By Ross H. McKenzie PhD P. Ag.; Retired Agronomy Research Scientist
Variable Rate Fertilizer (VRF) application involves the application of different rates and/or types of fertilizers on uniquely different soil areas within a field according to a pre-set field map that is developed based on various types of information. The objective is to optimize fertilizer inputs and crop yield; but VRF comes with challenges.
The first challenge for farmers is to decide if there is enough soil variation within their fields to warrant the investment of developing prescription fertilizer maps. Private agronomists typically charge from $5 to $15 per acre or more for VRF services. Costs vary depending on the technologies used, level of soil sampling and analysis and level of VRF information provided.
A significant portion of annual cropland across the Prairies of Western Canada has enough variability in surface soil physical and chemical characteristics within fields to warrant using some level of variable rate fertilization. As a general rule, fields with rolling topography have good potential for using VRF technology, while land with relatively uniform topography often does not have enough soil variation to warrant using VRF application.
On lands with variable soils and topography, there can be considerable variation in plant available soil nitrate-nitrogen (NO3-N), phosphorus (P), potassium (K) and sulphate sulphur (SO4-S) levels, and therefore could benefit by varying fertilizer types and rates across fields.
Once the decision is made to consider using VRF technology, the daunting challenge for farmers and crop advisors is to develop “effective” variable rate fertilizer prescription maps. To do this, uniquely different soil management zones must first be identified. The questions farmers must ask include the following:
- What are the most important soil factors that need to be delineated to identify site-specific management zones that have lower, medium and higher crop production potential on their land? These variable factors could include soil organic matter levels, depth of top soil, depth to subsoil, variation in soil texture, soil salinity, available levels of N, P, K or S, etc.
- What are the best approaches to identify site-specific soil/topography management zones within fields? Information utilized could include aerial photos, field topography map, soil texture map, soil organic matter map, crop yield maps and satellite imagery. Most important is field-specific farmer knowledge and experience, which doesn’t cost the $10 per acre charged by VRF companies.
- How variable are soil test N, P, K and S levels within a field and among the identified soil management zones? How does crop response to each of these fertilizers vary from year to year?
- How will you or your crop advisor determine the optimum fertilizer types and rates for each management zone to optimize economic returns for variable fields? For example, will you apply more fertilizer or less fertilizer on eroded knolls or less productive versus higher production areas?
Tools for generating management zones
From an engineering standpoint, equipment manufacturers have done a good job of developing seeding equipment with the ability to vary fertilizer rates utilizing global positioning. Unfortunately, from an agronomic standpoint the development of prescription fertilizer maps is a very technical and complex process. There isn’t a simple, easy process to generate prescription fertilizer maps for all soils and crops. There are many soil factors that affect crop yield potential, and these factors vary within a field and from region to region across the Prairies. A study by Dr. Raj Kohsla at Colorado State University showed that bare soil imagery plus topography plus farmer experience was the best method to determine soil management zones in terms of cost and accuracy.
Initially, to better understand soil variability on their farms, producers can use aerial photos of their fields, crop yield maps, provincial soil survey maps and their collective knowledge of crop production in their fields. This can be a good start, but often more detailed information may be needed. Industry agronomists have varying opinions on the best ways to develop fertilizer prescription maps. They utilize a range of methods to generate various types of field maps and each has advantages and limitations:
Crop yield maps – These can be generated when yield and geographic position data are recorded at harvest with your combine. Yield maps are very useful to show the higher, medium and lower yielding areas within a field. The challenge then is to understand what major factors contribute to higher or lower yield potential. Unfortunately, different yielding areas are not necessarily well correlated with differences in soil types and soil fertility levels. To complicate the process, yield maps often vary considerably from year to year, making it difficult to accurately delineate different field management areas.
Soil salinity maps – If slight to moderate levels of salts are a potential problem on your farm, developing a salinity map using EM 38 or Veris technology can be utilized. Then, fertilize rates can be
reduced – depending on the levels of soil salinity – to match lower crop yield potential. A good salinity map is a valuable tool if soil salinity is a problem in your fields.
