Soil and water

Soil Pores, Water and Productivity

A productive soil is permeable to water and is able to supply water to plants. A soil’s permeability and water-holding capacity depends on its network of pores:

• Large pores (macropores) control a soil’s permeability and aeration. Macropores include earthworm and root channels. Because they are large, water moves through them rapidly by gravity. Thus, rainfall and irrigation infiltrate into the soil and excess water drains through it.

• Micropores are fine soil pores, typically a fraction of a millimeter in diameter. They are responsible for a soil’s water-holding capacity. Like the fine pores in a sponge or towel. Micropores hold water against the force of gravity. Much of the water held in micropores is available to plants, while some is held so tightly that plant roots cannot use it.

Soil that has a balance of macropores and micropores provides adequate permeability and water-holding capacity for good plant growth. Soils that contain mostly macropores drain readily, but are droughty and need more frequent irrigation. Soils that contain mostly micropores have good water-holding capacity but take longer to dry out and warm up in the spring. Runoff of rainfall and irrigation water also is more likely on these soils.

What Affects Soil Porosity?

Several soil properties affect porosity, including texture, structure, compaction and organic matter. You can evaluate your garden soil with respect to these properties to understand how they affect its porosity. The only tools you need are your eyes, fingers and a shovel.

Clay particles are the smallest — about the size of bacteria and viruses — and can be seen only with a microscope. They typically have a flat shape, similar to a sheet of mica. Soils rich in clay feel very hard when dry, but are easily shaped and molded when moist. Although all of these particles seem small, the relative difference in their sizes is quite large. If a typical clay particle were the size of a penny, a sand particle would be as large as a house.

Soil texture directly affects porosity. Pores between sand particles tend to be large, while those between silt and clay particles tend to be small. Thus, sandy soils contain mostly macropores and usually have rapid permeability but limited water holding capacity. Micropores predominate in soils containing mostly silt and clay, creating high water holding capacity but reducing permeability.

Particle size also affects the surface area in a volume of soil. Surface area is important because surfaces are the most active part of the soil. They hold plant nutrients, bind contaminants and provide a home for microorganisms. Clay particles have a large surface area relative to their volume, and a small amount of clay makes a large contribution to a soil’s surface area.

Nearly all soils contain a mixture of particle sizes and have a pore network containing a mixture of pore sizes. A soil with roughly equal influence from sand, silt and clay particles is called a loam. Loams usually make good agricultural and garden soils because they have a balance of macropores and micropores. Thus, they usually have good water-holding capacity and moderate permeability.

A sandy loam is similar to a loam, except that it contains more sand. It feels gritty, yet has enough silt and clay to hold together in your hand. Sandy loams usually have low to moderate water-holding capacity and good permeability. Silt loams are richer in silt and feel smooth rather than gritty. They are pliable when moist, but not very sticky. Silt loams usually have high water-holding capacity and low to moderate permeability.

Clays and clay loams are very hard when dry, sticky when wet and can be molded into wires and ribbons when moist. They generally have high water-holding capacity and low permeability. Almost any texture of soil can be suitable for gardening, as long as you are aware of its limitations and adjust your management to compensate. Clay soils hold a lot of water, but are hard to dig and dry slowly in the spring. Sandy soils need more frequent watering and lighter, more frequent fertilization, but you can plant them earlier in the spring. All soils can benefit from additions of organic matter, as described below under “Adding Organic Matter.”

Many soils contain coarse fragments, such as gravel and rocks. Coarse fragments do not contribute to a soil’s productivity and can be a nuisance when you are digging. Don’t feel compelled to remove them all from your garden, however. Coarse fragments aren’t harmful, and your time is better spent doing other gardening tasks. The only time rocks are a problem is when you have nothing but rocks on your land. Then, water and nutrient-holding capacities are so low that it is difficult to grow healthy plants.

Clay — The smallest type of soil particle (less than 0.002-mm in diameter).

Sand — The coarsest type of soil particle (0.05- to 2-mm in diameter).

Silt — A type of soil particle that is intermediate in size between sand and clay (0.002- to 0.05-mm in diameter).

Soil — A natural, biologically active mixture of weathered rock fragments and organic matter at the earth’s surface.

Growing roses is more than just digging a hole in the ground and sticking a rose into it and expect to have great roses. Knowing and understanding the importance of soils and fertilizers is very important to growing roses and other plants.

Soil texture

Texture describes how coarse or fine a soil is. The coarsest soil particles are sand. They are visible to the eye and give soil a gritty feel. Silt particles are smaller than sand — about the size of individual particles of white flour. They give soil a smooth, floury feel. On close inspection, sand and silt particles look like miniature rocks.

Soil Structure

Percentages of clay, silt, and sand in the basic soil textural classes

Individual particles of sand, silt and clay tend to cluster and bind together, forming aggregates called peds, which provide structure to a soil. Dig up a piece of grass sod and examine the soil around the roots. The granules of soil clinging to the roots are examples of peds. They contain sand, silt, clay and organic matter. Aggregation is a natural process caused largely by biological activity such as earthworms burrowing, root growth and microbial action. Soil organic matter is an important binding agent that stabilizes and strengthens peds.

