Wheat planting is just around the corner in parts of Kansas, so now is the time to complete your soil sampling. This ensures you have solid information on which to base fertilizer decisions, especially important following drought conditions and lower-than-expected yields in some areas of the state.
Which nutrients should be tested?
The most important nutrients to focus on this year will depend on your location, nitrogen application strategy, and tillage system. Statewide, the nutrients most likely to generate a wheat yield response are nitrogen (N) and phosphorus (P).
Wheat is the most P-responsive crop grown in Kansas. While P removal by wheat is lower than with corn or soybeans, the relative yield response is often greater. As such, knowing your soil test P levels and corresponding fertilizer needs is key.
Additionally, low soil pH is becoming an increasing concern, particularly in fields with a history of high N application and low cation exchange capacity (CEC).
Beyond the “Big 3” (pH, N, and P), potassium (K) deficiency can occasionally be an issue, especially in southeast and south central Kansas. While wheat is less prone to K deficiency than crops like corn, soybeans, or grain sorghum, deficiencies can still occur. In most cases, K fertilization is focused on those rotation crops; meeting their higher K needs often reduces the risk of deficiency in wheat.
The 0-6 inch soil sample: Non-mobile nutrients and pH
A standard 0-6 inch surface sample is normally used to evaluate pH (more information about liming acid soils for wheat is available here: https://bit.ly/4mqcFtR), and non-mobile nutrients such as P and K. Phosphorus and K availability are buffered processes in Kansas soils. This means the soil contains significant quantities, and the K-State soil tests provide an index value of the amounts available to the plant, not a direct measure of total soil content. In most Kansas soils:
The buffering value for both nutrients varies with CEC and existing soil test levels:
In addition to pH, P, and K, consider testing for soil organic matter (SOM) and micronutrients like zinc (Zn):
The 0-24 inch soil sample: Mobile nutrients
While pH, SOM, P, K, and Zn are non-mobile and tend to accumulate in surface soils, the mobile nutrients nitrogen, sulfur, and chlorine (Cl) move through the soil and accumulate in the subsoil. For this reason, a 24-inch profile sample is strongly recommended before planting wheat, corn, or grain sorghum.
Nitrogen is likely to produce a yield response across Kansas. Contrary to popular belief, soil N accumulation isn't limited to western Kansas. Our dry winters mean N can accumulate throughout the state. Since rainfall typically peaks in June and July and drops off in fall and winter, N losses tend to be limited after harvest.
Wheat takes up most of its N before flowering — typically April in southeast Kansas and early May in north central Kansas. Following dry years, significant residual N may be present at planting. On the other hand, strong yields may leave behind less residual N than assumed. Either way, adjusting N fertilizer rates based on soil tests is critical. (See the companion article in this issue for details on the correlation between soil nitrate levels and wheat yield.)
Sulfur deficiency in wheat is increasing across Kansas, largely due to:
Though not as soluble as nitrate, S is still mobile and tends to collect in the subsoil. A 24-inch profile test is a good way to evaluate S needs.
Chlorine (chloride is the plant-available ionic form) is the third essential mobile element to consider. Chloride deficiency is normally found in the eastern half of the state on soils that do not have a history of potash (KCl) application. This includes many areas north of the Kansas River and across central Kansas wheat-producing regions.
Cl deficiency affects grass crops like wheat, corn, and grain sorghum, and is linked to reduced disease resistance. Kansas has well-calibrated Cl soil tests, so a profile sample can help determine whether application is needed.
Summary
Kansas wheat producers should use soil testing to guide fertilizer decisions, especially in a year with tight margins and varied yield outcomes.
Sampling now allows ample time to analyze the results and adjust your fall fertilizer plan accordingly.
For more information on soil sampling or to submit samples to the K-State Soil Testing Lab, visit: http://www.agronomy.k-state.edu/services/soiltesting/
Dorivar Ruiz Diaz, Nutrient Management Specialist
ruizdiaz@ksu.edu