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Soil is not a homogenous substance. Even after drying, grinding and sieving one sub-sample will differ from another. Through careful statistical analysis of a quality control sample, the variation from sample to sample has been determined for each parameter. While these variations are different for each parameter it is a good rule of thumb to use +/-5% for pH and +/-10% for all other parameters.
If your test result shows 10ppm nitrate-N then you should read this to mean nitrate-N is between 9 and 11ppm. Similarly, if the pH is reported as 5.3 you should read this as the pH is between 5.1 and 5.5 pH units.
Soil pH is taken in water at a ratio of 1:2 and indicates the pH of the soil-water matrix. Buffer pH is taken in water with a modified Mehlich buffer solution at a ratio of 1:2 and indicates the reserve acidity of the soil. Buffer pH is used to calculate lime requirement.
On your test report form you will see P0 and P1. These are values for water soluble (or labile) P and Mehlich III extractable P. The labile P is immediately available to plants while the extractable P may become available to the plants through weathering during the season. Generally you would use the M3 value to estimate P availability but should your crop be showing signs of P deficiency the P0 value may be useful. Keep in mind though, the addition of P fertillizers will not affect P0 very much in mineral soils since the added P is quickly bonded chemically to the soil. In organic soils this effect is much weaker and addition of P fertillizers may have an effect on P0. Remember, a soil test is a guide only and should never be used to the exclusion of good agricultural practices.
The Plant Science Lab uses the Mehlich III solution to extract P from the soil. The amount of P extracted by the Mehlich III solution has been shown to be equivalent to that extracted by the Bray 2 solution (Soil Sampling and Methods of Analysis, Carter et al, Lewis, p43
). If you are used to the Bray 2 extraction you can use the values reported using the Mehlich III confidently without any correction.
The Plant Science Lab uses ammonium acetate to extract NO3-N, SO4-S, Cl and P0. Mehlich III solution is used for all other analytes. |
The graphic presentation of elements near the top of your test report indicate whether the soil is deficient, marginal, optimum or in excess for each element listed. These recommendations are based on an average taken across many hundreds of samples. However, since soil type and prospective crop affect the availability of nutrients care must be used when using the graphical display. For example, your Boron level may be reported as optimum but if you are growing a crop that requires more Boron than usual you may still require the addition of more Boron.
The values used for the graphical display are as follows (in ppm).
EC: good=<1.5, caution=1.5-2.5, toxic=2.5-4.0, very toxic=>4.0
H+ (exchangeable acidity) is calculated from (6.50 - Buffer pH) / 0.25 for values of buffer pH below 6.50. For values above 6.50 H+ is 0. CEC is the sum of exchangeable cations (Na+, K+, Ca2+, Mg2+, Fe3+, Mn2+, H+) reported in cmol[+]/kg. CEC is a measure of a soils ability to retain nutrients. Higher CEC values mean a greater ability to retain nutrients rather than lose them to hydrological activity. BS (base saturation) is calculated from the sum of the base cations divded by the CEC times 100 Expressed in %. ([K]+[Ca]+[Mg]/(CEC))*100. |
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