What sort of soil test should be taken? How many? How often?
As we noted in the last installment, interpreting soil tests is a bit of an art. Deciding on the type, number and frequency of soil tests is another art, and an important part of making the whole process economical. Ultimately it will be a judgment call on the part of the grower, depending on:
a) The amount of land (acres) farmed
b) The amount of fertilizer anticipated to be used
c) The anticipated value of the crop
d) The cost of the testing program
e) The sensitivity of crop yield and quality to fertilizer applications
f) Past experience and the grower’s confidence in the test results
For instance, it is commonly recommended that you take a soil test for each block of 20-40 acres that is being cropped/farmed. This number is a very broad and general compromise based on the idea that for most crops, testing +/- 40 acres per sample can be very economical based on potential fertilizer savings alone. In the case of high value crops, such as fruit and vegetables, smaller testing blocks may be justified – the cost of fertility inputs and the risk associated with mistakes in fertility calculations are much greater. Again, for the individual farm, the size of the blocks tested and the frequency of testing is always a judgment call based on the factors listed above.
Many growers that believe in the value of soil tests will sample their soils every year. This allows them to see trends as well as a yearly ‘snapshot’. Others might test every second year or even less often. Another possibility would be to do comprehensive testing one year and a more basic set of tests the following year.
Part of the decision involves what nutrients (and other parameters) to test. A ‘typical’ or ‘standard’ soil test will report soil texture, pH, Nitrogen (N), Phosphate (P), Potassium (K), Sodium (Na), Calcium (Ca) and Magnesium (Mg). These are all major nutrients for plant growth. Additionally, Sulfur (S) might be reported along with Soil Organic Matter (OM), Cation Exchange Capacity (CEC), Sodium Absorption Ratio (SAR), Base Saturation, and various soil nutrient ratios.
A more comprehensive test will include the standard minor nutrients: Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), and Boron (B). These are sometimes referred to as micronutrients or trace elements. Testing can also be done for Molybdenum (MO), Cobalt (Co), Nickel(Ni), Aluminum(Al), Selenium (Se), Chromium (Cr), and more. Although these more ‘exotic’ essential trace elements are used in extremely small amounts by the plant, they are very important to some processes such as enzymatic and catalytic reactions that contribute to plant’s immune system responses.
The next installment will explore how to pick a soil lab that fits your needs.