way, it adds many of the nutrients you’d get from straight manure compost without all the bulk and bother.

Granular fertilizers

Granular fertilizers are probably the most common and easiest to use. They’re easy to apply by hand, but aren’t as quickly available to plants as liquid fertilizers.

Granular fertilizers are processed and made from a blend of different products. They’re often derived from materials such as soybeans or chicken manure (nitrogen), bones or rock phosphate (phosphorous), and greensand and wood ash (potassium).

Here are a few examples of granular fertilizers that feature specific nutrients:

  • Alfalfa meal: Made from alfalfa plants and pressed into pellet form, alfalfa meal is a good form of nitrogen and potassium fertilizer that stimulates plant growth. Roses are particularly fond of alfalfa meal fertilizer. Soybean meal is a similar plant-based granular fertilizer.

  • Corn gluten meal: Mostly sold as an organic herbicide for crabgrass and dandelion control, corn gluten meal has about 10 percent nitrogen.

  • Bone meal: This product is derived from animal and fish bones and is a good source of phosphorous (11 percent) and calcium (22 percent). It’s good for bulbs and root crops, but it may attract animals.

  • Greensand: Greensand is a mineral mined from old ocean deposits. It has a good amount of potassium (3 percent) and is slow to decompose, supplying this needed nutrient to your soil for years after you apply it.

  • Rock phosphate: This phosphorous-rich mineral comes in a hard form (20 percent phosphate) and a soft form (16 percent phosphate). The hard form breaks down slowly in the soil. Soft rock phosphate breaks down a little faster.

  • Limestone: Although lime is commonly used to raise the pH of soil for better plant growth, depending on the type of lime you use, it also adds calcium (46 percent) and magnesium (38 percent). Choose dolomitic lime if you need more calcium and magnesium in your soil and calcitic lime if you need just calcium.

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