Testing Your Soil
Why and How to Take a Soil-Test Sample
Why have a soil tested?
Does my soil have problems?
Does my crop need fertilizer?
What kind of fertilizer should I use?
How much should I apply?
A soil analysis can help farmers and gardeners answer these questions. A basic soil analysis provides information on two important soil characteristics:
Soil pH is a measurement on a scale from acid (low pH) to alkaline (high pH). Most soils are on the acid side of the pH spectrum. Good soils for crop production are often moderately acid, but some soils in Hawaii are acidic to the extent that crops grow poorly.Soil tests indicate pH problems and allow recommendations for correcting them.
Available nutrient levels in the soil determine how good crop growth will be. Testing for phosphorus (P),potassium (K), calcium (Ca), and magnesium (Mg) helps determine the need for soil amendments (phosphate, and lime or dolomite) and the right fertilizer formulations for the crop to be grown.In addition to the basic information on pH and levels of major nutrients, specialized soil analyses can help toinvestigate other factors that may limit crop growth:
Soil salinity can build up in coastal areas and in soils irrigated with brackish water or to which too much fertilizer has been applied.Nitrogen (N) is required in large quantities by most crops, and adding N is a basic part of most fertilizer programs. In special circumstances, N can be analyzed as total N, ammoniacal N (NH4-N), or nitrateN (NO3-N),but this is not usually done because N does not remain in the root zone for very long. Organic carbon (C) analysis, like N analysis, is useful only in special circumstances. Most soils
benefit from additions of organic matter.
Aluminum (Al) in soils can be toxic to plants if pH is low and the Al is too available to them. Knowing the soil’s pH and classification is the first step in predicting Al problems, and tests for “extractable” Al can then be done if necessary.
Micronutrient levels in the soil may be analyzed when crop symptoms suggest problems.
Micronutrients often measured include boron (B), copper (Cu),manganese (Mn), and zinc (Zn).These specialized soil tests usually are not called for unless crop growth problems have been observed or there are other reasons to suspect that they are needed. Taking a good soil sample.
Soil tests are done on a sample that is only a tiny fraction of a field or garden plot. Soil treatment recommendations assume that data from the analysis of that tiny fraction represent the entire area to be treated. Therefore, care must be taken to ensure that the soil sample truly represents the field or plot.If differences can be seen in the soil from various parts of the overall area to be sampled, each distinct subarea should be sampled separately. Differences in soil color or texture are obvious reasons for taking separate samples. Other reasons include differences in land slope,soil drainage,crop management history (different soil amendments or fertilizers), variations currently observed in crop growth, or variations in the natural egetation.Each soil sample analyzed should be a combination of 5 to 10 subsamples taken from the soil area of interest.
The subsamples should each be about the same amount of soil, and they should be mixed together thoroughly as they are collected. The final sample taken from this mixture is called a composite sample.
How large an area to sample? For home gardens,one sample that is representative of the garden plot is usually sufficient. For orchards or farms, even if no distinctly different soil types are noticeable,
large areas should be subdivided into sample areas of 2–5 acres and sampled separately.
What equipment do I need?
Map the area sampled if you are taking more than one sample. Mark each sampled area on the map with a label that you will also write on the sample bag.Spade or shovel (for specialized soil tests, tools should preferably be made of steel, because tools made of brass, bronze, or galvanized metal may contaminate samples with copper or zinc) Plastic bucket or large plastic bag for collecting and mixing subsamples Plastic bag to contain about 2 cups (1 pint) of the final, composite soil sample (thin plastic bags that can “breathe,” such as sandwich bags, are better than thick plastic bags for storing soil; brown paper bags can contaminate samples to be tested for boron) Waterproof marker to label the plastic bag to identify the sample.Collecting the soil sample.
For each distinct soil area you are sampling, take 5 to 10 subsamples and mix them together to obtain the final sample. Take the subsamples by selecting spots in a pattern that ensures a balanced representation of the whole area sampled.
Use clean tools to sample soil, a clean container to mix it, and clean bags to store it. Small amounts of contaminants, especially fertilizer or lime, can distort the analysis results.
How deep to sample?
The top 4 inches for lawns, turf, established pasture, and “no-till” fields.
The top 8 inches for conventionally tilled fields and garden plots
The top 8 inches plus a separate sample for the 8–24 inch zone for tree crops
The sampling method:
1. Clear surface litter and plant growth from the sample spot. Dig a hole about as wide as your spade and as deep as the layer you are sampling.
2. With the spade tip placed one inch outside the edgeof the hole, cut down to remove a slice of one side of the hole wall.
3. Keeping that slice on the blade of the spade, use a trowel, knife, or stick to cut away the sides of theslice, leaving a center section about 1 inch wide. This 1 x 1 inch vertical section of the soil is your subsample.
4. Place the subsamples in the plastic container, mix them together well, and remove about 2 cups (1 pint) of this mixture. This is your composite sample, to send to the laboratory for analysis.
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