Monday, July 5, 2010

The Soil in which we Grow

By Nina Berryman
This spring at one of our shareholder orientation meetings a shareholder was asking about the land on which we grow. There are multiple ways to answer this question. There is the micro analysis of what our soil consists of, and there is the historical analysis of how the land has been used. I am more versed in the micro analysis of the soil and would like to learn more about the historical use of the land. If anyone has grandparents who grew up in the area and might know what this land looked like 70 years ago, I’d love to talk with them! What I do know about the history of the land is that since Saul High School has been leasing the land from Fairmount Park it has either been maintained as turf grass, planted in flowers, or planted in peach trees. At one point there were also bee hives in the area where our lower hoop house is. I can write in more detail about the land in which your vegetables grow on the micro level. The first year we tested for heavy metals to make sure the soil was not contaminated. Soil is considered safe if it has less than 150 parts per million. The soil in our field is 38.37 ppm. Because we know no contaminants have been added to the field since we started farming here a year ago, we only needed to test for heavy metals once. Every year, as soon as the ground thaws we take a soil sample to determine what and how much amendments we want to add. We sample in the main vegetable field, our hoop houses, and the corn/potato field across the street. To do this we dig about twelve holes, randomly scattered around the farm and take a soil sample from each hole. We then mix the samples together in a bucket, spread that composite out on newspapers to dry for twenty-four hours, and then take a one cup sample from that sample. This sample we send to Penn State Extension Soil Lab. This year the pH was 6.8. pH is measured from 0 to 14, with 1 being very acidic and 14 being very basic. For vegetable production slightly acidic soil is ideal, so we are right on target. Some plants prefer more acidic soils such as blueberries and potatoes. Having soil that is slightly acidic increases plants’ ability to take up nutrients that are available in the soil and reduces the plants ability to take up toxins that might be in the soil. Also, when the pH is right, beneficial organisms are most active. If you have soil that is too acidic, adding limestone is the most common way to raise the pH to a more desirable level. If you have soil that is too basic, you can add sulfur to lower the pH.
Cation exchange capacity (CEC) is another important part of the soil analysis which is another measurement of the soil’s capability of retaining and supplying nutrients. CEC can be thought of as the micro-level transfer of nutrients in the soil to the plants roots. Organic matter provides most of a soil’s CEC capability. Clay, also provides a portion of a soil’s CEC capacity. If you have very sandy soil with low organic matter the CEC will be low. We add compost to our vegetable beds to increase the organic matter content. According to UMass-Amherst Department of Plant and Soil, a “balanced” soil has a 70% Calcium saturation (ours is 76.7), a 12% Magnesium saturation (ours is 18.0), and a 4% Potassium saturation (ours is 5.3).
NPK is a key part of the soil test as well. N, P, and K are the periodic table of contents abbreviations for Nitrogen, Phosphorus and Potassium. In essence, this is the food for our plants. Plants also need a lot of other nutrients, like calcium and magnesium, but NPK are the nutrients which plants need the most of. Nitrogen is very variable in the soil and very little remains in the soil after the growing season. It is necessary for almost every aspect of plant growth. For these reasons we apply a fertilizer with a relatively high percentage of nitrogen (5%).
Phosphorus enables plants to use the energy that they create from photosynthesis. According to or soil test our field exceeds the plants’ needs for Phosphorus, so we use a fertilizer with only 1% Phosphorus content.
Potassium enables plants to utilize Nitrogen and water. Our field also exceeds the plants’ needs for Potassium so we use a fertilizer with 1% Potassium content.
One major part of our soil analysis that we don’t need a laboratory to test is the soil type; that is, whether it is sand, silt or clay, or a mixture. Sand, silt and clay are different categories of mineral particle sizes, with sand being the largest, then silt, and clay being the smallest. Oh, and there is the category of…extremely rocky! In my opinion, a soil can be considered rocky when it has particles that are large enough to get stuck in your tiller tines! Some parts of our field are rocky, some are very rocky. Between the rocks, our soil is on the sandy side.

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