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Soil pH and Fertilizers

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Publication Number: IS372
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Soil pH and Fertilizers

The pH of a soil refers to how acid or alkaline the soil is. The letters “pH” mean “potential hydrogen.” The availability of nutrients is directly affected by soil pH. If the soil’s pH is too high or too low, some nutrients become insoluble, limiting the availability of these nutrients to the plant root system.

The acidity-alkalinity scale ranges from 0 to 14. Soils are referred to as being acid, neutral, or alkaline, depending on their pH levels. A pH of 7 is neutral, while a pH lower than 7 is acid, and a pH higher than 7 is alkaline (basic).

A logarithmic scale is used to measure a soil’s pH. That is, a change of one unit in the pH scale represents a 10-fold change in acidity or alkalinity. A soil with a pH of 5.0 is 10 times more acidic than a soil with a pH of 6.0 and 100 times more acidic than a soil with a pH of 7.0. This is one good reason to be very careful in trying to increase or lower soil pH. Some factors, including soil type and organic matter, affect the amount of materials needed to change soil pH.

Fertilizers and rain affect soil pH. Organic matter and soil microorganisms are a few other factors that affect soil pH. Agricultural limestone normally is used to increase the soil’s pH. Sulfur is normally used to lower the soil pH. But fertilizer and water normally change the soil pH more rapidly.

Plants have specific pH requirements for normal growth. Most ornamental plants grow well in a pH range of 6.0 to 7.0. But azaleas, gardenias, camellias, and related plants require a soil pH level between 4.5 to 5.5.

It is important to know the pH levels and nutrient requirements of your ornamental plants to ensure normal growth and success. The following table lists some woody and herbaceous landscape plants and their desired soil pH ranges.

Common Landscape Plants and Their Preferred Soil pH Ranges

Shrubs

Preferred pH

Azalea

4.5–6.0

Barberry

6.0–7.5

Buddleia (Butterfly Bush)

6.0–7.5

Camellia

4.5–5.5

Crapemyrtle

5.0–6.0

Deutzia

6.0–7.5

Euonymus

6.5–7.0

Flowering Almond

6.0–7.0

Gardenia

5.0–6.0

Hibiscus

6.0–8.0

Holly

5.0–6.0

Huckleberry

5.0–5.5

Hydrangea (blue)

4.5–5.0

Hydrangea (pink)

6.0–7.0

Japanese Quince (flowering quince)

6.0–7.0

Ligustrum

6.0–7.0

Lilac

6.0–7.5

Oleander

5.0–7.5

Philadelphus (English Dogwood)

6.0–8.0

Pyracantha (Firethorn)

6.0–7.0

Spiraea spp.

6.0–7.0

Tea Roses

5.5–7.0

Viburnum

6.5–7.5

Weigela

6.0–7.0

Trees

Preferred pH

Apple, Peach, Pear, Cherry

6.5–7.0

Dogwood

5.0–7.0

Elm

6.0–7.5

Flowering Crab Apple

5.0–6.5

Holly

5.0–6.0

Magnolia

5.0–6.0

Maple

6.0–7.5

Oak Group

 

Pin Oak

5.0–6.5

Scarlet Oak

6.0–7.0

Red Oak

5.0–7.5

Pine

5.0–6.0

Redbud

5.5–6.5

Vitex

6.0–7.0

Weeping Willow

5.0–6.0

Garden Flowers

Preferred pH

Amaryllis

5.5–6.5

Baby’s Breath

6.5–7.0

Balsam (Touch-Me-Not)

6.5–7.0

Begonia

5.5–7.5

Caladium

6.0–7.0

Candytuft

6.5–7.0

Canna

6.0–7.0

Carnation

6.5–7.0

Chrysanthemum

6.0–8.0

Cockscomb (Celosia)

6.0–7.5

Coleus

6.0–7.0

Cornflower

6.0–7.5

Cosmos

6.5–7.0

Daffodil

6.0–7.5

Dahlia

6.5–7.0

Day Lily

6.0–8.0

Easter Lily

6.0–7.5

Four-O-Clock

6.0–7.5

Foxglove

6.5–7.0

Geranium

6.0–8.0

Gladiolus

6.5–7.0

Hollyhock

6.0–8.0

Iris

6.5–7.0

Larkspur

6.5–7.0

Lupine

6.5–7.0

Marigold

6.0–7.5

Nasturtium

6.5–7.0

Narcissus

6.0–7.5

Pansy

5.0–6.0

Periwinkle

6.5–7.0

Petunia

6.5–7.0

Phlox

5.0–6.0

Poppy

6.5–7.0

Salvia

6.0–7.0

Shasta Daisy

6.0–8.0

Snapdragon

6.0–7.5

Sweet Alyssum

6.5–7.0

Sweetpea

6.5–7.0

Sweet William

6.5–7.0

Tuberose

6.0–7.0

Tulip

6.0–7.0

Verbena

6.0–8.0

Zinnia

5.5–7.5

The following table lists fertilizer materials that supply certain nutrients and change the soil pH. Use this table as a reference. It does not take the place of a soil test.

