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202106 Fresh Quarterly Issue 13 06 Put Your Soil To The Test
Issue ThirteenJune 2021

Put your soil to the test

Measuring is the first step in managing soil health. By Grethe Bestbier.

Is there life in my soil? What kind of life? Does my soil provide enough food for microbes and plants? Soil health tests provide answers to these and other important questions. Fresh Quarterly spoke to soil health experts to learn the why, what, and how of soil testing.

Why assess soil health?

The three components of soil health are the physical, chemical, and biological characteristics of the soil. The physical component refers to soil type, texture, and pore size, while the chemical component is all about soil nutrients, such as nitrogen, phosphorus, and potassium.

“While additive agricultural practices used in the past largely focussed on adding more chemicals, and conducting chemical tests, the entry of regenerative farming has placed the third — the biological — component of soil in the spotlight,” explains Sheila Storey, director of nemlab, NemaBio, and the Soil Health Support Centre. Today, the importance of measuring soil biodiversity, organic carbon, and microbial biomass is undeniable.

When all three components are in balance, the soil ecosystem functions optimally, microbial activity is promoted, and effective nutrient cycling takes place.

Nutrient cycling is the core of soil health, and involves many complex interactions, explains Karina Roodt, soil biologist at the Soil Health Support Centre. It’s not practical to test for every single thing, but the correct approach to testing can inform better decisions.

“Being able to monitor your soil year after year is extremely valuable,” says Roodt. “Within a single year you can already see whether there is change or not. It is a great way to track all the soil variables in an attempt to understand what is happening in the soil, and if progress is being made, and whether the soil food web allows for effective nutrient cycling.”

The different soil health tests

The Haney test

The Haney test was developed in Texas. It consists of three components: microbial respiration; water-extractable carbon and nitrogen; and acid-extractable plant nutrients.

The microbial-respiration or CO2-burst test provides an estimate of the amount of life in the soil. Organisms that consume oxygen during respiration produce CO2, or carbon dioxide. The amount of CO2 produced by a soil sample indicates how many living, respiring organisms it contains.

To do the test, water is added to a container filled with dried soil to simulate a rainfall or irrigation event which triggers microbial respiration. The test then measures the CO2 produced during a 24-hour period by microbes living in the soil. The amount of CO2 directly correlates with the quantity of microbes present —the microbial biomass — in the soil.

Results are reported as CO2 in parts per million. A result of 0–60 ppm is considered indicative of low microbial biomass, a result of 60–120 ppm is acceptable, and results above 120 ppm are excellent. Perennial crops yield higher numbers than annual crops, as the former are disturbed less often. More fertile soils will also produce higher numbers than degraded soils, and finer soils often have higher respiration rates than coarser soils, all other things being equal.

This simple test is done year after year to determine the increase or decrease in the soil’s microbial activity.

The water-extractable carbon and nitrogen component of the Haney test uses water to extract soluble organic carbon and the total available nitrogen. Soluble organic carbon is the nutrient source for soil microbes, and the amount present reflects the energy available to fuel soil microbial life. The amount of soil carbon that is water extractable is about eighty times smaller than the total soil organic carbon pool. A test result of 100–300 ppm is considered acceptable.

Water-extractable nitrogen is calculated by measuring the total water-extractable nitrogen and subtracting the portion that is inorganic nitrogen, which is present as ammonium and nitrates. Water-extractable nitrogen can be broken down by microbes, and made available to plants as inorganic nitrogen. It should make up approximately 40%–60% of total soil nitrogen.

The acid-extractable plant nutrient or H3A test uses a combination of organic acids that mimic the actions around the root that cause the release of nutrients for the plant to take up from the soil. The name H3A is not a chemical formula — it refers to the initials of the scientists who developed the test. Test results usually include phosphorus, potassium, calcium, magnesium, and sulphur, as well as certain micronutrients.

The volumetric aggregate stability test

Soil consists of aggregates, which are clusters of primary soil particles — sand, silt, or clay — held together by organic matter and minerals, and by organic substances produced by roots and soil organisms. Aggregate formation is aided by natural forces such as wetting or drying. Compacted soils tend to have more of the smaller aggregates, called microaggregates.

