A toxic alfalfa situation

It doesn’t matter what variety of alfalfa you plant. It doesn’t matter how much your seeder cost. It doesn’t matter if you nail the correct planting depth. It doesn’t matter if you poured the fertilizer onto the field. None of these things or a lot of others matter if the soil pH is too low for optimum alfalfa production.

All of this is why addressing soil pH is generally found in the first chapter of any alfalfa production handbook.

Low soil pH is a hindrance to alfalfa production because of aluminum. Without going too deep into the chemistry of the situation, aluminum becomes much more soluble in acidic soils, leading to greater plant uptake of the compound and a toxic situation for the plant.

Too much available aluminum has the effect of slowing and pruning alfalfa root growth. This, in turn, reduces the activity and ability of Rhizobia bacteria to inhabit root nodules and fix nitrogen for the plant. In fact, a visual plant symptom of a low soil pH is that alfalfa plants are chlorotic, which can be an indication that they lack nitrogen. Often, these areas appear in patches.

Aluminum toxicity is best mitigated by correcting a low soil pH with an application of agricultural lime. Best results are achieved when the application is made six months to a year before the alfalfa seeded. A lime application will also help make other essential fertilizer nutrients more available.

Other mitigation strategies

Research has shown that a lime application may not always be effective for correcting a low subsoil pH, causing aluminum toxicity to still be an issue. Gypsum (calcium sulfate) applications may help to resolve this situation.

Applied alone, gypsum will have no impact on soil pH. It must be used in conjunction with agricultural lime and a recommended fertilizer program.

Gypsum provides a high volume of calcium that will displace the aluminum on subsoil particles; this allows it to be leached below the rooting depths when enough moisture is available.

To ensure proper root growth, alfalfa needs moisture, an abundance of calcium at the root tip, and an absence of aluminum. The sulfate in the gypsum binds and precipitates out the aluminum. Coupled with the added calcium, this enhances the subsoil rooting environment, especially in fields where soil pH has been low for an extended period of time. A well-drained site is also important for sustained alfalfa production, which is true regardless of the native soil pH.

Research from the University of Georgia showed that alfalfa root growth increased 200% at a depth of 2.5 feet where gypsum had been applied. A subsoil soil test deeper than 15 inches is recommended to confirm if a gypsum application is warranted to reduce potential aluminum toxicity issues from low pH and high aluminum availability. Toxicity is likely when the subsoil pH falls below 5.5.

Perhaps someday we will have alfalfa varieties that are much more tolerant of acidic soils. That research continues to be done and will be especially beneficial where native soil pH tends to be low.