Fertilizing Alfalfa

Some farmers have commented that they will delay applying potassium fertilizer when the price is higher than what they expect they should pay and wait until the prices are reduced. In too many cases this may not be the best use of their resources. Agronomists know that each ton of alfalfa that is harvested removes 15 to 20 pounds of phosphorus and 50 to 60 pounds of potassium. An average yield of alfalfa can easily remove 60 pounds of phosphorus and 200 pounds of potassium. At these removal rates soil fertility levels can rapidly be reduced to levels that will not support high yields.

In addition to low soil fertility levels, conditions in the soil can affect the availability and uptake of nutrients. Low soil pH levels affect the ability of soils to allow most nutrients to be available to plant roots. When soil pH drops below 6.5 phosphorus and potassium uptake will be limited. At pH below 6.0, this effect can be dramatic. Frequently lime is referred to as a poor man's fertilizer for this reason. Another consideration for lime needs is related to the increasing transition to no-till establishment of alfalfa and corn. Lime does not move easily through the soil profile. As a result, a liming program that emphasizes frequent applications of lime is necessary to ensure that soil pH levels are maintained at optimum levels throughout the root zone for all crops grown.

Other factors can affect plant nutrient uptake. Alfalfa has a single large taproot that can easily reach soil depths of 2 to 3 feet. However, the plant still depends on smaller side roots and root hairs off these side roots to obtain plant nutrients and moisture. If plant roots are diseased or damaged by insects they will not be able to efficiently obtain soil nutrients. Soil compaction will also limit root growth. Forage harvesting requires many trips over a field for each harvest and multiple times throughout the growing season. Often, harvest is dependent on weather patterns, and harvesting between rain events can result in being on fields when soil conditions result in compaction. Multiply this number of trips by 3 to 5 years, and it becomes evident that surface and deeper compaction can become an issue. Take a close look at your plant roots. Twisted or misshaped roots can be an indicator of soil compaction challenges.

Ensuring soil fertility levels are near or above the optimum range should be the goal of forage producers. On dairy and livestock farms the use of heavy manure applications prior to alfalfa establishment will provide adequate nutrient levels for the first few years of the crop. However, by the second growing season, these nutrients can become reduced to levels that are below the optimum levels for top alfalfa production. Annual top-dressing of potassium after first cutting and in August is necessary to ensure high yields. Be sure to include soil testing to ensure you are applying only the amount and types of soil nutrients that are necessary to ensure high yields.