Soils
Much Ado About pH
By Don Julien
(Originally published in Rose Petals, October, 1998 newsletter of the Seattle Rose Society)
All summer we have been babying our roses, feeding, watering, deadheading, checking for insects and diseases, as well as enjoying the blooms. Our attention has been up close and personal. As fall arrives, it is time to step back and assess our bushes from a broader perspective. What did well or poorly, and why? If you notice that certain bushes did not perform well, there may be one of several culprits (e.g., the variety, water stress, crown or root gall or rot), but a common and hidden one is soil pH.
An out-of-balance soil pH will interfere with the plant’s uptake of various nutrients from the soil. This can be countered or hidden temporarily with foliar feeding, but a rosebush needs more than what can be provided in a spray. If you foliar feed (Miracle-Gro®, fish fertilizer, liquid kelp), your leaves may be dark green, and your blooms rich with color, but if the bush remains reluctant to grow to its expected size, or develop new canes, you may have a pH problem.
You often notice a pH problem as a nutrient deficiency. Most likely, leaves will turn a light green. Whether it happens to new growth or old growth, whether the whole leaf lightens or just the veins, or just the part between veins suggests what nutrient is missing. Another clue is weak or spindly growth. Finally, if a bush has been a magnet for disease and insects all season, you probably have a water and/or a nutrient problem. (If your bushes lack enough water, they also have not been getting enough nutrients.)
The temptation is strong to react to a perceived nutrient problem by dumping on more fertilizer. Resist that impulse. First, check for a water problem. Dig down about a foot with a trowel; if you are dealing with a rose in a container, pull it out. Water problems include too dry and too wet. If you squeeze the soil and water runs out, it’s too wet.
If water is not the problem, then a nutrient deficiency may be next. But the only way to tell if a nutrient is really low or missing is by a soil test. Before you go to the bother and expense (although it is a good idea to get a soil test every year or two or three), a simple test for pH may suggest that the nutrients are really there, but simply unavailable.
What is pH?
First a little bit of basic chemistry. We probably all remember that any solution is really a bunch of molecules running around; in the case of water, H2O…two hydrogen atoms bound to an oxygen atom. I remember in elementary school being told that when you dissolve something, such as sugar, in water, it doesn’t "disappear"; the sugar molecules just slip in between the water molecules.
But this explanation was a bit simplistic. Any solution does have a bunch of molecules running around, but many of those molecules split up into single atoms or smaller clusters, called ions. Ions have a small electrical charge that helps them bind back together in the original molecule, or to bind with other types of ions to form new molecules (remember chemical reactions?). The charges are positive or negative, based on the nature of the atom or atoms in the cluster. Molecules always split apart the same way, since certain bondings are weaker than others (Water always splits into H+ and OH-, never HH and O). The positive and negative charges act much the same as magnetic positive and negatives. Opposites attract; equals repel.
In plain old water, the number of molecules that split off into ions is always the same. As we add stuff to water, however, we bind up H+ or OH- ions so there may be more of one or the other floating around free. Scientists have developed a way to measure the number of free ions floating around, and use a term called "p" to express that number. So soil scientists may talk about pNa, pK, pOH, but the most common measure is pH.
In plain old water, the number of H+ and OH- ions are the same: 0.0000001 grams per liter, or 10-7, or expressed as a pH of 7. If the number of H+ ions increases to 0.000001 grams per liter, or 10-6, the pH is 6, 10 times a pH of 7. If the number of H+ ions increases to 0.00001 grams per liter, or 10-5, the pH is 5, 100 times a pH of 7. The more a pH reading various from 7, up or down, the number of H+ ions increases or decreases exponentially. This is important, because it demonstrates why it is much more difficult to correct an extremely high or low pH than one than is off slightly.
When you add something to water (in our case, it is generally a fertilizer that dissolves into the water in the soil), some of its molecules also separate into ions. Many of our nutrients are positive ions (also called cations): calcium, magnesium, sodium, iron, manganese, copper, zinc, potassium, and nitrogen (from NH4+, "ammonium"). Phosphorous, sulfur, boron, molybdenum and nitrogen (from NO3-, "nitrate") generally come from negative ions (called anions).
