It takes time and management to determine whether alfalfa has enough nutrients to produce high yields. And with the high costs of fertilizer, and the demand and higher prices for alfalfa, it’s more important than ever that hay growers analyze their crops’ nutrient needs. Then they can accurately decide whether fertilizing alfalfa pays or not.
That’s according to Steve Orloff, a farm advisor in northern California's Siskiyou County. Orloff spoke at the National Alfalfa Symposium last month.
“Last December, the price for fertilizer was more than twice what it was five years ago. And I hear a lot of prices for 11-52-0 now that are over $800/ton nearly triple previous prices,” Orloff said. “So you really have to question and look carefully: Do you need that fertilizer or not?”
The first step to analyzing fertilizer needs is to document what nutrients an alfalfa field may be deficient in. “There are several ways to do that. The first one is visual observation – just looking at the plants in the field and seeing if you can pick out nutrient deficiencies.”
That’s not the best method, he said. A number of nutrient-deficiency symptoms are similar to each other or look like water-logging or other plant-damage symptoms. Also, by the time a grower sees deficiency symptoms, yield is already affected.
Two other methods of diagnosing nutrient deficiencies in alfalfa: soil testing and plant tissue testing. If a grower opts to soil-test, Orloff would recommend doing so at planting and periodically during the lifetime of the stand. But know that soil tests recognize phosphorus and potassium deficiencies, but aren’t good at recognizing other deficiencies.
“Soil analysis is good for preplant assessment. It’s good for pH, salinity and for monitoring phosphorus and potassium. But a plant tissue test is by and large more accurate for all the nutrients, and especially for sulfur, boron and molybdenum.
Plant tissue tests should be taken at first cutting in two- to four-cut regions and at later cuttings of alfalfa grown in long-season growing areas like Central California. Tests should be taken every year or every other year until a fertilizer program is established.
Sampling should be done by dividing a field into poor, medium- and good-growth areas and then benchmarking those areas to come back to each year. “Our traditional University of California recommendation is to collect 40-60 stems from plants at random, and you don’t want all those stems from the same plant.”
Samples are divided into thirds. “The bottom third of the plant we discard and throw away. The middle third of the plant we separate the leaves and the stemsby rubbing off the leaves once the sample has dried. The leaves we analyze for sulfur and that mid-stem portion we analyze for phosphorus and potassium. Then the very top of the plant, the upper third, we analyze for boron and molybdenum,” Orloff said. Other states have different methods but this has worked well for California.
With tissue-test results, growers can determine if their soil is deficient, marginal, adequate or high in the five nutrients, and if they need to apply fertilizer.
One way to reduce costs in applying phosphorus is to apply a two-year supply every other year, he advised. “At planting, incorporate a high-analysis phosphorus into the top 2-4” of soil.
“A common question from growers is, ‘Can I broadcast phosphorus over an alfalfa crop and expect to get a response?’ You can. A lot of roots are growing very close to the surface and, even though phosphorus doesn’t move far in the soil, a topical application of it to established stands is effective.” In Orloff’s area of the country, phosphorus can be applied between October and February, but it takes 60-90 days to get a full response from the fertilizer. “So you want to fertilizer 60-90 days before that first cutting,” he added.
Visual symptoms of potassium deficiency are very definitive – white or yellow spots on leaf margins. Muriate of potash is probably the most economical source of fertilizer, but if a field also has a sulfur deficiency, potassium sulfate can be used. Again, 60-90 days are needed before first cutting to get the best fertilizer response.
Sulfur deficiencies are more difficult to diagnose by sight, and soil tests “are almost useless,” Orloff said. Typically, applying 200-300 lbs/acre of elemental sulfur at planting will last four to seven years in areas known to be deficient.
Orloff explained that he and colleagues are studying a way to sample for tissue testing using the bale cores used for forage analysis. For more on that method, watch for Hay & Forage Grower’s May issue.