Submitted by Mengistu Liramo Aja, Addis Ababa, Ethiopia
In short, the answer is yes, you can estimate the fertility status of soil without knowing the plant nutrient status. Traditionally this can be done through soil analysis, which provides an index of the plant availability of soil nutrients through a standardized test from a lab. Other soil tests can assess properties such as soil pH, cation exchange capacity, and organic matter content, which all contribute to the soil’s potential to supply the right balance of nutrients at rates that meet the demands of a productive crop.
Since soil analysis is not always feasible for a variety of reasons, one can gather a general sense of soil productivity/health by examining soil physical properties such as its texture or structure. The proportion of sand, silt, and clay has a large influence on how water is either held or drained in a soil profile, which ultimately contributes to its ability to support crop growth, especially under extreme stresses such as drought or flooding. It is easy to grab a handful of soil to get a quick assessment of whether soil can form stable yet friable aggregates. It is also possible to detect whether any dense and compacted layers are present that would influence root penetration and water movement. Another important indicator for soil fertility is its color, which is often analogous with soil organic matter. The darker a soil the higher the likelihood for the soil to have a higher soil organic matter content and vice versa.
Besides physical characteristics there are other biological indicators that are less commonly measured such as the diversity and activity of soil microorganisms. However, this assessment is commonly done under controlled conditions that measure soil respiration, or the release of CO2 as microbes work to mineralize sources of energy like organic matter. Soil microorganism populations play a key role in organic matter decomposition, mineralization, and the cycling of nutrients into forms that are plant available. On a more visible scale, it is even reasonable to make a general assessment of soil health by observing how populations of earthworms are thriving due to their role in soil aeration and organic matter breakdown.
Periodic field scouting of crop development is also a recommended way of assessing both plant and soil fertility status.
The topography of the farm as well as landscape features such as termite mounds or even trees could have substantive implications on soil fertility. On steep slopes, nutrients tend to accumulate at the bottom of the slope. It is worth noting that steep slopes are at high risk for soil erosion and water run-off. Termite mounds, trees, and sites of charcoal burning are some of the contributors to the high variability of soil fertility on African farms. For instance, termite mounds, provide micro-sites for the accumulation of nutrients and soil organic matter.
Periodic field scouting of crop development is also a recommended way of assessing both plant and soil fertility status. Checking plants for the symptoms of nutrient deficiencies can help to identify if one or more nutrients are limiting plant growth. It can also be useful to perform mini on-farm experiments that test how crops respond when nutrients are either omitting or included. Lastly, a system for keeping a record of crop yields can be a good tool for inferring how soil fertility is trending over time.
Even if one does not have access to a lab, you can form a good assessment of soil fertility/health without directly measuring plant, or soil, nutrient status. Being a keen observer and good record-keeper goes a long way towards ensuring you can identify key factors influencing soil health and fertility and enable better soil management practices for a specific field.
APNI Contributors: Gavin Sulewski, Sr. Editor and Dr. Ivan Adolwa, Scientist
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