Farming Magazine - September, 2013


Forages: A Good Time for a Soil Fertility Checkup

By Everett D. Thomas

Early fall is a good time for a soil fertility checkup, for several reasons that we'll discuss in some detail. The weeks prior to corn harvest for silage sometimes offer one of the only breaks in what often seems like a never-ending series of deadlines: Do spring tillage, seed forages, plant corn, then mow hay crops several times during the summer. For many farmers, September is a "no-mow" month for alfalfa, and most corn planted for silage isn't yet ready for harvest. What should you do now? Following are some suggestions.

Topdress alfalfa with potash

The combination of high potassium fertilizer prices and low dairy profit margins caused many farmers to reduce or even skip a year or more of potash applications to alfalfa fields. Where manure was applied, available soil potassium (K) levels may have been maintained, but just barely, since most farmers don't apply a heavy enough application of manure to supply the potassium needs of high-yielding alfalfa. Alfalfa and alfalfa-grass are heavy users of potassium, and available supplies of this essential nutrient can quickly become depleted. Right now, fertilizer prices may be as good as they're going to get for a while, and early fall is an excellent time for potash applications to perennial legumes. Potassium is antifreeze for perennials! Grasses are also heavy users of potassium, but they have the ability to "forage" for this nutrient much more efficiently than does alfalfa, so annual applications of manure may be all that perennial grasses need to meet their potassium requirements. Of course, rely on soil analysis to confirm this.

While potassium is the critical nutrient for alfalfa, make sure that the soil pH in your established alfalfa fields hasn't decreased due to the combined effects of crop removal, leaching and nutrient applications. Lower atmospheric depositions of sulfur are causing farmers to increase their use of sulfur-containing fertilizers, some of which are quite acidic.

Fall soil analysis

Not only is fall an excellent time to apply potassium fertilizers to alfalfa, but it's also the best time to sample soils for fertility needs. To lighten the load, I recommend sampling fields every three years (unless there's a good reason to sample more frequently) and taking soil samples on about one-third of your fields each fall.

Prime candidates for soil sampling are fields that will be seeded to alfalfa, since you want to have the soil pH close to 7 prior to seeding. A soil pH of 6.2 is fine for corn, but I prefer maintaining a pH of at least 6.5 while the field is in corn to make it easier to increase it to 6.8 to 7 just before seeding alfalfa.

If you suspect that soil pH may be very low, sample at least two years prior to seeding, since lime takes time. If soil pH is above 6, you should be able to correct it with a single fall application of lime, but if the soil is highly acidic it's better to split the application.

Why fall soil sampling instead of spring? Two reasons: First, as previously noted, the workload on a farm is often lighter in the early fall than it is during the rush of spring tillage and planting. There's also a technical reason: Research has shown that soil test levels, particularly of potassium, vary during a 12-month period and may be somewhat different soon after the soil thaws in the spring versus this time of year. Sampling at about the same time each year will avoid these seasonal differences; my suggestion is late summer through early fall. I prefer getting soil sampling done before it gets really cold because the nicer the weather, the more thorough a job you or someone else is likely to do. Cold weather can arrive early, especially in northern areas, and it's no fun trying to sample frozen soils.

A buyer's market for fertilizer?

Due to several factors, both national and international, the retail prices of all major nutrients were quite favorable this summer. Spring planting in much of the Corn Belt was greatly delayed, so once a weather window opened, some farmers decided to skip preplanting applications of fertilizer to save time. (In some parts of the Corn Belt, almost 50 percent of the corn crop was planted in a single week.) This reduced fertilizer demand from what was expected just prior to all that wet weather.

International demand for most fertilizers has been weak (urea in particular), while increased phosphate production in the Middle East and Morocco and increased imports of potash have combined to boost supplies of these nutrients.

I'm not sure how long this buyer's market will last - editorial deadlines make accurate predictions difficult - but fertilizer prices should still be favorable as you read this, though probably not as low as they were in July. If you'll need to apply potash to cornfields after harvest, price fertilizer now, order it ASAP, and take it either in the dealer's spreader for immediate application, in bulk or in 1-ton bags for later applications. Bulk storage is good for low and no-nitrogen fertilizers, especially muriate of potash (0-0-60), but I prefer the 1-ton bags for planter-applied fertilizers because they're easy to transport to the field and you can position the bag right over the fertilizer hopper for easy filling.

If you have "high and dry" storage and sufficient capital, you might consider buying some starter fertilizer, MAP (monoammonium phosphate) or potash for later use. Those 1-ton bulk bags are made of woven polyethylene, are often plastic-lined, and tie securely at the top. During my years as agronomist at Miner Institute, we occasionally stored "big bag" fertilizer over the winter, especially surplus low-nitrogen starter fertilizers. We even kept a bag or two of nitrogen fertilizer in good condition through a winter.

Buying and storing fertilizer now for later use involves some price risk if prices remain low. However, looking at all indications, as well as statements from fertilizer industry analysts, it would seem that chances are fairly slim that fertilizer will be as cheap in several months or next spring as it is now.

Ev Thomas has written our Forages column for 15 years and has been an expert contributor on a number of other topics.