Precision Farming, one of the latest 'buzz-words' in
agricultural circles, promotes variable management practices
within a field according to site or soil conditions. Variable
Rate Fertilization (VRF) is the cornerstone of Precision Farming:
fertilizer rates are varied across a field to minimize over an
under application. In short, VRF puts fertilizer where it can be
utilized by the crop. Theoretically, VRF is environmentally
sound, but if it doesn't increase yields, reduce costs, or both,
it'll never fly.
The technology: Global Positioning System using satellites,
yield monitors, automatic fertilizer rate adjustment, etc. is
being used to increase fertilization efficiency, but the science
behind VRF is less clear.
Would VRF make "cents" on your land? The simple answer is: variation of the factors affecting crop yield must be large and consistent enough to justify the extra costs of collecting information and managing parts of fields differently.
So which factors affect yield?
1. Topography;
2. Soil properties, such as organic matter, available nutrients, texture, salinity;
3. Weed, insect, and disease levels;
4. Cropping history; and of course,
5. Weather - namely rainfall and temperature.
Yield is determined by the factors most limiting to
productivity. Some people suggest that yield maps can be used as
the basis for varying fertilizer rates for the next crop, but we
must remember that yield mapping is helpful only if there is a
consistent yield variation pattern from year-to-year and
crop-to-crop, or you can identify the factors causing yield
variation. Is yield variability due predominantly to available
nutrient levels? In many instances, it is not. A definite benefit
of yield mapping, however, is to evaluate the results of your
fertilization strategy, whether it be conventional or
VRF.
Soil fertility and the amount of water available to the crop
are usually the two main factors affecting crop yield potential.
Topography is a primary factor contributing to soil variability,
and influences both fertility and available water. Thus, soils on
similar slope positions tend to have similar qualities. Fields
with significant topography are best suited to VRF. Yield
potentials are usually lower on hilltops (less fertile, less
water) and higher on mid-slope or level depressional area (more
fertile, more water). Therefore, fertilizer recommendations
should take into account both soil available N levels prior to
seeding and yield potentials (match crop N demands with soil plus
fertilizer N supply). However, unpredictability of weather
creates unpredictability of yield responses. In dry years, low
areas may yield best; in wet years, mid-slope areas may excel.
The question is whether soil fertility or water is limiting
yield.
To get started in VRF, you don't have to take the high tech
route. A less complicated approach exists that is just as
feasible and may give similar benefits. It is relatively simple
and inexpensive to manually adjust fertilizer rates on the
go.
Research conducted by the Saskatoon Research Centre at the Conservation Learning Centre suggests that VRF, based on soil organic matter and topography, enhance both fertilization efficiency and profitability. The costs of varying fertilizer rates according to topography should be similar to conventional fertilization. Thus for fields with significant variation in soils or elevation, VRF may be the next revolution since subsurface banding for enhancing the efficient use of fertilizers, and has the potential to improve the bottom line while being environmentally friendly.