How do the plants get it?
Now we turn to a little physics. Soil is made up of gazillions of tiny particles. These particles have rough edges. Think of one of them as a basketball. A basketball is not smooth, but has lots of tiny bumps all over it. The little bumps make it easier for the basketball player to grip the ball. The little bumps on a soil particle all have a slight negative charge.
If you drop a basketball in water, and then remove it, all the water drips off except for a thin film, held in place by the ball’s rough surface. Soil particles in "moist soil" have a similar film, which, of course includes molecules and ions. The slight negative charges on the little bumps attract the positive-charged hydrogen ions. Since the negative charge is slight, the hydrogen ions move on and off the "basketball" fairly easily. When we add fertilizer to the water, the nutrient cations can also cling to the "basketball," replacing some hydrogen ions.
Tiny root hairs grip the soil particles like a basketball player, and pluck the nutrient cations from the particle, not from the water in the soil. (Nutrient anions are generally taken up directly from the solution.)
What causes the pH problem?
Most often, our pH problems occur when the hydrogen ion count is high (the pH number gets lower, also called "acidic"). With an excess of hydrogen ions floating around, the ones gripping the little bumps are more reluctant to turn over a spot to the fertilizer cations. With some fertilizer molecules, the abundance of hydrogen ions in solution discourages the molecule from splitting altogether. The fertilizer molecules and ions soon leach out of the soil. If the pH gets below 5 or above 8, calcium cations bond with phosphate anions, depriving the plant of both.
In acidic soils (pH lower than 7), nitrogen, potassium, calcium, phosphorous and magnesium will become less available, and completely unavailable at a pH of 4. In extremely acidic soils, aluminum, manganese and iron can become toxic to the plant.
In alkaline soils (pH above 7), phosphorous, iron, copper, zinc, boron and manganese will become less available. Extremely alkalinity will release toxic amounts of molybdenum.
The two charts show the availability of nutrients at different pH’s. Most of our local soils are mineral aggregate (glacial till). The ideal pH for roses in these soils would be 6.5 to 7.0. But if you have amended your soil, adding large quantities of organic matter, the ideal pH will be lower, more in the range of 6.0 to 6.6 (your soil wouldn’t be muck, but that chart demonstrates the shift in nutrient availability with the increase in organic matter).
How do I find out my pH?
There are several ways to test pH. Garden centers and hardware stores have simple test kits that use indicator solutions. These tend to be horribly difficult to read, since you try to compare the color of a solution to a badly printed chart reflecting a pH range from 4 to 10. These are marketed to cover the widest range of garden pH use, so the indicator solution’s range is a broad one. There are indicator solutions that measure closer ranges of pH.
One application of a closer range of pH is with test strips. With these, you make a solution from your soil, dip the end of the strip in the solution, and then compare the strip color with a color guide on the package. The Bradenton-Sarasota Rose Society Bulletin published an article by Harold Walters that recommended a pH indicator strip that measured pH from 5.2 to 6.8, and the color chart is printed on each strip. The strips are available from Markson Lab Sales, PO Box 1359, Hillsboro OR 97123-1359, 1-800-942-8626. A box of 200 strips is $17.50 plus $5.52 shipping and handling. At the time, the catalog number was 2628-990 (5.2 to 6.8 range).
A cheap pH meter, about $20, will give you a quick reading, anywhere and anytime, with no fuss. The meters are considered relatively inaccurate, but if you are trying to get an idea of your pH within .5, it can be useful. The meter includes a probe that you push into the wet soil, and a reading comes up on the meter. Although many garden centers have carried them in the past, I’ve lately only seen them in the catalog from Charley’s Greenhouse Supply (www.charleysgreenhouse.com, 1-800-322-4707).
Finally, you can prepare soil samples, bring them to a meeting, and fill out a reply postcard (bring a stamp). Sydney Allison-Towle will take the samples home, test them on the Rose Society’s pH tester, and then send you the results.
What can I do to correct pH?
