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The Practical Gardener


Archive for June, 2011


Soil structure – Improving your soil 0

Posted on June 08, 2011 by Paige
soil in sand box

Sand box soil in need of improvement - Under layers of sand and weed barrier cloth. Compacted and receiving no organic matter (no leaf drop etc.)

So you can’t improve your soil by changing the texture (see last post), what can you do?

Well, first I need to clarify exactly what most people want from their soil – what exactly is the goal of “soil improvement”? Well, in most instances the ideal soil has the following properties:

1) Well draining – After a soil is saturated, a  certain amount of the water drains out  quickly so that there is both air and water in the soil pore spaces. Roots need both water AND air to function. Water-logged soils often lead to root rots and plant death.

2) Good water holding capacity – Ah, sounds a bit tricky to have soil that both drains well AND holds on to water well but that’s what we’re looking for.

3) Good infiltration rates – This is how quickly water enters the soil.  You don’t have to stand there with the hose turned to “slow dribble” to prevent water from running off or lose your soil to erosion as it is entrained by water during a storm.

4) Good nutrient holding abilities – Plants need nutrients, they are held in storage in the soil. You need adequate “storage” facilities.

5) Easily penetrable by roots – Okay, if you have good drainage chances are roots will also be able to move through the soil fairly easily.

If your soil is lacking one or more of these properties, you probably want to improve it. How?

Granular peds

Granular soil structure (good). (Picture from NASA soil science website.)

Well, I established in the last post that you can’t change a soil’s texture, but you can change a soil’s structure. Soil structure is the way in which the individual mineral grains and organic matter in the soil join together to form clumps or aggregates. You want your soil to form aggregates because you end up with a variety of pore sizes. Little pores are good at holding onto water and larger ones drain quickly providing air pockets for the roots.

Soil structure is easily destroyed and slowly built (excessive tilling, particularly when wet, and compaction are two ways to destroy soil structure). Clay soils tend to form aggregates because the clay pieces in it have a lot of – are you ready for it? – unsatisfied surface charges and they stick together like little magnets. Organic matter is also a fabulous glue. Fungal filaments, and various secretions and excretions by things living in the soil stick soil particles together.

sand box soil

Soil loosened with fork demonstrating massive structure (not good)

Sometimes in the top layer of soil, most of the soil aggregates are actually worm castings (worm poop). The soil passes through the worm’s gut (along with the organic material it means to eat) and everything gets altered and glued together. So keeping adequate organic matter in your soil (about 4-10%) hugely benefits soil structure and improves your soil.

Don’t just willy-nilly add piles of organics to your soil, however. (See Dr. Linda Chalker-Scott’s discussion of this.) Too much organic material can actually be bad for the soil. Get a soil test before you act. A variety of labs are available to do soil tests. I send mine to UMass Amherst because it’s cheap.

surface clay layer

Soil had a surface layer of clay (pale) that is the remnant of former sod (mesh netting still in place). This layer will inhibit infiltration. I worked to incorporate it into the rest of the soil.

To summarize:

To improve your soil, disturb it as little as possible (minimize compaction and tilling), maintain adequate organic matter and preserve soil life – those earthworms and other soil organisms will do a lot of the work for improving your soil if you let them – so minimize herbicide/pesticide use.

Soil texture – can you change it? 0

Posted on June 03, 2011 by Paige

Texture triangle

You often hear that adding organic matter to your soil will improve its texture. WRONG. You can’t, practically speaking, change a soil’s texture. Period. Don’t even try. Read the rest of this entry →



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