How Does Your Soil Fertility Stack Up?

It’s about as basic as you can get in alfalfa production: Make sure soil nutrients are at optimal levels to give your crop the best opportunity to grow and thrive, and achieve return on investment and yield potential goals.
 
There is indeed room for improvement in U.S. alfalfa production. According to the USDA, in 2016, the national average yield for alfalfa and alfalfa mixtures was 3.45 tons per acre. That’s about half of what we would aim toward when formulating a fertility plan for alfalfa growers. And we have alfalfa varieties that are capable of yielding 10-plus tons per acre in many of our environments. I’m convinced the way to bridge this yield gap is largely through fertility management.
 
It sounds like Soil Fertility 101, but sometimes alfalfa growers get caught up in other items pertaining to alfalfa management, and fertility gets shortchanged. In order to optimize the investment you’ve made in your alfalfa crop, it’s important to make sure it gets off to the best possible start. Here are some tips to help make that happen.
 
1. Optimize nutrient levels prior to planting.
Alfalfa is not a crop where we establish a stand and then play significant catch-up on fertility, so we want adequate fertility before we plant. In the life of a stand, stand loss occurs at a certain rate every year. Adequate fertility helps keep plant and stem counts elevated (stands naturally self-thin, especially in the year of seeding), so it’s imperative to establish fertility up front to help ensure robust stand development, plant health and strong yields.
 
For growers in the Midwest and eastern U.S., I recommend checking off three boxes for optimal soil fertility: pH, phosphorus and potassium. If baseline levels for these are not met, I would advise delaying planting until you can get fertility to appropriate levels. Any nutrients found to be deficient should be applied and managed to adequate levels well  before you plant a new alfalfa stand. Incorporating nutrients (before planting) into the root zone will allow greater distribution of macronutrients such as phosphorus and potassium.
 
For growers in the West, things are a bit different. “By and large, western alfalfa growers consider phosphorus their biggest nutrient concern, followed by potassium and sulfur,” says Steve Orloff, farm advisor and country director, University of California Cooperative Extension. “Growers take pH into consideration, but it is generally not a problem here. Rainfall amounts are lower, so as a whole, low-pH soils are rarer in this area.” 
 
Of course, it’s easy to lump all Midwestern or Eastern or Western alfalfa growers into three large groups, but that wouldn’t be accurate, says Orloff. “Recommendations need to be specifically targeted to where growers farm and what soil types they have,” he says. “Growers need to do soil and tissue testing, and keep accurate records of the nutrients they apply each year — ideally in a spreadsheet so they can monitor trends.”
 
2. Perform soil and tissue testing.
Work with your trusted agronomic advisor to take soil samples where you want to plant alfalfa. If you live in the Midwest or the East and are planting next spring, you should be soil sampling now, if you haven’t already, so you can assess results and take corrective measures prior to planting.
 
Orloff says that doing a pre-plant soil test is very important, and recommends doing soil tests annually. When high soil variability is suspected, periodic grid sampling is helpful to achieve the most accurate results, and then using site-specific management (SSM) to apply the most fertilizer where the soil is the most deficient.
 
“Many growers will just get a composite soil sample for the whole field and base their fertilizer decisions on that,” he says. “But, depending on the area, soils can vary widely. You’re better off doing a grid sample, or at least dividing the field into different zones — high-, medium- and low-producing — sampling them separately and fertilizing them accordingly.”
 
He also recommends tissue sampling to see how nutrients are being taken up into the plant. It’s important to realize that tissue samples may give erroneous values if a crop is stressed. For example, if an alfalfa plant is stressed for water, it’s not reaching its full growth potential because water is limiting. In this situation, nutrients in the plant will concentrate, so the grower would think the plant had adequate or even high nutrient levels.
 
Furthermore, soil testing can be somewhat inaccurate in testing sulfur, boron and molybdenum, says Orloff. “That’s why it’s good to have a combination of both soil and tissue testing and track levels over time,” he notes. The recommended method for plant tissue analysis is to collect 40 to 60 stems at 10 percent bloom.
 
3. Optimize genetic potential.
Soil fertility maintenance is important to make sure the alfalfa genetics you paid for are being used to their fullest potential. When you remove dry matter from fields after a cutting, you are removing phosphorus and potassium, as well as other macro- and micronutrients. Be sure you are applying nutrients not only to maintain soil fertility, but to replace what you are taking away in dry matter.
 
If your yield goal is the national average of 3.45 tons, you can fertilize to hit that target. But you are leaving lot of money in the field if you’ve planted a variety with genetics that are capable of producing a lot more yield with optimal moisture and temperatures, planted into the right soil type. As always, we want to fertilize according to yield potential, not average yield.
 
“Growers need to be certain the fertility level is high enough so that the crop can live up to its genetic potential,” says Orloff. “Many times what will happen is you may have a very high-yielding variety, but the plant can never reach its full yield potential because there’s a limiting nutrient or water that is more limiting than the genetic potential of the alfalfa variety you’ve planted.”
 
You’re making a significant investment in alfalfa genetics, and you want them to deliver a return. Even the best genetics can’t take you over the finish line if you haven’t taken some of the basic steps needed to ensure soil fertility.
 
4. Manage saline or sodic soils.
In Western geographies, salinity can be a significant issue in alfalfa production. In California, higher-value crops such as almonds, pistachios and some vegetable crops are being planted in the best soils, with alfalfa relegated to more marginal soils with high salinity. Alfalfa is actually more salt-tolerant than previously thought, but these types of soil challenges still need to be managed, says Orloff.
 
“Saline (high-salt) soils and sodic (high-sodium) soils need to be managed differently; and some soils can be both saline and sodic,” he says. “If growers suspect salinity problems in their soils, they need to have their soils analyzed and find out what the values are. Fortunately, alfalfa breeding companies are developing varieties that have improved salt tolerance, and that is the wave of the future.”
 
5. Plan ahead.
In alfalfa production, planning is key. Orloff offers these tips to help prepare for your next planting.
  • Know how much fertilizer you will need and when in order to get the best price.
  • Get soil samples from a field before you plant new stands, and leave ample time to interpret them and apply nutrients as needed.
  • Depending on your soil type and nutrient deficit, plan to make applications anywhere from two to six months before the first cutting to derive the full benefit.
 
Alfalfa can germinate and grow under fairly low soil fertility, but it will not thrive. It will also be more susceptible to threats, including nematodes, insects, disease, or other abiotic and biotic stresses. Don’t let this happen to your high-value alfalfa crop. Get the most out of your investment by nurturing it to deliver its full potential.

 
Optimum levels for pH, phosphorus and potassium
pH 6.5 to 7.0
Phosphorus 60-plus lbs. per acre
Potassium 250-plus lbs. per acre
 
Jennings J. Establishing Alfalfa for Forage. University of Arkansas Division of Agriculture, Research and Extension, University of Arkansas System. FSA15-PD-3-13RV. PDF accessed Aug 24, 2017.