Download the comprehensive manual :
For quick guides to each protocol, see the links above at right under “protocols”.
The soil health tool kit assembles a number of methods to assess soil health in the context of projects and networks working with smallholder farmers. Here on the website we describe the protocols and manual for soil pH, permanganate-oxidizable soil carbon (POXC or active carbon), particulate organic matter (POM), soil aggregate stability, and soil available phosphorus. In the downloadable manual (links above), there are additional protocols to estimate soil moisture, measure soil texture and assess soil macrofauna, as well as a demonstration/test for soil organic matter using hydrogen peroxide. We are currently working on validating, improving, and simplifying these tests, and adding tests for soil water infiltration and soil respiration.
Overall sampling strategy for soils
Strategies for sampling need to adequately represent a farm field. 5 to 10 sampling locations within each field are generally employed, though as few as three are sometimes used if a large number of fields are being sampled. In general soil is cored to the depth of the plow layer (0-15 cm or 0-20 cm) where roots are most abundant – thus the zone that plants depend most on in terms of biological processes and soil fertility. It is also possible to substitute a traditional soil corer with a square-tipped shovel or even a machete, that can slice the edge of a hole cleanly, followed by trimming the slice with a knife to create a “core-like” square section of the soil e.g. 4 x 4 cm. This is more time-consuming but no worse for rigor than a soil core from a corer (which can cost hundreds of dollars).
However if assessing soil macrofauna is of interest (see section 6 below), it is necessary to excavate larger volumes (e.g. 20×20 area, x 20 cm depth), and then this becomes the strategy for also gathering soil for testing of soil carbon, physical health, and fertility – no corer or corer is needed, just some straight spades or square-tipped shovels, for precisely excavating.
It is likely that finding chemical reagents will be a serious challenge to address in many regions. Starting early and identifying potential suppliers in larger cities is fairly important. Below are the five major chemicals required for the available soil phosphorus (Olsen P) and active C tests. See each test protocol (at top right) for the exact amounts, recipes for reagent solutions, etc.
For soil available phosphorus (P) testing using the Olsen Extraction:
- Small molybdate blue reagent packs that are supplied by Hanna Instruments, the company making the colorimeter (Hanna Low Range Phosphate reagent , Hanna Instruments catalog number 93713-03 is enough packets for 300 tests)
- Sodium Bicarbonate (NaHCO3): Cean baking soda from the supermarket usually works but needs to be tested by preparing Olsen solution and testing it without mixing with soil to calculate a blank value (see method) . If analytic grade NaHCO3 is available, this is excellent. 1.05 grams are required per sample analysis. Each kit requires about 350 g, figuring a little extra for calibration, testing etc.
- Sodium Hydroxide (NaOH): lye from a hardware store usually works, or analytic grade. Very little of this reagent is needed, just under 10 g per kit for 300 samples.
- Sodium Bisulfate – also called Sodium Hydrogen Sulfate (NaHSO4) — this is a much safer and easier to dose alternative to sulfuric acid or hydrochloric acid. It is sold cheaply as a swimming pool chemical to lower pool pH in the U.S, and it is worth checking that means of purchasing in a capital city in Africa, like Nairobi etc. About 150 g is required for 300 tests (0.45 g per test) so it is not out of the question to import it, but is good to find locally. Dilute sulphuric acid sold as battery acid can also work, though sodium bisulfate is safer.
For the Active Carbon Test:
- No additional reagent packs are required, like the phosphate reagent packs above.
- Potassium Permanganate (KMnO4) — only 64 mg is needed per test or 20 g for 300 tests. This means that if absolutely necessary, a small plastic container of KMnO4 can be flown in with travelers to the region.
- Calcium Chloride (CaCl2; Magnesium chloride, MgCl2 may also work). 0.3 g are needed per test or 90g for 300 tests.
For the pH test, pH buffers or calibrants for the 4.0 and 7.0 level are needed to calibrate the meter, and can usually be found in chemistry or medical supply stores.