Marin Master Gardeners
University of California
Marin Master Gardeners

Marin IJ Articles

Good gardens start from the ground up

November 28, 2009
Martha Proctor

As a dietitian I advised individuals on ways to make better food choices to achieve a healthier lifestyle. Now as a Master Gardener, my focus is on improving the soil to produce healthier plants.

This is the first of three articles taking some of the mystery out of soil, particularly the soil we encounter in Marin County.

Soil, the loose upper 6 to 8 inches of earth in which plants grow, is teeming with microorganisms (animal), the residue of live and dead plants (vegetable) and the decomposed rock from which the soil originated (mineral). Two other important components of soil are air and water. Soil supplies plant roots with a source of air, water and nutrients plus insulates them from extreme changes in temperature. Top soil is the layer most influenced by climate and most enriched by the addition of organic matter.

Good gardens start from the ground up, so a better understanding of soil will help you build the best soil for the plants you intend to grow. Two important elements are soil texture and structure.

Soil texture

Soil texture refers to the relative proportions of sand, silt and clay mineral particles in a soil. Soil texture affects plant growth because it determines the tilth, or fitness of the soil as a medium for growing plants, and its nutrient and water-holding capacities. The proportion of sand, silt or clay is an important determinant of a soil's properties.

Texture is important because the size of the three particle types affects the size of the pore spaces among these particles, which, in turn, influences the amount of air and water the soil can hold.

Soil texture is grouped into three general categories: coarse (sandy soils), medium (loamy soils) and fine (clay soils). Clay is composed of the smallest particles, silt is a bit bigger and sand has the largest particles.

Soil in which sand, silt and clay are in roughly equal proportions, or loamy soil, contains a variety of particle sizes so it provides a good balance of air and water to the soil. Loamy soil allows good penetration by plant roots and it drains well.

Unfortunately, most gardens do not contain an ideal mixture of the three particle types. Very coarse, sandy soils dry out rapidly and are difficult to maintain at a high fertility level. Silty soils have a slower water infiltration rate, but a higher water-holding capacity than sandy soils.

Clay soils (generally more than 40 percent clay), common in Marin County, retain more water, have slower air and water movement and hold more mineral nutrients than sandy soils. Because of the smaller size of clay particles, soil in which clay predominates can easily become compacted.

When soil becomes compacted, the pore spaces inhabited by soil microbes collapse, which adversely impacts the movement of air and water through soil and limits the growth and expansion of plant ro0ots. In soils throughout California, retention of plant nutrients is correlated with the amount and kind of clay in the soil.

Clay soils amended with appropriate proportions of organic matter, however, are some of the most productive farmland soils on earth. Thus it is best to modify clay and sandy soils if gardening is to be successful.

Soil structure

Soil structure refers to the arrangement of the particles of sand, silt, clay and organic matter into clusters within soil. A cluster - a clump of soil particles held together in a unit - functions as a single large particle. Clustering is very weak in sandy soils. Soil structure is greater if clay and organic matter are present in soil, as they act as binding agents.

Plant growth is strongly influenced by soil structure because the structure affects pore space and a plant's access to moisture and air. The fertility of the soil, its porosity and the density of roots within it also contribute to soil structure.

Good-quality topsoil has a crumb-like structure. Tiny pores between soil particles allow water from the soil surface to infiltrate so water is retained but excess amounts drain down to allow space for air circulation. Plant roots grow best when air spaces between soil particles are maintained. Soil texture and structure can be modified by using soil conditioners.

Soil conditioners act to improve soil aeration, drainage, moisture-holding capacity and tilth. Commonly used soil conditioners include compost, peat moss, sawdust, wood chips, composted animal manures, green manure crops, coarse sand and perlite. By incorporating coarse, rather than fine sand, and organic matter into a garden soil, the gardener can, over time, produce a desirable loamy soil.

The next article in the series will address the essential nutrients that plants depend on to thrive and the importance of the soil's pH in the plant's ability to utilize nutrients.

DIGGING IN

Soil texture

- Refers to the proportions of sand, silt and clay mineral particles in soil

- Affects plant growth because it determines the fitness of the soil as a medium for growing plants, and its nutrient and water-holding capacities

Soil structure

- Refers to the arrangement of the particles of sand, silt, clay and organic matter into clusters within soil

- Strongly influences plant growth because the structure affects pore space and a plant's access to moisture and air

The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 499-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato.

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