Soil and pH
- What is pH?
- What is the best soil pH?
- Why is pH so important?
- How does dolomite differ from lime?
- I need to have some soil tested in an azalea bed in front of my home before I fertilize. How do I get this done?
- How do I take a soil sample correctly?
- What are the best type fertilizers to use?
- What does 100% organic mean?
- My soil is too alkaline (basic). How can I lower the pH?
- I am trying to determine which plants like an acid soil and which prefer an alkaline site. Are there guides to the proper acidity for plants?
- A soil test showed our yard needed potassium. Where do I find this nutrient for use in the landscape?
- What should I do with soil that was in pots after plants declined or was left after transplanting? Can I mix it with fresh soil or just dump it?
What is pH?
pH is a measure of acidity or alkalinity. pH is expressed by a number on a scale from 0-14. A neutral reading is 7. Any reading below 7 represents an acidic condition and the smaller the number the more acidic the condition. Any number above 7 indicates an alkaline condition and alkalinity increases as the number on the scale increases.
What is the best soil pH?
Optimum soil pH will vary from plant to plant, but considering all factors, a pH between 5.5 and 7.0 is generally accepted as the best range.
Why is pH so important?
The pH of the soil has a direct influence on what nutrients are available to plants. If the pH is above or below the recommended range for that plant, then nutrients may not be soluble (absorbable by plants) or they may be so soluble that they leach or become phytotoxic. Therefore, the plant can show signs of nutrient deficiencies or toxicity even when the correct amount of fertilizer is applied to that plant. pH also influences the activity of soil microorganisms. Compost piles depend on the decomposing activity of bacteria and fungi. Therefore, lime is usually added to compost piles to keep the pH in a range suitable for maximum bacterial action.
When should soils be limed?
Lime is added to the soil when a soil test determines that the soil pH is too acid. It is the carbonate ion which neutralizes the acid. It results in carbon dioxide and water, if the reaction goes to completion.
How does dolomite differ from lime?
Dolomite is lime. There are various forms of lime that are used to increase soil pH. Standard lime contains mostly calcium carbonate. Dolomitic lime is a mixture of calcium and magnesium carbonates. Whenever liming is recommended, dolomite is often suggested since it will adjust the soil pH and also add magnesium, which is often deficient in Florida's sandy soil.
I need to have some soil tested in an azalea bed in front of my home before I fertilize. How do I get this done?
Soil testing for major nutrients of phosphorus and potassium plus secondary nutrients like calcium and magnesium can be obtained through your local extension office. The office supplies test kits you use to send samples to the University of Florida soil-testing laboratory. The cost for the test is $7.00 and in return you get an analysis plus fertilizer recommendations.
Besides the nutrient analysis, you receive a soil acidity test, often referred to as a pH test, plus recommendations for adjustment if needed. The pH test may be the most important part of the soil analysis, as azaleas grow best in a pH range of 4.5 to 5.5. Many local soils and those near homes and concrete walkway are much higher. At these higher pH levels, the azaleas often turn yellow and are weakened by poor growth.
If you wish, a pH test can be made using kits available through many garden centers, and they and local extension offices offer this test for free or a small fee. They also suggest soil treatments to adjust the soil acidity to a pH best suited for azaleas.
How do I take a soil sample correctly?
Using a trowel, dig a V-shaped hole in the soil four to six inches deep. Remove a 'slice' from one side of the hole. Collect similar "slices" from several spots in the area you wish to plant. Mix all the individual "slices" together. One-half pint of this mixed sample is sufficient for testing purposes. The sample should be free of all plant debris and rocks, but should include all the "dust" (fine particles).
What are the best type fertilizers to use?
A complete fertilizer is best for most plants. A complete fertilizer means that all three major elements - nitrogen, phosphorous, and potassium - are available in that fertilizer in some proportion. It is also recommended that a fertilizer containing other elements such as iron, manganese, sulfur, etc. be applied once a year. These nutrients are listed on the label as secondary and/or minor plant nutrients. For a "long lasting" fertilization select a fertilizer containing some percent "Water Insoluble Nitrogen".
What does 100% organic mean?
This term causes a lot of confusion. It is used in fertilizer advertisements because many people incorrectly associate "organic" with "slow release" in the context of fertilizers. However, this is not necessarily the case.
- Nitrogen which is WATER-SOLUBLE is readily available to plants but it is also readily able to leach. As such, it can be lost to plants and can contribute to environmental pollution. Nitrogen which is NOT water soluble requires time in the soil to become water soluble and is thus considered slow release and less likely to be lost to leaching.
- WATER INSOLUBLE nitrogen can come from natural organic materials (e.g. seed meals, sludge, dried blood, etc.) as well as from certain forms of manufactured (synthetic) organic materials (e.g. urea-formaldehyde, isobutylidine diurea [IBDU], etc.). These are the SLOW RELEASE nitrogen fertilizers.
- WATER SOLUBLE nitrogen can be either organic (e.g. urea) or inorganic (e.g. various nitrate and ammonium salts). It can be natural and manufactured.
Florida fertilizer law, in trying to protect the consumer, requires that the various forms of nitrogen be specified on the fertilizer tag. This gets pretty complicated because terms like "water soluble organic N and or urea N", "water insoluble N", and "synthetic organic N" are needed. Many consumers do not readily understand these terms.
My soil is too alkaline (basic). How can I lower the pH?
Alkaline soils are a result of natural soil characteristics or excessive applications of lime. Adding elemental sulfur can lower the pH of over-limed soils. 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. Plants that are tolerant of high pH should be used to avoid continuing problems.
I am trying to determine which plants like an acid soil and which prefer an alkaline site. Are there guides to the proper acidity for plants?
Let's start by saying most plants prefer a slightly acid soil. If ever in doubt, you can usually be successful growing the common landscape plants in a soil with a pH between 5.5 and 6.5. A few that grow best in a more acid soil below pH 5.5 include azaleas and blueberries. Some that like the pH 7, a neutral soil, or slightly above include the herbs of borage, sage, tarragon and thyme.
Usually references do give a general guide to the soil acidity desired. They often mention a plant needs an acid or slightly acid soil, and you can assume this is in the 5.5 to 6.5 range. Only in a few instances is a more detailed discussion of a plant's acidity requirement needed. Having the soil acidity at an exact pH is not that important for most plants.
A soil test showed our yard needed potassium. Where do I find this nutrient for use in the landscape?
Potassium, or potash, is the third number in the analysis on the fertilizer label. It can be added separately or as part of a complete fertilizer product applied during a normal feeding. The nutrient helps promote plant vigor, disease resistance and cold hardiness.
Garden centers offer potassium in bags as either muriate of potash or potassium sulfate. Usually only small quantities of these concentrated products are applied. Follow the label when supplying this nutrient to deficient soils as a one-time treatment.
Perhaps an easier way to add the potassium is through normal feedings. Select a fertilizer high in this nutrient such as a 15-0-15 or a similar product. This fertilizer has the needed nitrogen (15) for green plant growth, no phosphorus (0) because horticulturists find it is plentiful in many local soils and the extra potassium (15) for the landscape. Using one of these products at feeding time can keep the potassium at the level needed for good plant growth.
What should I do with soil that was in pots after plants declined or was left after transplanting? Can I mix it with fresh soil or just dump it?
Residual soils are best added to compost piles. Composting helps eliminate pests that might be in the soils and could affect plants added to the containers. If you do not have a compost pile, the soil can be applied to well-established shrub beds with plants different from the containers. Scatter the soil across the surface of the bed where it will be subjected to the weather, and potential pests will be destroyed.