Soil that has good physical characteristics, or good soil structure, is said to have good tilth. What this means is that air and water can come and go easily but there is always enough of each remaining in the soil to meet the needs of the soil organisms and the plants.
Soil with good tilth is easy to work with hand tools. Plant roots have an easy time growing through soil with good tilth. The soil becomes a moisture regulator - in times of drought it holds moisture, in times of heavy rain it manages to maintain enough air to prevent plants from drowning.
Soil with good structure tends to encourage water to penetrate deeply rather than run off so it greatly decreases erosion.
Part of the soil structure is determined by the parent material of the mineral component of the soil. Different particles bind together or aggregate in different ways. For example clay soil tend to make overlapping horizontal layers while sandy soils tend to be very granular. The spaces where air and water could live line up very differently in these two types of soils.
For the clay the layers don't line up, they are a maze that prevent water from freely flowing. The soil is also sticky so it is tough for these layers to shift. Thus many clay soils have a tendency to be poorly drained.
By contrast the sandy soil has almost no structure, the porous areas can shift very easily - too easily really - and water has a tendency to drain through very rapidly taking any fertilizer with it.
Cultivation and irrigation practices can have a negative effect on soil physical characteristics over time. For growing most plants we are looking for a soil that doesn't compact easily, allows water to penetrate easily while still holding moisture well.
Nine times out of 10 the best way to improve soil structure is with compost - it is the ultimate soil conditioner.
What makes compost unique is that it is more than just organic matter, it is an entire ecosystem. So when the compost community is introduced to your soil they put the site under construction.
The soil organisms aggregate the soil - something like the way we might build a house - using long strings of organic matter. To maintain structure that works requires constant activity on the part of the soil organisms and constant additions of organic matter.
In natural systems the organic matter comes naturally, in the garden it comes from you and I with the ideal input being compost.
The first trick with clay soil is to only work it when the moisture conditions are just right - too dry or too wet and it is verging on impossible. Just right is when you take a handful of soil and form it into a ball. It should keep its ball shape but easily break apart if you drop it or push your finger into it.
Work in lots of compost for these results.
• It improves the drainage while maintaining the water holding capacity of the soil
• It interrupts the aggregation of the clay particles so that the soil has a more granular structure
• It helps maintain the soil's porosity so that air and water can move freely through the soil
• It makes the sticky soil more friable or workable
• It reduces the bulk density of the soil helping it resist compaction
• It helps the roots penetrate the soil.
Sandy soils are less fussy about when they are worked than clays. Add your compost just about anytime.
• It greatly improves water retention by anywhere from 10 to 1000 times
• It helps keep fertilizers and nutrients in the root zone, prevents them from being leached into the ground water
• The humus helps bind together particles in the sandy soils to make a better aggregation.
Often when we have a problem in the garden or on the farm we look to soil tests to tell us about nutrient deficiencies in our soil. But research is showing that soil structure, especially the organic matter in the soil is at least as important.
Because organic matter enhances the water and nutrient holding capacity of the soil it has a beneficial effect on crops.
It has a huge potential benefit to coping with natural disasters that seem part of these times. Plants have a chance with both flood and drought.
As an added bonus the humus in the compost acts as a kind of carbon sink thus reducing CO2 levels that contribute to climate change.