Whether the sudden sharp drop in air temperatures across Kansas this month affects the wheat crop that has already emerged to any degree depends on several factors.
It might only mean risk of cold injury for fields that were sown relatively early and already had a good above-ground development. These plants wouldn’t have had the chance properly to acclimate to the cold temperatures and are more susceptible to winterkill.
The moisture level in the topsoil will be important, as dry soils will get colder more easily than wet soils. Soil moisture was generally good in most of the state except for southwest Kansas. The cold temperatures will be more likely to cause injury to wheat in fields that were planted early, had some significant growth during September and early October, and are now showing drought-stress symptoms.
There was a severe cold snap in mid-November 2014 that contributed to winter injury on some wheat and in many cases resulted in winterkill. In 2014, the crop was sown relatively early and had good development in the fall because of warm temperatures in October. These warm October temperatures caused drying of the topsoil and enhanced the potential for cold damage.
The weather during the fall of 2014 also was warm, which likely provided too few cold enough nights to have allowed the wheat to develop cold hardiness. The extent of the unusually large and rapid drop in temperatures from well above normal to well below normal is a concern in those early planted fields, which would be more susceptible to injury from the recent cold snap. The first thing we’ll see is burn down of the wheat from these cold temperatures, but if the crown below the soil surface remains alive, the wheat should be fine. We likely won’t know with certainty the extent of the damage until at least a couple weeks of warmer temperatures occur.
Excess nitrogen
Taking 24-inch soil profile-Nitrogen samples in the fall has been a recommended practice for making a nitrogen recommendation for winter wheat for many years. However, because of the mobility of nitrate-N in the soil, soil test values observed in the fall may be completely different from values observed in the spring, particularly on soils prone to leaching. Because many producers wait until spring green up to make their nitrogen application, does soil sampling in the fall for nitrate-N really provide useful information for nitrogen management in wheat? That is a legitimate question.
Analysis of yields taken from K-State research plots that received no nitrogen fertilizer shows a strong positive relationship with fall soil profile nitrate-N. Wheat yields increased rapidly as soil nitrogen levels increased to about 80 pounds soil nitrogen per acre, and then leveled off. We found that at low soil nitrate levels, wheat yields responded well to applied fertilizer. We also found that when fall soil profile nitrate-N levels are greater than 80 to 100 lb/acre, it is unlikely the site will respond to additional fertilizer nitrogen applied in the spring.
In short, a strong relationship was found between wheat yield and fall nitrate-N levels from 24-inch profile soil test analyses when no nitrogen fertilizer was applied. Although new practices have been developed to improve nitrogen management in winter wheat, soil sampling in the fall for nitrate-N remains an important practice to manage nitrogen efficiently and can result in considerable savings for producers.
When soil sampling for nitrogen is not done, the K-State fertilizer recommendation formula defaults to a standard value of 30 lb/acre available nitrogen. In this particular dataset, the average profile nitrogen level was 39 lb N/acre. However, the nitrogen level at individual sites ranged from 11 to 197 lbs N/acre. Most recommendation systems default to a standardized set of nitrogen recommendations based on yield goal and/or the cost of nitrogen. Without sampling for nitrogen or using some alternative method of measuring the soil’s ability to supply nitrogen to a crop, such as crop sensing, the recommendations made for nitrogen will be inaccurate, resulting in a reduction in yield or profit per acre and increased environmental impact.
Failure to account for the nitrogen present in the soil wastes a valuable resource and can result in excess foliage, increased plant disease, inefficient use of soil water, and reduced yield. Soil sampling in fall for nitrate-N can have a significant impact on nitrogen recommendations for winter wheat, thus improving nitrogen management, and is strongly recommended.
Ryan Flaming is a Kansas State Research and Extension agent for Harvey County. Agriculture is his specialty. The Harvey County Extension Office can be contacted at 284-6930.