Soil texture maps – It is generally assumed that field areas with higher clay content have a higher water holding capacity and therefore have higher crop yield potential. Soil texture maps are generated from information collected using an EM 38 or Veris. Each technology attempts to measure the apparent electrical conductivity of soil through the use of sensors as they are transported across a field. The premise is that clay textured soils are better conductors than sandy textured soils, so clay soils give a higher sensor reading versus sandy soils. However, readings are also higher on wetter versus drier soils, and when soil salts are higher versus lower. There is no way for these instruments to distinguish the causes of higher versus lower readings. These technologies should not be used when soils are frozen because frozen soil moisture (i.e., ice) will not cause the instruments to respond in the same way as liquid soil moisture (i.e., water). Therefore, the ability to develop accurate soil texture maps is questioned by some researchers and agronomists in terms of their accuracy and validity.
Soil organic matter and pH maps – Besides soil texture, some machines utilize on-the-go mapping of other soil properties such as pH and organic matter. Near Infrared (NIR) spectral measurements are claimed to correlate with soil organic matter, soil carbon, soil pH buffer capacity and soil moisture. Considerable research needs to be conducted in Western Canada to determine the potential for using these new sensors, and this will take some time.
Satellite imagery maps – The maps can help identify the higher versus lower productive areas of a field. For example, Near Infrared satellite imagery is being used to assess plant growth. Higher relative biomass production areas in a field are assumed to be associated with higher crop yield potential. Several companies use imagery to delineate crop management zones within a field. Imagery information can change considerably during a growing season and is often variable from year to year. This makes interpretation of imagery information a challenge. Therefore, imagery from a number of good crop production years must be collectively utilized to identify areas within a field with consistently higher or lower productivity. Satellite imagery has similar limitations to utilizing crop yield maps. The major factors that contribute to the differences in crop biomass potential within a field must be determined. It is important to determine if the differences in biomass are related to differences in soil types and soil fertility, or other crop production factors.
Topography maps – These can be very useful for developing fertilizer prescription maps. Topography is a major soil-forming factor affecting how variable soils have developed on variable landscapes (see “Understanding Soil Variability”, Top Crop Manager, December 2013, Vol. 39, No. 17, pages 18-20). When a combine with a yield monitor collects global position information, elevation data can also be collected, and elevation maps can be developed. One of the best programs to generate a very good topography map was developed by Dr. Bob MacMillan (retired), and is called LandMapR. Dr. Mike Duncan (NSERC Chair Precision Agriculture and Environmental Technologies, Niagara College) recently updated the program. This free program is an excellent, simple and effective way to develop a field topography map. Figure 1 shows a map developed from elevation data collected at harvest from an irrigated field near Lethbridge, Alta. The topography map very nicely delineates upper, mid and lower slope positions, which can be closely related to uniquely different soil areas.
Developing your own management zones
Each different mapping process has advantages and limitations. Gathering multiple layers of information can be very useful. But more layers of information can also become a daunting process to sort through, especially if information is conflicting.
If the major issue causing soil variability is rolling, variable topography, an effective and simple way to get started to achieve the goal of developing a good fertilizer prescription map is to have a good topography map. Then compare this to your yield map. This will often reflect the uniquely different soil areas in a field, which is critical.
The next step is to soil sample each soil management area to assess soil fertility and other soil characteristics. Often, treating upper, mid and lower slope positions separately as different management zones is a good starting point. Tables 1 and 2 (below) show soil sample analysis results of an irrigated and a dryland field respectively near Lethbridge. Each field was sampled based on topography versus sampling based on satellite imagery. The analysis shows very clear soil fertility differences among upper, mid and lower slope positions. Soil sampling based on topography can often provide a relatively easy and cost-effective way to delineate different soil areas within a field. Using satellite imagery based zones can also be very useful but has limitations.
The final step in the VRF process is to decide the productivity level of each soil management zone to plan an economical fertilizer plan for each zone. This will be discussed in the next issue of Top Crop Manager.