Soil pH: Making Adjustments to Boost Fertility

Raising soil pH is relatively inexpensive. Lime is the product of choice but there are two basic types of lime: high-calcium and dolomitic.

Soil pH adjustment may seem like a pretty straightforward operation, but there are many things to consider before undertaking such a bold step with soil chemistry. The first step is determine the direction you need to go and the products to use to achieve your goal. I cannot stress enough the importance of getting a good soil test. I’ve heard people say that based on the type of weeds or the fact that moss is growing means the soil pH needs adjusting. Assuming those statements were true, which direction and how much adjustment should be made? Without a good soil test it is pure and simple guesswork.

Generally I prefer to see soil pH of 6.2 plus or minus two-tenths on soils with exchange capacities of 10 or higher. This does not apply to high organic matter soils with organic levels above 15 percent. These soils may slide down to a pH level of 5.5 or a little less and still have enough calcium and  magnesium for adequate plant growth. At the low pH levels I become more concerned with manganese toxicity in the soil solution.

I have experienced manganese toxicity on soils that have 5-8 ppm manganese levels on the standard soil test extracted with the Mehlich III. On light exchange capacity soils, I prefer pH to be 6.5 plus or minus two-tenths. There are two reasons for this slight increase in pH. The first is due to the low buffering capacity of  sandy soils, which could result in a significant drop in the pH over a short period of time from manure or nitrogen applications as well as leaching. Secondly, it is especially difficult to maintain enough soluble calcium in solution in light, sandy soils.

For most soils the tendency is to see soil pH levels drop over time. This is primarily due to the loss of cations by crop removal, erosion and nitrogen displacement. Calcium, magnesium and potassium are the major cations affected by this removal process. Soils consisting of coral and calcareous sands have such large volumes of calcium in the base makeup of the soil that a drop in the soil pH may not be seen in one’s lifetime. It is for this reason that these calcareous soils, which may have pH levels around 7.5, cannot economically be adjusted downward.

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Raising Soil pH

Raising soil pH is the most common practice and is relatively inexpensive. Lime is the product of choice. There are two basic types of lime,  high calcium and dolomitic. High calcium lime will normally test around 30 percent calcium and 3-5 percent magnesium, and dolomitic lime will test around 21 percent calcium and 12 percent magnesium. There are those who will say lime is lime so use whatever is closer to the farm. I prefer to balance the cations using Albrecht’s ratios of 65 percent calcium and 15 percent magnesium base saturation. On soils with exchange capacities greater than 15, base saturations of magnesium greater than 20 tend to increase the tightness of the soil, resulting in more compaction issues and grass control problems.

Depending on soil test data and the balance of the cations, either calcitic or dolomitic lime will be selected. If the magnesium base saturation is below 15 percent I start off with an application of dolomite lime. Assuming  2 tons of lime is needed to adjust the pH, the first ton would be dolomite and maybe a year later a ton of calcium. Lime applications need to be considered more like yearly fertilizer applications instead of a once every 3-5 year project. Putting on lime will impact the availability of potassium and especially phosphorus. Trace elements may also be affected by liming. Even though a soil may need 2 or 3 tons of lime, I prefer to limit most of my lime applications to 2,000-3,000 pounds-per-acre unless aggressive tillage will be performed. Since most farmers are going to no-till or minimum till, incorporation of the lime is very limited. This will reduce the solubility of the lime since the pH in the band at the surface will be rapidly increased.

This layering of the lime could also affect the results of your next soil test if you are sampling deeper than what the lime has impacted in the soil. In the lab the soil will be ground and thoroughly mixed. Your results will show the soil as if it were a homogeneous mix when in fact you may have a layer of unreacted lime that was dissolved by the extracting solution, leading you to believe the nutrients are all available. This stratification is a real concern that is not being addressed in our no-till practices. Some new watershed data shows phosphorus levels in the water as high, and in some cases exceeding, the 1990 levels when no-till and minimum were not widely accepted practices. These issues could be alleviated with some aggressive tillage, even moldboard plowing every five or six years following wheat. This program would require immediate leveling and planting a good cover crop to prevent soil loss.

The turf industry suffers from the same issues of stratification but only through aggressive core aeration and nutrient balancing of the top dressing materials can this be minimized.

Besides considering the type of lime to be applied to the soil, the fineness of the lime is equally as important. I would prefer to see the lime ground to the point where 60-80 percent of the lime will pass a 60-mesh screen. The finer the lime, the more reactive the lime will be. The more reactive the lime, the smaller the applications should be but more frequent applications will be required. Fine limes do have one major drawback. The finer the limes the dustier they are and harder to spread in windy conditions. Spread widths should be adjusted to the finer portion of the lime and not the coarse particles.

Lowering Soil pH

Adjusting the pH downward with sulfur is recommended for calcareous soils and not for soils with a high pH as a result of sodium. High sodium levels as a result of irrigation water need to be flushed out through watering and possibly the use of gypsum rather than sulfur.

Lowering pH is not generally done on a large-scale basis in general agriculture due to the cost factor. I have done it where a couple of acres were going to be planted to blueberries, gardens and turf situations. In agriculture situations where a farm has been over-limed, we generally increase phosphorus applications, preferably  through starters and foliar feeding, along with foliar feeding traces. The use of more acidifying nitrogen sources during corn rotations is also beneficial. I prefer to lower pH levels over a period of a couple years so I can retest and monitor the progress. Attempting to lower the pH of calcareous sand-based soils is not feasible or economical.

By Bill McKibben. This article was first published in the March 2011 issue of Acres U.S.A.

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