Farm Ireland

Monday 18 December 2017

Tillage: Harness the invisible power of soil's biology

Gerry Bird

As tillage farmers, we often underestimate the contribution that soil flora and fauna can make to our crops. At the National Compost Conference recently, the emphasis was on the role that compost can play in maintaining vital soil processes.

The main areas of interest were in nutrient supply, organic matter maintenance, moisture retention, soil structure enhancement and contribution to soil biology. The topic of soil biology generated a good discussion.

Soil biology is essentially concerned with the living part of the soil including fungi, bacteria, worms, beetles, slugs, leatherjackets and lots more.

Earthworms are probably the most obvious soil inhabitant and are generally looked on as beneficial. Despite the bad press they get, slugs, symphilids, and nematodes all play beneficial roles in organic matter breakdown.

The problem with these latter organisms is that they sometimes find it difficult to differentiate between the crop and organic debris.

Soil organic matter requirements, as outlined by the Department of Agriculture, are mainly concerned with maintaining soil stability and minimising soil erosion, which has a negative impact on water quality.

Organic matter is the only soil component that the farmer has any control over. The percentage of organic matter in soil influences the activity of the vast army of soil microbes. A living soil, with the correct balance of fungi, microbes, various insect groups and worm populations, acts to make nutrients available to the crop.

Growers using organic manures are extremely dependent on good soil biological activity. The process of these beneficial soil activists is regulated by the soil temperature, pH level, oxygen and moisture content.

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A compacted soil with excessive water levels and low pH values responds poorly to applied slurry, sludges or composts, and some growers actively apply these products on to damaged land believing that this practice will solve the problem.

Compacted fields have a specific collection of soil biology. However, due to the lack of air, excessive water, low pH and partially decomposed organic material, species of bacteria and fungi are not beneficial to growing crops.

Applied organic slurries and crop residues contain nutrients, N, P and K, but the bulk of this fertiliser is not water-soluble and it has to undergo a natural breakdown to make it available.

Bacteria break down organic nitrogen to ammonium, then nitrite and more bacteria continue the process to nitrate. Perennial crops, such as grass and fruit, mainly use the ammonium form of nitrogen; cereal crops, oilseed rape and potatoes are annual crops and favour the nitrogen in mainly a nitrate form.


A compacted soil will have a high percentage of anaerobic bacteria, which will mainly transform ammonium to nitrite, which is not a plant friendly form and a waste of nutrient potential. The process of organic manure breakdown under these conditions will be very slow, with lots of organic acids created.

The soil pH in these circumstances will often be low, so earthworms are scarce, and there will be no worm channels, so water movement is restricted. With no oxygen in the soil due to water saturation, a bad situation gets worse!

The worms create an environment and food for bacteria and fungi through soil aeration, soil casts and body slime. Other bacteria and insects then eat these fungi and bacteria.

At each stage of the process, nutrient is released into the soil solution and is eventually transformed into plant-available fertiliser. The area around soil roots is teeming with fungi and bacteria all dissolving, recycling and storing plant available nutrient.

The rate of the breakdown of organic matter is also dependent on the ratio of carbon to nitrogen content of the manure, compost or returned straw. Straw has a high N:C ratio so the soil bugs use a lot of available nitrogen to breakdown the straw, which sometimes results in a temporary nitrogen deficiency in the crop.

The nitrogen absorbed by the microbes becomes available as they die or are eaten by other species, so the nutrient remains in the field but might be too late for a critical crop stage.

Tillage operations all impact on the soil biology, depending on the type of seedbed created. Fertiliser and pesticide applications can alter the type and rate of biological activity.

Growers should be aware of the powerful unseen workforce continuously active below the surface and the benefits of a healthy soil web.

Gerry Bird is a crop consultant and a member of the ITCA. Email:

Indo Farming