Organic matter will lower pH slightly to moderately. Compost is more neutral, but peat, sawdust, conifer needles or chippings will reduce pH more dramatically. Sulfates will generally lower pH; ammonium sulfate (a common source of nitrogen in fertilizer mixes), iron sulfate (Ironite®), magnesium sulfate (epsom salt) will reduce pH, although potassium sulfate is neutral. Sulfur is also a direct way of reducing a high pH. As you can see, many products we add to our soils as nutrient supplements have the added effect of reducing pH. This, plus the naturally acidic nature of our local soils, is why we rarely have a problem of high pH, other than from a mistake made while raising pH.
Raising pH is not a fast process. This is why we recommend you attend to it in the fall. The most common method of raising pH is by adding lime to the soil. There are several kinds of lime. Do not use builders lime or quicklime; they will fry your plants. The preferred limes for garden use are hydrated lime and ground lime.
Hydrated lime will act faster, changing the pH within 2 to 3 months, but can burn roots. It is also caustic, so handle it with gloves. Hydrated lime would be useful in building a new bed (or in a vegetable bed) that won’t be used until spring.
Ground lime is slower, taking up to 6 months to work its magic, but is safe for use around existing plants. There are two types of ground lime, calcitic and dolomitic. Calcitic lime is 70% calcium carbonate and 15% magnesium carbonate; dolomitic lime is 50% calcium carbonate and 40% magnesium carbonate. Dolomitic lime is generally easier to find, and adds both calcium and magnesium to your soil. You might want to look for calcitic lime, however, if you have been applying heavy doses of epsom salt (magnesium sulfate) to your beds. (Normally we recommend one or two applications of epsom salt per season; some people make four or five…a "heavy dose.")
The chart indicates how much lime should be added to different types of soil to change the pH up to a pH of 6.5. A pound of lime is one cup. Do not overdo an application: no more than 6 lbs. per 100 sq ft per application. If your soil needs more, make two applications a couple of months apart.
To Raise pH
The following chart shows the number of pounds of ground limestone needed per 100 sq ft to raise soil PH to 6.5. If the amount is large (more than 6 lbs), add it in stages.
An out-of-balance soil pH will interfere with the plant’s uptake of various nutrients from the soil. This can be countered or hidden temporarily with foliar feeding, but a rosebush needs more than what can be provided in a spray. If you foliar feed (Miracle-Gro®, fish fertilizer, liquid kelp), your leaves may be dark green, and your blooms rich with color, but if the bush remains reluctant to grow to its expected size, or develop new canes, you may have a pH problem.
You often notice a pH problem as a nutrient deficiency. Most likely, leaves will turn a light green. Whether it happens to new growth or old growth, whether the whole leaf lightens or just the veins, or just the part between veins suggests what nutrient is missing. Another clue is weak or spindly growth. Finally, if a bush has been a magnet for disease and insects all season, you probably have a water and/or a nutrient problem. (If your bushes lack enough water, they also have not been getting enough nutrients.)
The temptation is strong to react to a perceived nutrient problem by dumping on more fertilizer. Resist that impulse. First, check for a water problem. Dig down about a foot with a trowel; if you are dealing with a rose in a container, pull it out. Water problems include too dry and too wet. If you squeeze the soil and water runs out, it’s too wet.
If water is not the problem, then a nutrient deficiency may be next. But the only way to tell if a nutrient is really low or missing is by a soil test. Before you go to the bother and expense (although it is a good idea to get a soil test every year or two or three), a simple test for pH may suggest that the nutrients are really there, but simply unavailable.
What is pH?
First a little bit of basic chemistry. We probably all remember that any solution is really a bunch of molecules running around; in the case of water, H2O…two hydrogen atoms bound to an oxygen atom. I remember in elementary school being told that when you dissolve something, such as sugar, in water, it doesn’t "disappear"; the sugar molecules just slip in between the water molecules.
But this explanation was a bit simplistic. Any solution does have a bunch of molecules running around, but many of those molecules split up into single atoms or smaller clusters, called ions. Ions have a small electrical charge that helps them bind back together in the original molecule, or to bind with other types of ions to form new molecules (remember chemical reactions?). The charges are positive or negative, based on the nature of the atom or atoms in the cluster. Molecules always split apart the same way, since certain bondings are weaker than others (Water always splits into H+ and OH-, never HH and O). The positive and negative charges act much the same as magnetic positive and negatives. Opposites attract; equals repel.
