Most crop plants are adversely affected by shallow soils. Alfalfa is one that is especially sensitive to soil depth.
Alfalfa depends on its deep root system to supply water during dry periods. The roots of
alfalfa have been observed growing as deep as 30 feet. While it is unlikely that they need to grow that deep, alfalfa roots are capable of effectively utilizing eight feet or more of soil depth. Since water is such a critical factor, the following example is given to show how soil rooting depth can affect the amount of water available to an alfalfa crop. For this example, we will consider a typical silt loam soil following a good rain which saturated the soil. First of all, not all of the water held by the soil is available to the crop. The part that is available is called the available water capacity (AWC) of the soil. We will assume that the average AWC of our soil is 1.8 inches of water per foot of soil depth. If the soil were two feet deep, it would supply 3.6 inches of water to the growing crop. Based on a requirement of 5.5
inches of available water to produce a ton of alfalfa hay, this soil could produce only 0.65 tons of alfalfa hay per acre. If the soil were eight feet deep, it could supply over 14 inches of water to the crop. This soil could produce 2.5 tons of alfalfa hay per acre.
From this example, it is easy to see why a deep soil has a greater alfalfa yield potential than a shallow soil does. Fortunately, we usually get additional water supplied to a growing crop through rain or irrigation. This helps to reduce the adverse effects of the shallow soils low water supplying capacity. In low rainfall which lead to insufficient water supply, the benefits of a deep soil become apparent. Soil depth also affects the nutrient availability, but not to the same extent that it does water. This is due to the fact that nutrients are concentrated in the surface layer of the soil. still, there are nutrients in the deeper soil layers that are available to alfalfa roots. These may be secondary and minor elements which are needed only in small quantities. A deep soil gives the alfalfa roots a better chance of finding the nutrients they need.
Source: SOIL AND FERTILITY CONSIDERATIONS FOR GROWING ALFALFA
Monroe Rasnake, Extension Agronomy Specialist, University of Kentucky