When and Why Top-Dress Fertilize Alfalfa

Warren C. Thompson
National Forage Specialist: America’s Alfalfa

There are two reasons to fertilize alfalfa after it has been established. 1) increase yield and 2) improve persistence. Once the stand is established and the land is committed to alfalfa production, what else is important?

So often, I have seen folks who are so proud of the fact that they are at last growing the crop, they forget to feed the crop! As a result, they are often disappointed in yield and thriftiness. Here is rule number one: any time a crop is harvested and hauled off the land, fertility is hauled off as well. If it is not replaced at the crop removal rate or higher, crop performance and survival (in the case of perennial forages) both will drop.

What fertilizers? Seldom is nitrogen needed to top-dress alfalfa; for once established, the legume produces it’s own nitrogen. Phosphorus and potassium are the main ingredients that are needed to grow healthy plants. In areas where Boron is low, (that is usually where limestone is needed to correct the soil pH) it should be added annually at rates that range from 2 to 4 pounds per acre of (actual B) These rates vary by region. Check with your County Agent or University for the best rate in your area.

How much P2O5 (phosphate) and K2O (potash)? There is no better guide than a current soil test and valid recommendations. Just keep in mind that every time you haul off a ton of hay, you are removing about 12-15 pounds of P2O5 (phosphate) and 60 pounds of K2O (potash), about 100 pounds of lime and ½ pound of Boron. In some areas, sulfur and magnesium are needed but their use and recommendations are not uniform. Soil tests and plant tissue tests will help tell you about these two and maybe others. But phosphate, potash, and lime + Boron are the mainstays for top yields.

When is the best time to top-dress fertilize an established alfalfa crop? I had a friend at Clemson University a long time ago who had a way of handling this question very appropriately. His stock answer was, “don’t put it off, put it on”! In most places except on sandy soils, the best time to top dress alfalfa is in the fall. Why? Fertilizers, fertilizer spreader trucks, and rental buggies are both more plentiful and prices are usually more favorable than in the spring. Also, the ground is firm making it easier to apply and ‘easier’ on the stand. This timing helps to stabilize and feed the plants during the winter months and provides better winter survival potential and earlier spring growth.

Through the years, many have wondered if potash would leach when applied in the fall or even late winter. In recent yeas, we see little if any evidence of leaching, even potash leaching, except perhaps on sandy soils. On these sandy soils, most recommendations call for split applications just before and during the producing season. How about split applications other than these soils? Chances are you will be doing more harm to the alfalfa stands by driving over the fields than you will be helping. If you get the fields treated for a whole years’ production, leave them alone is my suggestion

What about fertilizing where you are grazing alfalfa? It depends on the grazing systems you are using AND results of soil tests. It just makes sense that when cattle graze, they remove very little of the fertility in the process of producing bone, meat tissue, and milk as most of the plants are returned to the soil as manure. The problem is manure distribution. When the animals are confined to small paddocks in large numbers for short periods and re-cycled with new growth, it just make sense that the manure will be pretty well distributed. But if the animals are allowed to roam over large areas for extended times, two things will happen: forage and animal performance per acre will take a hit and uniform manure distribution will be terrible and of passing value in maintaining uniform fertility. In this last case, top dressing will be required to maintain stands and hold high yields.