Optimum Soil pH Ranges of Common Nursery Crops

Originally compiled by Dr. Willard T. Witte, Associate Professor

while serving as the U T Extension Nursery Specialist in 1978

Updated by Mark Halcomb, Area Nursery Specialist in Middle Tennessee

and Dr. Donna Fare, Research Horticulturist, USDA-ARS

INTRODUCTION

Soil reaction, or pH, may be either acid, neutral, or alkaline. This is measured on a pH scale running from nearly 0 (strongest acid) to 14 (strongest alkali or base). Neutral is 7, slightly acid 6.1 to 6.9, moderately acid 5.6 to 6.0, strongly acid 5.1 to 5.5, very strongly acid 4.5 to 5.0, and extremely acid below 4.5. Most Tennessee nursery soils range between 4.5 and 6.5 with occasional higher values.

The soil pH has an important influence on the availability of nutrients in the soil and on the growth of different kinds of plants. Information in this article has been assembled from many sources to help explain the effect of soil pH on nursery plants. The list should be used as a guide rather than an exact measurement of preferred pH range for a plant. Hopefully it will aid in managing the pH of nursery soils.

SOILS TEND TO BE ACID

Tennessee soils naturally tend to become acid in reaction with pH values from 4.5 to 6.5. This is due to the removal of base-forming ions (Ca++, Mg++, K+) and subsequent replacement by acid-forming ions (H+, Al+++). The following conditions enhance or speed up this acidifying process:

1. Application of acid forming fertilizers which produce large amounts of H+ ions, such as urea, ammonium and sulfate fertilizers.

2. Removal of base-forming ions by crops.

3. Removal of base-forming ions by leaching and erosion.

4. Decomposition of organic matter which produces carbonic acids.

By adding base-forming ions (that is, the Ca++ and Mg++ in lime and Dolomite) the acid forming ions are displaced and the soil pH rises. Lime recommendations are made according to the pH reported on soil tests and are based on past experience with Tennessee soils.

SOIL CHEMISTRY AFFECTED

The chemical availability of nutrients may be affected by pH as follows:

1.	Manganese availability increases as pH drops with toxic symptoms occurring on some plants at pH 5.0 or below. In some soils in Tennessee this is an important factor, as manganese is concentrated heavily enough to exist as "pellets". These look like shot gun pellets on the soil surface after a rain.

2.	Availability of phosphorus, one of the major plant nutrients, decreased as pH rises or falls from about 6.3.

3.	Availability of copper and zinc is greatest between 5.0 and 6.0.

4.	Calcium, magnesium, and molybdenum become less and less available as pH drops from 7.0 to 4.0.

5.	Activity of soil microorganisms affecting organic matter decomposition and nutrient availability (especially nitrogen) may be greatly reduced as soils become more and more acid.

In summary, these are all good reasons for keeping the soil pH between 5.5 and 6.5 for most plants, but there are some exceptions.

PREFERRED pH RANGES

Every kind of plant is believed to have a most suitable soil reaction, ie., preferred pH range for its best and fastest growth. A few kinds of plants grow best in a rather narrow pH range, while most plants fortunately tolerate a wide pH range. Rarely does any one kind of plant grow equally well in both distinctly acid and distinctly alkaline soils. We are all familiar with the gardeners rough classification of plants into "acid-
loving" and "lime-loving" and there is quite a bit of truth in this in this grouping provided we don't carry it too far.

Acid loving generally refers to plants that do well between pH 4.0 and 6.0 with a preferred range of perhaps 5.0 to 5.5. This group contains all the ericaceous plants: heath, heather, azalea, rhododendron, mountain laurel (Kalmia), leucothoe, huckleberry, bearberry, pernettya, andromeda, blueberry, etc. The acid loving group also contains some of our choice small native flowering trees as fringetree, silver bell, sourwood, yellowwood, franklinia, and stewartia which are adapted to the acid organic conditions of the forest floor. Acid loving plants, especially the ericaceous ones, are often said to be lime-intolerant. This is thought to be due to the inability of their roots to take up iron under neutral or alkaline conditions even though iron may be present in the soil. Most of us have seen the iron chlorosis of azaleas grown in alkaline soil - the new leaves yellow except for the veins, or in severe cases all the new growth on one or more branches may turn completely yellow. Another feature of ericaceous plants is thought to be a tolerance of the large amount of aluminum ions present in strongly acid soils.

On the other side of the pH scale are the lime lovers, typified by plants that thrive or at least tolerate alkaline conditions and just won't do as well in strongly acid soil. Some examples that come to mind are lilac and mockorange; and some people would put elm, quince, linden, and some junipers in this group. Many desert plants grow in alkaline soils but it is not clear whether they require it or just tolerate it better than other plants. It is hard to pin down a pH range for this group, but somewhere around 7.0 to 7.5 or higher would seem about right. The main point to be made is that most of these plants grow perfectly well in slightly acid 6.5 soils but can also tolerate more alkalinity than most other plants.

A third group could be broken out for those plants which do best in moderately acid soils of 5.5 to 6.5. This includes the majority of nursery crops.