Material

Analysis N-P-K

Rate of application per 100 square feet

Speed of Reaction

Effect on pH

Dry

Liquid

Ammonium Sulfate

20-0-0

1⁄2–1 lb

1 oz per 2–3 gal

Rapid

Very acid

Sodium Nitrate

15-0-0

3⁄4–11⁄4 lb

1 oz per 2 gal

Rapid

Basic

Calcium Nitrate

15-0-0

3⁄4–11⁄2 lb

1 oz per 2 gal

Rapid

Basic

Potassium Nitrate

13-0-44

1⁄2–1 lb

1 oz per 3 gal

Rapid

Neutral

Ammonium Nitrate

34-0-0

1⁄4–1⁄2 lb

1 oz per 5 gal

Rapid

Acid

Urea

45-0-0

1⁄4–1⁄2 lb

1 oz per 5–7 gal

Rapid

Slightly acid

Mono-ammonium Phosphate

11-48-0

1 lb

1 oz per 3 gal

Rapid

Acid

Di-ammonium Phosphate

18-46-0

1⁄2–3⁄4 lb

1 oz per 4–5 gal

Rapid

Acid

Triple Superphosphate

0-46-0

1–21⁄2 lb

Insoluble

Medium

Neutral

Superphosphate

0-20-0

3–5 lb

Insoluble

Medium

Neutral

Potassium Chloride

0-0-60

1⁄2–3⁄4 lb

1 oz per 4–5 gal

Rapid

Neutral

Potassium Sulfate

0-0-50

1⁄2–1 lb

Not advisable

Rapid

Neutral

Complete Soluble (mixtures)

20-20-20

20-5-30

12-12-12

Not advisable

1 oz per 3–5 gal

Rapid

Various

Complete Dry (mixtures)

10-10-10

5-10-10

2 lb

2–3 lb

Relatively insoluble

Various

Various

Various

Various

Limestone

None

5–20 lb

Insoluble

Slow

Basic

Hydrated Lime

None

2 lb

Relatively insoluble

Rapid

Basic

Gypsum (calcium sulfate)

None

2–5 lb

Insoluble

Medium

Neutral

Sulfur

None

1–2 lb

Insoluble

Slow

Acid

Epsom Salts (magnesium sulfate)

None

8–12 oz

1 oz per 5 gal

Rapid

Neutral

Aluminum Sulfate

None

Not advisable

1 oz per 5 gal

Rapid

Very acid

Urea Formaldehyde

38-0-0

3–5 lb

Slow

Slightly acid

Magnesium Ammonium Phosphate

7-40-6

Variable

Slow

Neutral

Dried Blood

12-0-0

2–3 lb

Medium

Acid

Steamed Bone Meal

Usually

5 lb

Slow

Basic

Castor Pumice

5-1-1

3–5 lb

Slow

Cottonseed Meal

7-2-2

3–4 lb

Slow

Acid

Hardwood Ashes

0-1-5

3–10 lb

Medium

Basic

Hoof and Horn Meal

13-0-0

2–3 lb

Slow

Seaweed (Kelp)

Usually

2–3 lb

Slow

Linseed Meal

5-1-1

3–5 lb

Slow

Acid

Soybean Meal

6-0-0

3–5 lb

Slow

Trace Elements

3–6 oz

0

Iron Sulfate

8–12 oz

1 oz per gal

Chelated Iron

1–2 oz

1 oz per 25 gal

Borax

1⁄2 oz

Copper Sulfate

1–2 oz


Information Sheet 372

Reviewed by Keri Jones, PhD, Laboratory Coordinator, Plant and Soil Sciences.

Copyright 2020 by Mississippi State University. All rights reserved. This publication may be copied and distributed without alteration for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.

Produced by Agricultural Communications.

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Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. GARY B. JACKSON, Director

Department: Plant and Soil Sciences

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Authors

Portrait of Dr. Keri Denley Jones
Laboratory Coordinator
Soil Testing Lab