Soil depth

Soil depth refers to the depth of the topsoil plus subsoil and can be easily determined with a shovel. Soils can be classified as being deep or shallow as follows:

	Depth (Topsoil + Subsoil)
Deep soils	90 cm +
Moderately deep	50-90 cm
Shallow	25-50 cm
Very Shallow	Less than 25 cm

Actual vs. Usable Depth: There's often a big difference between actual depth and usable depth, because the factors listed below can also limit root penetration:

• Excessive subsoil compaction.

• Hardpans and claypans ( explained in the drainage section).

• Poor drainage.

• Excessive subsoil acidity (very low pH).

• Potential rooting depth of the crop itself; some are naturally much deeper-rooted than others. (See Table 5-1 in Chapter 5 on water management.)

• Overly shallow watering can restrict depth of roots, since they will not grow into dry soil.

The Value of Deep Rooting

Deep rooting isn't necessarily essential for good crop yields. Some shallow soils can produce excellent yields if well managed. However, there are benefits to encouraging deep rooting:

• Better drought tolerance.

• Better nutrient uptake since the roots explore more soil.

• In irrigated crops, deeper rooting allows more water to be applied per application and more time between waterings. This can be very helpful in areas where farmers use furrow irrigation and receive water from the main ditch on an erratic schedule.

How to Encourage Deeper Rooting

• Use raised beds or ridges since they actually increase soil depth and provide a double layer of topsoil. However, they're not suited to dry conditions, because they dry out too fast.

• Double-digging will help encourage root growth into previously uninviting subsoil.

• Avoid overly shallow watering; this is most likely to occur on clayey soils because of their high water-holding capacity.

• Fertilizer use will stimulate deeper rooting.