As farmers in the upper Great Plains of North America adopt one pass seeding practices on their farms, placing the majority of the crop’s nitrogen requirements between every second seed row is a methodology of direct seeding that is growing in popularity. Recognized early as a low risk method of nitrogen fertilization for direct seeding, and utilized on many acres since the early 90’s, we now have a large variety and volume of experiences on how well this seeding practice works. At the same time, there is still much to be learned and understood regarding mid row nitrogen placement as well as other fertilizer nutrients.
In actuality, the weedy space between the seed rows is typically the non-fertilized inter-row, and the inter-row space with little to no weeds is the location of the banded nitrogen. I have seen many fields over the years, where the weeds in the non-banded inter-row are growing at a faster rate and greater density than the weeds in the nitrogen-banded inter-row.
In this photograph , we can see a very visible difference between neighbouring crop inter-row spaces. This is from a farm located near Kalispell, Montana taken in spring 2006. The crop is spring wheat seeded with a Bourgault 5710 air hoe drill on 9.8” spacings and disc-type mid-row banders. 50 lbs. of 11-51-0 was seed-placed, and 240 lbs. of a urea/potash blend placed in the mid row to give 95 pounds of actual N and about 10 lbs. of actual potash per acre. The nitrogen was placed approximately ¾” deeper than the seed. Moisture was considered to be good at the time of seeding.
The photo, taken by the local Agrologist, shows a flush of Russian thistle in the non-banded inter-row space. It is safe to assume that the weed seed density would have been relatively even in the field. So; why is the Russian thistle not growing where the nitrogen has been placed? A logical explanation is that the toxicity of the nitrogen band was great enough to poison and/or desiccate the weed seeds in close proximity to the fertilizer band.
Agriculture Canada researchers at Brandon report that seeds and roots developing within 2 inches of the nutrient band are at risk from lethal or sub-lethal effects emanating from the nutrient band and Alberta Agriculture researchers have documented that this risk increases with wider row spacings that tend to result in higher concentrations of nutrient in the bands at the same rates of nutrient per acre. It is well documented and understood that the process of converting nitrogen fertilizer in the soil from an ammonia based compound to the nitrate form, although essential for the healthy growth of a crop, is a very toxic reaction to the emerging seedling if is too close to the band or the band is in a very high concentration. Nitrogen fertilizer, in the presence of the Urease enzyme, needs a period of time to transform from its initial ammonia (NH3) form to ammonium (NH4) to nitrate (NO3); which is the usable form of nitrogen for annual crops. Ammonium is not very mobile in the soil, so will remain in a 3” to 5” band around the initial placement as it is converting to nitrate. Any seeds caught in this ammonium band, or ammonia retention zone, will be damaged or killed by the toxic and desiccation effects of the nutrient band. This risk can never be eliminated, but can be reduced in wet conditions and exaggerated in drier conditions.
We must remember and appreciate that fertilizer nutrients placed in bands in the soil create zones in and around the band that are extremely toxic to germinating seeds or developing roots. The higher the concentration rates of the fertilizer band, the longer will be the time interval between application and the roots ability to enter the nutrient band. Fertilizer bands only work well with mobile nutrients such as nitrogen or sulphur where the roots can access the nutrients as the nutrient moves away from he band in the soil solution. This also why placing non-mobile nutrients such as phosphate in the nitrogen band denies the crops access to those nutrients until later in the growing season; often too late to be of benefit for the crop in that growing season. Non-mobile nutrients should be placed in the seed row in accordance with the recommended rate guidelines.
One criticism that I have heard attributed to mid row nitrogen application is it that can promote and support weed growth over the nitrogen band, showing up as heavier weed density in every second row spacing. I have personally used mid-row nitrogen since 1993 and I have seen fields and photos of fields where one row space will be almost devoid of weeds, and the row spaces on either side are relatively populated with weeds. Since the nitrogen band is placed between every second row, it would be understandable to conclude the weeds are growing over the nitrogen band, thriving on the available nutrient from the fertilizer band and competing with the crop.
Lyle Cowell, agrologist with the Saskatchewan Wheat Pool in Tisdale, has also noted over the years that weed growth is usually less in the row space where the nitrogen has been placed.
"There may be a number of different reasons for this, I’m not sure exactly what…. nitrogen toxicity…. or some tillage from the mid row banders, but I have seen this over and over again… that there are less weeds over the mid row band.” Lyle also notes, “And what you have here [non-banded inter-row space] is that there is no nitrogen fertilizer here, so the weeds that have emerged are nitrogen deficient…. we have good nitrogen access by the crops that is placed safely away from the band.”
It is always difficult for a producer to determine what has caused the damage to his emerging crop, or if damage is even present, without having a controlled test strip in the same field. Few farmers have the time or the resources to do this, and rely on our input and studies to help guide them in their decisions. Checking for weed growth on the nitrogen band is one example that can be easily monitored. Producers who use the mid row method of nitrogen placement can flag the nitrogen bands right at the time of seeding at various locations in their field. Prior to spraying, they can check the flagged inter-row spacings and compare them to the neighbouring inter-row spacing on either side. Again, factors including soil composition, soil moisture and fertilizer rate will affect the difference seen between the two inter-row spacings and must be considered in your observations and comparisons.
By doing this simple test, at the very least, producers will be able to put their minds at ease that their ever-increasing investment in fertilizer is supporting their crop and not going to the weeds.