Soil pH

pH is a familiar term. It measures the acidity or alkalinity of a solution. The pH scale runs from 0 to 14, with 7 as the neutral point. The pH decreases from 7 as a solution becomes increasingly acidic. It increases from 7 as the solution becomes increasingly alkaline.

Battery acid is very acidic at a pH of 0.0. Common household vinegar is quite acidic at 2.0. Coffee is around 5.0. Pure water is neutral at 7.0. Household ammonia is alkaline at 10.5. Drain cleaner is extremely alkaline at 14.0.

Soil’s pH is one of the invisible factors that determines plant health. The symptoms of unhealthy pH are serious, but their cause isn’t always obvious.

Soil acts as a reservoir for water and minerals. Water forms a film on the inside surfaces of soil pores. Mineral ions, dissolved in that water, stick to charged sites on soil particles. There, they can be taken up by roots. But the solubility of these ions depends on the soil water’s pH. At acidic pH’s, their solubility decreases, and metal ions like iron and aluminum leach from minerals and take up space that could otherwise be occupied by nutrients like magnesium, calcium, potassium, nitrogen, and phosphorus. Phosphorus is particularly troublesome. Its solubility is low even under ideal conditions, but it’s also a critical nutrient. At over-acidic pH’s, it forms insoluble complexes with iron, which are no use to plants.

For most plants, the ideal pH is slightly acidic. At a slightly acidic pH, most nutrients reach their maximum solubility. This includes key micronutrients like boron, iron, and molybdenum. However, when the pH drops too low, these nutrients (as well as aluminum, which isn’t a plant nutrient) become too soluble, and can rise to toxic levels.

The issues that arise from acidified soil aren’t limited to plants themselves. Soil microbes, which are essential to liberating nutrients from soil and pulling nitrogen from the air—are much less efficient at extreme pH’s, or they can die off altogether.

(It should also be noted that too high a pH (a soil that’s too alkaline) is harmful, too.)

This all leaves two obvious questions: What makes a soil acidic? And what can be done about it?

Soils can acidify naturally. When water passes through them, it carries away some of the mineral ions. And plants consume mineral ions as they grow. Mineral ions tend to be alkaline, so their loss drives the pH down. On top of that, root and microbe metabolism produces carbon dioxide, which is also acidifying.

And, unfortunately, nitrogen fertilizers, useful though they are, tend to acidify soil over the long term, as they decompose into usable nitrate and nitrite. In urban and polluted areas, there’s also the problem of acid rain and acidic runoff.

Some soil problems are difficult to treat. It’s hard, for instance, to remove lead from contaminated soil. It’s labor-intensive to remedy soil compaction. But acidic soil is relatively easy to treat. It’s usually done by liming: applying alkaline minerals like limestone, chalk, dolomite, or slaked lime, to push the pH back up. This has the side-benefit of supplying calcium and magnesium, which are essential plant nutrients. Care must be taken not to over-lime, of course. That requires testing the soil’s pH, which can be done with inexpensive soil pH testers, with test strips, or with test kits.

It’s also important to adjust planting and management practices, where needed. Care must be taken not to over-water, and to limit the application of acidifying nitrogen fertilizers where possible. And, in ornamental plantings, it helps to choose plants that don’t mind acidic soils. Some plants even prefer their soils acidic. These include ferns, azaleas, some pines, oleanders, and blueberries. (For more detailed and specific information, the US Department of Agriculture has a fairly comprehensive database available here.)

pH problems are easily missed, since, without testing, the symptoms are vague and easy to misinterpret. But when the problem is found, it’s also a relatively easy fix.

Mycorrhizae

We’re made of dirt. We eat plants that grow in dirt. Foods like eggs, meat, and dairy come from animals that eat plants. The iron in our blood, the calcium in our bones, and the phosphorus in our DNA all come from soil.

Getting those nutrients from the soil into our bodies, though, isn’t a trivial matter. Some nutrients, like water, potassium, and nitrogen move easily through the soil. Nutrients like iron and phosphorus, on the other hand, are prone to oxidation and mineralization, and are a lot harder. Phosphorus is vital for all life, and especially for plants, but it’s not very mobile. Once a root has absorbed all the phosphorus around it, the zone of depletion remains depleted for a long while, since the phosphors is slow to diffuse in and replenish it.

And that brings us to the point of this post: mycorrhizae. Mycorrhizae are fungal symbiotes that colonize plants’ roots. Their relationship with plants is ancient: fossil mycorrhizae have been found dating back at least 400 million years. [1] The symbiosis is very intimate. Plants allow mycorrhizae to penetrate their root cells, where they form structures called arbuscules and vesicles, which allow them to exchange water and nutrients with the host plant. And plants, in turn, willingly exude substances for mycorrhizae to feed on, and accept chemical signals from mycorrhizae that alter their root growth to better suit the mycorrhizae.

Mycorrhizae, like many fungi, grow filaments called hyphae. Mycorrhizal hyphae act as an extension of the host plant’s root system, dramatically increasing its reach and its surface area. In the case of phosphorus, mycorrhizal hyphae increase the volume of soil roots can draw from. They also excrete organic acids and enzymes that convert phosphorus into a mobile form. [2] In some cases, mycorrhizae alone can supply as much of 80% of a plant’s phosphorus demand.

Phosphorus, though, is only one of the vital nutrients. Mycorrhizae have been documented to help absorb a slew of others, including nitrate, ammonia, ammonium, calcium, iron, potassium, and zinc. [3]

All other things being equal, mycorrhizae-colonized plants tend to be larger, healthier, more productive, and more resistant to stress and disease than plants grown in sterile soil.

This last point is important: mycorrhizae can provide significant protection against disease. [4] First of all, since they grow so close to the roots, they create a physical barrier. Second, fungi (mycorrhizae included) are constantly at war with other microbes, and so they’ve got powerful chemical defenses. They’re also in chemical communication with their hosts, so they can mobilize those defenses when the host sends out a distress signal.

Unfortunately, the modern world isn’t always kind to mycorrhizae. Soils see a fair bit of abuse in urban and agricultural soils. Construction, foot traffic, vehicle traffic, and heavy equipment compact the soil. This crushes the soil’s pore spaces, which makes the soil more prone to water-logging and oxygen deficiency, and more difficult for roots and hyphae to penetrate.

In agriculture, soil also sees very intensive use. That often leaves it dry and depleted, and so, to keep their plants healthy, farmers must resort to chemical fertilizers and pesticides. When over-applied or mis-applied, chemical fertilizers can chemically damage the soil, which can kill beneficial microbes. And broad-spectrum fungicides often kill mycorrhizae just as easily as disease-causing fungi. The result is a soil that might have all the nutrients a plant needs, but lacks the mycorrhizae that help plants absorb them.

But the news isn’t too bad: In the last few decades, understanding of the role and importance of mycorrhizae has grown rapidly. And with the rising interest in organic food, and with the increasing cost and regulation of chemical fertilizers, farmers, landscapers, and plant-care professionals are looking for ways to help make plants more resilient and self-sufficient. Mycorrhizae allow plants to grow and be productive with less labor and chemicals. With our growing population and growing demand for food, that’s more important than ever.

Citations

[1] Four hundred-million-year-old vesicular arbuscular mycorrhizae.
[2] Roles of Arbuscular Mycorrhizas in Plant Phosphorus Nutrition
[3] Nutrient uptake in mycorrhizal symbiosis
[4] Arbuscular mycorrhiza protect an annual grass from root pathogenic fungi in the field.