When many of us in agriculture speak of nutrient management, we fail to differentiate between those nutrients that are mobile in the soil and those that aren’t. Soil particles (sand, silt and clay) have a negative charge. Most major, minor and micro nutrients have a positive charge and cling to the soil. Generally they are only moved off site through erosion. Of the three major nutrients – nitrogen, phosphorus and potassium – nitrate nitrogen has a negative charge and moves freely in the soil profile, chiefly with water movement in the soil. Of the minor nutrients – calcium, magnesium and sulfur – sulfur also has a negative charge and moves in the soil profile. In the micro nutrients, only chloride has a negative charge.
In the Corn Belt, we are concerned with nitrate nitrogen and sulfur. Since water movement and weather are such major factors in nitrogen management, sampling must be done at deeper depths of one to four feet as opposed to surface sampling – generally six to eight inches for P & K. Since the soil characteristics determine the water holding capacity of the soil, testing for nitrate nitrogen is best done by zones of differing soil types. To correctly identify changes in soil type across the landscape, an accurate soils map needs to be made by electricomagnetivity.
Enough cores need to be taken from each zone to get an accurate picture of the zone. Zones over twenty acres should be subdivided. Cores or collection points should be geo-referenced in order to increase repeatability at the next sampling cycle. This adds to the quality of the sampling procedures. After an EM map, a number of other data layers can be added to the final map, including topography, ASNT, yield, OM, etc…
The fact that soil mineralization contributes nitrogen to the growing crop has been known forever. Extension Services have never used mineralization in any of their recommendations formulas. Mineralization potential favors from a number of factors, including the amount of OM in the soil, but not from that alone. Mineralization varies in the Corn Belt with temperature and moisture. Dry, cold conditions inhibit mineralization of N, while warm, moist conditions increases it. Acidic soils which inhibit soil microbial activity depress mineralization. Soils that are compacted or are saturated lessen oxygen in the soil, which lowers microbial activity which lowers mineralization. All of these factors make giving any general recommendation about the amount of mineralization at any time in any location impossible. This does not mean the sharp agronomist and progressive farmer cannot use the information successfully.
Nitrogen Management is the most difficult nutrient to manage in the Corn Belt. Look at the complexity of the diagram in N monitoring. When adding manure applications to the mix, N management becomes even more complex, but because of economic and environmental demands, it is also more demanding. Split applications are better than one single application. N applied closest to the plant’s use is best as with side dressing and fertigation.
Variable rate nitrogen application maps need numerous layers to correctly assess N needs. Layers may include previous crop, mineralization potential, soil productivity, organic matter, potassium levels, calcium levels, population, variety, yield goal, plus timing and method of application. Vigorous debate occurs among agronomists as to which layers apply and how much influence each layer should have on the final map.
Precision Phosphorus & Potassium
These two major nutrients, along with most of the minor and micros, vary across the landscape from soil type to soil type. They also vary within a soil type. Further, they vary from past manure practices (mostly get rid of it), small fields being combined into larger fields and all the other hand of man influences. Since these past influences cannot be seen walking in a field, sampling for immobile nutrients by zone (if small enough) is good, but grid sampling is better in the majority of the Corn Belt. (Link to grid sampling) In zone sampling, there is no standard on what data should be used to create the zone. Bare ground satellite imagery is used, but it is a poor guess at best. There is no disagreement on 2.5-acre center point grid sampling. It is a standard. The only differentiation on grid sampling is how many cores to take. Research done by Dr. Robert Miller of Colorado State University and Director of Soil Testing Lab Certification Program, established the standard of at least eight cores in soil that has been tilled and ten to eleven cores in no till situations. MISS takes eleven to twelve cores in all circumstances.
There are numerous computer programs that use the nutrient data collected in the field and make application maps for various brands of variable rate application spreaders. Many progressive fertilizer dealers have that capability in house. Some choose to sublet that work. Various levels of complexity can be used, depending on the farmer’s wish and desire. MISS offers recommendation services, for any and all nutrients, either in house or subcontracting to companies such as MAPS, Crop Metrics, SST or Premier Crop.