In plain old water, the number of molecules that split off into ions is always the same. As we add stuff to water, however, we bind up H+ or OH- ions so there may be more of one or the other floating around free. Scientists have developed a way to measure the number of free ions floating around, and use a term called "p" to express that number. So soil scientists may talk about pNa, pK, pOH, but the most common measure is pH.
In plain old water, the number of H+ and OH- ions are the same: 0.0000001 grams per liter, or 10-7, or expressed as a pH of 7. If the number of H+ ions increases to 0.000001 grams per liter, or 10-6, the pH is 6, 10 times a pH of 7. If the number of H+ ions increases to 0.00001 grams per liter, or 10-5, the pH is 5, 100 times a pH of 7. The more a pH reading various from 7, up or down, the number of H+ ions increases or decreases exponentially. This is important, because it demonstrates why it is much more difficult to correct an extremely high or low pH than one than is off slightly.
When you add something to water (in our case, it is generally a fertilizer that dissolves into the water in the soil), some of its molecules also separate into ions. Many of our nutrients are positive ions (also called cations): calcium, magnesium, sodium, iron, manganese, copper, zinc, potassium, and nitrogen (from NH4+, "ammonium"). Phosphorous, sulfur, boron, molybdenum and nitrogen (from NO3-, "nitrate") generally come from negative ions (called anions).
How do the plants get it?
Now we turn to a little physics. Soil is made up of gazillions of tiny particles. These particles have rough edges. Think of one of them as a basketball. A basketball is not smooth, but has lots of tiny bumps all over it. The little bumps make it easier for the basketball player to grip the ball. The little bumps on a soil particle all have a slight negative charge.
If you drop a basketball in water, and then remove it, all the water drips off except for a thin film, held in place by the ball’s rough surface. Soil particles in "moist soil" have a similar film, which, of course includes molecules and ions. The slight negative charges on the little bumps attract the positive-charged hydrogen ions. Since the negative charge is slight, the hydrogen ions move on and off the "basketball" fairly easily. When we add fertilizer to the water, the nutrient cations can also cling to the "basketball," replacing some hydrogen ions.
Tiny root hairs grip the soil particles like a basketball player, and pluck the nutrient cations from the particle, not from the water in the soil. (Nutrient anions are generally taken up directly from the solution.)
What causes the pH problem?
Most often, our pH problems occur when the hydrogen ion count is high (the pH number gets lower, also called "acidic"). With an excess of hydrogen ions floating around, the ones gripping the little bumps are more reluctant to turn over a spot to the fertilizer cations. With some fertilizer molecules, the abundance of hydrogen ions in solution discourages the molecule from splitting altogether. The fertilizer molecules and ions soon leach out of the soil. If the pH gets below 5 or above 8, calcium cations bond with phosphate anions, depriving the plant of both.
In acidic soils (pH lower than 7), nitrogen, potassium, calcium, phosphorous and magnesium will become less available, and completely unavailable at a pH of 4. In extremely acidic soils, aluminum, manganese and iron can become toxic to the plant.
In alkaline soils (pH above 7), phosphorous, iron, copper, zinc, boron and manganese will become less available. Extremely alkalinity will release toxic amounts of molybdenum.
The two charts show the availability of nutrients at different pH’s. Most of our local soils are mineral aggregate (glacial till). The ideal pH for roses in these soils would be 6.5 to 7.0. But if you have amended your soil, adding large quantities of organic matter, the ideal pH will be lower, more in the range of 6.0 to 6.6 (your soil wouldn’t be muck, but that chart demonstrates the shift in nutrient availability with the increase in organic matter).
How do I find out my pH?
There are several ways to test pH. Garden centers and hardware stores have simple test kits that use indicator solutions. These tend to be horribly difficult to read, since you try to compare the color of a solution to a badly printed chart reflecting a pH range from 4 to 10. These are marketed to cover the widest range of garden pH use, so the indicator solution’s range is a broad one. There are indicator solutions that measure closer ranges of pH.
