Assessment of soil health: a new approach
Regardless of what soil analysis is being considered, sampling always comes first. For basic basic agrochemical analysis (nutrient content analysis), most scientific universities recommend taking soil samples at a depth of 0-15 cm, 0-20 cm or 0-25 cm (but no deeper than 0-30 cm). For soluble nutrients such as sulfates or nitrates, samples can also be taken from other depths to better understand their concentrations and refine recommendations. There is also an opinion that with constant soil analysis (analysis of the same plot from year to year), it is better to take samples in the same period of the year. This will help track the changes that occur with the soil.
When taking a composite (or mixed) sample, we simply randomly move around our field, taking samples at different locations and mixing them in a clean plastic bucket. There is also a zonal type of sampling, but for it it is important to correctly define the zones. The source of such information can be a yield map or a map of the chemical composition of the soil, or data on the relief and its changes.
If you mix two different soil samples together (for example, one with high productivity and one with low productivity), then of course you will end up with a medium productivity sample. From the point of view of agrochemical analysis, such samples should not be properly mixed, they should be analyzed separately to identify problem areas and understand the essence of this problem. To obtain one representative sample, it is necessary to take 10-15 spot samples from the sampling area and mix them in a clean plastic bucket.
If we consider the main components of soil condition assessment (chemical, physical and biological) and imagine them as three circles, then the place of their collective intersection will be what is called “soil health”. Chemical components include some specific parameters (such as pH and organic matter) and the presence of available nutrients (macro- and micronutrients) in the soil; to physical ones – stability of aggregates (sieve method) and soil water retention; to biological ones – respiration of biomass (release of CO2) and activity of the soil microbial community.
From the perspective of the basics of “soil health”, the amount of CO2 released is an indication of the microbial biomass in the soil, and carbon is the food source of this biomass. When the amount of released CO2 gives high values (for example, two hundred or three hundred mg of CO2), and the amount of carbon, on the contrary, is low, it means that there is a large microbial community in the soil that needs a lot of food. But low carbon means we need to find extra carbon to feed all that microbial biomass. In this case, a good solution would be to use cover crops.
One of the chemical assessments of soil condition is the analysis of available cations and soil saturation with these cations (or base saturation). Ammonium acetate is used for this analysis, which provides insight into some physical parameters of the soil. It is known that calcium and magnesium in the soil play a very significant role, they also help in the formation of chemically stable aggregates.
The total amount of nutrients, which can only be analyzed by digesting the sample, is another chemical parameter that is considered in the context of “soil health”. Because it is known that microbes can use some of these insoluble nutrients for their existence, thereby making them available to plants. So, for example, the top 20 cm of soil can contain from 1000 to 2000 kg of total phosphorus, the content of total potassium has an even greater amount, but almost all of them will be in forms inaccessible to plants. But the more microbes and mycorrhizal fungi there are in the soil, the more likely it is that some of these nutrients will become available.
A water extract (aqueous extract) is also used separately for chemical components, in order to obtain information on water-soluble carbon and nitrogen that may be available to the microbial community. Also, one of the important parameters of “soil health” is the ratio of carbon and nitrogen. It is generally accepted that the ideal ratio of carbon to nitrogen should be from 8 to 15; for ordinary agricultural soils, it is most often in the range of 10-20.
Recommendations for nitrogen fertilizers are usually based on the content of nitrate nitrogen (as the dominant form), mechanical composition and amount of organic matter. Recommendations based on “soil health” also take into account the amount of ammonium and organic nitrogen. But you can also use information about nitrate nitrogen, which is at a depth of 30-60 cm and 60-90 cm, to obtain recommendations. It is accepted to consider that the amount of nitrate nitrogen that can be found in deeper parts of the soil (below 20 cm) can be about 10 kg/ha. This value is taken into account when making nitrogen recommendations, if no additional tests for nitrogen content are performed.
The stability of the soil aggregate is one of the important functions of the soil related to the soil structure. Soils with a good structure allow air and water to pass more easily and hold water better. Air movement in the soil is very important, both for the life of aerobic microbes and for the growth of roots, which also need oxygen. Additional water infiltration tests show how much water can remain and be retained in the soil when the soil receives it through irrigation or rain. But with a significant excess of water in the soil, there will most likely be a lack of oxygen for normal plant growth and the functioning of the microbial population. So, for example, an analysis of iron content in the soil can provide the following information: is there an excess of water in the soil? Because an excess of water in the soil leads to a deficiency of oxygen, which in turn leads to very high concentrations of iron.
The stability of the soil aggregate determines the amount of sand – the largest fraction in the soil. Soil aggregates, as a physical component of the soil structure, allow water to enter the soil more easily and quickly. Good aggregate stability means that the soil can hold more moisture. So this test shows how good your soil structure is at absorbing and holding water
In general, we can say the following: the more various parameters we can check, the better we will understand how healthy our soil is. Therefore, the main idea of these studies is that “soil health” is always more than one test!
This article is based on information kindly provided by the laboratory Ward Laboratories, Inc.