Monitoring Soil Nutrients Using Management Units
Natural soil variability is a fact that growers must deal with on cropland. Yield potentials, nutrient requirements and management practices vary within fields whose margins are delineated largely on survey boundaries rather than production potential.
Pacific Northwest cropland fields have contrasting areas with distinctive soil properties, production potentials and limitations. If these areas can be delineated for management purposes, then knowledge of these distinct conditions can be used to develop special production strategies.
One management option is the adjustment of fertilizer rates for the particular soil, in order to match yield with available water and improve fertilizer use efficiency. Tillage and other cultural management options could also be adjusted for the particular soil and crop production conditions.
Doing the above is called Site-Specific farming. It measures what is different, records the differences at distinct and specific locations, and then directs differences in management or inputs based on the information.
A map is made for the field showing the differences. This map can be created using yield, electrical conductivity (EC), soil survey, infrared photo, weeds, insects, diseases or probably any criteria that impacts crop performance. To make it useable, allowing just three areas of difference keeps the map from being cluttered.
David Mulla at Washington State University studied two sites, one at St. John of 59 acres which has sharply rolling landscape and a site of 40 acres near Colfax with a broad gentle slope. The St. John site had shallow topsoil or exposed subsoil on the ridge, deep topsoil on the slope and bottom. Colfax exhibited no visual differences in soil productivity. Both sites were conventionally farmed wheat-summerfallow systems.
Mulla studied the nutrient requirements of the sites. Uniform fertilizer applications used by the farmer and variable rate fertilizer applications were compared along parallel transects 2000 feet log and 400 feet wide. Standard WSU soil sampling and tests were used to develop fertilizer recommendations for both systems
Mulla created his map using topography, top, slope and bottom. A fertilizer recommendation was developed for each topographic or management unit or specific-site, whatever term you want to use. These recommendations resulted in purchasing $11.20/A less fertilizer product at St. John and $31.77/A less product at Colfax.
There is an emerging axiom: fertilize using the 50:80 rule. This rule states 80 percent of your yield comes from 50 percent of your land. Therefore, fertilize appropriately.
A description of using Management Units is in a bulletin from Oregon State University, EM 8920-E. Title is: Monitoring Soil Nutrients in Dryland Systems Using Management Units. This is more intense management than uniform fertilizing. So what are we looking for? Actually we are looking for patterns in soil nutrients over several years, probably three at a minimum. To get the most accurate pattern, one needs to soil sample in the exact spot, every time he takes a soil sample.
There are two types of soil sampling methods-grid and zone sampling. Dr. Dave Franzen at North Dakota State University studies both methods. Grid sampling uses a systematic method to direct where samples are taken. Often they are one per acre. Franzen has used grid sampling on a 5 acre grid. He found this less dense sampling grid seldom represents actual field fertility levels as measured by one acre grid samples. One sample per acre is not reasonable because it is too expensive for anyone to use.
Zone sampling is based on the idea soil fertility patterns are related to some logical reason. North Dakota research has shown nitrogen levels are often related to landscape patterns. Soil sampling based on landscape patterns used only an average of 5 samples in a 40 acre field in North Dakota research. This method was as accurate as the one sample per acre method. This finding means accurate soil sampling can be economical in wheat country and not confined only to high-value crops.
Stewart Wuest and other Pacific Northwest Scientists conducted soil sampling for organic matter, not nutrients. They took samples from a 60-square mile county and from a 1.25 acre field plot. The county data is from, Whitman County, Washington and the field plot near Pullman. Soil samples were taken on a grid pattern from 220 locations across the county and likewise 220 locations in the field plot. Within the 1.25 acre field plot the soil carbon measurements varied from about 0.5% to 3%. Countywide the variation was similar. The variation within the two samples sites was greater than the variation between the two sites. This is why site specific soil sampling can be so productive.
Trends are what give greatest accuracy in nutrient management. This means I need to go to the exact spot each time I pull a sample. One may go out into his 300-500 acre field and think he is on the same spot he pulled the previous years sample. The above paragraph shows if you are lucky enough to be on the same 1.25 acre part of the field you pulled last sample from, you have one chance to be correct and 219 changes to be wrong on that 1.25 acre site
This variability points out why sampling must be conducted at the same exact site from year to year in order to achieve any kind of accuracy with nutrient patterns over time. This accurate sampling requires GPS locating to give site specific information.
Interesting observation have occurred. In North Dakota David Franzen assumed higher rates of N would bee required in more productive soils. However, management zone use called for lower N rates on productive soils and higher rates needed in less productive soils. What was finally determined is that the highly productive soils were higher in organic matter and releasing its own nitrogen.
The long term advantage for dryland wheat will be what Mulla pointed out: precise nutrient application and a reduction in fertilizer expense. Additional benefits will be nitrogen use efficiency. Other areas can reduce nitrate leaching, volitization, and benefit water quality.