1) applying different rates of a fertilizer nutrient like nitrogen across the entire land area.
2) application of a fertilizer nutrient only to certain areas within the field.
Appropriate rate variations can be difficult to sort out given the variability we typically encounter in Saskatchewan: soil, climatic, economic fluctuations usually large! Interactions affecting response to applied nutrients like nitrogen can be difficult to predict.
Variable rate is most applicable to those nutrients like nitrogen and phosphorus in which a range of application rates can be set out according to differences in such measured parameters as level of available nutrient and moisture availability across the field.
In the on-off approach, problem areas where deficiencies are suspected are located in the field through visual inspection or using yield maps produced from the combine yield monitor. Soil and plant testing inside and outside the affected area is used to first determine whether or not a nutrient limitation is responsible for the low yield and then subsequently the nature and extent of the limitation.
The limitation is then addressed by applying the appropriate fertilizer nutrient to the deficient area at a single rate: the applicator turned on in the deficient area and turned off outside the area.
The "on-off" approach is especially useful for addressing productivity limitations caused by deficiencies of lesser elements like sulfur, copper and boron. Deficiencies of these nutrients tend to be patchy, often found in localized areas within a field: i.e. eroded knolls, sand or gravel lens. Furthermore, rather than a range of rates associated with different soil test levels, the recommendations often take the form of a single recommended rate when the test level is below a critical value i.e. apply or don't apply. There are economies to be realized by applying a micronutrient like copper to the 10% of the field that actually needs it rather than applying it across the entire field.
Crop response to added fertilizer nutrient at a particular location in the field depends on:
Both of the above approaches enable the construction of soil fertility maps which can be used as part of the basis for variable rate application. Direct measurements have an advantage in that they can provide a direct assessment of supply and balance of all plant nutrients while remotely sensed information such as soil organic matter content may be applicable in reliably predicting nutrient supply for only a few nutrients. However, direct measurements, particularly intensive grid sampling and testing, can be expensive. Both approaches are currently being researched and evaluated for their suitability in precision fertilization across North America.
A simpler, more conservative and perhaps more realistic approach to providing field fertility information in Western Canada would be to use inexpensive remote sensing imagery in combination with direct fertility measurements. In this approach, remote sensing information such as an aerial photograph may be used to divide the field into individual management units based on identifiable differences in landscape or past management. Direct measurements of soil fertility in selected examples of the management units would then be made which could be extrapolated to all management units identified and located through remote sensing.
For example, one may go into a farm field and based on landscape position, make direct soil fertility measurements on five randomly selected knolls, five mid-slopes, and five depressions within the field. The information that is provided on the relationship between landscape position and soil nutrient availability can then be extrapolated to the entire field area using the remotely sensed information on landscape distribution across the field .
For example, knolls are usually drier than depressions due to precipitation and snowmelt run-off along with increased evaporation. Conservation tillage practices can help reduce this source of variability by encouraging water retention within the upper slope landscape units. Lower stored soil moisture on the knolls may limit yield potential and nutrient requirements such that despite lower soil available nutrient levels, the same or even lower rates of fertilizer would provide adequate crop nutrition compared to mid-slopes and depressions. For this reason, assessments of depth of moist soil at different slope positions are likely to be useful in developing good site-specific fertilizer recommendations in Saskatchewan.
An important soil chemical factor affecting response to applied fertilizer is the presence of sub-soil salts. In locations in the field where salts approach the surface, the effective rooting volume and water available to the crop is greatly restricted, such that these areas are much less responsive to added fertilizer and therefore represent locations where fertilizer rates may be significantly reduced.
Other biological limitations to plant growth and overall nutrient requirements may come also into play in a farm field. The effects of insect and disease damage and weed competition should also be taken into consideration. For example, high wild oat infestation in depressional areas will restrict the crop yield response to added nutrient. As well, fertility-disease interactions need to be accounted for. The role of chloride in disease suppression in cereal crops may have landscape dependency, in terms of both the effect of landscape on soil chloride content and the effect of landscape on disease severity in the crop.
It is important that agronomic research keep in step with the technical advances being made in information gathering so that the information collected can be properly interpreted and put to best use by producers.