Residential Lawn & Garden

Fertilizer and pH FAQs

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What is pH?

pH is a measure of soil acidity (sourness) or soil alkalinity (sweetness). A number expresses pH on a scale from 0 to 14. A neutral reading is 7. Any reading below 7 represents an acid soil and the lower the number the more acidic the soil. Any number above 7 indicates an alkaline condition and alkalinity increases as the number on the scale increases.


What is the best pH?

Optimum pH will vary from plant to plant, but a pH between 5.0 to 7.0 is generally accepted as the best range for most plants.


Why is pH important?

The pH of the soil governs what nutrients are available to plants. If the soil pH is above or below the recommended range (5.5 - 7.0), nutrients may not be soluble (absorbable by plants) or they may be so soluble that they become Phytotoxic. Therefore, a plant can show signs of nutrient deficiencies or toxicity even when the correct amount of fertilizer is applied to that plant.


When should soils be limed?

Lime is added to soil when a soil test determines that the soil pH is too acid. Lime reacts with water and releases calcium that neutralizes the acidity of the soil, thus raising the pH of the soil.


How does dolomite differ from lime?

Dolomite is a type of lime. There are various forms of lime used to increase soil pH. Standard lime contains calcium. Dolomitic lime contains calcium and magnesium. Whenever lime is recommended, dolomite is suggested since it will adjust the soil pH and also add magnesium which is usually deficient in Florida's sandy soil.


My soil is too alkaline (sweet or basic). How can I lower the pH?

Alkaline soils are a result of natural soil characteristics or excessive applications of lime. The pH of over-limed soils can be lowered by adding elemental sulfur. Never apply more than 5 to 10 pounds of sulfur per 1000 square feet per application. Iron sulfate, aluminum sulfate, or ammonium sulfate also has soil acidifying properties. Naturally alkaline or calcareous soils are common in coastal counties. It is difficult, if not impossible to lower the pH of these soils. Nutrient deficiencies in plants growing on calcareous soils should be treated by nutrient foliar sprays. Better yet, use plants which are adapted to coastal conditions (i.e. alkaline soils and/or salt) -- See "Salt Tolerant Plants for Florida"


How do I take a soil sample correctly?

Using a trowel or shovel, dig a V-shaped hole in the soil four to six inches deep. Remove the entire soil wedge. Collect similar "wedges" from several spots in the area you wish to plant. Mix all the individual "wedges" together. One pint of this mixed sample is sufficient for testing purposes. The sample should be free of all plant debris and rocks. It is also very important for the soil to be completely dry.


What will a soil pH test tell me?

A soil pH test will tell you how acidic or alkaline your soil is. It will not tell you the fertilizers you need, what diseases are present in the sample, or any other reasons why plants won't grow in an area. Homeowners who want a more complete soil test can send samples to the University of Florida's soil testing laboratory in Gainesville. This test measures and makes recommendations for pH, phosphorus, potassium, magnesium and calcium. Nitrogen is not measured. Forms and mailing boxes for this test are available from the receptionist at the Extension Service.


What is the best type of fertilizer?

This depends entirely on the type of plant and the circumstances. The numbers on the bag (6-6-6, 10-10-10, 16-4-8, etc.) represent the percent of nitrogen, phosphorous, and potash in the product. It is also recommended that the fertilizer also contain trace or micro-nutrients such as iron, manganese, sulfur, etc. These nutrients are listed on the label as secondary plant nutrients. For a "long lasting" fertilizer, select one that contains "slow-release" nitrogen or one that has all of the fertilizer encapsulated.


What does "100% organic" mean?

When the term "organic" is used on a fertilize label, it means that all or part of the nitrogen is in an organic form. This form must be identified as synthetic-organic and/or natural-organic, and the respective percentage or each must be specified (for example: 70% synthetic, 30% natural). Synthetic organics are organic compounds which are chemically synthesized such as Urea Nitrogen. Synthetic organic nitrogen is identified in the guaranteed analysis as Water Soluble organic nitrogen and/or Urea Nitrogen. Natural organic nitrogen comes from natural sources such as manure, seed meal, sludge, dried blood, etc. The percent of natural organic material is listed under Water Insoluble Nitrogen. Recently, many forms of water insoluble nitrogen have been developed (sulfur-coated urea, ureaformaldehyde, Isobutylidine dilurea-IBDU, etc.). These sources act like natural forms in that the nitrogen is slowly released.


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