WHAT TO DO?

So what does all this mean to the nursery grower, landscaper, or gardener? Most obvious is that one should not attempt to grow acid loving and lime loving plants side by side - one or the other is going to suffer. It would seem to make sense to group the plantings into two or three groups according to the preferred pH range of the plants, such as strongly acid - 4.5 or 5.0 to 5.5; moderately acid - 5.5 to 6.5; and slightly acid to alkaline 6.5 to 7.5. Then the soil could be managed to bring the pH near the desired range. In Tennessee this would usually mean lime or dolomitic lime applications. Some landscape situations would require application of powdered sulfur to lower pH for acid loving plants, as where mortar from masonry or cement work has sweetened adjacent soil.

To keep track of pH changes in the nursery, soil tests should definitely be taken and pH adjusted to the preferred range for the plants before they are lined out. Thereafter, it is a good idea to take a sample every year or two, especially on long term crops, and to keep a record for each block in production. Lime applications to adjust pH to proper levels are one of the cheapest and most effective ways to aid the growth of nursery crops.

RELATIVE SALT TOLERANCE

Nearly all fertilizer materials are salts of various compounds. These dissolve in the soil water and become available to plants. Too much salt in the soil will burn the roots by drawing water out of the fine root tissue, Some plants are quite sensitive to this "fertilizer burn" and are classed as having low salt tolerance. They need light fertilizer applications, and care must be exercised when using the readily soluble, or "hot" fertilizers such as ammonium nitrate, or concentrated liquid fertilizers.

Other plants can tolerate high levels of soluble salts in the soil, so can withstand heavy fertilizer applications. The majority of the plants in the list that follows have been classified by nursery researchers as having either low, medium, or high salt tolerance.	Nsy pH text 2-2002 Fertility disc

Desired pH and Salt Tolerance of Common Nursery Plants by Common Name

Common Plant Name	Scientific Name	Desired pH	Salt Tolerance
Abelia	Abelia x grandiflora	6.0 - 7.0	Medium
Almond, Flowering	Prunus triloba	6.0 - 7.0*	Low
Althea (Rose-of-Sharon)	Hibiscus syriacus	6.0 - 7.5	High
Andromeda	Pieris japonica	5.0 - 6.0	Low
Apricot	Prunus armeniaca	6.0 - 7.0	High
Arborvitae	Thuja occidentalis	6.0 - 7.0*	Medium
Ash, Green	Fraxinus pennsylvanica	*6.0 - 7.0*	Medium
Azalea	Rhododendron obtusum, etc.	*4.5 - 5.5*	Low
Barberry	Berberis thunbergi	*6.0 - 7.5	Low
Beech	Fagus sylvatica	*6.0 - 7.0*	Low-Medium
Birch	Betula spp.	*5.0 - 6.0	Low-Medium
Butterfly Bush	Buddleia davidii	6.0 - 7.5	?
Boxwood	Buxus sempervirens	*6.0 - 7.0*	Medium
Cedar, Red	Juniperus virginiana	5.0 - 7.0*	Medium
Cherry, Flowering	Prunus serrulata	5.0 - 7.0*	Medium
Cotoneaster	Cotoneaster spp.	*6.0 - 7.0	High
Cottonwood	Populus deltoides	5.5 - 7.0	Medium
Crapemyrtle	Lagerstroemia indica	5.0 - 6.0	Medium
Crabapple, Flowering	Malus varieties	6.0 - 7.0	Medium
Cypress, Bald	Taxodium distichum	5.0 - 6.5*	Low
Dogwood	Cornus florida	*5.0 - 6.5	Low
Deutzia	Deutzia gracilis	6.0 - 7.0*	Low-Medium
Elm, Chinese	Ulmus parvifolia	6.0 - 7.0*	High
Euonymus, Creeping	Euonymus fortunei	*6.0 - 7.0	Med - High
Euonymus, Burning Bush	Euonymus alatus	*6.0 - 7.0	Low
Fir, all types	Abies spp.	*5.0 - 6.0	Medium
Firethorn	Pyracantha coccinea	6.0 - 7.0	Medium
Forsythia	Forsythia x intermedia	6.0 - 7.0	High
Franklin Tree	Franklinia alatamaha	4.0 - 6.0
Gardenia	Gardenia radicans	5.0 - 6.0	Medium
Ginkgo	Ginkgo biloba	6.0 - 7.0	High
Hawthorn	Crataegus spp.	6.0 - 7.0*	Medium
Hemlock	Tsuga canadensis	*5.0 - 6.0	Medium
Holly-grape	Mahonia aquifolium	5.0 - 6.0	Medium
Common Plant Name	Scientific Name	Desired pH	Salt Tolerance
Honeylocust	Gleditsia triacanthos	5.5 - 7.0*	Med - High
Honeysuckle	Lonicera spp.	6.0 - 7.0	Med - High
Holly, American	Ilex opaca	4.0 - 6.0	Medium
Holly, Japanese	Ilex crenata & varieties	5.0 - 6.5	Med - High
Holly, Foster #2	Ilex & attenuata 'Fosteri'	5.5 - 6.0
Hornbeam, American	Carpinus caroliniana	6.0 - 7.0	Low
Horsechestnut	Aesculus hippocastanum	6.0 - 7.0	Med - High
Hydrangea (for blue flower)	Hydrangea macrophylla	4.5 - 5.5	Medium
Hydrangea (for pink flower)	Hydrangea macrophylla	6.0 - 7.0	Medium
Hydrangea, oak leaf	Hydrangea quercifolia	?5.5 - 6.0?	?
Ironwood	Carpinus caroliniana	6.0 - 7.0	Low
Ivy, English	Hedera helix	6.0 - 7.0	Med - High
Juniper	Juniperus species	5.0 - 7.5	Med - High
Juniper, Creeping	Juniperus horizontalis	4.0 - 6.0	Medium
Larch	Larix decidua	6.0 - 7.0	Low
Leucothoe	Leucothoe fontanesiana	5.0 - 6.0	Low
Locust	Robinia pseudoacacia	6.0 - 7.0	High
Lilac	Syringa vulgaris	6.0 - 7.0*	Medium
Linden	Tilia spp.	6.0 - 7.5	Low
Magnolia, deciduous	Magnolia soulangeana	*5.0 - 6.0*	Medium
Magnolia, Southern	Magnolia grandiflora	5.0 - 6.0	Low
Maple, Japanese	Acer palmatum	?5.0 - 7.2?	?
Maple, Silver	Acer saccarhinum	6.0 - 7.0*	Medium
Maple, Sugar	Acer saccharum	*6.0 - 7.0	Low
Maple, Red	Acer rubrum	*6.0 - 7.0	Low - Med
Maple, Norway	Acer platanoides	6.0 - 7.0*	Medium
Mimosa	Albizia julibrissin	5.5 - 6.5	High
Mockorange	Philadelphus coronarius	6.0 - 8.0	High
Mulberry	Morus spp.	6.0 - 7.0*	High
Nandina	Nandina domestica	5.5 - 6.5	High
Oak, Pin	Quercus palustris	5.0 - 6.5	Med - High
Oak, Scarlet	Quercus coccinea	6.0 - 7.0	High
Oak, Red	Quercus rubra	*6.0 - 7.0	High
Oak, White	Quercus alba	5.5 - 6.5*	Med - High
Peach	Prunus persica	6.6 - 7.0

