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PLANT NUTRIENTS, SOIL FERTILITY and
GRID SAMPLING
This section of the Precision Farming course looks more intensively at the heterogenous nature of soil and at its importance as a factor in the limitation of yield potential of any crop. Early American settlers were first introduced to Precision Farming by friendly indians who taught them corn production and soil fertility practices such as placing a fish
(or two) under each hill of corn to supply a slow release nutrient source of N-P-K and trace elements. The rates were no doubt based on visual assessments of soil depth and organic matter and past performance of the field.
As mechanical tillage machinery and methods revolutionized agriculture in the 19th and 20th centuries, the field became the major unit of production. Justus von Liebig’s theories of the chemical basis of plant growth coupled with his now famous “Law of the Minimum” led the way for chemical analysis of soil. Total chemical analysis of soil left much to be desired from a practical viewpoint because early tests (and some today) bore no relationship to plant growth response since they did not measure plant availability or uptake of the nutrient ions by the crop.
By the 1930s and 40s many state experiment stations were establishing soil testing laboratories and urging farmers to assay their soil nutrient status. Most recommendations went to great lengths to assure that the sampling technique produced a random representative sample of the field. Many recommendations (Missouri for example) suggested one sample per 20 acres but this was often compromised to accommodate larger field sizes.
Following the end of World War II, fertilizer, especially nitrogen, became more generally and economically available. Agronomists and fertilizer sales reps (including The Potash Institute) brought Liebig’s Law of the Minimum back into the philosophy of soil nutrient management, but by this time the list of known essential elements had expanded to sixteen. Many agronomists and the Potash Institute (currently the Phosphate and Potash Institute) expanded the idea of the limiting element, often portrayed as a barrel with one short stave or as a chain with a weakened link, to all management factors. Many researchers decried the approach as non-scientific or too muddled by interactions difficult to measure or assess. Two approaches to soil nutrient status were practiced. One attempted to build soil fertility levels by application of organic matter and amendments so that crops would not experience nutrient limitations.
The second was to supply the nutrients needed (or removed by each crop) irregardless of the soil nutrient status. Reduced profit margins subsequently brought crop production managers back to a more practical approach of supplying only those elements that limited yield. Environmental concerns also made blanket applications of nutrients, pesticides or water politically, socially and biologically incorrect.
In the 1990s the availability of GPS and GIS to agricultural applications suddenly made it possible to manage very small units (field, peds or ?) rather than managing the field as an average. Reawakened interest in the Law of the Minimum and soil testing held out the prospect of increasing yield while decreasing costs. In many instances soil differences and yield monitors justified the expense of the GPS, GIS technology as increased yields were achieved with fewer chemical inputs. Additionally, the environmental cost of over-application was lessened especially in the case of nitrate and phosphorus fertilization.
New rules were developed for frequency and location of soil sampling to demonstrate the potential for cost savings. Many early
Precision-ag practitioners advised sampling on a grid with each square representing 4 acres or 2 acres or <=1 acre. The grid pattern was quickly refined by practice and modified into diamond, random or other grid sampling schemes. Efforts to reduce costs and careful analysis of the data has had the effect of looking at combinations of sampling or soil zones based on soil type, yield maps, irrigation, drainage, salinity or other relevant parameters. This is the point at which precision ag now finds itself. In the future scientists envision having probes mounted on tillage equipment which will map the entire fields for soil texture, pH, salinity or chemical parameters. The Veris technology represents one of the early ventures into this type of soil mapping. Very often the new map becomes the basis for more complete soil testing by zone.
Soil testing methods remain the subject of research and controversy. Recent discussions in California indicate that on Western soils, physical properties, salinity and compaction may be as important to yield as the chemical parameters which have played such a prominent role in the adoption of precision agricultural technology in the Midwest.
Bringing the results of soil tests and yield monitoring together via map stacking and other data analysis tools (SST) has demonstrated that every field is not likely to present easy
“textbook” answers. Dr. Pierre Robert (Univ. Minn.) and others have emphasized that agronomic sleuthing or artistry and experience may become the most valuable tool required to take advantage of the precision ag technology. In the not too distant future, however, precision agriculture and SST will be providing a wealth of information that will literally turn every field properly tested, harvested and analyzed into a multifaceted research plot. Many software packages such as SST have the capability to do multiple regression analysis of the stacked map data to help us delineate the yield limiting factor.
Variable rate application technology will then make possible tremendous crop yield increases since most genetic yield potentials are much higher than our current average yields.
Dr. Edwin C. Seim.
Read: Chapter 4. Soil Sampling and Analysis , The Precision Farming Guide For Agriculturists. 1997. Deere and Company, Moline, IL.
If available, please also check the following:
Site Specific Handbook, Fifth edition 5/97.
Ag-Chem Equipment Co., Inc.; Minnetonka, MN.
www.agchem.com
Useful sources of soil fertility basics and recommended practices include the following:
The Western Fertilizer Handbook
California Fertilizer Association, Sacramento, CA.
Tisdale, S.L. & W.L. Nelson. Soil Fertility and Fertilizers, 3rd edition or later, 1975- .
MacMillan Publ. Co., New York, 694p.
Kinsey, N. & C.Walters. 1993,1995. Neal Kinsey’s Hands-on Agronomy. Acres, USA. Metairie, LA.
Check the website for the Phosphate and Potash Institute at www.ppi-far.org/ and go to Guidelines 1-10. Please read SSMG-1 through SSMG-5. .
Another site that demonstrates the ideas presented in this segment can be found at
http//w3.aces.uiuc.edu/InfoAg/GIS/. At
www.farmresearch.com/Midwest/mn17/default.htm Dr. George Rehm is developing a summary of a project designed to evaluate and demonstrate the potential value of intensive grid soil sampling and variable rate phosphorus fertilization for corn and soybean production. This is an ongoing study and you may wish to follow it throughout the growing season. |