Water stable aggregates are important because they allow for soil porosity, which in turn underpins gas diffusion, water infiltration, and drainage. Water stable aggregates also help to control erosion, and can resist disruptive forces such as tilling.

The Soil Health Support Centre offers the volumetric aggregate stability test to characterise the aggregate profile of soil samples. In this test, the soluble particles within the soil are dissolved in water, and the remaining stable aggregates are mechanically removed, and the aggregate stability percentage is determined. Cultivated soils will often yield a low stability compared to more natural soils.

The phospholipid fatty-acid test

The phospholipid fatty-acid test is a molecular test that provides a profile of the soil microbial community. Phospholipid fatty acids are the building blocks of cell membranes, and can be used to identify and quantify microbes. The test results indicate the number and types of microbes in the soil, providing a snapshot of the microbial activity of a soil sample at a particular moment. Therefore, it should be repeated under similar circumstances as far as possible.

The phospholipid fatty-acid test gives the ratio of fungi to bacteria, and of predators to prey. When these ratios increase, it shows that there are more microbes consuming other microbes, and this translates into more nutrient cycling in the soil. It also indicates that there is more competition in the soil, which means fewer opportunities for pests and diseases to thrive.

The bio-indicator test

Soil nematodes are microscopically small organisms that occur in soil moisture, and provide various ecosystem services, like breaking down organic matter, redistributing nutrients, sequestering carbon, and improving soil structure. Nematodes are good indicators of soil health, and the nematode faunal profile is used to assess the soil food web.

The profile is compiled by identifying all free-living nematodes within the soil sample to family level, and classifying them into one of at least five feeding groups. The nematodes within each family are then further classed according to their life strategy, as either explosive colonisers — opportunists — or conservative persisters —survivalists. Colonisers can rapidly take advantage of an increase in nutritional resources, whereas persisters are more common in stable ecosystems with complex food webs. Colonisers tend to be more tolerant of pollutants than persisters.

Next, various indices are calculated to draw up a nematode faunal profile. The indices are used to measure soil disturbance, for example pesticide or fertiliser application, or mechanical tillage. The structure index reflects recovery from stress, while the enrichment index indicates a disturbance caused by the addition of a resource, such as organic matter. Soil disturbance can lead to a reduction in the ecosystem services mentioned above.

Enzyme activity tests

Enzymes are proteins that catalyse biological reactions. Soils contain a whole complement of enzymes that are derived from plants, animals, or microbes. The three most commonly studied enzymes are β-glucosidase, urease, and phosphatase, which are active in the cycling of carbon, nitrogen, and phosphorus, respectively. These enzymes are particularly sensitive to management-induced changes in the soil, which makes them useful in assessing soil improvement, fertility, productivity, and degradation.

The soil health services offered by the ARC Infruitec-Nietvoorbij include the measurement of β-glucosidase, urease, and phosphatase. The laboratory also conducts the AI3 soil-alteration index, which quantifies the balance between the activities of the three enzymes. This method offers a practical way to determine whether soil management practices are successful, and points out possible fertility and soil health problems. Because enzymes are very sensitive to chemicals, especially heavy metals, this test has the added benefit of indicating soil pollution.

What should I know about soil health testing?

Deciding which tests to conduct can be a challenge, and various factors like cost, aims, and test sensitivity, should be considered. The Soil Health Support Centre combines the results of the Haney analysis with the volumetric aggregate stability test. These results can be supplemented by looking at the community profile of the soil, using the phospholipid fatty-acid test or the nematode bio-indicator test.

All testing options have their advantages and shortcomings, which should be weighed against each other to make the best decision with your available resources. While combining the Haney analysis with a phospholipid fatty-acid test yields plenty of data, it is an expensive route. On the other hand, the nematode bio-indicator test is more affordable, and very sensitive, but yields less quantitative data.

Moreover, testing biological components comes with its own challenges. Working with a living system means that conditions and activities are always changing. That is why it is important, when repeating soil health tests, to conduct them at approximately the same time of year. To enhance test accuracy, Roodt also advises producers to stick with a single laboratory to conduct their tests year after year, as different extraction and analysis techniques may impact results.

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