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By Doug Shoup, Southeast Area Crops and Soils Specialist, after Kansas State Extension eUpdate No. 234
For optimum alfalfa production, soils should have a pH in the range of 6.5-7.5. If soil pH levels are either below or above those levels, stand establishment and production can be reduced. In Kansas, soil pH changes dramatically from east to west. Generally, soil pH is relatively low in the east and excessively high in the west.
The following case is from Osage County in the summer of 2009, and was in consultation with Rod Schaub, Osage County Agent. The field was planted in the fall of 2008 and fertilized with 100 lbs of 18-46-0. Two adjacent fields, separated by a grass waterway, were planted and have been managed the same for at least the last 10 years (Figure 1).
However, one field had a good stand of alfalfa while the other field had a large portion that was weak and dying. After visiting with the producer, it was determined that there were no herbicide, disease, or insect issues that would have caused a difference between the fields.
The field with the poor stand was not uniformly poor. There was a range of plant conditions in this field, from healthy to weak. Along the edge of the waterway in the poor alfalfa field, plants were noticeably healthier. After examining the roots, we found that nodulation was reduced severely in the weak alfalfa while the healthy alfalfa had fair nodulation (Figure 1).
Using a hand-held pH meter, measurements were taken across both fields. In the field with poor stands, we measured the pH level starting with the strip of relatively well-established alfalfa adjacent to the waterway and then 50 and 100 feet away from the waterway (Figure 2). Where the alfalfa was in better condition, the pH was 6.1. In the weaker alfalfa 50 and 100 feet away from the waterway, the pH was 5.9 and 5.7, respectively. In the small adjacent alfalfa field with good overall establishment, the soil pH was 6.4.
While taking the pH reading in the good field, we noticed a large amount of gravel in the soil. The producer remembered that the small field was the storage site for a large gravel pile when road construction had been done more than 20 years ago. We concluded that the differences in pH on these fields was caused by the gravel, and windblown dust from the gravel.
This field problem is a good reminder that it is very important to take a soil sample and correct any pH or fertility problems prior to establishing a new stand of alfalfa. Alfalfa is one of the more sensitive legumes to lower pH levels, as well as phosphorus and potassium deficiencies. The hand-held pH meter did a good job of showing the sensitivity of alfalfa to a gradient of lower pH levels.
Although the alfalfa was able to establish and produce nodules in the marginal pH soil of 6.1, the nodules were generally fewer, larger, and more malformed, as shown in the photos in Figure 1.
The field conditions above illustrate the impact of low pH on alfalfa production. In these conditions, lime application is required for proper growth and production.
- To view Kansas State Extension’s eUpdates, click here or go to http://tinyurl.com/8a2zayp.
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