Chapter 2. Make the pH Work for You

One of the principal influences for good or bad in soil is its pH. This used to be the province of scientists and chemistry students, but over the past few years it has become part of the home gardener's everyday world. In many, many cases, pH is the key to proper plant growth, and a pH reading can tell you much about what is going on beneath the surface of your garden.

Soil pH can be a highly technical subject, but for us it need not be. Actually the pH scale is just as easy to work with as the thermometer scale. You don't have to know thermodynamics, heat transfer and other aspects of temperature to understand what happens to your plants when thermometer readings drop to or below 32° Fahrenheit. Similarly, without knowing a thing about hydrogen and hydroxyl ions, logarithm exponents or other technical details of the pH theory, you can make practical use of soil pH elements that your plants (and the soil organisms) need.

Direct and Indirect

Effects of pH are both direct and indirect. Direct effects, while not numerous, can be critical. In the case of a soil that is too acid or too alkaline, there can be (1) toxic effects on the plants themselves, and (2) an unfavorable balance between acid and alkaline elements needed by plants.
Indirectly the pH can have an effect on one or more of the following:

1. Availability of essential elements
2. Activity of soil microorganisms
3. Solubility and potency of toxic elements
4. Prevalence of plant diseases
5. Competitive ability of different plant species
6. Physical condition of the soil (when lime is used to raise the pH)

The pH scale is a measure of balance between acidity and alkalin­ity of soil solutions. The scale is simplicity itself, being a series of numbers starting at 0.0, the most acid, and running in tenths up to 14.0, the most alkaline. The neutral soil reaction on the scale is 7.0, the mid-point where acid and alkaline elements are in balance. (Soil reaction refers to the degree of acidity.) Gardeners do not use the entire pH scale, since reactions from 4.0 to 9.0 are just about the limits for plant growth.

Each full step or unit up or down on the scale (say from 6.0 to 7.0 or 7.5 to 6.5) represents a tenfold increase or decrease in the degree of soil acidity. For example, a soil solution with a pH of 6.0 is ten times more acid than one with a pH of 7.0.

SOILS CLASSED BY pH

Years ago, Dr. Edgar T. Wherry devised a classification of soils by degrees of acidity; it is still useful but should be qualified by the fact that many plants spill over into two or more classifications while some are relatively sensitive to pH. Under his system, soils are classi­fied as follows:

Superacid: Bogs, largely of sphagnum origin, with a pH range of 3.0 to 4.0. Only a few plants thrive under these superacid conditions. Because bacteria and fungi cannot function at this low reading, organic matter breaks down slowly or not at all. (It is interesting to see that two plants which do well in superacid soils—pitcher plants and sundews—do not rely upon soil for nitrogen, but are carnivorous.)

Mediacid: Bogs of sedge and sphagnum where no run-off from lime-bearing soils drains in. The pH is from 4.0 to 5.0. Broad-leaved evergreens thrive on moist mediacid soils, while hemlock, spruce and oaks grow on somewhat drier areas.

Subacid: Older gardens and fields from which lime has been all but exhausted, resulting in a pH of 5.0 to 6.0. Also includes old upland woods and some swamps.

Minimacid: Gardens and fields which are limed from time to time; woods on soils over limestone; old untilled grasslands or soils under oaks. The pH ranges from 6.0 to 7.0.

Minimalkaline [including Neutral]: Marshes and lowlands into which water drains from lime-rich soils. Contain debris from lime­stone ledges and cliffs, and leaf mold from hardwood forests ex­cept, under most instances, from oaks. The pH is from 7.0 to 8.0.

To the above classification we might add a group for gardeners who live in the Great Plains area where rainfall is too light to leach out alkalizing chemicals, resulting in alkali- and salt-sick soils typ­ical of such regions with a pH of from 8.0 to 9.0.

Two in One

Dr. Wherry linked minimacid and minimalkaline soils into a broader class he called Circumneutral. This may be a somewhat bookish word, but it does convey the impression of a wide range of plants that will thrive in a wide range of soil reactions, from 6.0 to 8.0. It has been my experience, however, that even these tolerant plants—including most annuals, perennials and so on—respond better to a narrower range of pH, say from 6.0 to 7.3 or from 6.0 to 6.9. Under these less alkaline conditions the plants7 leaf color is better, even though there is no other sign that soil pH has affected growth.

My gardening preference is for plants classed as circumneutral at a pH range of 6.0 to 6.9. Within this range all the food elements they need are available in highest concentration except perhaps for iron, zinc and copper, which are, however, present in large enough amounts for normal growth.