Common Plant Name	Scientific Name	Desired pH	Salt Tolerance
Pear, Callery	Pyrus calleryana	5.5 - 7.0*	High
Phlox	Phlox paniculata	6.5 - 7.0
Photinia	Photinia x fraseri	5.5 - 6.5
Pine	Pinus spp.	*5.0 - 6.5	Low
Plum, Purple-leaf Flowering	Prunus cerasifera	*6.5 - 7.5
Poplar, Lombardy	Populus nigra 'Italica'	6.0 - 7.0	Low
Poplar (White, Grey, Silver)	Populus alba & others	6.0 - 7.0*	High
Privet, Common	Ligustrum vulgare	6.0 - 7.0	Med - High
Privet, Glossy	Ligustrum lucidum	6.0 - 7.0	Med - High
Quince, Flowering	Chaenomeles speciosa	6.0 - 7.5	High
Redbud	Cercis canadensis	*5.5 - 6.5*	Low - Med
Rhododendron	Rhododendron hybrids	5.0 - 6.0	Low
Rose, Tea	Rosa hybrids	5.5 - 7.0	Low - Med
Rose, Rugosa	Rosa rugosa	6.0 - 7.0	High
Serviceberry/Sarvis	Amelanchier spp.	5.0 - 6.0	?
Sourwood	Oxydendrum arboreum	*4.5 - 6.0	?
Spirea	Spirea thunbergi & others	6.0 - 7.0	Low
Spruce, Norway	Picea abies	*5.0 - 6.5	Medium
Sweet Gum	Liquidambar stryaciflua	6.0 - 7.0	High
Tamarix	Tamarix pentandra	6.0 - 7.0	High
Tupelo (Sour Gum)	Nyssa sylvatica	6.0 - 7.0	Medium
Tulip Poplar	Liriodendron tulipifera	6.0 - 7.0	Medium
Viburnum	Viburnum spp.	6.0 - 7.5	Low
Vitex (Chaste-Tree)	Vitex agnus-castus	6.0 - 7.0	Low
Walnut	Juglans spp.	6.0 - 7.0	Low
Weigela (Beautybush)	Weigela florida	6.0 - 7.0	Medium
Willow	Salix spp.	*6.0 - 7.0	High
Willow, Golden	Salix alba 'Vitellina'	*6.0 - 7.0	Medium
Yew	Taxus spp.	6.0 - 7.0	Medium
Yellowwood	Cladrastis lutea	*5.0 - 6.5	?

*Asterix on right hand side of column indicates tolerance of a more alkaline soil reaction.

*Asterix on left hand side of column indicates tolerance of a more acidic soil reaction.