Optimal pH and an abundance of microbes: good soil health starts here

Much has been said and written about soil health, but what does it mean and what difference does it make to agricultural productivity?  

Dr Katharina Kötting , Director Innovation and Technical Marketing Agriculture for Omya, says healthy crops start with healthy soils. “Soil health is the capacity of the soil to act as a living ecosystem.  Our soil must be able to store and provide water and nutrients, sequester carbon and provide a suitable environment for living organisms. Then it will be able to support plant growth at the level required for today’s growing population. Soil health is essential to food security, so it has a huge impact.” 

Factors contributing to soil health include structure, organic matter content, pH, biodiversity, water and nutrient management. The three key variables, namely physical, chemical and biological, and the interactions between them allow soils to function efficiently and productively, sustaining maximum crop productivity.  

Soil pH: the master variable 

One of the most important variables is soil pH. As Dr Koetting explains, pH is much more than a chemical characteristic. “We call pH the ‘master variable’ in soil health because it has such a big influence on soil properties. Soils can range from highly acidic with a pH as low as 3, to extremely alkaline at pH 10. The target pH should be between 6.0 and 6.8 for ideal cropping conditions. Once it gets to the extremes, crops are bound to fail.” 

pH is a measure of the concentration of hydrogen ions in the soil – the greater the concentration, the more acidic the soil and its corresponding negative impact on soil health. “Soil pH can be measured easily in the field or through laboratory soil analysis,” says Dr Koetting. “There is some natural seasonal variation in pH, due to seasonal changes in soil temperature and humidity, fertilizer application, organic matter decomposition and plant activity.  It is important to note that the main causes of soil acidification are ammonium fertilizer applications, uptake of nutrient ions by crops, and leaching of nitrates from the soil.” 

At optimal pH, the soil ecosystem operates in balance, providing the best possible environment for living plants and organisms. The essential nutrients required for crop growth become available at a pH of 6.5, below this fertilizers are bound to the soil and unavailable to the crop.  

The importance of micro-organisms to soil health 

Just one teaspoon of soil can contain more than one billion bacteria and up to 1km of fungi. Microorganisms such as these are vital to soil health because they decompose organic matter, break down pollutants, improve soil structure, and promote water retention. They also form symbiotic relationships with plant roots, promoting nutrient absorption and protecting plants from pathogens. 

Dr Koetting notes, “Without soil microbes, soil would be less fertile and less able to support crop growth. They play a key role in maintaining a balanced ecosystem, which is so important to soil health. However, their activity is strongly influenced by soil pH.  An example is that of nitrifying bacteria which participate in the conversion of ammonium into nitrogen available to plants, as the pH increases from 6.2 to 7 there is a 55% increase in active bacteria. Soil microorganisms function best at neutral pH, and if the soil is too acidic or too alkaline, then the processes they drive are impaired.” 

It’s not just microorganisms that are essential for soil health, says Dr Koetting. “Don’t forget about earthworms’ contribution to soil health and structure too. A good population of earthworms would be around 120 – 140 worms in a square meter of topsoil. In a year, that number of worms would pull down and incorporate up to 6 t/ha of organic matter from the surface. It’s worth noting that wormcasts can contain on average more than 40% more nitrogen, phosphorus, potash and organic C than bulk soil. Earthworms are doing a great job for farmer fields.”   

It is recognized that earthworms do not do well in acidic soils. Studies show that reducing soil pH will reduce the number of earthworks.  To maintain a healthy earthworm population, growers should aim for a pH above 6. 

Soil management to improve the living environment  

Growers should start by testing their soil pH. Where pH has dropped below 6.5, soil is regarded as acidic and should be corrected unless the rotation includes acid tolerant crops such as potatoes. Most growers traditionally use calcium carbonate as the best option for soil pH correction. There are many calcium carbonate products in the market, and it is worth bearing in mind that that one important parameter to determine effectiveness of products is how fine the product is ground during the initial process.   

Dr Koetting explains, “The finer the product is ground, the more reactive it is and the faster it can neutralize the soil. Micronized particles have a larger surface area which can react with the excess hydrogen ions in the soil. Unfortunately, finely ground products are notoriously difficult to apply. For that reason, Omya has developed granulated products which contains micronized calcium carbonate helping growers have a versatile product that is not dusty and can be applied precisely and avoiding the loss of product to drift. 

Omya Calciprill can be applied with granulated fertilizer equipment. It can also be blended with fertilizers to provide the right balance of nutrients and maintaining or correcting the soil pH according to the soil requirements.  

Omya Calciprill also brings benefits to soil structure, says Dr Koetting. “Omya Calciprill granules break down very quickly in soil moisture and disperse rapidly through the soil profile. As the carbonate reacts with the hydrogen ions, calcium ions are released. The positively charged calcium ions promote that soil particles clump together to form larger more stable structures, reducing the soil bulk density. These changes in the soil structure are essential for improves soil porosity, water-holding capacity and reducing soil erosion and compaction.” 

Omya Magprill is a similar granulated product containing micronized dolomite. It enables growers to provide magnesium nutrition alongside calcium and is ideal for soils displaying magnesium deficiency as well as low pH.  

Dr Koetting concludes, “Omya Calciprill and Omya Magprill can be used together or alone to correct soil pH and to provide a healthy environment for macro and microorganisms as well as for improving soil structure and nutrient availability. We know how vital soil health is to crop productivity and this is a simple way to ensure that soils are kept healthy and suitable for crop growth.” 

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