Organic matter, a vital soil ingredient that we will discuss later, has an important effect on pH. When present in the soil in generous amounts it "buffers" the bad effects either of a too acid or a too alkaline soil. For this reason, plants growing in a soil high in organic matter will often do well even though the pH reading is nearly a point either way from the ideal range. As will be seen by consulting the plant pH preference list which appears in the Appendix, most plants commonly grown in gardens do best within a pH range of 6.0 to 6.9. Only those which require an acid soil (rhododendrons and blueberries, to name two, usually called ericaceous plants) require a lower pH.

Here, then, is a key to better plant growth—keep the soil pH between 6.0 and 6.9 and keep up the organic content.

Then and Now

Grandfather knew nothing about the pH scale. Even if the theory had been invented in his day he probably would have called it "book rubbish." Nevertheless, he knew enough about soil reaction to spread load after load of marl or ground limestone on his fields every third or fourth year. He did this to "make the soil sweet and keep the land up," as he phrased it. He knew that when the lime began to "dis­solve" his crops grew better and he made more money. Too, the soil would be in better "tilth" and would turn more easily under the plow.

In this simple but important chore he was repeating what genera­tions of farmers since Roman days had done before him—overcom­ing a too-acid soil with lime. Although ignorant of chemistry, he knew a soil was "sour" if sorrel grew well, or "sweet" if clover and alfalfa thrived. Sometimes, if in doubt about the time to lime, he would touch a grain or two of soil to his tongue. If it had a soapy taste he knew it had some lime in it but if it tasted acid or sour he laid plans to supply the missing element. Plant growth improvements seemed so directly connected with these applications that he thought of limestone as "rock manure," supplying something the plants had to have. Today we know that while lime does supply calcium, usually the indirect effects are much more important than the direct. This in no way detracts from the soundness of grandfather's methods—he got results.

You need know little more than he did in order to use pH cor­rectly. True, you have the help of modern soil testing equipment much more accurate than the human tongue, and you can regulate the actual pH range more accurately. But without any scientific background you can correctly apply lime and fertilizer on acid soils, or if your soil is too alkaline you can apply sulfur to bring down the pH.

LIME AND SULFUR ALTER pH

As mentioned, when growing all but a limited number of plants (acid-loving species like blueberries, mountain laurel [kalmia], hollies, camellias, azaleas and rhododendrons), you should strive for a soil reaction somewhere between 6.0 and 6.9. Ordinarily, a reading of 7.3 is as high as your garden soil should be allowed to go if you are growing the usual mixture of annuals, perennials, vegetables and shrubs. For many plants, even this is a trifle high. Growth would be better if sulfur were used to lower the reading.

It is difficult to make exact recommendations for amounts of chemicals needed to raise or lower soil pH. Light soils require lesser and heavy soils need greater quantities of acidifying or alkalizing agents. A soil high in organic matter has a different requirement than one low in organics. If the organic matter in the soil has been re­duced to humus, the "buffering" effect of the humus usually in­creases the amount of pH alteration material needed.

The only sensible way to solve the problem is to treat the soil and recheck the pH reading after two weeks, after a month, and again after two months. If not enough material was applied, simply add more. If too much, there is no harm in using sulfur to undo the effects of limestone, and vice-versa.

Here are some suggested amounts:

To raise the pH of light sandy loams one full point (i.e., from 5.5 to 6.5) add 35 pounds of ground limestone to 1,000 square feet. On a medium loam soil, apply 50 pounds, and on a heavy clay loam, 70 pounds. (Either agricultural limestone or the fine chips used for top-dressing driveways can be used.)

To lower the pH of light sandy loams one full point (i.e., from pH 6.0 to 5.0) add 10 pounds of dusting sulfur per 1,000 square feet. In medium loam soil, add 15 pounds, and to heavy clay loam, 20 pounds. (Ordinarily dusting sulfur is perfectly satisfactory; no need to pay a premium for special grades.)

Within the 6.0 to 6.9 range, all foods needed by the majority of shrubs, annuals, perennials and other "average" garden plants are available in the soil in soluble form, provided the foods are present in the first place. Bacteria thrive and do their vital work better in this pH range, and certain potential poisons, such as aluminum, are locked up so they cannot injure plant roots.

Pay particular attention to the above phrase, "provided the foods are present in the first place." No matter how pH is juggled up or down, it cannot make available any food element that is not present. For example, plants may show by certain signs that they are not taking up iron from the soil. If the pH is high, we might suspect that iron is present but locked up in insoluble form. If, however, plants still show a deficiency of iron after sulfur has been applied to lower the pH, then we know that iron is lacking and must be supplied in a form plants can absorb.

