| SUBSCRIBE MARKETS WEATHER LINKS HOME |
|
It's time to check your soil's fuel gaugeDo you know how to take a representative sample from your soil and how often to sample? It's key to managing the nutrient levels in your fieldsby KEITH REIDSoil sampling is absolutely key to managing the fertilizer or manure you apply, since it is the only way to tell what is in your soil. Nutrients in the soil are not visible to the naked eye, so it is possible to have a soil in beautiful tilth, with lots of organic matter, but which is yielding far below its potential because of nutrient deficiencies.Soil testing is a two-part process, which involves collecting a sample that represents the field and then having it analyzed to provide good information about how much nutrient is available from the soil. In this article, I'm going to concentrate on how to collect a representative sample. The first rule in soil sampling is to take enough cores. Nutrient values in soil can vary considerably over small distances, either from natural variation in the parent material, or from uneven application of manure or fertilizer. This man-made variability may be accidental (inconsistent spreading width, equipment malfunction) or intentional (banded fertilizer). If the variation is within the rooting zone of a plant, it won't care since it will pull the nutrients from where they are most available. The important thing is to make sure that the sample represents the average nutrient availability within that zone, not the high levels from within a fertilizer band and not the low levels from the inter-row area. How often should you sample? Soil tests will change slowly over time, as nutrients are added as fertilizer or manure and removed by crop harvest. Soil sampling should be frequent enough to pick up these changes before they have a significant impact on fertilizer or lime requirements, without incurring the expense of soil analysis more frequently than it is needed.
In a standard cash crop rotation, once every three years is adequate. In permanent pasture or forage-based rotations, it might be appropriate to stretch this to once every five years. More frequent testing is required where the nutrient-holding capacity of the soil is low, and where nutrient removal or nutrient additions are large. This means that sandy soils should be sampled more frequently than medium- or fine-textured soils. So should fields that receive high rates of manure or fertilizer and those growing crops with high nutrient removal (corn silage, processing tomatoes, high-yielding legume forages).
The extreme solution in small fields is to divide each field up into smaller grids and analyze each one separately. This may, or may not, provide useful information. Sampling in small blocks can uncover variations in soil nutrient levels based on past management. The most common mistake, however, is to dump the results from point soil tests into a geo-statistical program and allow the software to draw a map of the results. These programs depend on the correlation between neighbouring sample results so they can predict the values between the sampling points, and this requires much closer sample spacing than most farmers can afford.
The compromise that appears to work very well is to use all the knowledge we have about a particular field (topography, yield maps, past management) to divide the field into management zones, and then sample these as separate blocks.
In the next issue, we'll follow what happens when your sample arrives at the soil test lab, and what you can do to ensure accurate results from the analysis. BF
|