Question: I’ve heard many people talk about checking the pH of the soil for the lawn, garden and landscape. Is it really that important or some test to make more money. Marcus, Bryan, Texas
Answer: Marcus, do not shy away from the term pH because it sounds mysterious and hard to understand? If you do, you’re missing a chance to grow better plants with less effort and at a lower cost.
Gardeners don’t hesitate to refer to the thermometer when frost threatens. They know that when the mercury skids to 32 degrees certain plants will be killed even though they may have no knowledge of heat loss by radiation, the theory of ice crystallization and other scientific principles involved. They go merrily about the task of cooking an egg and completely ignore the theories of kinetic energy, coagulation of protein, etc. Yet, faced with a simple measuring device like the pH scale, many gardeners are nonplused.
The Neutral Point of pH – 7.0
Actually, there are fewer scientific phenomena involved in using pH than in boiling an egg or in freezing water. Try to think of pH as a scale of 14 steps, reading from 0.0 (the most acid) to 14.0 (the most alkaline). Half way between, at 7.0, the acid and alkaline elements are evenly balanced, so we call this the neutral point.
If we have a soil that reads over 7.0, we can make it acid by adding some acidifying material like sulfur. If it’s too acid, we can add lime and raise the pH.
How a Plant Feeds
But how does all the pH numbers affect the feeding of plants? Before we try to understand the effect of pH on plants, we should understand how a plant eats or feeds. Remember that a plant can’t tuck a napkin under its chin and sit down to a meal of potatoes and roast beef with all the trimmings. Instead, plants live on a thin “soup” or solution of nutrients they drink up from the soil. If this soup contains the foods plants need, they flourish. If not, they grow poorly.
These nutrients must be in relatively simple form. Plants cannot use elaborate forms of nitrogen such as protein. If bacteria attack the proteins in plant tissues and break them down into simpler forms, plants can take them up. The “organicultists” to the contrary, plants can’t distinguish between nitrate nitrogen which comes from bacterial decay of meat and that out of a bag of ammonium nitrate.
Plant Food Elements In Solution
Since these plant food elements are in solution, they are highly sensitive to the effect of other chemicals. With a few cents worth of lime, for example, a garden patch can be made so alkaline that iron, which stays in solution only in an acid soil, will be locked up or fixed so completely that plants will starve for this element. Or by making the soil too acid or too alkaline, phosphorus can be made unavailable to plants, even though the soil contains hundreds of pounds per acre of this element.
Importance of pH – The Why?
Here is why pH is so important: Only within a relatively narrow range of pH are all the food elements that plants need for good growth available in large enough amounts for good growth. Without attempting to tell why this is so, let me state that if the pH of a given soil can be kept at 6.0 to 6.9 (slightly acid) every element that plants need will be available, provided it is in the soil in the first place.
Phosphorus, as already mentioned, is highly sensitive to pH. At readings over 7.3, it becomes fixed and cannot go into solution to feed plants. At readings of 5.0 or below, it is again locked up. I have actually seen limed fields where plants showed deficiency symptoms in soils that contained ten times as much phosphorus as the plants needed for normal growth – if they could get at it.
Iron is particularly sensitive. In over-limed soils it is not uncommon to see plants with the yellow mottling that shows they are not getting enough iron. Chlorosis, as this condition is called, is perhaps the most common single deficiency disease of plants. Oaks, in particular, are sensitive to it, Only last year, heavy rains in one part of the Middle West washed so much alkaline material out of upland soils that oaks growing lower on the hillside began to suffer from iron chlorosis.
Potash is not too sensitive: pH readings may go as high as 8.0 before it is locked up. It shares this point with aluminum. At pH readings below 5.0, aluminum is the culprit in making phosphorus unavailable, locking it up in insoluble form.
Nitrogen and pH
The effect of pH on nitrogen is indirect. As already mentioned, organic matter is not available to plants until it is broken down into simpler forms. But in acid soils, the bacteria responsible for this breakdown do not work well. They, too, require a pH of 6.0 to 6.9 for best results. In acid soils fungi, which do not always yield the best end-products of organic breakdown, take over for the bacteria. If for some reason a soil cannot be treated to overcome acidity, feeding with nitrate nitrogen in inorganic form will give better results than using organic manures or composts, since nitrate nitrogen is not affected by pH.
Too much calcium can definitely be bad. In addition to limiting the supply of iron, it also locks up manganese, an important element in the manufacture of foods by plants from inorganic chemicals.
Magnesium (which should not be confused with manganese in spite of its similar spelling) acts much like calcium at a pH over 7.0 and makes manganese and iron unavailable.
Understanding The Power of Soil pH
All this means that if we want our plants to be well fed without the addition of excessive amounts of plant food elements to the soil, we must juggle the pH between 6.0 and 6.9.
If soils are too acid (readings of 5.9 and below) add hydrated lime for quick results, ground limestone for long lasting effects. The pH can be increased about one full point (say from 5.5 to 6.5) by applying seven pounds of ground limestone to every 100 square feet of ground. On lighter sandy soils, use only three and one-half pounds to get the same effect. Hydrated lime works faster. but should not be used too close to planting time. Five pounds will treat 100 square feet, or two and one-half pounds on lighter soils.
Alkaline soils with pH readings of 7.0 and above need acidifying. Ammonium sulfate is good because it both reduces pH and provides the plants with nitrogen. Six pounds of ammonium sulfate to 100 square feet will cut the pH one full point. Three pounds will be enough on sandy soil.
If the soil is naturally low in iron, iron sulfate can be used for acidifying. Eighteen pounds of iron sulfate to 100 square feet will reduce the pH one full point; use half that amount on lighter soils.
If applied in a ring about 1 foot away from plants, sulfur can be used to lower the pH. This is the best method for treating a gradual increase of alkalinity due to surface water running into specially acidified soils. Make a shallow trench around the plant and trickle in sulfur through which surface water must run to reach the plants. It is hard to give a formula for this type of treatment, but where large areas of soil are to be treated, two pounds will reduce pH a full point on heavier soils, one pound on sands.
by C Milton
