Precision Agriculture
Introduction and Background
Agricultural Evolution. From the beginning of cultivation, and the earliest settlement of land for food production or farming, it is easy to imagine that land units were selected for cultivation based upon uniformity of the unit relative to surrounding land. In fact, on the broadest scale, whole regions were identified for cultivation based upon their relative uniform suitability compared to the surrounding regions. Hence, the Central Great Plains region of the United States was readily converted from prairie grassland to cultivated cropland. Forested areas, rock outcrops and small mountainous areas were obvious exceptions. Within this Great Plains region, pioneer farmers laid-out fields based upon the economies of scale, which were severely restricted by their limited resources, and demands of diversified agricultural systems. Early farms were designed to be self-sufficient and included fields to provide feed, food, and cash crops. The Homestead Act provided the settlers with 160 acres to develop within this self imposed system or framework. Consequently, field sizes were often much smaller (10 to 40 acres) than is common today (80 to 160 acres). By comparison, the average farm size in Oklahoma today is about 480 acres, and the average size wheat field in Northcentral Oklahoma is 80 acres.
Field and farm size increases have paralleled advances in agricultural technology and rural standards of living. Increases in the size of field equipment naturally led to a desire for, and increase in, the size of fields. This in turn led to an increase in the amount of land a farmer could manage. As a result, fields that were initially small and whose boundaries were determined by a perceived uniformity in characteristics like slope, texture, and productivity potential, and which were used for the production of different crops (e.g. pasture, perennial legumes like alfalfa, annual row crops like corn, cotton, soybeans and sorghum; solid seeded annuals like wheat) were combined into larger fields that could most efficiently be managed as larger, independent units.
Inherent Variability. The manner in which native land was developed for agriculture was related to inherent variability among farms in communities and among fields within farms. As example, some land was selected for cultivation because it was level and free of rock outcrops and trees. Other land may have been selected for livestock pasture (or ranching) because it was not level, was partly forested, had rock outcrops or could provide water for livestock. These differences still exist today.
Even the casual observer can easily distinguish gross natural or inherent variability in the landscape. For example, the Stillwater surroundings are dominated by forested uplands and a few cultivated creek-bottoms. The Enid, Oklahoma area is primarily made up of flat, level cultivated land. On a finer scale, or resolution, natural variability has been delineated by identifying and mapping different soils (US Soil Survey). In the normal process of soil surveying, units as small as five acres may be recognized. Consequently, within the average field of 80 acres several different soil series may exist. It is important from the standpoint of Precision Agriculture to understand that soil surveys that recognize different soils based on morphological features do not necessarily identify soils with different production capacities or that have different crop production input needs. Also, lines on soil survey maps that delineate different soils should not be expected to reliably separate soils that are uniquely different, even in morphological differences, because actual inspection of soil morphological features was not performed on every five acres. Soil scientists delineated soils based upon perceived differences associated with observations of changing vegetation, slope, drainage patterns, etc., and their understanding of the local influence of the soil forming factors (vegetation, parent material, slope, climate, time). Nevertheless, some of these morphological differences among soils do strongly impact crop production and the crop response to production inputs. Most noteworthy are soil texture, soil depth, soil organic matter content and slope.
Acquired Variability. Additional field variability results from how fields or soils within fields have been managed over time. An obvious example of this is found as a result of farm consolidation and increasing field size over time. In the early years of converting native land to cultivated land, field size was much smaller than today. As farm and field size increased, several small fields that had different management histories were consolidated into one larger field. In extreme cases old abandoned farmsteads (buildings, trees, livestock feeding pens, etc.) were removed or destroyed and the area incorporated into an existing field. The new field acquired new variability, usually in excess of existing natural or inherent variability. Another example of acquired variability is related to terracing and its influence on field management. Terraces on fields with extreme slopes may be too close to allow uniform treatment of the field, especially as it relates to application of fertilizers or ag chemicals by large commercial applicators (60-foot wide application). An example of soil test variability for a 20-acre field in Noble County is illustrated below.
