Comprehensive Soil Tests
Growers are used to taking soil samples and having them analyzed for available nutrients, pH, and total organic matter by a university or commercial lab. In arid regions it is common to also determine whether the soil is saline (too much salt) or sodic (too much sodium). This provides information on the soil’s chemical health and potential imbalances. To get the most benefit from soil tests, sample soils frequently (at least every two years) and keep good records. Evaluate whether your soil test values are remaining in the optimal range, without adding large amounts of fertilizers. Also, make sure that you do not end up with excessive nutrient levels, especially phosphorus and potassium, due to over-application of organic materials. If your soil test report includes information on cation exchange capacity (CEC), you should expect that to increase with increasing organic matter levels. And, as discussed in chapter 20, soil CEC increases following liming a soil, even if there is no increase in organic matter.
The traditional soil test does not, however, make a comprehensive assessment of the health of a soil, which fact probably fed the “chemical bias” in soil management. In other words, the widespread availability of good chemical soil tests, although a very useful management tool, may also have encouraged the quick-fix use of chemical fertilizers over the longer-term holistic approach promoted in this book. The Cornell Soil Health Test was developed to provide a more comprehensive soil assessment through the inclusion of soil biological and physical indicators in addition to chemical ones (figure 22.3). Those indicators were selected based on their cost, consistency, and reproducibility and their relevance to soil management. The Cornell Soil Health Test also considers indicators that represent important soil processes. It provides information on four physical indicators:
- aggregate stability (relates to infiltration, crusting, and shallow rooting),
- available water capacity (relates to plant-available water),
- surface and subsurface hardness (relates to rooting), and
- soil texture (relates to most soil processes and is important for interpretation of other measurements);
and four biological indicators:
- soil organic matter content (relates to many soil processes, including water and nutrient retention),
- active carbon content (relates to organic material to support biological functions),
- potentially mineralizable nitrogen (relates to ability of organic matter to supply N), and
- root health (relates to soil borne pest problems).
In addition, nine chemical indicators, which indicate nutrient availability and pH balance and are part of the standard soil test, are included. Altogether, the Cornell Soil Health Test measures seventeen indicators related to relevant soil processes. The sampling procedure involves taking in-field penetrometer measurements and using a shovel to collect a disturbed sample, which is then submitted to a soil testing lab. A few indicators are especially noteworthy. The aggregate stability test is an excellent indicator of soil physical quality because aggregation is critical to many important processes such as aeration, water flow, rooting, and mobility of soil organisms. The test uses simulated rainfall energy to evaluate the strength of the aggregates, similar to conditions in the field. We have seen that soil management has a strong effect on aggregate stability, as seen in figure 22.4. Under organic management 70% of the aggregates of the silt loam soil remained on the sieve after application of energy from a rain simulator, while for a similar soil under conventional management only 20% of the aggregates remained.
Active carbon is a relatively new indicator that assesses the fraction of soil organic matter that is believed to be the main supply for the soil food web and, during its decomposition by organisms, provides nutrients for plant uptake. Ray Weil of the University of Maryland has shown that it is easy to see changes in this test as management changes so that the results of the test can provide an early indication of soil health improvements. (It takes a long time to determine an increase in the total amount of organic matter in the soil.) Active C is assessed as the portion of soil organic matter that is oxidized by potassium permanganate, and the results can be measured with an inexpensive spectrophotometer (figure 22.5).
Another noteworthy indicator is the bean root rot bioassay, which provides a highly effective and inexpensive assessment of root health and overall disease pressure from various sources (plant-parasitic nematodes; the fungi Fusarium, Pythium, Rhizoctonia; etc.). Figure 22.6 shows examples from soil from a conventional field with bean root tissue containing lesions and decay, while the roots from beans growing in soil from an organic field are mostly white and are therefore more functional.
A soil health test report provides an integrative assessment and also identifies specific soil constraints; see figure 22.3. This particular report is for a soil that had been under intensive vegetable production for many years. For each indicator, the report provides a measured value and the associated score (1 to 100), which is basically an interpretation of the measured result. If scores are low (less than 30), specific constraints are listed. An overall soil health score, standardized to a scale of 1 to 100, is provided at the bottom of the report, which is especially useful for tracking soil health changes over time.
The test report in figure 22.3 is somewhat typical for traditionally managed vegetable fields in the northeastern part of the U.S. It shows the soil to be in excellent shape in terms of the chemical indicators but severely underperforming with respect to the physical and biological indicators. Why is that the case? In this situation, the farmer was diligent about using the conventional (chemical) soil test and keeping nutrients and pH at optimal levels. But intensive vegetable cropping with conventional plow tillage without cover crops caused an unbalanced soil health profile for this field. The test identified these constraints and allows for more targeted management, which we’ll discuss in the next chapter.
Microbial Soil Tests
Soils also can be tested for specific biological characteristics—for potentially harmful organisms relative to beneficial organisms (for example, nematodes that feed on plants vs. those that feed on dead soil organic matter) or, more broadly, for macro and microbiology. Since networks of mycorrhizal fungal filaments help plants absorb water and nutrients, their presence suggests more efficient nutrient and water use. Total and active bacterial and fungal biomass and various associated ratios are now offered on a commercial basis. These indicators tend to be sensitive to soil management and provide information on how biological functions are performing. Soils that are low in both bacterial and fungal counts are assumed to be biologically deficient and would gain from a variety of organic amendments.
The relative amounts or activities of each type of microorganism provide insights into the characteristics of the soil ecosystem. Bacterial-dominated soil microbial communities are generally associated with highly disturbed systems with external nutrient additions (organic or inorganic), fast nutrient cycling, and annual plants, while fungal-dominated soils are common in soils with low amounts of disturbance and are characterized by internal, slower nutrient cycling and high and stable organic matter levels. Thus, the systems with more weight of bacteria than fungi are associated with intensive-production agriculture (especially soils that are frequently plowed), while systems with a greater weight of fungi than bacteria are typical of natural and less disturbed systems. The significance of these differences for the purposes of modifying practices is unclear, because there is no evidence that one should make changes in order to change the amount of bacteria versus the amount of fungi. On the other hand, modifying practices causes changes to occur. For example, adding organic matter, reducing tillage, and growing perennial crops all lead to a greater ratio of fungi to bacteria. But we generally want to do these practices for many other reasons—improving soil water infiltration and storage, increasing CEC, using less energy, etc.—that may or may not be related to the ratio of bacteria to fungi.
Other Tests and Measurements
Many other measurements can be made, either in the field or the laboratory: infiltration capacity, volume of large pores, etc. As we are writing this book, promising new molecular techniques, like microarray analysis, are being developed that allow for targeted biological analysis. Making such measurements in a meaningful way is challenging, and we recommend the involvement of a scientist or extension agent if you want to pursue more sophisticated methods.
There are geographical considerations to soil health assessment as well. High salinity and sodium levels should be assessed in arid and semiarid regions (especially when irrigated) and lands prone to coastal flooding. In some regions, soils may have high acidity (low pH) in the subsoil that limits root proliferation into deeper layers, and samples from the deeper layers may need to be chemically analyzed. If there are concerns about soil contamination, as may be the case in urban or industrial areas, or when sewage sludge or dredged materials have been applied, tests for heavy metals or other contaminants are recommended. Laboratories that do these types of tests are listed in the “Resources” section at the end of the book.