Soils: December 2003

Actually, I lied in the title of my article. There are actually quite a few things I'd rather have for Christmas than your soil samples, so if any of you are in a particularly giving mood, contact me and I'll send you a list.

However, I would also like your soil sample. I'd like to show you some reasons in this article why sending in samples could be a Christmas gift to yourself, as well.

We recently conducted an analysis of our cooperators' soil test data from 1999 to 2003. The results were pretty interesting, and they point out the necessity of collecting soil samples in order to do a good job of fertilizing and liming.

Soil pH and Liming
As Figure 1 shows, about 10 percent of the samples sent to us from cooperators are very acidic, with a soil pH of 5.0 or less. Another 15 percent of the samples fall in the range of pH 5.1-5.5. About 22 percent more of samples sent to us have a soil pH of 5.6-6.0.

Just what does that mean to you? We recommend liming bermudagrass when the soil pH falls below 5.0. That means that only 10 percent of the samples sent to us would need lime if bermudagrass was the crop. However, about 47 percent of the samples sent to us have a soil pH of 6.0 or less. We recommend liming forage legumes when the soil pH drops below 6.0. So, about half the samples sent to us would need lime if the crop to be grown was alfalfa or another forage legume. About 25 percent of the samples sent to us have a soil pH of 5.5 or less. At this soil pH level, we recommend liming almost any crop except bermudagrass.

Let's see how this might affect you. If you're growing alfalfa or forage legumes and you don't soil test or lime, there's almost a 50 percent chance that your soil is too acidic for optimum growth. This means your fertilizer won't work the way it should, and your yield potential is limited by soil acidity.

If you're growing winter pasture and don't soil test, there's a 25 percent chance you need to lime in order to achieve the best yields. Are you one of the 25 percent? How can you tell?

Does that mean you should automatically lime? No! According to the percentage of samples that come through our lab, if you lime without a soil test, there's a 50 percent chance you don't need the lime on legumes and a 75 percent chance you don't need lime on winter pasture. You would be wasting money, and even taking a chance of harming the crop, by over-liming. On the other hand, if you don't soil test and don't lime, there's a good chance you're limiting yields with acid soils. The obvious answer to the problem is to always soil test every field at least every three years!

Soil Phosphorus
Figure 2 shows that about 21 percent of the samples that come through our lab are very deficient in soil test phosphorus (<10 lbs. soil test P per acre). Plants grown on soils with this soil test P level will not produce high yields unless you apply fairly high amounts of phosphate fertilizer. You can apply all the nitrogen you want and the plants will not respond because they are very deficient in phosphorus.

About 70 percent of the samples sent to us in 1999-2003 required some amount of phosphate fertilizer (total of all samples <44 lbs. soil test P per acre). The soils that are very deficient in P would require more phosphate fertilizer than soils that are less deficient in P. About 30 percent of the soil samples sent to us would not require any phosphate fertilizer at all. How do you know where your soils fit? Once again, always soil test every field at least every three years!

Soils in our service area of southern Oklahoma and north-central Texas generally need potassium less often than they need phosphorus. Figure 3 shows that less than 5 percent of samples we received were very deficient in potassium and about 50 percent needed some amount of potassium fertilizer (total of all samples <220 lbs. soil test K per acre). For the last time (at least in this article), to know your soil's nutrient status, always soil test every field at least every three years!

Remember, be good to your plants, because they know if you're naughty or nice to them.