Terrace # 6
Terrace # 5
pH = 4.6
Terrace # 4BI = 6.8
pH = 5.5N = 16
BI = 6.7P = 65
Terrace # 3pH = 5.4N = 12K = 310
BADBI = 6.8P = 32
Terrace # 2SPOTN = 20K = 423
pH = 5.77.3P = 31
Terrace # 1BI = 6.9--K = 522 FIELD FIELD
pH = 5.3N = 2067 AVERAGE RANGE
BottompH = 5.2 BI = 6.9P = 2322 pH = 5.2 pH = 4.6-5.7
BI = 7.0 N = 10K = 397343 BI = 6.8 BI = 6.6-7.0
pH = 4.9N = 13 P = 44 N = 14 N = 10-20
BI = 6.6P = 54 K = 415 P = 49 P = 23-93
N = 10K = 354 K = 408 K = 310-522
P = 93
K = 435Extra
Extra Lime, no PExtra P2O5 Lime
A strict interpretation of the average soil test values would result in recommending 1.2 ton/acre of effective calcium carbonate equivalent (ECCE) lime, 61 lb N/acre (35 bushel yield goal for wheat grain, where the requirement is 2 lb N/bushel yield goal), 13 lb P2O5/acre, and no K2O. However, if terraces were treated individually according to soil tests, lime would range from 1.9 to 0.7 tons ECCE lime/acre, N from 50 to 60 lb/acre, P2O5 from 0 to 37 lb/acre, and K2O would not be required for any terraces.
Conventional Treatment of Variability. As we consider Precision Agriculture, and everything it involves and implies, we might benefit by first becoming familiar with how variability is currently being treated in production agriculture. Most production inputs are applied at a constant rate over entire fields. Field size (average for wheat 80 acres) is strongly influenced by farm size (Waggoner Ranch in Texas had a 7,500 acre wheat field) and field shape by legal survey of land (sections, quarters, etc.) and physical features (rivers, roads, etc.).
Within field variability is generally not measured or treated in commercial agriculture today. In the case of soil pH and nutrient management, where soil testing is used, farmers are advised to collect 15 random samples from the field and mix them in a bucket to obtain a composite for the field. This gives them a reliable estimate of the mean, but no indication of the variability in the field. Extremes in variability could result in some of the field receiving excessive inputs and some of the field not receiving adequate inputs.
Perhaps the greatest failure in treating variability today is not failure to recognize it within fields, but failure to recognize it among fields and failure to even recognize it at all, at the field level. Examples of this are found in the results of the OSU “Free Wheat” soil test for the summer of 1996, where it was found that for participants in Garfield County (Enid) 67 % of the farmers that sampled five fields fertilized all five the same and the average field size was 135 acres! Forty two percent of the farmers tested their fields less frequently than every three years, with four percent indicating they had never tested their wheat fields. The following table shows an example of a farmer who had not tested fields for at least 15 years. All but one field had acquired adequate levels of available N and P from the standard practice of treating all fields with the same fertilizer input each year. These same fields had apparently acquired a lime deficiency (soil pH below 5.5) sufficient to cause yield reduction from aluminum toxicity. Although great variability existed among the five fields that totaled 349 acres, all fields had been treated the same. Farmers who soil tested on a regular basis were similarly found to treat all fields the same, even when large differences were found in the results among fields.
Should every managed production input for a field be considered at variable levels? The answer to this question, at least in part, depends upon whether or not there is a high probability of increasing the production profits from applying an input to portions of the field at a different rate. If areas of the field can be identified and marked for different rates, those rates can be applied to areas of only a fraction of an acre without additional cost to the farmer. This was being done on an estimated 10 % of the acreage in 1999 for fertilizer and lime application and 20 % of the acreage for herbicides (personal communication, Clyde Coop, Medford, OK). However, the question still remains as to the basis for determining different input rates or levels. In the case of lime and fertilizer, it is not obvious that a reliable soil test is the basis for identifying different rates.