This is because the actual yield potential isn’t known until we are part way through a cropping season, but nutrient application rates usually need to be decided on before planting. The following are management techniques used individually, or in concert, to reduce risk from in-season weather variability.

1. Plan for a realistic but above average crop. This means having fertilizer resources in place to supply sufficient nutrients for an above average yielding crop.  In this manner the farmer can realize above average yields if the weather turns out more favorable than normally experienced. If less than optimum weather is experienced the extra nutrients will be residual in the soil and this allows a slight reduction in fertilizer rates the following year. The soil acts as a buffer to supply additional nutrients when needed, or to store unutilized nutrients if that occurs. This is especially effective for less mobile nutrients in the soil such as P and K. It is difficult to apply these two nutrients as an in-season top-dress application and actually achieve sufficient movement through the soil to penetrate into the root zone of the crop.
2. For more soil mobile nutrients, such as N and S, effective in-season applications can be possible. This allows flexibility in the amount of N and S applied depending on how growing conditions proceed. One strategy is to apply a base amount of N and S before or at planting and then be ready to top-dress or side-dress additional N or S if needed. If a less favorable year happens then no additional N or S is applied. This is especially useful for N as it is usually wise to avoid having excess amounts of residual plant available N, in the nitrate (NO3-) form, after crop harvest. Nitrate in the soil over the fall, winter and early spring can be susceptible to leaching and or denitrification losses. However there needs to be forward planning to insure that sufficient N and S fertilizers can be obtained from the retail supplier if needed. This may involve pre-purchasing fertilizer, but not taking delivery unless needed. If not used there may be a business arrangement to have a future credit of N and S fertilizers for the next growing season. If arrangements are not made a farmer may be disappointed to find out there is a shortage of needed in-season fertilizer and other farm operations will be supplied first.

One odd trend in the Northern Great Plains Region is that there seems to be greater than average early growing season precipitation in the southern areas (e.g. Montana, North Dakota, and southern Manitoba, southern Saskatchewan and southern Alberta), while in the more northern areas (e.g. north-west Alberta or the Peace River area) the early growing season precipitation seems lower. It is almost as if the north and south areas have made a flip-flop in precipitation amounts received, as shown in the Alberta Agriculture and Rural Development figure below for 30-May-2011 showing soil moisture supplies for a projected spring wheat crop. Red areas are below normal moisture conditions and blue areas are above normal moisture conditions.

Figure 1. Soil moisture departures for 30-May-2011 compared to thirty-year normals. AgroClimatic Information Service, Alberta Agriculture and Rural Development, http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/cl12944 One challenge is that no one knows whether or not this trend will continue, or could switch back to more normal conditions. Either way, plans need to be put in place to have fertilizer supplies ready for the 2012 crop year. By planning for a better than average year a farmer will always be ready to achieve higher than average yields if the weather so cooperates.