All the steps outlined below should be conducted in the spring when you are planning and planting. That's not to say they can't be done throughout the year but Spring of course is best. Identifying a defect in your soil in mid summer, once your plants have gone through their active growth spurts of spring is like closing the barn door after the animals have escaped.
1. Soil Structure
Dig a hole 6 to 8 inches deep in order to extract an intact section, breaking it apart by hand will help determine what soil type you are working with. Ideally, the soil should be comprised of various sized crumbs that will retain shape under light pressure. If it takes too much pressure to break them apart, your soil is too hard. If they compress as opposed to crumbling your soil has too much clay.
Soil that is too hard is generally lacking in organic matter. The moist important part of soil structure is organic matter. Open, porous soils do not inhibit the free movement of moisture and and allow for aeration of the soil allowing the plants to develop a healthy productive root system. This is why you'll frequently hear advice that you should amend your soil with organic matter or well rotted compost.
Soil compaction is the result of pores being squeezed out of the soil usually due to pressure applied to the soil surface but particularly when the soil structure is poor to begin with. Without sufficient porous soil plant roots develop poorly, largely due to inadequate aeration and moisture circulation. Fertilizer does not penetrate to where the roots can absorb it, Earth Worms and other beneficial organisms are unable to move through the soil uninhibited further contributing to the degradation of the soil.
Professionals will check for compaction with a tool designed specifically for that purpose - a compaction tester. You can devise your own simple compaction tester with a a metal coat hanger and snips. Cut off the straight section at the base of the hangar, as a long a section as possible. Plunge this section into the ground, should you hit any obstructions such as rocks or tree roots try again. Mark the depth at which the wire begins to bend, the sooner it bends, the more compacted your soil is. At least a foot of easily penetrable soil is optimal.
3. Soil Organisms
Determining the natural subterranean insect activity in yor soil is a no brainer. Simply dig up a few clusters of soil here and there, you should be able to find some earth worms, as well as other unwanted buggers such as grubs, centipedes, ground beetles and even some fungi. If your soil is devoid of organic and insect activity , chances are it won't be to the liking of your plants either.
A thriving population of diverse bacteria and fungi, insects, grubs and worms is one of the more easily visible signs of soil quality. Earthworms are one of the best signs you can find in your garden soil. No earth worms means the soil does not have enough of the organic matter upon which they feed. Earthworms also enrich the soil, earthworm casting are basically a chem trail of rich fertile organic matter laced with beneficial organic material and plant nutrients. Earth worm activity aerates the soil as well, which will increase water infiltration.
4. Root development
Root health of plants already growing in the garden area is a good sign of the soil structure. Pull up a plant or even a weed and check the root development. You should find fine hair like strands which are signs of a healthy plant.
Soft or discolored browning roots are indicative of poorly drained soil, possibly due to compaction or clay content. Poorly developed roots or stunted retarded root development can indicate nematodes or other root feeding pests.
5. Water Infiltration
Testing the water infiltration of your soil can be done in a number of ways. The most obvious way is to soak you soil thoroughly and see how long it takes to seep into the ground. If you have standing puddles at low points for any substantial length of time you have poor water infiltration. Water sitting on the surface and not properly infiltrating the soil means the roots are not getting it, and it is also conducive to the development of harmful bacteria and fungi.
You can also test water infiltration with an empty coffee can. Remove the bottom and push it into the ground allowing several inches to remain above the surface.
Fill the can with water and mark the water level, then gauge how long it takes for the water to drain into the soil. Repeat this a few time and the absorption rate should slow as the soil becomes saturated. When the soil is saturated and absorption rate becomes consistent anything slower than 3/4 inch per hour indicates poor water infiltration due to soil compaction.
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