Although there are many factors contributing to successful farming operations, any farmer or gardener will tell you one of the most important elements is the soil. The soil is the base for all plant life. In addition to being the anchor matter for the plant roots, it also provides water and nutrients for the plant. Plants grown in good soil will perform better and experience fewer problems with insects and disease.
Soil by definition is made up of sand, silt and clay particles derived from rock broken down over thousands of years by climatic and environmental conditions (rain, glaciers, wind, rivers, animals, etc). Sand particles are defined as particles between 2.00 – 0.05 mm in diameter (USDA) and they feel gritty in your fingers. Silt particles are particles that are between 0.05 – 0.002 mm (USDA) and feel similar to flour. Clay particles are particles smaller than 0.002 mm (USDA) and feel sticky in your fingers when wet and clump to the point that you can not see an individual particle without a microscope. These particles are derived from a number of different types of rock so defining something as sand, silt or clay is about sizing the particles, not determining their original source. The amount of each of these components characterizes your soil. For instance, if you have a lot of sand, your soil will drain quickly and in contrast if you have a lot of clay particles, your soil will often be compact and retain water.
In addition to the sand, silt and clay, you will also find nutrients, organic matter (decaying plant and animal material), and pore space (open space that holds air and water) in soil. These characteristics also impact the growing conditions for your plant. Just like people need vitamins, plants need certain nutrients for proper growth and development. The available nutrients affect plant growth below and above the ground and especially impact fruit production. Organic matter influences nutrient and pore space content. As plants and animals decay they release additional nutrients and create new pore space. The pore space is important to soil structure (the arrangement of the particles in relationship to each other). In an optimal situation about 50 % of the volume of the soil is pore space with half of that filled with water and half filled with air (the other 50% is the sand, silt, clay and organic matter). Roots need air as much as they need water and the plant can actually suffocate or drown if completely emerged in water for extended periods of time.
What is the best kind of soil? Well there is no such thing as the perfect soil, but there is a perfect soil for a particular plant. Each plant likes different conditions. In general, common garden plants prefer a well-draining loam (a soil that is composed of approximately 40 percent sand, 40 percent silt, and 20 percent clay with plenty of organic matter and ample pore space) however there are plants that grow better in sandy conditions and others that grow well in compact, clay soils.
Farmers begin by analyzing their soil and then determine the best crops to grow. Although soil can be amended and improved (adding organic matter is common practice to improve drainage and nutrient content) on a large scale it is best to pick out a crop that will grow well in the soil available to you. With your school farm, you may have more control over the soil you use (especially if you are using container gardens, raised bed gardens or indoor gardens), but the following activities will help you to practice analyzing soils and discussing different soil properties including soil texture, drainage and nutrient content.
For more background information on soil visit:
- NRCS Soil Website: http://soils.usda.gov/
- NASA’s Soil Science Education Home Page: http://soil.gsfc.nasa.gov/index.html
Soil Texture: Soil texture is the way soil feels and it is determined by the amount of sand, silt and clay particles present. Here are two activities to determine the particle make up of your soil. If you are starting an outdoor in-ground or raised-bed garden, try the activities using the soil from the prospective plot.
If you are going to create container gardens or indoor gardens, most likely you will be using a soilless potting mix (these mixes are usually made from peat moss, vermiculate or perlite and are called soilless mixes because they do not include sand, silt or clay), so ask students to bring in soil samples from home or use soil from your playground area.
Ribbon Test: Take a small clump of soil and add water until it makes a moist ball. Next rub the soil together between your fingers. If the soil makes a nice, long ribbon, then it has a lot of clay in it (thus sticks together well). If it crumbles in your hand, then it has a lot of sand. If it is somewhere in between, then you probably have a good mix (a soil with a good mix of all 3 components is called a loam). Although this test does not give you an exact percentage of each component, it provides a general description and it can be used in the field due to the ease of implementation (all you need is a little water).
Shake It Up: Invite students to further explore different soil components by creating “mudshakes” and watching components settle out. For each soil sample, have students fill a clear plastic container about two-thirds full of water, then add enough soil to nearly fill it to the top. Also add a pinch of laundry detergent to help the soil components separate well. Shake the container vigorously then observe it over the next couple of days as the particles settle into layers. Ask students to hypothesize about the composition of the different layers. The larger particles (sand) are heaviest and will settle at the bottom, followed by silt with the last full layer being clay. The clay may stay suspended and cloud the water for a long time. Organic matter will float on or just below the water surface.
Once the container has settled, compare the results to your ribbon test. How do they compare? Measure the height of each layer and than translate that into percentages for each component (height of each component divided by height of the sample). Use the Shake It Up worksheet to help with your evaluation.
