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Sources of Variation in Pastures that Affect Soil Analysis
Good soil sampling techniques are essential for the soil analysis to be meaningful. Pastures tend to be highly variable with respect to soil fertility, soil depth, and general production potential. Simply obtaining 15 to 20 probes for each management unit, as is recommended for most cultivated fields, may not be the best method of soil sampling pastures. Figure 1 shows the variation in phosphorus index of seven pastures from 1989 to 2003, all sampled during March of each year. The variation from year to year far exceeds the error expected due to laboratory techniques and makes interpretation of the analyses a difficult task.
Figure 1. Variation in phosphorus index of seven pastures at the Eastern Research Station, Haskell, OK, from 1989 to 2003. Each point represents the analysis of one pasture (15 to 20 probes, 5 to 6 inches deep. A phosphorus index of 65 indicates 100 % sufficiency.
Some of the Sources of Variation in Soil Fertility in Pastures
Pasture - Even though each pasture on a farm has some common characteristics and appearances, pastures differ appreciably from one then another aspects.
Topography - Several aspects of topography may be in the same pastures.
Hills and Valleys - Tops of hills typically have the least soil depth and fertility may be highly variable; whereas, the valleys tend to have deeper soils and tend to be somewhat more moist. If hills and valleys have been fertilized similarly, more plant growth in the bottoms may have made them somewhat less fertile.
Drainage Areas - natural and man made drains are part of the pastures.
Terraces Channels, Tops, and Back Slopes - The terraces are many decades old, and they were reinforced during the last 20 years.
Time since Grazing - If animals have grazed a pasture since the last rain, one to a few probes will, at random, include an invisible urine spot. Sampling soil after a at least one rain should decrease the probability of a probe including a urine spot.
Fertilizer History - If a pasture was fertilized recently the soil test may be affected. If the fertilizer was applied non-uniformly the soil sample will not be useful.
Number of Probes - Traditionally 15 to 20 probes makes an adequate soil sample to determine nutrient needs. More probes may improve the accuracy of the sample in highly variable pastures. The optimum number of probes for variable pastures is unknown.
Sampling Depth - All probes should go six inches deep, but sometimes it is impossible because of hard layers in the soil.
Representativeness of Probes - Every effort should be made to take probes from every area in the pasture in relation to its importance to the grazing and fertilizing program.
Cattle Traffic Patterns - Animals do not travel throughout pastures randomly. They follow certain paths; consequently, using nutrients from some areas more than others. More urine and manure accumulates where animals rest, water, and feed. Disproportionate number of samples from resting, watering and feeding areas will tend to skew the nutrient levels higher than the true average of a pasture.
Hay Feeding Spots tend to be higher fertility than the surrounding area because the wasted hay has nutrients that decompose and become part of the soil and an accumulation of manure and urine.
Haying - Harvesting forage as hay export nutrients from where the forage grew. If hay production is an important of the pasture operations, the soil fertility is decreased and the need for fertilizer is increased.
Runoff from Corrals - As expected, runoff from corrals contains nutrients from manure, urine, and hay fed in the pens. This adds to the fertility level of a pasture. Areas that catch much runoff from corrals should be fertilized separately from the rest of the pasture. Sometimes it may be efficient to avoid fertilizing areas just below corrals.
Soil Movement - When soil moves, nutrients move with it. This can cause a decrease in nutrients when soil is carried mechanically or via erosion from an area. Where soil is deposited, by similar actions, fertility may be higher.
A New Study:
We have initiated a project to evaluate the various sources of fertility variation in pastures. This may lead to an improved way to collect soil samples from pastures. In this study we will use the natural variation in the pastures and attempt to minimize the variation by several treatments and sampling procedures.
John Caddel and Jesus Santillano,
Plant and Soil Sciences Department
