In-Row Starter Fertilizer for Wheat

J.H. Grove—Soil Fertility Researcher
C.A. Knott—Extension Grain Crop Specialist
K.S. Rod—Agronomy Graduate Student
E.L. Ritchey—Extension Soil Specialist

With thinking about wheat planting has come the usual, and less usual, questions about fall soil fertility. This article is prompted by recent questions about one of the less usual topics, in-furrow starter fertilizer for wheat. What, how, why/when and why/when not?

For wheat, in-furrow starter fertilizer can be applied in a number of ways, depending upon residue management (no, minimum or full tillage), equipment (conventional drill or air seeder), and product (dry or liquid). Placement can occur behind a coulter but ahead of seed delivery, with seed delivery, or after seed delivery but usually before the press wheel. Chosen products most always provide phosphorus (P), usually contain nitrogen (N) and occasionally also provide potassium (K), sulfur (S) or micronutrients. This article will emphasize P because wheat’s need for fall N alone can more easily be provided for with a broadcast N fertilizer
source. 

In general, closer seed-fertilizer proximity increases P use efficiency, reduces P fixation in some soils, and can improve early crop growth and vigor with stressful planting conditions. Due to a fertilizer’s salt index, care must be taken to avoid overly high in-furrow starter rates, depending upon the chosen fertilizer material, that will delay/reduce germination and emergence. Phosphate sources vary considerably in their salt index, with diammonium phosphate (18-46-0)
being about 3 times higher than triple-super (0-46-0), and the other common P sources falling in between. The different placement tactics described above result in different degrees of seed-fertilizer contact. Soil moisture at planting and soil texture also complicate predictions of any salt effect. In general, in-furrow starter P rates range from 5 to 20 lb P2O5/acre.

A review of recent research reports from other states (Virginia, Nebraska, Kansas and Oklahoma) finds no value to in-furrow starter for wheat when planting dates are early or optimal (little stress). Late (cold soil) planted wheat tillering, growth and vigor were enhanced by in-furrow starter use, but yield was not always increased because late planted
wheat generally experienced later grainfill that sometimes coincided with hotter weather.

Soil pH and P levels also played a large role in yield responses to in-furrow starter P. When soil pH was low (below 5.2) there was a greater probability of a yield benefit to infurrow
starter P. When soil test P was medium-high (actual numerical value depends on the extractant being used), there was little chance of a yield benefit and no chance of a positive return on investment. Recent work in Kentucky, by Carrie Knott and Katherine Rod, further confirms this observation. Across 6 site-years of data, with Mehlich soil test P ranging from 49 to 100 lb P/acre, two wheat varieties (Pembroke 2016 and Pioneer 26R53), and two seeding rates
(35 and 56 seeds/ft2), wheat yield without in-furrow starter P averaged 78 bu/acre, and 73 bu/acre where 42 lb infurrow starter P2O5/acre (as 0-46-0) was used for October
planted wheat. With November planted wheat, they found yields of 64 bu/acre (no starter) and 66 bu/acre (with the 42 lb in-furrow starter P2O5/acre).

In summary, in-furrow starter P for wheat is of no value when planting on time into fields whose soils have been well managed as regards soil pH and soil test nutrient levels. This
is an important consideration, given that planting while applying starter materials can significantly slow the planting process – driving establishment of a greater portion of the
crop into a less favorable fall growth period, especially when fall weather conditions are already challenging.