Monday, February 22, 2010

Learn something new from a soil analysis

Because students are working to expand the community organic garden to take up most of the space within the fenced area at the arboretum, we mailed off soil samples to the Timberleaf Soil people in California. The test called for a general application of colloidal rock phosphate to build the phosphorous reserve, along with various specific quantities of alfalfa meal depending on what crop we intend to plant. For example, 4.1 lbs. of alfalfa meal per 100 sq. feet is recommended for cabbage plants, but only 3.4 lbs/100 sq. feet for kale and 1.8 lbs/100 sq. feet for spinach. Alfalfa meal supplies nitrogen to the soil for plant uptake, but not all plants require the same levels of nitrogen in the soil. According to the Timberleaf soil analysis, nitrogen mineral recommendations are the most challenging due to the high degree of variability in soil types, climate, time of the year, soil moisture, biological activity, type of organic matter applied and the plant varieties. The best course of action is to keep records of the type and amount of nitrogen applied, the crop we plant, weather conditions, and the time of year applied. That way, you can compare results from various growing seasons over time and judge for yourself what your average nitrogen requirements are.
We are low in trace minerals zinc, manganese, iron, copper, and boron. In order to remedy this, the analysis recommends applying, per 100 sq. feet, 1.5 oz. of 36% zinc sulfate, 4.7 oz. of manganese sulfate, 12.9 oz. of iron sulfate, .7 oz. of copper sulfate, and 1 oz. 14.9% boron. Because boron can be toxic in larger quantities, it is important to pay attention to the measurements and split applications several months apart. Further ammendments include 7 oz. of potassium sulfate per 100 sq. feet and 2.5 oz. of 90-95% sulfur.

It will take an estimated 3-4 years to reach balanced soil fertility.
The organic matter content of the red clay under the grassy patch is only 2.1%. The desired organic matter content is 5-6%, so we will apply a healthy measure of compost prior to planting. The soil is acidic, with a pH of 5.5 and a buffer pH of 6.8

The soil test taught students about Cation Exchange Capacity (CEC). Cations, which include calcium, magnesium, potassium, and sodium, are all positively charged, or basic minerals. The CEC tells us the quantity of cation nutrients the soil can hold. Good soil structure, or "tilth", means exceptional capactiy to hold, store, and release nutrients in a biologically available way. Good soil acts like a sponge for air, water, and nutrients!
So, soil contains many clay and organic particles in it. These soil particles have many negatively charged sites on their surfaces, which attract the positively charged mineral cations, holding them loosely to the surface of the particles. The roots of plants can exchange hydrogen ions in return for these cations. The CEC tells us the sum total of negatively charged sites in the soil particles and express it as a number - so in theory, a 20 CEC soil can hold twice the nutrients of a 10 CEC soil. We have a CEC of 4.2, so the way to increase that is by adding compost, thereby integrating more organic material into the soil.

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