Wednesday, September 24, 2014

Clay Soil Amendments for Durham County: the Recommended Materials

Dried manure from livestock offers great soil amending
properties, but surprisingly not much in the way of fertilizer.
Durham gardeners quickly learn that heavy clay soil needs amending to properly grow every plant from vegetables to landscaping hardwoods. But what type of materials are best? Commercial nurseries and big box home improvement stores stock bags labeled "soil amendment," however, are these products really effective or just another marketing scheme?
Here are the top recommended soil amendments from the NC Cooperative Extension Office and considerations about other commonly (and mistakenly) used materials.

Compost (Humus)
Compost, or humus, is decomposed plant material. It makes an excellent organic amendment for clay soils. Almost, any plant-derived material will make good compost. Your lawn, your trees, and your household are excellent sources of FREE raw materials. Creating compost from them takes just a little collecting and a few months' time. 
  1. From your lawn and garden: save yard waste, grass clippings, spent annuals, etc. 
  2. From your trees: save raked-up leaves in fall, and grind them up by making several passes over them: with the lawn mower. Leaves will decompose much more rapidly when ground up. 
  3. From your household: save vegetable peelings, canning wastes, coffee grounds, etc. (but not cooked food or animal by-products). 
“Cold” composting simply means piling your organic debris somewhere and letting nature take its time decomposing it…takes a year or two, depending on the materials in the pile. Putting mulch on your beds and letting it decompose there is a simple form or cold composting. 

"Hot" composting means building a pile that contains both nitrogen-rich and carbon-rich materials, keeping it moist, and turning it regularly to encourage the microbes that carry out decomposition. The pile heats up from the microbes’ activity, and the elevated temperature is usually hot enough to kill weed seeds and disease pathogens that may be in the pile. 
Get Your Compost!
  • Start a compost pile. Your environment, your plants, and your landfill will all benefit. Compost is one of the best soil amendments available and it’s free.  Detailed information on Composting is available from the Durham County Extension Center.
  • Municipal Compost & Mulch. Call the Solid Waste Management office of the City of Durham and ask about free compost and mulch made from shredded and decomposed yard waste. The City’s material varies - if you can, tell what individual pieces used to be, it's still too fresh to be used as a soil amendment, but can be used as mulch.
  • Purchased Compost (Humus). Few homeowners have sufficient quantities or compost to amend heavy clay soil. Compost can be bought in bags at gardens centers, or by the truckload from companies that sell mulch or topsoil. 

Pine Bark Soil Conditioner
Finely- ground nuggets sold as "pine bark mulch" is an excellent soil amendment, and has the advantage of being a native and renewable resource. A pea size grind (1/4 to ½ is ideal.  The nuggets sold as “pine bark mulch” are too big.  Brands and names of products vary, so look before you buy. 
Pine bark soil conditioner is available in bags or by the truckload. Look in the Yellow Pages under “Soils.”  Ask suppliers to describe what they sell, as some places mix it with soil. Straight pine bark is a better deal.  There are other products on the market call “soil conditioner” but many are too fine to provide the needed pore space.  

Composted Manure
Well-aged manure is an excellent soil amendment material that provides some minor nutrients as well, though its fertilizing capacity is often over-estimated, most analyze at about 1-1-1 .
Some examples:   





Cow Manure, dried  




Hen Manure, fresh




Horse Manure, fresh




Once it is sufficiently composted, manure has no ‘barnyard smell.’  A load or fresh manure in your driveway, however, may raise some concerns among your neighbors.   

Gravel is useful as a soil amendment for improving drainage. The best size is a pea gravel called “78". Which is about 3; 8" diameter. It is a permanent soil addition that does not' break down. It does not, or course, add nutrients to the soil; you still need organic material for that. As an added bonus, sharp gravel in the soil seems to deter tunneling moles and voles.  A similar material is expanded shale, sold under brand names such as "Perma-Till." Expanded shale is lighter weight than gravel, but more expensive. 
Putting gravel in the bottom of a planting hole is NOT a good way to improve drainage in poorly - drained clay soils. A planting hole in such soils can form a "bathtub without a drain," and gravel at the bottom will not drain the water. It is better to incorporate amendments into the soil, and raise the level of the bed above the existing soil.
NOT Recommended:  Peat, Moss
Peat moss is acceptable for houseplants, for starting seeds, and for amending sandy soils: these applications put peat's water-retention characteristics to good use. Peat moss does not perform well as a soil amendment in clay; it first turn; the bed into a soggy bog, and then decays rapidly, leaving the soil as sticky as when you started. 
NOT Recommended: Sand
Sand is not a good amendment for clay soils. Any mixture less than 70% sand in 30% clay actually packs more densely that straight clay. This makes a readily compactable soil that isn’t fun to garden in. Add a bit of water and make your own bricks. 
NOT Recommended: Gypsum
Gypsum "clay buster" sold in garden centers is useful in alkaline clay soils, but is not effective on our type of clay.
NOT Recommended: Fresh Manure
Fresh animal manure (in addition to being fragrant) is too salty to use near plants. It will dry out roots and cause burned edges on the leaves. Compost it until it no longer smells like the barnyard; once decomposed it makes an excellent soil amendment. Manure is also a good nitrogen source in the compost pile, to offset higher-carbon materials like dried leaves and plant stems. Don't use human waste or pet wastes, as these can transmit diseases to humans. Cow, horse, rabbit and chicken manure are fine. 
NOT Recommended: Fresh Wood Chips or Sawdust
Wood chips can take years to decompose, and wood needs a lot of additional nitrogen to balance its high carbon content. If wood chips or sawdust are decomposing in your garden soil, they will take nitrogen from the soil to the detriment of your plants. 
  • Hardwood chips break down faster than pine chips. Loads of wood chips from the city or from tree cutting crews may contain large log chunks that will break down even more slowly than chips. Some types of wood may raise soil pH undesirably. 
  • Wood chips are sometimes used as barn bedding for farm animals; the manure mixed in will provide additional nitrogen for decomposition. (Owners of horse stables may be glad to have you take their barn waste away, but don't till it into your garden until it's well rotted.) 
  • Decomposed sawdust is a good soil amendment. 
  • Of course, undecomposed materials can be used on top of the soil as mulch.
MAYBE: "Topsoil"
Purchased topsoil is not necessarily better than amending your native soil with organic materials Sometimes the "topsoil" you purchase is not much different from your existing soil; it's just not compacted yet. Sometimes it contains too much sand. Unsterilized topsoil may contain weed seeds. 
Topsoil is offered in many formulations; you can sometimes request a topsoil mixture with extra organic material added. Purchased topsoil is fine for raised beds, though the caveats above still stand. Ask suppliers to describe what they sell. Better yet, visit the yard yourself. 

Whether you use purchased topsoil or make your own amended soil, be sure to mix the new materials thoroughly with the native soil. If new soil is just spread over the existing so 1, plants will not root into the clay underneath, and the plants will dry out in hot weather. 

It's worth the work to amend your soil!
Durham's heavy clay soil CAN become good soil. Adding organic material and relieving compaction makes a world of difference to your plants.

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