Soil Drainage: Soil drainage is a critical factor when determining good crops for a particular site. Although having water available is certainly important, too much water causes the plants to suffocate and also promotes many fungal diseases. Although there are some plants that grow well in boggy soils, most production crops (vegetables, row crops, fruit trees) need good to excellent drainage. Below are two experiments to test the drainage of soil or potting mix for your prospective school farm.
For an Outdoor Garden: Dig a 12-18 inch deep hole in the proposed location for your school farm project (a post hole digger will work well). Fill the hole with water. If the water drains within a few hours, then the drainage is excellent, if it empties within 24 hours, then the drainage is acceptable and if it takes longer than that, then you have poor drainage.
For Indoor, Raised Bed or Container Gardens: Obtain a collection of different types of soils and potting mixes from local garden centers and landscape supply companies. When building raised beds, you usually order garden soil by the truck load from landscape supply companies. Many times the companies will have multiple blends of soil with varying amounts of compost and organic matter for you to choose from. Check to see if they will donate samples of the different types of soil for you to test. Indoor and container gardens usually use soilless potting mixes or garden soil amended with peat moss. A variety of types of potting soils are available at all garden centers.
Fill 6-inch, plastic pots (make sure they have drainage holes) with the different types of soil and potting mixes you were able to obtain. Additionally, fill one pot with sand to use for comparison. Add water to your pots until it emerges from the drainage holes. This is a sign that the soils are completely saturated. Wait 30 minutes to make sure all excess water has drained.
One at a time, hold each pot over a plastic tray or clear plastic bowl (the tray or bowl needs to be able to hold up to 1 cup of water). Measure out 1 cup of water and slowly pour it over each pot of soil. Record the amount of time it takes before water begins to emerge from the drainage holes. Continue to hold the pot over the tray or bowl until it stops dripping and then measure the amount of water in the tray or bowl (pour back into the 1 cup measuring cup).
Use the Drainage Experiment Worksheet to collect the data and ask students to hypothesize what this means about drainage. The faster the water began to drain, the better drainage the soil or potting mix possesses. Also the closer the end water measurement is to the original 1 cup of water, the better the drainage of the soil or potting mix.
Generally you want your soil to hold moisture, but not stay too wet. Based on this information, what soil or mix do they think is best? For further exploration, give the students a chance to feel each pot of soil and record whether or not it feels wet or moist. An additional method to test moisture is available by using a moisture meter which uses sensors to detect water levels (many garden catalogs have these available or you may be able to borrow one from a local gardener).
Nutrient Content: Plants receive nutrients by absorbing them through their roots. The soil nutrients come from decaying plant and animal matter. As they decompose, the nutrients are released into useable form into the soil.
Plants have 6 macronutrients (nutrients they need in large quantities: nitrogen, phosphorus, potassium, sulphur, calcium and magnesium) and 8 micronutrients (nutrients they need in small quantities: iron, zinc, copper, molybdenum, boron, manganese, chlorine and nickel) essential to their growth and development.
Obtain a do-it-yourself soil test kit. These kits are available at local garden stores and from garden catalogs. Using your prospective soil, test the nutrient content of your soil (most kits only test for pH and the big 3 nutrients: nitrogen, phosphorus and potassium). Write the results of each test on the chalkboard.
If you are planting an indoor garden, you can use these tests on your chosen mix too, however some mixes contain slow release fertilizers which will not read properly. For soilless potting mixes, a better option is to check the bag for nutrient content and if you do not find information about it, contact the company for the details. Do not be surprised to find out your soilless mix does not contain any nutrients. Often times this is the case so the grower can have complete control over the amount and timing of nutrients supplied. So if you are using a potting mix, you may want to practice soil testing using a sample from your playground.
What do the results say about your soil’s nutrient content? Does your soil have the nutrients needed for healthy plant growth? Will you need to supplement your soil with additional nutrients through fertilizer or compost?
Do-it-yourself soil kits vary in their accuracy. For more accurate results, obtain a soil test kit from your local Cooperative Extension office or from http://www.css.cornell.edu/soiltest/newindex.asp and send a test sample to the Soil Nutrient Analysis Lab. Share the results with your class and compare to the results from the do-it-yourself kit. How do the results compare? How reliable were the do-it-yourself soil test kits?
For Further Study: The Natural Resources Conservation Service (NRCS) has developed soil surveys which include soil maps and other information for farmers and ranchers to help with land use and management. Originally these surveys were published in hard copy by the U.S. Department of Agriculture and could be obtained from your state or local NRCS office. To ease distribution, these surveys are being added to the NRCS Web site or distributed on CD. Check with your local NRCS office or http://soils.usda.gov/survey/printed_surveys/ to determine the availability of your local soil survey.