This time of the year I get a few phone calls every week regarding soil sampling. “I have taken the samples; what should we test for?” I have been promoting more comprehensive testing for a number of years. It adds value to the results and increases our understanding of the vineyard soils. The Basic Soil test (pH, Olsen P, Calcium, magnesium, potassium and sodium, CEC and Soil Volume weight) only gives us a limited amount of information. For instance, the Olsen P test has shown to have some problems on soils where Rock Phosphate has been used, or where lime has been applied recently. This is where a second opinion (Resin P test or Mehlich P test) may be useful.
If Olsen P levels are low, is it because an overall low phosphorus status in the soil, or is there an issue with low availability of phosphorus (low pH, low soil biological activity). Please remember that the basic soil test extracts nutrients that are “more or less available” to plants. They don’t extract all nutrient (reserve levels). Knowing what reserve phosphorus levels are, can help decide whether to focus on improving availability or to apply high rates of phosphorus fertiliser instead.
In addition to promoting more comprehensive soil analyses, I should have made some suggestions as to what we should test for on our website. This is the first step.
For a vineyard without a history of comprehensive testing (including new vineyards), I generally recommend the following set of analyses:
1 Basic Soils test (pH, CEC, SVW, BS, Ca, Mg, K, Na and Olsen P).
2 Sulphate sulphur – most viticultural areas in New Zealand have low sulphur status. Sulphur is important for vine nitrogen metabolism, plays a role in aroma precursors and in vine defence against pathogens.
3 Soil Totals (Total P, S, Ca, Mo, Cu, Cd, Se and Co). This test measures the total (reserve) levels of phosphorus, sulphur, calcium, molybdenum, copper and some other trace elements. It will tell us a lot about the underlying fertility of the soil. The total sulphur test may be a more reliable indicator of sulphur status than other sulphur tests. Copper status is important; this test will pick up soils with very low copper (which can affect vine nitrogen metabolism and lignification) and soils with high copper (due to repeated copper applications). High copper can affect soil biology and vine nutritional status. Molybdenum is an important element affecting nitrate reduction in vines. Low levels can affect flowering and fruitset in prone varieties (especially Merlot and Gewürztraminer).
4 Organic Soil profile (Organic matter, carbon, available and total Nitrogen, Available to Total Nitrogen ratio, C/N ratio). Monitoring carbon levels (organic matter) should really be part of the basic test for vineyards. It is a reflection of the quality of soil management. Available nitrogen levels vary considerably throughout the year (as a result of soil moisture and temperature effects on soil biology), but total nitrogen is quite stable and again is an indication of underlying fertility. The ratio between available and total nitrogen can be an indication of soil microbial activity responsible for nitrogen mineralisation. Other factors however may also play a role. The Carbon to Nitrogen ration can also be an important indicator of soil problems.
5 Mehlich 3 (Mehlich phosphorus, iron, manganese, zinc, copper, boron, cobalt and aluminium). This is a very cost-effective way to get a second available phosphorus test, and an indication of the trace element status of this soil. It is about the same price as the Hot water Boron test on its own. Although the latter is a superior extraction, the Mehlich test will still give an indication of boron status.
In soils where comprehensive tests have been done in the past we normally recommend the basic test, with added sulphate sulphur , organic N profile and sometimes Mehlich 3 extractions.
Subsoil samples are different from topsoil samples. They can have their own requirements, especially in cases where salinity may be an issue. The recommended tests also depend on the sample depths. Feel free to contact me to discuss this in more detail if you are planning subsoil sampling.
