Phosphorus Soil Test Change Following the Addition of Phosphorus Fertilizer to 16 Kentucky Soils

When applying phosphorus to soils it is important to know how much the soil test P changes with the addition of various rates. Soils are different in how they respond to varying rates of application, and only limited information is available for Kentucky soils

The Effect of Drying Soil Samples on Soil Test Potassium Values

Extreme temporal and spatial variability of soil test potassium values (STK) was measured on small plots (12-ft x 40-ft) being used for a STK correlation and calibration study on a Crider soil in Larue County, Kentucky. Twelve periodic samplings of the 20 small plots in this study over a period of 18 months showed as much as two-fold temporal differences in STK within individual plots, many of which had received no potassium (K) fertilizer during the study. Spatial variability of STK also varied as much as two-fold among the individual small plots at any given sampling time for similar treatments. Such differences of STK values from the same site could cause wide variations in recommendations for rates of K fertilizers needed. Several possible sources of this variability were considered for further investigation

Drawdown of Soil Test Phosphorus and Potassium levels by Alfalfa

Alfalfa hay production removes large amounts of phosphorus (P) and potassium (K) from soils. Because of this, there is always interest in the reduction of soil test phosphorus (STP) and soil test potassium (STK) levels by high-yielding alfalfa. Periodic soil sampling for 2 years during an on-farm· fertilizer study on a high yielding alfalfa field provided an insight into this

Precision Agriculture: The Effect of Variable Rate Fertilizer Application On Soil Test Values

Use of variable rate fertilizer spreaders (VRS) is available to farmers in many areas of Kentucky. For use of VRS, a soil fertility map must be prepared for the field to be spread which requires subdividing the field into subunits. Each subunit is then soil sampled separately. A common procedure in commercial use is to grid a field into 2.5 acre blocks and to take a composite sample of 6-8 cores along the perimeter of a circular radius of 60-80 ft from the center of each block. Each block receives a separate fertilizer recommendation based on results from the soil test. With this information, a VRS can be programmed to apply the recommended rate of fertilizer on-the-go to each specific block as it drives across the field. The objective is to direct the amount (or kind) of fertilizer to soil test variations which occur within the field. This approach assumes that the result from each soil sample of each block uniformly represents the soil test level for all the area within that block. It also assumes that the VRS applies fertilizer (amount and kind) uniformly across its swath width and along the pathway it is driven across each block. These assumptions may be questionable. We conducted a study to measure soil test levels within blocks of a field which had been soil tested on a grid, before fertilizer was applied and at harvest, 6 months later. The objective was to define soil test variability within blocks before and after VRS fertilization. This information should provide insight into the effectiveness of on-the-go VRS fertilizer application in lowering soil test variability between individual blocks

Precision Agriculture: A Field Study of Soil Test Variability And Its Effect on Accuracy of Fertilizer Recommendations

Use of precision agriculture techniques in Kentucky during the past several years has generated interest in how to soil sample a field for use in programming computer-driven, on-the-go, variable rate fertilizer spreaders (VRS). The advantage achieved by VRS is related directly to variability of soil test (ST) values within a specific field and the accuracy of how they represent the field. Since variability of ST values commonly exists on a small scale, a very intensive sampling procedure (grids of one acre or less in size) would be required to accurately describe the nature and extent of such variability within a field. The cost of sampling and analysis on such a scale would be prohibitive to most commercial producers. For this reason, many fertilizer dealers offering VRS to customers recommend sampling on a 2.5 acre grid (330x330 ft), and use this 2.5 acre unit as the basis for VRS within a field. Use of this procedure to vary fertilizer rates within a field is based on the assumption that variability of ST values within each 2.5 acre block is less than that for the field as a whole. And further, this assumes that ST values are fairly uniform across the swath width of the spreader (about 60 ft) and along the 330 ft pathway of the spreader as it is driven through each 2.5 acre block. Both these assumptions are questionable. Wells (1) reported variation in ST phosphorus (STP) of nearly two-fold across and along 40 ft wide spreader swaths in a 3.4 acre field which was intensively sampled in Shelby Co., KY. Recommendations for phosphate fertilizer rates among the 162, 8 x 20 ft blocks sampled in that study varied from 0 to 110 lbs/A. If the entire 3.4 acre area had been fertilized with a uniform rate based on results from one composite sample taken randomly from within the 3.4 acres, the entire area would have received 80 lbs P2O5/A. Application of the uniform 80 lb rate to the 3.4 acres as compared to the rate which would have been required for each of the 162 areas sampled within the 3.4 acres, would have resulted in only 31% of the area receiving the correct rate. Additionally, 39% would have been underfertilized and 30% would have been overfertilized. Such variability within a 3.4 acre block would not likely be overcome by use of a VRS programmed with the capability to vary rates every 2.5 acres

