Kansas State

 Before producers convert a field to long-term no-till, they often apply and incorporate lime if the soil test shows a need for it. That’s because it is not possible to incorporate lime after the field has been converted to no-till, unless the producer is willing to do some rotational tillage.

 But several years after the initial lime application, it is not unusual for the soil pH to decline again, especially in the upper 2 to 3 inches of soil. Why is that?

Some of the factors include rainfall and root activity, which is related to crop productivity. But these factors act relatively slowly over time, and have very little short-term effect on pH. The most significant factor that causes soil acidity is the process of nitrification. The article below (No. 2) goes into more detail on this process.

In no-till, all or part of the nitrogen fertilizer is often surface-applied. As nitrification occurs year after year in the upper layers of the soil, this will lower the pH significantly. As a general rule, for every 100 pounds of N fertilizer applied, 500 pounds of “typical” agricultural lime will be needed to neutralize the acidity produced from nitrification.

When the pH gets too low for optimal crop production, lime must be applied to reduce the effect of toxic aluminum on plant roots, to maintain good conditions for microbial activity, and to get the best performance from some of the soil-applied herbicides. Most growers prefer to surface-apply the lime if the ground is to remain in long-term no-till. How effective will surface-applied lime be, and how long will it take to start increasing the soil pH?

Lime applied on the soil surface reacts as quickly in the soil as lime incorporated by tillage, but only in the upper surface inch or so of the soil. Surface-applied lime moves down through the soil at a very slow rate, so changes in pH occur very slowly below that surface inch, if at all.

K-State research on liming rates and methods on no-till soils

No-till lime research was done by Chad Godsey and Ray Lamond of K-State from 2000 through 2004. They applied lime to two soils in Cowley County and one in Marshall County. All of the soils had been in long-term no-till production, and had a pH less than 6.0 in the top 6 inches of soil. Both agricultural lime and pelletized lime, at equivalent ECC rates, were surface-applied to these soils. The rates were based on the amounts needed to raise the pH in the top 6 inches to 6.8, using the SMP buffer, as determined by soil tests at the K-State Soil Testing Laboratory. Lime was applied in the following amounts:

* Full rate, one-time application

* Full rate, applied in four consecutive years using a quarter-rate each year

* Half-rate, one-time application

* Quarter-rate, one-time application

* None

Individual limestone rates were on an ECC basis at all sites. The ECC of the commercially available limestone was 55% and that of the pelletized limestone was 86%.

Over the five years of the studies, surface application of limestone was effective in raising the soil pH in the surface 2 to 3 inches of soil, but not at lower depths. Below 3 inches, there was no significant change in pH after 5 years. The observed downward movement of limestone was largely dependent on the amount of limestone applied to the surface, and did not differ between the two types of lime. When the full rate was applied, the change in soil pH was about the same whether the lime was applied all in one application, or in four annual applications of a quarter-rate each.

Although the soil pH was increased by the lime applications, the application treatments failed to increase the pH to 6.8, when measured by a 0-to-6-inch soil sample, which was the goal. However, the pH of the top inch of the soil was increased to over 6.8 at all the sites when full rates of lime were applied. It should be noted that this research was conducted in areas with an average annual rainfall of 32 to 40 inches.

Final comments

It is critical to closely monitor soil pH in no-till systems. The top few inches of soil may become extremely acidic due to the surface application of N fertilizer. However, soil surface pH can also become too high if a large amount of lime is applied at one time and left on the soil surface. This can affect nutrient availability in the upper soil levels, microbial activity, and herbicide performance or carryover. It is best to apply small amounts of lime more frequently to maintain soil pH in a no-till system.

How often should lime be applied in a no-till situation? A general guideline for lime applications in no-till is: half the rate, twice as often. This depends on several factors. A coarse-textured soil with a low CEC does not require a lot of lime to correct soil pH, but may need to be limed frequently. A finer-textured soil with a high CEC requires a large amount of lime to initially correct pH, but it may be several years before another lime application is needed due to its high buffering capacity.

The frequency of lime applications needed also depends in part on how much nitrogen fertilizer is being applied and the yield level of crops being produced. In general, the higher the nitrogen rates and yield levels, the more frequently lime will be needed. Due to the variation in buffering capacity of soils, lime applications should always be guided by soil tests.

The bottom line is that there are beneficial effects of surface application of limestone to acidic no-till soils even though the immediate effect may only be in the top 1 to 2 inches.

-- Dave Mengel, Soil Fertility Specialist

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-- Dorivar Ruiz Diaz, Nutrient Management Specialist

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