Soil Secrets

By Steve Reiners, Professor and Chair, Horticulture Section, School of Integrative Plant Science, Cornell University, Cornell AgriTech. This is part of a series of columns that he wrote about vegetable gardening during the pandemic.
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April 16, 2021

Last year, I wrote a column on how to prepare your soil for the upcoming year.  That got me thinking about the soil itself and the amazing things that go on beneath our feet.  And since the key to a good garden is a healthy soil, let’s start with understanding what soil is.

Soil can be divided into its four components - minerals, organic matter, water, and air.  The percentage of water and air in the soil is constantly changing.  After a heavy rain, water fills the air space or soil pores.  The more water, the more waterlogged the soil. 

As the soil dries out, air replaces the water in the soil.  A good soil drains well and does not remain waterlogged.  Without air in the soil, beneficial microbes and roots die.  How quickly a soil drains is related to the mineral type and organic matter components of the soil.

The mineral components

The mineral portion is composed of sand, silt, and clay.  Sand particles are the largest of the three.  They range in size from 1/50 to 1/500 of an inch.  This may sound very small. But in the context of soil, these particles are huge. 

A sandy soil drains quickly.  That’s great in a wet spring,but not so great in a dry summer.

Silt is smaller than sand and has a silky feel to it.  Sand and silt differ mostly in size. 

Clay particles are extremely small.  Clay also has a slight electrical charge, which enables it to grab on to nutrients in the soil and hold them for uptake by plant roots.  A soil high in clay drains slowly but can hold on to nutrients more effectively than sandy soils.

No soil is made up of only one of the components. Soils are a mix of these three.  A loam is a nice soil since it has a mixture of about 40% sand, 40% silt, and 20% clay.  It holds water and nutrients well due to the presence of clay while the sand ensures good drainage and plenty of oxygen for the roots.

The proportions of sand, silt and clay in your soil is referred to as the soil’s texture. What is the texture of your soil? Most of our soils in the Finger Lakes tend to be higher in silt and clay, making them loams. 

Within the mineral component of soil, we can also add the nutrients plants need.  There are 12 nutrients that plants absorb through their roots. Some are needed in large amounts, such as nitrogen, phosphorus and potassium. Some are needed in tiny amounts like boron and molybdenum.  Each is essential, whether needed in large or small quantities.

Benefits of organic matter

In addition to the mineral component of soils, perhaps the most important building block is the soil organic matter.  This usually makes up 1% to 4% of the soil.   The organic matter represents the accumulation of partially decayed plant and animal residues.  The material is constantly being consumed by soil microorganisms, which in turn also die and contribute to the organic matter. 

Organic matter is responsible for the loose, friable soil conditions we want.  It increases the amount of water a soil can hold (very valuable in a sandy soil) and improves drainage in a clay soil.  It helps soil form aggregates. These "crumbs" are groups of soil particles that are bound together and improve the soil's tilth. Organic matter serves as a slow-release source of nutrients needed by plants, and is the main source of food for soil microbes.  Take away the organic matter and you essentially sterilize the soil!

I mentioned that clay carries a slight negative electrical charge.  So does organic matter.  This ability allows soils high in both to attract positively charged essential nutrients like calcium, potassium and magnesium. The total charge is referred to as the soil’s Cation Exchange Capacity (CEC).  Soils with a high CEC retain nutrients and are less likely to see them leach away in heavy rain events, like they might in low CEC sandy soils.

Improving your soil

What can you do to improve your soil?  Try putting a 1-inch layer of compost on your soil every spring.  Limit the use of rototillers.  Although they can make fast work of soil prep, they can destroy soil structure, break down the soil aggregates and burn off organic matter. 

Add organic matter by growing a cover crop like clover or buckwheat in a garden bed and turn it into the soil.  Use organic mulches like straw that will slowly rot while keeping weeds to a minimum and conserving water.

If sand helps with drainage, why not add some sand to a soil to balance out the clay and improved drainage?  In theory, this does work but it is a bit impractical.  On a 10- x 10-foot garden, you would need 400 to 500 pounds of sand to notice a difference.  Focus on increasing organic matter for the best way to improve your garden.

Don't forget pH

The pH of a soil is a measure of the soil’s acidity. Most soils will have a pH between 5 and 8.  Vegetables do best in a soil with a pH of roughly 6 to 6.5, or just slightly acid.  What is so special about this pH?  This level represents the best compromise.  At that pH we have the greatest availability of all nutrients needed by plants.

Limestone is typically added when soil is acidic and needs to be raised. Sulfur can be added to reduce the  pH of soils that are above 6.5, or for plantings of acid-loving plants like blueberries.  Always have your soil pH checked with a soil test before adding either.  And know your soil type as the amount of lime or sulfur needed depends on it.

Learn more about your soil:

  • Visit the Cornell Garden-Based Learning website for information about how to take a soil sample and submit the test to Agro-One Services to learn about the nutrient levels, pH, organic matter content and texture of your soil.
  • Visit the Cornell Soil Health Lab website to learn more about their battery of tests that measure the biological as well as the physical and chemical properties of your soil.
  • Get the SoilWeb smartphone app.  It can sense your location, identify the soil type and provide you with information about the of soil you have, its texture, drainage, and best uses. (App Store | Google Play)

In the garden this week.

It’s wet and cold out there. Soil temperatures remain in the low 50’s.  You can continue to plant peas, spinach and Swiss chard and other cold-tolerant crops outside now. But don’t rush things.  Temperatures for the next week look seasonable but we’re not out of the woods for frost for at least another month.  Use the time to clean up your garden, take a seed inventory, and prepare your garden tools.