Soil Fertility

Taking the Long View
By Everett D. Thomas

Another column on soil fertility? I'm sure that's what some readers are thinking, since the January column was on this general topic, and there's no topic I've written about more often during the 15 years or so of doing this column. There's three reasons for this. First, fertilizer is the biggest crop input cost on most farms - the ones that grow crops, that is. Second, fertilizer has a huge impact on crop yield and quality. Third is the environmental impact, both on and off the farm, of improper nutrient use. Underfertilization results in lost yield and quality, while overfertilization implies money spent on fertilizer that could have been better used somewhere else on the farm, not to mention the impacts on ground and surface water quality.

Soil analysis: Consistency is the key

Forage analysis for minerals is a relatively straightforward process: The sample is dried, ground and analyzed for the various minerals. What you see is what you get. While there's textbook data indicating the relative availability of many essential minerals, this isn't included on forage analysis reports.

Soil analysis is much more complex. The dried soil sample is treated with a chemical extractant to provide an index or measure of the relative availability of the minerals in the soil. However, there are several different soil extractants used by the various soil testing labs, and one isn't necessarily any better than the other. However, the various soil extractants can result in hugely different analytical numbers: A soil test phosphorus (P) reading of 20 parts per million (PPM) would indicate a high soil test P level using one commonly used extractant, but a very low soil test P using another common extractant.

Imagine a farmer using one soil testing service in 2011 and another in 2012. In 2011, he notes that one cornfield has a soil test P of 30 PPM, which the soil test report indicates is quite low. He therefore increases his nutrient applications, applying a heavy rate of manure, plus an increased rate of starter P. He resamples the field that fall, but this time the sample is sent to a different soil test lab, one using a different soil extractant. Now the soil test comes back at 3 PPM, still a low reading, but now a fraction of what it was before he applied all that manure and fertilizer. What happened?

The two soil test labs used different soil extractants. Both accurately determined that the soil was deficient in P. However, by only looking at "the numbers," the farmer might conclude that his soil fertility plummeted, but this isn't the case at all. In fact, his plant-available amount of P probably changed little. That's because applying manure or fertilizer often doesn't result in an immediate increase in soil test values, especially (but not limited to) P. Applied nutrients, including the nutrients in commercial fertilizer, manure and plowed-down crop residues, must react with the soil and go into solution before becoming available to plant roots. Other soil minerals, as well as either high or low soil pH, can affect soil test values as well as inhibit plant uptake. Applying nutrients to the soil isn't like salting a steak. That's why it's so important to choose a reliable soil test laboratory, and then stick with that lab, year in and year out.

Soil analysis versus fertilizer recommendations

It would be simple to say that the soil test numbers aren't important, that all you need to do is rely on the fertilizer recommendations generated by the soil analyses. Unfortunately, this isn't true, and this holds for both university soil test labs - there are somewhat fewer of these now - and commercial labs. I've seen some simply terrible fertilizer recommendations, not because some fertilizer dealer is trying to sell something the farmer doesn't need, but because of a lack of information.

Some soil test lab information sheets don't ask for any manure application data, and on a dairy or other livestock farm, this is a critical failing. And national or regional soil test labs don't have enough (or any) information on local conditions or the yield capability of the soils on that farm. Then there are the varying philosophies of fertilizer use, some that are highly questionable.

For example, a few months ago, a farmer asked me to evaluate the fertilizer recommendations made by a commercial soil testing lab, since he thought they seemed very high. In reviewing them, I noted that the soil analysis for one micronutrient was consistently high, but the fertilizer recommendation called for applications of this expensive, rarely deficient micronutrient for each of the next three years.

A photo or a moving picture?

A soil analysis indicates the fertility the day the sample was taken, but not whether it's increasing, decreasing or relatively unchanged. This is important information. Even if you've been following fertilizer recommendations made by a responsible agronomist, that doesn't mean the recommendations are right for your farm. For instance, if your soil test potassium (K) levels have been decreasing from the desired level (I recommend maintaining soil test K at least at medium levels), regardless of the fertilizer recommendations, you should consider increasing your K rate (as manure or commercial fertilizer), or at least have a serious discussion with someone qualified to evaluate your situation.

Years ago, Miner Institute used this type of information to increase its K application rates. We had over 10 years of soil analysis data, all by the Cornell University soil test laboratory. The data showed that soil test K was generally low, and in spite of following university soil fertility recommendations it wasn't increasing. We therefore increased the application rate of K fertilizer, and only then did soil test K levels begin to increase. So did forage K concentration, which was also quite low.

One fertilization philosophy aims at maintaining soil test levels in the medium range, while another wants nutrient levels to be fertilized to the high range and kept there. Either can work, though the latter is more expensive to achieve. However, as important as knowing where soil fertility is today, it's also important to know in which direction it's headed.

Ev Thomas has worked as an agronomist in New York for 45 years. He has written our Forages column for 15 years and has been an expert contributor on a number of other topics.