By Allan Foster, PAg, Range Management Extension Specialist, Tisdale
Hay production removes large amounts of nutrients from a field and if these nutrients are not replaced by either manure or commercial fertilizers, hay yields will decline.
For example, a two ton per acre alfalfa hay crop will remove about 116 pounds of nitrogen, 28 pounds of phosphorus, 120 pounds of potassium, and 12 pounds of sulphur. Most of the nitrogen will be supplied by symbiotic nitrogen fixation but the rest of the nutrients will come from the soil. A one and one half ton per acre grass hay crop will remove about 51 pounds of nitrogen, 15 pounds of phosphorus, 64 pounds of potassium, and 12 pounds of sulphur, all coming from the soil.
The amount of nutrients removed by mixed stands of grass and alfalfa will fall between the amounts removed by pure stands and will depend on the percentage of each of the grass or legume component. This is why fertility requirements for mixed stands generally suggest that if the stand contains less than 25 per cent legume it should be managed as a pure grass stand. If the legume component is greater than 75 per cent, it should be treated as a legume stand.
Over time deficiencies of the primary nutrients (nitrogen, phosphorus, sulphur, and potassium) will reduce hay yields, especially if these fields are used for long-term hay production. Where hay stands are rotated with annual crop production, the nutrients released from organic matter, and the excess fertilizer used for the few years of annual crop production, can be captured by the subsequent forage crop and deficiencies are not as noticeable.
The nutrients most often deficient in alfalfa are phosphorus and sulphur. Potassium is generally adequate in most soils but can be limiting on light-textured gray soils. Poor responses of alfalfa to one-time low application rates of phosphorus may be due to phosphorus being tied up in the soil near the surface and not readily available to the crop.
Grass hay crops will almost always respond to nitrogen fertilizer. Phosphorus is often the next limiting nutrient. The determining factors for the size of the response to nitrogen are spring and summer rainfall and the amount of nitrogen that actually enters the soil. Poor responses to broadcast urea can often be attributed to atmospheric loss when weather conditions are warm and dry after spreading.
Some nitrogen fertilizer is important to gain the full yield benefit of the grass component of a mixed stand. However, too much nitrogen will encourage grass production at the expense of the legumes. Nitrogen fixation activity in the legume will also be decreased.
The first sign of nutrient deficiency in a hay stand is generally lower than expected production from plants that appear stunted and spindly. Taking a soil test in the spring or fall is the quickest method of determining the nutrient status of the soil and whether or not a nutrient deficiency is the problem.
For more information on this or other related topics, please contact your local Saskatchewan Agriculture Regional Office or the Agriculture Knowledge Centre at 1-866-457-2377.