Because plants tend to remove calcium from the soil as they grow, which in turn lowers pH, lime is closely tied in with our use of the pH theory. To a considerable degree, proper lime application (as­suming supplies of plant nutrients are ample) becomes the key to our success with garden soils. This does not mean that the indis­criminate use of lime year after year is the right way to run a garden. Too much alkalinity can do as much harm as too little. This is why no "rule of thumb" can be set up that will work all the time in every garden. The only safe guide is an actual test of soil reaction.

MAKING THE pH TEST

There are several methods of making a pH test. The most accu­rate, and one that will probably be used if you send soil samples to your state agricultural experiment station, is an electrical "bridge" which checks the reaction by electrical resistance. This is an expen­sive piece of apparatus and one that few amateur gardeners are likely to buy.

While this device gives extremely true pH readings, I strongly recommend that gardeners use home test kits despite the probability of less accurate results. There are a number of reasons why. First, since most stations charge for each sample submitted, the economy-minded gardener's usual practice is to mix soil from several sites into one sample and submit it for an "average" test. If the soil through­out the garden is uniform, this average test method is satisfactory. Such a situation, however, is quite unusual. For example, black dirt used as topdressing over backfill around most speculative (develop­ment) houses may come from piles of earth scraped from half a dozen different sites.

My own vegetable garden, while on land graded nearly a hundred years ago, is an example of how much the soil in one plot can vary. In one area it lies over an old creek bed that was filled-in in 1868 to make a level building plot. Since this was before bulldozer days, I can just imagine an old-fashioned horse-drawn scoop cutting down the hill on which the house was built, partially filling the creek with this earth. In another spot I find prairie soil of a different character. In one corner of the plot, tons of coal ashes were used in a mixture with some black soil and manure from a barn that once occupied the site.

All this soil history has been revealed gradually. Over a period of years I have double-dug the entire garden, uncovering everything from old barn footings to a buggy dashboard and an 1850 whiskey bottle. Incidentally, I unearthed a midden of undecayed chicken bones and rabbit skulls, which merely confirmed for me again the fact that bone (a good source of phosphate) resists decay for decades.

In this one garden, a small section filled with old eroded woods soil had an acid pH of 5.8, while the other end of the garden, where ashes predominated, tested 7.8. Obviously, if I had mixed these two to get an "average" sample I would have received a report of no value to me in working with either the acid or alkaline soils.

If you have your own test kit, however, various sections in your garden can be tested and treated individually. Even though such kits cannot be expected to be much more accurate than within two or three fractions of a point, they are much better in practical applica­tion than the more accurate electrical bridge tests of a single "average" sample.

Best Kit for You

I recommend kits which use a special liquid that, when dropped onto a crumb of soil, turns color according to the degree of acidity or alkalinity of the sample. This can then be compared directly with a color chart to get the reading. Some kits of this type supply a small china dish to hold the sample while others use glass tubes. A low cost unit which uses strips of wax paper is perfectly satisfactory once you learn to juggle the folded paper and read it against the chart.

Be sure to buy a unit that gives readings in direct pH figures, not in some mythical A, B, C system or in Roman numerals. Kits of the latter type are often sold cheaply or given away, but usually have to be used with a special product. Many of these products incorporate undesirable chemicals (such as aluminum sulfate used for acidify­ing) which you want to avoid.

Your first step in making a pH test is to get a uniform sample of the plot being tested. Don't use surface soil (roots rarely grow there) but dig down six inches. Avoid large lumps of organic matter unless you have a true organic soil such as peat or muck.

Soil should be moist for several days before you test it. (Drought affects the pH by killing off large numbers of bacteria, releasing organic acids which result in a false reading.) If the sample of soil used is allowed to dry for an hour or so in a shaded spot, it will give a clearer reading when the liquid is run through.

Over Sixty

Do not test cold soil. Cold inactivates bacteria, resulting in a false reading. Wait until soil temperature (not air temperature) has been above 60 degrees for at least two weeks, then test.

In making a test, don't neglect the subsoil, unless you are the lucky owner of a four-foot-deep black prairie loam. We forget that if surface soil is only six to ten inches deep, most roots of many crops will grow through that upper layer and get the majority of their nourishment from the subsoil. In checking soil where deep-rooted trees and shrubs are growing or will be planted, perhaps only sub­soil need be considered.