The spaces between peds are a soil’s macropores, which improve permeability, drainage and recharge of air into the soil profile. The pores within peds are predominantly micropores, contributing to the soil’s water-holding capacity. A well-structured soil is like a sponge, allowing water to enter and soak into the micropores and letting excess water drain downward through the macropores. Good structure is especially important in medium to fine textured soils, because it increases the soil’s macroporosity, thus improving permeability and drainage.

Compacted soil resists root penetration and water movement.

Compaction and Loss of Structure

Soil structure is fragile and can be damaged or destroyed by compaction, excessive tillage or tillage when the soil is too wet. Loss of organic matter also weakens structure. Compaction squeezes macropores into micropores and creates horizontal aggregates that resist root penetration and water movement. Compaction often occurs during site preparation or house construction, creating a difficult environment for establishing plants. Protect your soil from compaction by avoiding unnecessary foot or machine traffic. Tilling when soil is too wet also damages soil structure. If you can mold a piece of soil into a wire or worm in your hand, it is too wet to till. If the soil crumbles when you try to mold it, it is dry enough to till.

Compacted soil resists root penetration and water movement. Structural damage caused by human activity usually is most severe within the top foot of soil and can be overcome by proper soil management. In some soils, there is deeper compaction resulting from pressure from ancient glaciers. Glacially compacted subsoils (a type of hardpan) are common in the Puget Sound area, where the compacted layer often begins 18- to 36-inches below the soil surface. Where the land surface has been cut, leveled or shaped for development, the compacted layer may be much closer to the surface. This layer looks like concrete and is so dense and thick that it is nearly impossible to work with. If your garden has a glacially compacted layer close to the soil surface, consider using raised beds to increase soil depth.

Organic Matter

Adding organic matter is the best way to improve the environment for plants in nearly all soils. Organic matter helps build and stabilize soil structure in fine-textured and compacted soils, thus improving permeability and aeration and reducing the risk of runoff and erosion. When organic matter decomposes, it forms humus, which acts as a natural glue to bind and strengthen soil aggregates. Organic matter also helps sandy soils hold water and nutrients. See “Adding Organic Matter” later in this chapter for information on amending soil with organic matter.

Slope, Aspect, Depth and Water

Slope, aspect (direction of exposure) and soil depth affect water availability and use in a soil. Choose plants that are best suited to conditions on your property. Ridge tops and side slopes tend to shed water, while soils at the bottoms of slopes and in low areas collect water. Often, soils that collect water have high winter water tables, which can affect the health of some plants. Soils on ridge tops are more likely to be droughty. Site aspect also is important. South- and southwest- facing exposures collect the most heat and use the most water.

Soil depth also affects water availability by determining the rooting zone. Soil depth is limited by compacted, cemented, or gravelly layers, or by bedrock. A shallow soil has less available water simply because the soil volume available to roots is smaller. Dig below the topsoil in your garden. The deeper toucan dig before hitting a restrictive layer, the greater the soil volume for holding water.

Water Management in Your Garden

Soils and Irrigation

Most gardens in the Northwest require summer irrigation. Themed for irrigation varies, depending on soil water-holding capacity, weather, site aspect, the plants grown and their growth stage. In most cases, the goal of irrigation is to recharge the available water in the top foot or so of soil. For sandy soil, 1-inch of irrigation water is all you need. Any more will leach (move downward) through the root zone, carrying nutrients with-it. A silt loam or clay soil can hold more than 2-inches of water, but you may need to irrigate more slowly to prevent runoff.

Wet Soils

If your soil stays wet in the spring, you will have to delay tilling and planting. Working wet soil can damage its structure, and seeds are less likely to germinate in cold, wet soil. Some plants don’t grow well in wet soil. Raspberries, for example, often become infected by root diseases in wet soil and lose vigor and productivity. A soil’s color gives clues to its tendency to stay wet. If subsoil is brown or reddish, the soil probably is well drained and has few wetness problems. Gray subsoils, especiallythose with brightly colored mottles, often are wet. If your soil is gray and mottled directly beneath the topsoil, it probably is saturated during the wet season. Sometimes, simple actions can reduce soil wetness problems. For example:

• Divert runoff from roof drains away from your garden.

• Avoid plants that perform poorly in wet conditions.

• Use raised beds for perennials that require well-drained soil and for early-season vegetables.

Investigate whether a drain on a slope will remove excess water in your situation. Installing drainage can be expensive, however. When considering drainage, make sure there is a place to drain the water. Check with local regulatory agencies to see whether there are restrictions on the project.

Craig Cogger (cogger@wsu.edu), ‘Guide to Soil and Fertilizer, Parts I & II’, March & April 2014. Thorny Issues. Dan Simmons (puyalluprose@comcast.net), ed. Puyallup Rose Society