Source: Kansas State University Extension

By Dave Mengel, Soil Fertility Specialist,
and Dorivar Ruiz Diaz, Nutrient Management Specialist

Wheat planting is just a few weeks away, so now is the time to get your soil sampling done to have good information on which to base your fertilizer inputs.

This is particularly important now since 2016 wheat prices are currently lower than most of us would like and efficiency in production will be critical.

Which Nutrients
Should Be Tested?

The most important tests and nutrients to focus on this year depends in part on where you are located, the choices you make when applying N, and your tillage system.

The nutrients for which wheat is most likely to show responses statewide are nitrogen (N) and phosphorus (P). Wheat is the most P-responsive crop we grow in the state, and while P removal with wheat may be less than with corn or soybeans, the relative yield response is often the highest. So knowledge of P soil test levels and fertilizer needs will be valuable.

In addition, low soil pH is becoming a problem in fields everywhere in the state, especially fields with a history of high rates of N application and relatively low cation exchange capacity. 

In addition to the “Big 3” of pH, N, and P, potassium (K) deficiency in wheat can also be found in some areas of southeast and south central Kansas. Wheat is generally less prone to K deficiency than many of the rotation crops commonly grown, such as corn, soybeans or grain sorghum.

Generally, the focus of a K fertilization program is with the rotation crops, and meeting the higher K needs of corn and soybeans minimizes the chance of a K deficiency in wheat.

0-6 Inch Soil Samples

A standard 0-6” surface sample is normally used to test for pH and the non-mobile nutrients such as P and K. Phosphorus and K along with soil pH are buffered processes in our Kansas soils.

This simply means that the soil contains significant quantities of these nutrients, and the soil tests we commonly use provide an index value of the amounts available, not a true quantitative measure of the amounts present.

In the case of P, most Kansas soils have a “buffer factor” of about 18. This means that if you remove 18 pounds of P₂O₅ in harvested grain, the soil test will drop 1 ppm. The same works in reverse, if you add 18 pounds as fertilizer or manure, the soil test value goes up 1 ppm. The buffering value for K is around 8 pounds K₂O per 1 ppm K soil test.

The buffering value for both P and K varies based on soil cation exchange capacity and the soil test levels. On high CEC soils, especially those soils with high clay content, the buffering capacity goes up, so the soil test levels will change more slowly. But on low CEC soils the buffering capacity can be much lower, and soil test levels can change rapidly. The same situation occurs with soil test levels.

On soils with low soil test P or K levels, it will require more P or K to raise the soil test than at high soil test levels.

In addition to requesting the standard soil tests of pH, P, and K from the 0-6 inch surface sample, producers might also want to monitor soil organic matter levels and micronutrients such as zinc (Zn). Zinc is not a nutrient commonly found deficient in wheat production. However it is important for corn and grain sorghum. Thus including it in your sample package would be helpful for planning for these rotation crops.

Soil organic matter (SOM) is an important source of nutrients such as N and S. When calculating the fertilizer needs for both these nutrients, SOM is taken into consideration. For wheat production, 10 pounds of available N and 2.5 pounds of sulfur (S) is credited for every 1% SOM in the soil.

0-24 Inch Samples

In addition to pH, SOM, P, K, and Zn -- all of which are non-mobile in soils and accumulate in the surface -- N, S, and chloride are also nutrients which can provide significant yield responses when deficient in soils.

Since all three of these nutrients are mobile in soils and tend to accumulate in the subsoil, we strongly recommend the use of a 24-inch profile soil sample prior to growing wheat, corn, or grain sorghum.

Nitrogen is a nutrient, which is likely to provide yield response statewide. One common misconception is that the accumulation of N in the soil profile only occurs in the drier, western half of the state.

However with our dry winters, N can accumulate in the soil statewide in many years. Rainfall tends to peak in Kansas in June and July, with a rapid decrease in monthly precipitation in August and September. Rainfall totals are generally lowest in December and January.

Wheat takes up the majority of its N prior to flowering. In southeast Kansas that is in April, and in north central Kansas it is in early May most years.

In many years, especially following dry summers, significant amounts of N can be present in soils at wheat planting. In addition, N mineralization will continue in the fall until the soils freeze.

Since many producers in southeast Kansas use poultry litter as a fertilizer source, N mineralization is common in these fields and should be considered when making fertilizer decisions.

Sulfur deficiency is increasing across the state in wheat production also. Unheard of 10 years ago, S deficiency is now common in northeast and north central Kansas, and the area where these deficiencies are found is growing.

There are two primary causes: the reduction in sulfur deposition from the atmosphere seen over the past 2-3 decades, and the reduction in S content in many P fertilizers. While not as soluble as nitrate, S is also a relatively mobile nutrient, which accumulates in the subsoil. The S profile soil test has been well calibrated in Kansas and is a good way to determine S needs.

A third essential mobile element to be considered for wheat production with profile soil testing is chloride (Cl). Chloride deficiency is normally found in the eastern half of the state on soils that do not have a history of potash (KCl) application. In general this includes many areas in eastern Kansas, north of the Kansas River, and the central corridor of wheat production.

Chloride deficiency is associated with grass crops, wheat, corn, and grain sorghum, and is correlated with the plants ability to resist plant disease. Again, the profile soil test for chloride is well calibrated in Kansas and should be considered.

Summary

In summary, crop producers in Kansas should consider soil testing to help in making fertilizer decisions.

Wheat producers specifically, should use surface 0-6 inch samples to determine the need for lime on low pH soils (special sampling procedures are needed for continuous no-till fields, but that will be discussed in a later article), P, K, SOM and Zn.

They also should be using 24-inch profile soil tests for N, S and Cl. Now is the time to get those samples taken, to ensure there will be enough time to consider those test results when planning your fall fertilizer programs.