Thursday, September 29, 2011

Too Expensive to Call it DIRT...

Seawall & Raising the Island - 1905
We've started constructing the beds in our new veggie garden and the emphasis is now on soil preparation. Galveston County is an interesting place when it comes to soils. We have everything from super-heavy clay to pure sand. That means there's not a one solution fits all approach. In our case it's pure sand - mixed in with a lot of !@#$. Since we're only a few blocks north of the Seawall much of our "soil" is actually fill, brought in to raise the island following the 1900 storm. Also, we have a good deal of oyster shell that was used as a base for the house that once sat on the garden site. Add in a bunch of crushed concrete, broken glass, bits and pieces of wood and other construction debris and you've got what might be called an "urban soil."

That means we'll be bringing in some pretty pricey soil or bed mix to get things going. We've decided to construct shallow raised beds and fill them with the bed mix - NOT incorporating (i.e. rototilling) this fill in with the existing soil. That would only dredge up a bunch of weed seed and add more sand to the mix. We don't need any more sand!  Over the next few months we'll add more organic matter/compost to get a little better water and nutrient holding capacity.

I've already been asked if we plan on having the soil tested. Soil testing is often recommended by horticulture professionals and while I agree that the results of a soil test can be informative - I just don't always agree with WHAT's being tested. 

For the most part, gardeners place way too much emphasis on a soil's CHEMICAL characteristics (i.e. nitrogen, phosphorus, potassium, etc.). Although things like nutrient content, pH and EC are important - it's actually the soil's PHYSICAL attributes that have a much greater influence on plant growth. That's because these properties control things like aeration, drainage and water holding capacity. All super important - ESPECIALY in our area.

One reason there's not more interest in evaluating the physical characteristics of garden soil is because there aren't any "straight-forward" tests to run. Also - unlike a chemical analysis, there's not a convenient place to send soil to have a physical analysis run. 

That being said - here are 3 quick little tests you can do yourself to get a handle on a soil's physical characteristics. 

First of all, take a close look at the soil. Determine if the primary content is sand, clay, organic matter, etc. Make an estimate of particle size distribution - important for aeration/drainage and water holding. Moisten a sample and rub it between your thumb and forefinger. This too will provide a "feel" for texture.

Hole Test:
One of the best ways to determine a soils aeration, drainage and water holding capacity is to conduct a “hole-test.” Here are the basic steps:

Using a post-hole digger, sharp shooter or other shovel, dig a hole 6” – 8” in diameter and 18" - 24" deep.

Fill the hole approximately ½ full with water. Note the time.

Determine how long it takes for the water to drain from the hole.

se the following guidelines to interpret results from the hole-test:
·        < 15 min = Excessive drainage. Consider adding organic matter to increase the soil’s water holding capacity.

·        15 – 30 min = Adequate drainage and water holding proper­ties. Modifications not required for planting trees, but adding organic matter will still benefit most landscape plantings for flowers and shrubs.

·        >30 min = Poor drainage. Consider raised beds or incorporat­ing coarse textured soil amendments (compost, bark mulch, expanded shale) to increase aeration and drainage.

Let's assume that, based on a hole test (or perhaps visual observation), it is determined that the addition of organic matter is desired. The next challenge is to determine what source of organic matter to use. Peat moss, bark, compost, sand, etc. Or - perhaps some combination of any or all of these? That's when the following evaluation might come in handy.

Evaluating Organic Soil Amendments:
Use duct tape to cover the drain holes on a 1 gallon nursery container. Next - fill the container with the compost or other organic matter (OM) to be evaluated. NOTE: leave about a 1" reservoir at the top of the container.

Starting with a FULL 1 gallon container of water, SLOWLY pour the water in to the container filled with OM. As water displaces the air in the OM you will see air bubbling from the surface. Continue pouring until water begins to "puddle" on the surface of the OM. 

Note approximately how many ounces of water have been poured in (A). 

Next - holding the container with OM over a large bowl, CAREFULLY remove the duct tape and collect all of the water that drains out. Don't squeeze or force water out. 

Note approximately how many ounces drained out (B).

A = the total pore space of the OM.

B = air space of the OM.

A - B = water holding capacity (C).

Example: Poured in 100 ounces. 50 drained out. Water holding capacity = 50%

OM suitable for amending most landscape/garden soils C = 30% - 50% of A.
More than 50% will tend to hold too much water - if it ever rains again.

Remember - these are only evaluation "guidelines" not hard and fast rules. More often than not the amount of organic matter added is determined by time, effort required and (of course) $$$. Amending a soil can be expensive and it's not one of the more glamorous aspects of gardening. But getting started with soil that has the correct physical attributes in an important first step for long term success.

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