Marin Master Gardeners
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Marin Master Gardeners

Marin IJ Articles

Know your soil before you plant

October 31, 2014
Wendy Irving

Many of us love the experience of autumn in the garden — a time to prepare for the coming cold months, but also to plant cool season vegetables, new shrubs and trees, and spring flowering bulbs. The days are warm, the nights are cool, and the fog disappears as the inland and coastal temperatures equalize — a wonderful time to be in the garden, with your hands deep in dirt. And your new plants will thank you for the valuable jump start on next year's growing season.

New planting in the fall, as at any time of the year, means knowing your soil. As a newly certified Marin Master Gardener, one of the first things I learned was the importance of a healthy, stable soil as the foundation, not only of my garden, but of land the world over. Without nurturing and protecting the soil, we gamble with our environment and the very quality of our existence.

But, back to my little patch of dirt. I learned that I need to know my type of soil, its pH, what kinds of plants it will support, or how I may need to amend it to grow the plants I love.

As with many of us in Marin, my garden soil is mostly clay, although I have amended it over the years with compost and mulch. Soil texture types are grouped into three categories: coarse (sandy soils), medium (loamy soils) and fine (clay soils). You can identify your soil by grabbing a handful when it is moist — clay soils feel smooth and will stick together firmly when you squeeze it. It retains more moisture and holds more nutrients than gritty, sandy soils, which will fall apart in your hand.

Some of the finest agricultural lands in the world are clay, amended with organic material (compost).

Home gardeners strive for a loamy soil, a combination of sand, silt and clay in roughly equal amounts. Loamy soil, with tiny pores between the particles, allows water and oxygen to go where they need to go, encouraging root growth while transporting essential nutrients through the soil and up into the plant.

Speaking of essential nutrients, easy-to-use home soil testing kits allow you to test your soil for key factors like pH, and the presence of three vital elements for plant health — nitrogen, phosphorus and potassium. A few simple steps give you a basic knowledge of your soil.

Soil pH measures the acidity or alkalinity of your soil, which is key to the effectiveness of the nutrients in your soil. While the scale is broader, most soils fall between 4.0 and 9.0. A neutral soil is 7.0, with lower numbers more acidic and higher numbers more alkaline. Most plants prefer a range from 5.5 to 7.5, a range in which nutrients and micro-organisms are abundant. The bulbs you will be planting now like a neutral soil at 7.0. Most cool- season vegetables, such as spinach, radishes and peas, prefer a slightly more acidic soil in the range of 6.0, as do many ornamentals. If your soil pH needs amending, fall and winter are good times to do it. Add lime to raise the pH and aluminum sulfate or sulfur to lower it.

If you need a more in-depth analysis of your soil, you can send samples to a lab for testing. Call the Master Gardener help desk at 473-4204 for more information on professional testing labs and home testing kits, as well as hints on how to best obtain your samples.

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