Protect the soil, boost profits

How the soil food web retains and cycles nutrients to maintain soil fertility.

A microscopic view of an amoeba feeding. They search out and consume nutrient-rich bacteria and fungi and excrete nutrients utilised by plants.
Photo: Courtesy of Foundations for Farming

In our previous series we introduced you to the amazing soil food web. This complex ecosystem teems with organisms, such as bacteria, fungi, protozoa, nematodes and mites. In fact, more than a billion organisms can exist in 1g of healthy soil.
Bacteria and fungi work to break down organic material. A second level of organisms, such as nematodes, arthropods (which includes insects, arachnids and crustaceans) protozoa (small single-cell life forms that tend to exist as parasites) annelids (earthworms) and molluscs (snails and slugs), feed on the bacteria and fungi.

As each level feeds on the level below it, a waste product is produced – think of it as ‘microscopic manure’. This natural fertiliser is created on a daily basis, in proportions which exactly suit the type of plant life which dominates that area. The soil food web is healthiest in undisturbed soil, or where organic methods of farming are practiced. Here, the soil isn’t seen simply as a sterile medium, only there to hold up the plants, while inorganic fertilisers are fed to the crops.

Creating fertility
Many farmers throughout the world are finding that input costs increase, while productivity diminishes. The solution is found in further mechanisation and the planting of larger areas. Unfortunately this isn’t a sustainable solution. In an organic system, by contrast, the microscopic creatures we’ve mentioned ‘create’ fertility constantly, every minute of every day. This microbial manure is always available and ideally suited to the requirements of the plant. Let’s take a closer look at two of the most important functions the food web fulfils; nutrition retention and cycling.

Retention
Organic matter introduced into the food web as compost or mulch is decomposed by bacteria and fungi. The nutrients are then taken into the bodies of these organisms and immobilised, meaning they cannot be leached from the system. Fungi also retains calcium, and in an experiment done by Dr Elaine Ingham we see how important it is to have fungi in our soil food web. Four containers filled with sterile sandy loam were inoculated in different ways.


Treating the soil as a plant growing medium, while feeding inorganic fertilisers to the crops, is proving expensive – and unsustainable. Farmers are now learning to ‘grow’ their soil before sowing their crops.

The control container was not treated, the second had 5% organic matter added, the third a bacterial culture and the fourth a fungal culture. A calcium source (crushed oyster shell) was added on the top, then 1l of water was poured through each container. The water was collected and the calcium content measured. The results were so astounding the first time the experiment was conducted it was believed a mistake had been made and calcium hadn’t been added to the pot holding the fungal culture. But the results were confirmed.

The sterile parent material in container 1 retained no calcium. The organic matter was able to hold 2% of the calcium, the bacteria 5%. The fungi retained 100% of the calcium introduced into the pot. Fungi can minimise acidity found in conventional farming systems. Annual soil pH tests are used to calculate soil lime needs. Leaching and the addition of fertilisers throughout the season then lowers the pH, necessitating the addition of more lime. A function of fungi is to reduce the leaching of lime from the soil and prevent fluctuating pH levels.

Cycling
Nutrient cycling is the process in which nutrients are released for plant consumption, when bacteria and fungi are eaten by organisms higher up in the food chain. Protozoa, bacterial- and fungal-feeding nematodes, micro-arthropods and earthworms are primary nutrient cyclers. They consume bacteria and fungi and excrete nutrients in a form easily taken up by plants.

To understand this process, consider the following:

  • Protozoa, voracious feeders, can consume up to 10 000 bacteria a day.
  • Protozoa have a carbon/ nitrogen ratio of 30:1.
  • Bacteria have a carbon/ nitrogen ratio of 5:1. 

In order to create one protozoal protein, six bacteria need to be eaten (6 x 5 carbon molecules). However, only one nitrogen molecule is required, which means that five nitrogen molecules are released for every six bacteria consumed. Therefore, if the protozoa eats 10 000 bacteria a day, it will release at least 8 000 nitrogen molecules a day.

Healthy soil contains more than 50 000 protozoa/g of soil. Daily, then, 500 million bacteria will be consumed in every gram of soil and up to 400 million nitrogen molecules released per gram of soil per day. As we can see there is far more happening in healthy soil than we imagine.

Foundations for Farming (FFF) is a Zimbabwe-based NGO that teaches conservation agriculture practices to farmers, based on Christian principles. FFF provided the information for this article. Visit www.foundationsforfarming.com.