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A load of rot

Carbon plays a vital role in the productivity of a soil. In its organic form, carbon acts as a nutrient, supplying energy to the microbes which grow in the soil. Organic carbon also regulates soil moisture and quality.

Agriculture has seen an estimated 55 Petagrams of carbon lost from the world’s soils. This loss has occurred due to cleared soils losing carbon in the form of gas, through soil erosion, and through harvesting the carbon previously stored in the soil in forms such as crops and meat.

This problem of soil carbon loss is increasingly well recognised and efforts are being made to replenish agricultural soils using soil amendments and soil conditioners, where soil amendments are any substances applied to a soil to enhance plant growth and soil conditioners are a type of soil amendment which contains materials that can measurably improve specific soil physical characteristics or physical processes for a given use or as a plant growth medium. Soil amendments and soil conditioners are generally made from materials high in carbon, and can be applied to soils as a means of increasing their carbon concentrations.

A big question is whether soil amendments are a fad similar to those seen in human health and nutrition. There are questions, for example, as to whether superfoods actually benefit human health. So too are there questions as to whether soil amendments result in measureable and beneficial outcomes for agriculturalists.

Soil amendments as fertiliser

A range of soil amendments are currently touted on the market. Products include composts, biosolids (ie treated sewage), biochar, fly ash, seaweed extract, vermicast (worm compost), and animal manure. Many of these products are advertised with their nutrient contents at the fore.

Research shows, however, that even high nutrient soil conditioners like composts and meat, blood, and bone meal would need to be applied to farms in quantities far outside what is economically viable in order to meet the nutritional needs of a crop. This is in part because many of the nutrients within soil amendments are locked up in complex organic molecules and are only made available to plants after microbes break down these molecules. This process takes months to years.

The concentration of nutrients in soil amendments also plays a role. Soil amendments simply do not contain the highly concentrated nutrients that commercial fertilisers do. Chemical fertilisers also contain nutrients in a form which is largely available to plants immediately, especially those plants which were bred after the invention of chemical fertilisers in the mid-20th century and are used in cropping today.

When soil amendments benefit a soil

Although soil amendments are an uneconomical way to meet the nutritional needs of a crop, their long term application can increase the soil’s nutrient status under certain conditions. One study, for example, showed that organic carbon concentrations in a soil lasted one hundred years after regular and consistent application of manure over a preceding twenty year period. Such improvement of a soil’s nutrient status as a result of using soil amendments depends on a range of factors.

If the proportion of nitrogen to carbon in soil amendments is relatively high, for example, microbes appear to convert organic carbon to mineral carbon, making it largely unavailable for further microbial uptake. The benefit of soil amendments in terms of soil carbon is therefore less when amendments with high nitrogen concentrations are used over those proportionally lower in nitrogen.

Microbial populations are stimulated by soil amendments and microbial populations can increase in the soil after amendment application. This increase in microbe numbers can also mean more organisms converting the carbon applied to soils back to gaseous carbon (carbon dioxide or methane). The carbon applied to soils within amendments can be fluxed right back out of the soil as a gas and within months, soil carbon concentrations can be the same as before soil amendments were applied.

Temperature and rainfall also play a role in whether soil amendments will improve a soil’s nutritional status. Soil organic carbon increases with increasing precipitation, for example, but decreases with increasing temperature. It is not surprising then that the peat bogs of the northern hemisphere occur in cold, wet places, and the low carbon soils of Australia occur in hot, dry places.

Other benefits

Perhaps one of the most surprising benefits of soil amendments is that plant growth can be stimulated despite the lack of available nutrients. Research suggests that this is because cytokinins and perhaps auxins – plant hormones which stimulate plant growth – present in the humic substances which occur in many soil amendments stimulate crop growth and production.

Chemical fertilisers also appear to be made more available to plants when applied in concert with soil amendments. In addition, evidence suggests that clay-rich soils will have improved quality when soil amendments are applied, although there are exceptions to this.

The verdict

The two scientific research articles on which this blog is largely based state that only very limited research has been conducted on soil amendments and agriculture. They also state that this may be due to the very complex nature of such research – soil amendments vary enormously in terms of their content, and their chemical and physical attributes. The soils to which soil amendments are applied also vary, often a great deal in a small area. This makes robust, conclusive science relating to the impact of soil amendments on soils difficult to undertake.

The limited availability of research funds also means risky and complex research which may not lead to publication of articles in high-ranking journals can be less desirable than research where data is more easily attained, analysed, and published. Research into soil amendments certainly is complex and risky in terms of the potential publishability of results.

In any case, there is consensus that the nutritional status of a soil will increase in the soil if inputs exceed outputs. The amount of nutrient in soil amendments, however, means that they are an uneconomical way of meeting the inputs needed in commercial agriculture and they are therefore not effective fertilisers. Soil amendments can improve soil nutrition in the long term if the amount of nutrient added is not later removed in greater amounts through erosion, cropping and harvesting, and as gases to the atmosphere.

In terms of carbon, amendments applied to soils in environments with cool, wet, climates and the right clay content will flux gaseous carbon in lower concentrations. Carbon will be fluxed much more rapidly in environments which are dry and hot.

Due to the higher output of carbon in its gaseous forms, soil amendments must be applied in greater quantities in hot dry environments than cool wet environments if soil carbon concentrations are to be improved. Soils which have had regular application of amendments over decades or longer have been found to have higher carbon concentrations that surrounding non-agricultural land, showing that soil amendments can effectively improve soil carbon in the long term.

There is evidence that despite their lack of nutritional benefit, soil amendments improve plant productivity through the stimulation of plant growth and by allowing an increased availability of any chemical fertilisers applied to the soil.

So the verdict? Soil amendments are not a replacement for commercial fertilisers. They do provide important benefits to soils outside of soil nutrition, however, in their ability to build soil depth, carbon, and stimulate plant growth.

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