Several years ago I saw a good example of why subsoils should be checked. A friend of mine north of Chicago had some magnificent oaks growing at the foot of a steep hill on his property. Heavy wash­ing rains fell all spring, and suddenly my friend noticed that the oak leaves were beginning to turn yellow. A tree man sprayed them with an iron solution and they turned green for a while but soon reverted to yellow.

Tests of the surface soil around the oaks showed it was fairly high in pH, about 6.0, but low enough so that some iron would stay in solution. When, however, we checked the subsoil, we found it tested 7.5. This we diagnosed as a temporary alkaline condition pro­duced by lime washed out of the upper part of the hill by the heavy rains and carried down the hill, along a gravel layer just under the surface, to the roots of the oaks.

Holes bored around each tree and filled with ferrous ammonium sulfate soon brought about improvement in leaf color and tree growth. Drains to lead rain-wash from above into side channels, away from the oaks, prevented further trouble.

HOW pH AFFECTS NUTRIENTS

Now that you have a pH reading, the next problem is what it tells you. We want to know how pH affects food elements in soil.

Food elements needed by plants may not be available to them even though present in soil. Phosphorus is an excellent example. Vital to all life, it enters into every phase of plant growth, from beginning to end. It is a major ingredient in cell nuclei, and carries over in chromosomes to the succeeding generation. It is essential to photosynthesis, one of the most critical processes in the entire world by which energy is captured from the sun and used to run the or­ganic "motors" of every living cell—that is, to produce food.

Oddly enough, for all its essential nature, phosphorus is not read­ily available in soils except within a rather narrow pH range. When a pH of 5.0 or lower is reached, phosphorus is chemically trapped by aluminum compounds and converted into highly insoluble, fixed forms which are unavailable to plants. Iron is captured in a sim­ilar way.

When pH goes up and calcium is present in generous amounts, phosphorus reacts with it to form other highly insoluble compounds. Superphosphate is commonly applied to the soil to supply phos­phorus. If it contains any amount of fluorine (as is sometimes the case), and the soil pH is 7.8 or above, fluorapatite, the most insoluble of all phosphate compounds, is formed.

We need not go into the chemistry and interactions of all elements essential to plant growth; the important fact is that availability of nutrients in soil depends so directly on pH that adjusting this factor is something every gardener should know how to do. All of the min­eral elements plants need for growth are available between readings of 6.0 and 6.9. Even above and below this range, the minerals are available to a certain extent, so that if small errors are made in read­ing the tests, it is not too serious.

Here, then, is a basic principle in managing soils—keep pH be­tween 6.0 and 6.9 for all plants classed as circumneutral and you won't go far wrong. This assumes, of course, that the vital food elements were either in the soil when you tested it, or that you will supply them.

Plants with Different Needs

When you examine the list of plants and their soil preferences given in the Appendix, you will see that some of them do best at pH readings below 6.0 while a few are able to tolerate alkalinity above 7.5 to 8.0. Why these exceptions?

Here is one of those mysteries that makes soil such a fascinating study. These plant exceptions need nutrient elements which, accord­ing to the pH theory, should not be available at the readings listed for them. The answer is that soil is not a uniform, homogeneous mass, like a great blob of plastic with every molecule made up of identical atoms. Instead, soil is a composite: a great macrocosmos and micro-cosmos rolled into one. Within the same grain of soil can be found acid and alkaline elements existing side by side, "buffered" from attacking each other by a series of checks and balances that allow them to act according to their individual reactions.

Many soil grains are actually "sandwiches" with an acid core and a layer of alkaline material on either side. In acid soil, not all alkaline elements are neutralized, and vice versa. This is particularly true if the soil contains a good percentage of organic matter, the most powerful "buffer" of all.

Acid-soil plants owe their ability to grow at low pH readings to the fact that their roots can tolerate some free aluminum. At readings below 5.5, aluminum is set free unless it combines with phosphorus. This does not mean that the ability of plants to resist aluminum is unlimited: many growers of camellias, rhododendrons, azaleas and other ericaceous plants keep applying aluminum sulfate to acidify soils only to find that their plants deteriorate. The roots may be blackened and injured to a point where they can no longer take up nutrients.

Acid-Soil Plants Use Ammonia

The various acid-soil plants do not need their organic foods broken down as completely as do plants that require a more alkaline soil. The forms of bacteria and fungi that break down protein to release nitrogen are sensitive to pH. At readings below 6.0 only forms that break the pH down to ammonia are active. At higher pH readings, other organisms carry the breakdown further to produce nitrate nitrogen which all plants can use.

