Soil Testing is Serious Business

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

When you plan to seed a field of alfalfa or top-dress an established field, always to pull soil test to check on pH, phosphate and potash needs. Timing is extremely important especially when it comes to pH. Applied limestone will not change the pH overnight for it takes 12-18 months to elevate a pH level one full point (6.0 to 7.0). Most professionals and experienced farmers like to have the pH at 6.5 and above at alfalfa planting time. Phosphate and potash can be applied for new plantings anytime up to and including seeding time. Most farmers apply these at or just before the seed goes in the ground especially when you are using a starter fertilizer containing 20-30 pounds of nitrogen per acre.

What is the best time to top-dress fertilize established fields? More producers, nation wide are changing from spring to late-fall applications each year. Mainly, it’s a matter of convenience, the soil is drier and usually fertilizer prices are lower and the nutrients you really need are ordinarily more available. Also some very reliable studies have shown that fall applications helps winter survival with very little if any loss of nutrients unless by soil erosion. Also, growers have found that this early treatment helps to get the crop off to a great start in the spring.

Taking soil samples does require some thought and planning. Here’s the way I like to see it done when you are dealing with a multi-acreage planting. Pull at least one soil sample from each acre (two or more is better) and mix these ‘sub-samples’ to make the composite sample for the area. If there are multiple soil types in the field, better make two or more tests, dividing them by soil type. If the field has been combined with another field, keep the sub-samples and the composite sample separate. After mixing the sub-samples, use about a pint of soil and label it for your records and the records of the laboratory. If you have doubts as to where to deliver the sample, call your County Agent or your fertilizer dealer.
 

Remember, you are going to use a very small amount of soil that is going to represent 435,600 square feet, 4-6 inches deep (for 10 acres). Besides, you are committing this property to several years of production and a sizable investment in time and money. So take your time to be sure what you deliver to the laboratory is an accurate representation of the soils you are committing to this very important crop. The success you will have with your alfalfa crop can certainly depend on it.