By Brent McGhie, Butte County Master Gardener, September 20, 2013
To grow, plants must absorb nutrients from the soil. Not only must these nutrients be present in sufficient quantities, they must also be available for uptake by the plant. Nutrient availability is primarily determined by soil texture and pH. Soil texture is the amount of sand, silt, clay and organic matter in the soil. Soil pH is a measurement of the degree of soil acidity or alkalinity; this measurement has the greatest effect on nutrient availability in the soil.
pH is measured using a scale from 0 -14, with 0 being the most acidic, 7 being neutral and 14 being the most alkaline. Most plants grow best in a pH range of 5.5 to 7.5 (medium acidity to very slightly alkaline). In this pH range, essential nutrients are available in chemical forms that plant roots can absorb. At higher or lower pH levels, some nutrients form solid precipitates that cannot be dissolved in water, and thus are not available for absorption by roots.
Soil pH can also affect the solubility of harmful mineral elements. For example, in very acidic soils (low pH) the essential elements of nitrogen, phosphorus, potassium, sulfur, calcium magnesium, and molybdenum may be unavailable, while toxic levels of aluminum and manganese might be present. Soils with high alkalinity can reduce availability of nitrogen, phosphorus, iron and manganese resulting in deficiencies, and they may contain excessive levels of soluble salts or sodium, both of which are harmful to plants.
When a soil’s pH is too low (acidic) it can be adjusted upward with the addition of wood ashes or lime. Some forms of lime that can be added to the soil include calcium carbonate, calcium hydroxide, calcium oxide, or dolomite. Sulfur is used to lower the pH of alkaline soils (those with pH above 7.5). Elemental sulfur, gypsum, ammonium sulfate, and Epsom salt are sources of sulfur normally available for purchase.
Here in Butte County, low soil pH is most likely to be encountered in the foothill areas including Paradise, Magalia, east Oroville, and Forbestown. There are limited areas on the valley floor near Gridley and Biggs, and on the south and west margins of Durham and Chico where more alkaline soil conditions (above pH 7.5) are found. If you feel that pH and nutrient availability may be an issue in your garden soil, it is a good idea to have the soil tested. Simple pH test kits are available in garden stores; alternatively, soil samples can be analyzed by commercial Agricultural Laboratories, which can be found listed in the yellow pages of the phone book under Laboratories-Analytical, or in online directories.
Soil texture affects the soil’s ability to retain water and nutrients. Generally speaking, soils with large amounts of clay or organic matter tend to hold water and nutrients more effectively than sandy soils. Sand does not hold nutrients very tightly, so as water drains through sandy soil, it tends to carry nutrients along with it. This process, known as leaching, carries nutrients out of the root zone and makes them unavailable to plants. Clay, on the other hand, has the ability to attract and hold nutrients in the soil and thus fewer nutrients are lost when water drains through clay soils. However, too much clay in the soil can cause other problems. For example, clay soils have a tendency to be waterlogged during wet weather and rock hard when they dry out.
If a soil has too much sand or clay, the most cost-effective remedy is to add organic matter. Organic matter in the soil mimics the positive effects of clay (improved water and nutrient retention) without the disadvantages. Organic materials are most effective when the soil is amended at least 30% by volume. The effect of adding organic material is not permanent because it decomposes with time. Coarse materials decompose more slowly and last longer in the soil. Also, organic material should be composted before incorporating it into the soil because microbes feeding on uncomposted material can actually remove important nutrients from the soil.