Acid-soil plants—rhododendrons, camellias, kalmia, blueberries— are able to use ammonia nitrogen because mycorrhizae on their roots do the converting of ammonia into nitrate form. (For a full explana­tion of the role of mycorrhizae, see Chapter Eight on Microorgan­isms.) Plants in the circumneutral group, on the other hand, must depend upon soil bacteria to carry out this final nitrogen conversion stage. Since these bacterial forms work best in the soil at a pH of 6.0 or higher, circumneutral plants do not survive in acid soil.

Alkaline and Alkali Soils

At readings above pH 7.3 to 7.5, there is sharp reduction in the number of species which will grow well. Most of those that do sur­vive are plants with relatively light green foliage, such as alpines and plants from relatively dry areas with bright sunshine. Probably they are able to manufacture their food with less chlorophyll than is needed by plants from areas where sunshine is less intense. For this reason, alpines and others need less of the elements such as iron, cop­per and manganese that become unavailable in highly alkaline soils.

Another factor involved is organic matter. Since at a high pH it does not break down rapidly, it tends to accumulate. Particularly is this true in less humid climates, where soils with high pH are usually found. The effect of organic matter is to buffer high alkalinity. It absorbs certain food elements so they are still available to plants in spite of high pH. Most alpines will be found growing in pockets where organic matter has been trapped, while on the Great Plains, shrubs like the buffaloberry (Shepherdia) grow along water courses where flood waters deposit richer soils.

About the only direct effect of too high a pH is aluminum toxicity. At readings above 8.5, aluminum is released and will seriously injure plant roots (just as it does at pH 5.5). Aluminum is also harmful be­cause it makes phosphorus unavailable to plants. Among the vegeta­bles which are seriously injured by even small amounts of free alu­minum are lettuce, onions and beets.

LIME: EFFECTS ON PH AND SOIL CONDITION

Ground limestone is a dual-purpose mineral. We apply it primarily to overcome acidity but, in addition, we receive a bonus in soil con­ditioning (see Chapter Eleven) that alone is often worth the cost and effort of application. An application of ground limestone can give a heavy clay loam a loose, friable, well-aerated character. Such a soil allows water to penetrate readily without running off, and turns easily under the plow.

This change in condition is due to an electrical-chemical-physical reaction known as flocculation. In this reaction, each lime particle acts as an acid radical to attract several clay particles. These clumps of clay with a lime core form crumbs, which make the soil much more porous than it was before treatment.

The number of particles attracted in a clump varies with the type of clay. In general, clays from northern climates produce larger crumbs. For example, I have seen photographs taken with an elec­tronic microscope which showed eight clay particles clumped around a single lime core. Theoretically, then, a northern clay soil treated with enough lime to attract all the clay it contains should be eight times as permeable as it was before treatment. This degree of im­provement is, of course, never reached in practice.

On some southern clays I have found that an application of as much as half a pound of ground limestone to a square foot (500 pounds to 1,000 square feet) has in most cases loosened them to a surprising degree without raising the pH excessively. If you use anything like this much lime, I recommend that you make a soil pH test in a month or so, just for safety. If the test shows the pH is too high, sulfur can be used to reduce the reaction without seriously affecting the crumbs of clay and lime.

pH and the Competitive Ability of Plants

Logically then, rhododendrons and other ericaceous plants (broad-leaved evergreens) thrive in acid soil because their roots can take up the form of nitrogen available to them in acid soil. Such plants also have a high iron requirement; iron is an element which becomes less and less plentiful as alkalinity increases. Here, then, is a key to the competitive ability of various species of plants. We expect to find rhododendrons and azaleas ablaze on acidic granite ridges in the Great Smokies, while clovers thrive on lime-rich soils in the Middle West. We seldom see clover growing well in New England except where farmers have applied lime freely. Thus pH has a way of deter­mining the appearance of our native landscape by favoring one group of plants over another.

Chapter Digest

Soil pH is a scale of acidity-alkalinity that is neither complex to under­stand nor difficult to use. Relative acidity affects many aspects of soil use and plant productivity. Make pH tests of separate samples of soil from different parts of your property; do not mix the samples. Use a home "do-it-yourself" test kit if your budget cannot stand the cost of professional testing of numerous samples. Investigate first, however, for your state agricultural station may offer the service free. Armed with test results and the knowledge of pH presented in this chapter, you can manipulate applications of lime, sulfur, organic matter and fertilizer to best advantage so that your soil will favor desirable plant growth.

Are You Ready To Move Onto The Next Lesson? Click Here...