One application of a closer range of pH is with test strips. With these, you make a solution from your soil, dip the end of the strip in the solution, and then compare the strip color with a color guide on the package. The Bradenton-Sarasota Rose Society Bulletin published an article by Harold Walters that recommended a pH indicator strip that measured pH from 5.2 to 6.8, and the color chart is printed on each strip. The strips are available from Markson Lab Sales, PO Box 1359, Hillsboro OR 97123-1359, 1-800-942-8626. A box of 200 strips is $17.50 plus $5.52 shipping and handling. At the time, the catalog number was 2628-990 (5.2 to 6.8 range).
A cheap pH meter, about $20, will give you a quick reading, anywhere and anytime, with no fuss. The meters are considered relatively inaccurate, but if you are trying to get an idea of your pH within .5, it can be useful. The meter includes a probe that you push into the wet soil, and a reading comes up on the meter. Although many garden centers have carried them in the past, I’ve lately only seen them in the catalog from Charley’s Greenhouse Supply (www.charleysgreenhouse.com, 1-800-322-4707).
Finally, you can prepare soil samples, bring them to a meeting, and fill out a reply postcard (bring a stamp). Sydney Allison-Towle will take the samples home, test them on the Rose Society’s pH tester, and then send you the results.
What can I do to correct pH?
Organic matter will lower pH slightly to moderately. Compost is more neutral, but peat, sawdust, conifer needles or chippings will reduce pH more dramatically. Sulfates will generally lower pH; ammonium sulfate (a common source of nitrogen in fertilizer mixes), iron sulfate (Ironite®), magnesium sulfate (epsom salt) will reduce pH, although potassium sulfate is neutral. Sulfur is also a direct way of reducing a high pH. As you can see, many products we add to our soils as nutrient supplements have the added effect of reducing pH. This, plus the naturally acidic nature of our local soils, is why we rarely have a problem of high pH, other than from a mistake made while raising pH.
Raising pH is not a fast process. This is why we recommend you attend to it in the fall. The most common method of raising pH is by adding lime to the soil. There are several kinds of lime. Do not use builders lime or quicklime; they will fry your plants. The preferred limes for garden use are hydrated lime and ground lime.
Hydrated lime will act faster, changing the pH within 2 to 3 months, but can burn roots. It is also caustic, so handle it with gloves. Hydrated lime would be useful in building a new bed (or in a vegetable bed) that won’t be used until spring.
Ground lime is slower, taking up to 6 months to work its magic, but is safe for use around existing plants. There are two types of ground lime, calcitic and dolomitic. Calcitic lime is 70% calcium carbonate and 15% magnesium carbonate; dolomitic lime is 50% calcium carbonate and 40% magnesium carbonate. Dolomitic lime is generally easier to find, and adds both calcium and magnesium to your soil. You might want to look for calcitic lime, however, if you have been applying heavy doses of epsom salt (magnesium sulfate) to your beds. (Normally we recommend one or two applications of epsom salt per season; some people make four or five…a "heavy dose.")
The chart indicates how much lime should be added to different types of soil to change the pH up to a pH of 6.5. A pound of lime is one cup. Do not overdo an application: no more than 6 lbs. per 100 sq ft per application. If your soil needs more, make two applications a couple of months apart.
To Raise pH
The following chart shows the number of pounds of ground limestone needed per 100 sq ft to raise soil PH to 6.5. If the amount is large (more than 6 lbs), add it in stages.
Pounds of Lime per 100 sq ft
Current pH
Sandy Loam
Loam
Clay Loam
4.0
11.5
16
23
4.5
9.5
13.5
19.5
5.0
8
10.5
15
5.5
6
8
10.5
6.0
3
4
5.5
To Lower pH
The following chart lists the pounds per 100 sq ft needed of various products to lower pH one unit.
Soil
Sulphur
Aluminum Sulphate
Iron Sulfate
Light sandy soil
½
2½
3
Silt loams & clay loams
2
6½
7½
