Monday, March 21, 2011

Soil Analyses

Soil Analysis and Fertility

 
Horticulturists routinely provide soil analyses on behalf of contractors, landscape architects and homeowners. Personally I think the process is invaluable especially for new projects. It allows the client an opportunity to really make a difference in the final product. As they say, “you only get one chance to make a good first impression”. The same could be said for landscape projects: you only get one really good opportunity to alter the soil pH prior to planting. Even with prior planning the impacts are often minute.

Soil tests are also helpful in determining the basis for plant decline, poor growth and vigor, or inferior yields on fruit and nut trees. In fact, many of the disorders that we encounter in the field are related to less than satisfactory soil conditions, and soil testing can assist you during the site examinations. I would venture to say that over 50% of the trouble calls we encounter are soil related.

Of primary importance is the selection of a quality laboratory to perform the analysis. We often are asked to interpret the reports of a variety of labs and the data presented in the reports can be confusing and quite variable. It is critical that the lab work is conducted by a facility that is geared to perform agricultural analyses. Do-it-all labs may not have the expertise to conduct the tests in a meaningful and reliable manner for agronomic purposes. 

The quality of the soil sample is also quite critical. Occasionally we receive requests to pull a single soil sample for a 10 acre parcel. One sample would hardly be representative of the entire site, so decisions must be made as where landscaping will be placed as opposed to the sites for parking or structures. This can help to pinpoint the best locations for sampling. Take enough samples to make the analysis worthwhile.

Soils that are imported or stockpiled on site are often analyzed for suitability. Here too the size and method of the sample collection is the key to the accuracy of the analysis. Imagine pulling one grape off a single vine in a 100 acre field to determine sweetness for the entire crop. It would be similar to taking one fruit jar of soil from a 100 cubic yard pile for submittal to the lab. This is why composite sampling is important to ensure that the makeup of the sample is representative of the stockpile.

Soil fertility analyses can provide a lot of detailed information, such as pH, percent organic matter, nutrient holding capacity, the actual fertility levels for each element requested (such as nitrogen, phosphorus, and potassium). Even undesirable components such as soil salinity and excessive levels of micronutrients (such as boron) can be determined. Both salinity and nutrient toxicity can be more difficult to mitigate than nutrient deficiencies.

If you desire the lab can also run additional tests to determine texture (percentage of sand, silt and clay), bulk density, and water infiltration rate to name a few. These and other test procedures can assist the designer or contractor in the determination of best management practices for the soils prior to and after construction.

Once the lab results are in then you need to analyze the results. This requires a lot of familiarity with soil chemistry. Often the soil lab will provide generic recommendations for modifying the soil. For the inexperienced this can be very helpful. But to really interpret the reports you should consider having someone knowledgeable about the soils for the region go over the numbers. Agronomists and experienced horticulturists will be able to provide you detailed information regarding the cultural practices and products to help remedy the most difficult soils.

It is often said that “prescription without diagnosis is malpractice”, and making determinations about fertility without soil testing is pretty similar. If we do not know what the specific needs are for a particular plant type or soil then any recommendations regarding the addition of nutritional supplements would be made without sound judgment. So, the next time you encounter an odd plant disorder or embark on a new landscape project be sure to include a soil analysis in the package. Clients may not appreciate the science involved, but in the long run it could make a huge difference.

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