Alfalfa removes 10-12 lbs of phosphate and 60 lbs of potash per ton of hay, so annual applications of phosphorus (P) and potassium (K) fertilizer are often needed to maintain good stand vigor and longevity, says Dorivar Ruiz Diaz, Kansas State University Extension nutrient management specialist.
Alfalfa also shows responses to some secondary and micronutrients, and in Kansas, sulfur and boron can often limit yield potential and should be monitored periodically, he says.
“First of all, for new seedings, it is important to build up soil-test P and K to the optimum range before seeding, because this is the only opportunity (for the life of the stand) to mix the nutrients through the topsoil. At seeding, small amounts of P and K can be banded below the seed, making sure to allow adequate separation between the seed and the fertilizer band,” Ruiz Diaz says.
On established stands, dryland alfalfa can be topdressed with P or K fertilizer in the fall, early spring or even after the first cutting. Irrigated stands can also be fertilized at those times, or after any cutting because moisture can be supplied to make the topdressed fertilizer available to plants.
Phosphorus is a relatively immobile nutrient in the soil, but can still be effective if broadcast on the soil surface in an established alfalfa stand, he says.
“Broadcasting phosphorus has proven effective on soils low in phosphorus because alfalfa has roots near the soil surface.”
Although P and K fertilizer can be beneficial to alfalfa if applied in the early spring or after the first cutting, if soil tests show the nutrients are needed, producers also should remember the importance of early fall applications, Ruiz Diaz adds.
“For established stands, the critical time when alfalfa needs the most P and K is usually in preparation for winter. To boost the winterhardiness of the crop, a good supply of P and K needs to be added before the critical fall growth period. Early fall fertilization when there is still some plant growth (four to six weeks before dormancy) is usually more effective in increasing winter survival, because nutrients are supplied in time for adequate uptake.”
When high nutrient application rates are needed to boost soil fertility, splitting the total required amount into two or more applications is recommended in order to avoid salt injury and luxury consumption beyond the alfalfa nutritional requirement.
Lime is also important for alfalfa, but should be applied before seeding and during land preparation for planting. Alfalfa grows best in a soil pH range of pH of 6.5 to 7.5 and when needed, lime should be applied to reach a pH of 6.8.
It’s not necessary, or desirable, to apply nitrogen to established alfalfa stands.
“Nitrogen fertilizer applied to well-nodulated alfalfa will only stimulate grassy and broadleaf weeds, and may reduce stand longevity,” says Ruiz Diaz.