Effect of Magnesium and Sulfur Fertilization of Alfalfa

In response to concerns that high yields of alfalfa need to be fertilized with sulfur (S) because of soil depletion of S and less S entering the soil from atmospheric fallout, studies were conducted during 1998-1999, to test for S response by alfalfa. Additionally, alfalfa was tested for magnesium (Mg) response because previous hay analysis from the study site had shown very low levels of Mg

Potassium Soil Test Correlation and Calibration for Burley Tobacco Grown on an Allegheny Loam Soil

Burley tobacco removes large amounts of potassium (K) from soil. A 2,600 pound/A cured leaf crop removes around 200 lbs K/A/yr, with about 110 lbs of that in the leaf and 90 lbs in the stalk. Because of such a heavy soil demand for K, growers are always concerned that application of fertilizer K be sufficient for top production. The University of Kentucky\u27s Soil Testing Laboratory (Division of Regulatory Services) provides a statewide soil testing service. The Mehlich-3 soil extractant is used by the UK lab, and soil test K values from use of this extractant (reported as lbs K/A) are categorized as follows for burley tobacco: Very High, over 450; High, 450-301; Medium, 300-201; Low, 200-91; Very Low, less than 91. The amount of potash fertilizer (K2O) recommended varies from 400 lbs/A for soil test K levels below 91 to 0 when soil test K levels are above 450

The Effect of Rate and Source of Potassium Fertilizer on Cured Leaf Yield of Burley Tobacco and Leaf Content and Soil Test Levels of Potassium and Magnesium

In response to questions being asked by tobacco growers about the effectiveness of sulfate of potash magnesia (SPM; 21% K2O and 11% Mg) as coinpared to sulfate of potash (SP; 50% K2O), field studies were conducted during 1993-1994 to compare the two potassium (K) sources for use on burley tobacco. Any effect of SPM on yield of tobacco should be due to Mg since the only difference between the two sources in kind of nutrient contained is the presence of magnesium (Mg) in SPM. To compare the two K sources, we selected field sites low enough in soil test K levels that normally would result in increased tobacco yields due to application of fertilizer K

How Accurate Are UK\u27s Nitrogen Recommendations for Corn?

Average corn yields produced on soils with high yield potential have steadily been increasing in Kentucky during the past several years. Yields from such soils in years with adequate amounts of rainfall well distributed over the growing season (May-September) may average 180 to 200 bushels per acre. A bushel of corn with crude protein content of 8 to 9% contains about 1.3 to 1.4% total nitrogen (N) on a dry matter basis. This is about 0.6 to 0.7 lbs total N per bushel of corn (at 15.5% moisture), or 108 to 126 lbs N per acre for a 180 bu/A crop. Some growers question that UK\u27s fertilizer N recommendations will support such yield production

Small Scale Temporal and Spatial Variability of Potassium Soil Test Values On A Crider Soil

An on-farm, small plot study conducted in 1996, on a Crider soil in Larue County, Kentucky, resulted in unanticipated wide variability of soil test potassium (STK) values between spring and fall sampling. Because of this, the small plots were sampled monthly over a period of time with the objective of determining if such variability in STK values was real