Tilth

Tilth - A Foundation to Sustainable Agriculture

by George W. Powell, Ph.D., P.Ag.

Soil is the 'living - not living' skin that binds mineral elements, air, water and life into an interactive ecosystem. In agriculture, crop and livestock production is rooted (sometimes literally) in the productive and regenerative capacity of the soil. And a soil's tilth is a measure of the sustainable productivity of soil. By using practices that build and sustain soil tilth, therefore, you are contributing to productive, profitable and sustainable agriculture. Simply put, tilth is important to agriculture because soils and crops are foundational to most of agriculture.

Tilth is a very old word, based on a much older concept. It is from the same origin as the verb "to till," and originally meant land that is suitable to cultivate. The modern meaning of tilth is the 'biophysical condition of the soil, especially in relation to its suitability for growing a crop'. Tilth relates to both the type of soil aggregates present and their stability. And it applies to any agricultural setting, including crop lands, pastures and rangelands.

Tilth Factors

Soil tilth is not strictly related to any one condition, but reflects the interplay of six soil properties:

Structure;
Depth;
Density;
Organic content;
Root networks; and,
Animal activity.

Soil Structure

Soil structure is a very important factor in tilth because it determines three important soil-based factors for crop growth:

1. Air space - where mores pores distributed throughout the soil profile equals better growth potential:

- - Gives you an earlier start to growing season (because soil warms faster in the spring); and,
  - Gases flow freely to and from the atmosphere, including: oxygen flows in and carbon dioxide out; nitrogen gas moves to root nodules to be converted to ammonium, a biologically useful form of nitrogen.

2. Water movement and retention - with the goal of having well distributed, large enough soil pores to allow water to infiltrate and transmit through the soil (with dissolved nutrients), but small enough to hold onto surplus water for later use. This influences:

Infiltration - ease with which surface water enters the soil column;
Permeability - ease with which water moves through the soil; and,
Water holding capacity.

3. Physical matrix - For optimal crop production we want a soil matrix firm enough to provide good root-soil contact, yet loose enough to allow easy root growth for 'scavaging' water and nutrients.

Soil Texture

In order to understand soil structure, one must first understand soil composition and texture. Soils are composed of three primary elements: mineral, organics and pores (which can be occupied by air and water). The relative of mineral, organic and pore space must be balanced to optimize crop growth.

In the mineral component of soil, the 'texture' refers to the relative proportions of soil particles of varying diameter in any given soil: sand (0.05 to 2 mm), silt (0.002 to 0.05 mm), and clay (<0.002 mm). Soil texture is categorized (e.g. sandy-clay or silty clay loam) by referencing the various proportions of three soil particles onto a soil texture triangle.

"Coarse-textured" (a higher proportion of sand), "medium-textured", and "fine-textured" (a higher proportion of clay) are other adjectives that are often also used to describe soil texture. Coarse-textured and fine-textured soils are also referred to as "light" and "heavy" soils, respectively.

Soil Texture Triangle

Soil Aggregates

Soil structure relates to the dominant size and shape of soil aggregates. Aggregates form when biological and chemical processes bind soil particles together with varying amounts and arrangements of pores (empty spaces) between them. Soil scientists have classified soils into six basic aggregates:

Granlular;
Prismatic;
Massive;
Single grain;
Blocky; and,
Platey.

Clay content is a very important factor in soil aggregates and structure; more so than silt and sand. This is because soils with greater than 15 percent clay exhibit plasticity when they are wet. That is, they can be moulded or shaped into different arrangements and they can also form structures (plates, blocks, or columns) when drying because of particle adhesion. The type of aggregate formed then influences crop growth:

Large aggregates (plates, columns, large blocks) have a poor structure for crop growth because the pore space within the aggregates is too small and the space between aggregates is too large;
Medium to small aggregates best overall for supporting crop growth; and,
No aggregates (sand & gravel, muck) are also poor for crop growth.

Soils can often also be mixed with organic and inorganic coarse fragments that do not form aggregates. These include:

Coarse woody debris; and,
Rocks, in increasing size from pebbles, gravel, cobble, stones to boulders;

Soils that are unstructured or poorly structured, coarse, platey or smeared with few or fine pores, are indicators of low tilth for agriculture. High tilth soils are well structured, with either blocks or large granules with abundant pores.

2. Soil Depth

In order to support robust crop growth, soils need to be deep enough to create a positive medium over the full depth of the rooting zone. The deeper the rooting profile is, the more nutrients and soil water a crop can access.

As a rule of thumb in the Cariboo region, where soils are geologically 'young', soils with a shallow production layer (< 50 cm) are indicative of lower tilth, whereas, soils greater than 50-cm deep indicate higher tilth.

3. Soil Density

Soil density refers to how closely packed soil particles are within, and between, individual soil aggregates. Density is a direct measure of the amount of pore space in a soil and reflects both the physical make of the soil (texture and structure) and the degree of compaction.

Dense, heavy soils, have much less space in their matrix for air or water to support plant growth, than loose, light textured soils. Dense, compacted or cemented soils are therefore indicators of poor tilth, and uncompacted, loose, soil aggregates easily broken into small lumps, are indicators of higher tilth.

4. Organic Content

Soil organic content includes both living beings, from the microscopic (bacteria, fungi) to the macroscopic (insects, worms), and also undecomposed to fully decomposed (humus) dead organic matter.

Organic content is a very important determinant of soil tilth because is can have such a profound impact on the other tilth factors. Organic matter in the soil decomposes to humus and humic acids. Humic acids directly affect the aggregation and binding of mineral particles and soil structure, and also enable many complex chemical transformations in the soil making nutrients available for crop uptake. Humus therefore has a key influence on soil chemistry.

Organic matter alone, however, doesn't tell the whole story. The breakdown from undecomposed organics to humus is hindered when soils become saturated with water. Because of this, low organic content (i.e. pure mineral soil) or continuously saturated, very high organic soils (e.g. muck, peat) are both indicators of poor tilth. Whereas, high organic content throughout the soil profile, with low to moderate saturation, are indicators of higher soil tilth.

It is important to remember that soil organics must be continually replenished in agricultural settings. This is both to maintain the processes of decomposition that generate humus, and also to supply plants with organic nitrogen, phosphorus, sulphur and micronutrients. Therefore, the soil surface and the top of the soil column should contain leaf litter or other undecomposed organic matter.

5. Root Networks

Plants extend their roots throughout the soil to anchor themselves in place, and also to find and gather water and nutrients. The amount of roots throughout the soil profile is a key indicator of soil tilth, with a strong positive feedback loop. Good tilth promotes robust plant growth above and below-ground. And good root growth improves soil structure by:

Decompacting mineral soils and breaking apart 'bad' aggregates;
Providing pathways for air and water to descend into lower levels of the soil; and,
Directly and indirectly increasing the amound of soil organics.

6. Animal Activity

Animals, from the smallest of worms and insects, up to rodents and small mammals, all have a positive outcome on building and maintaining soil tilth.

Soil based-animals burrow tunnels and churn through the soil, breaking apart compaction and large aggregates along the way. Soil critters also consume larger bits of undecomposed organic matter, and excrete it throughout the soil profile. This hastens the decomposition process and release of plant nutrients trapped in the organic matter and improves the distribution of humus throughout the soil matrix.

As a general rule, tilth improves proportionally with increasing levels of soil animal activity. And as with root networks, animal activity in the soil can create positive feedback loops: the more animal activity present, the better the resulting soil tilth. The better the tilth, the more soil life, food webs and animal activity it can support.

Photos in this article are from Unsplash and Wikimedia Commons

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