Wednesday, 7 January 2015

Soil Moisture Basics Defined

For this first post I will be introducing some basic soil and soil moisture principles. Many of the terms defined here will be used through the posts on this blog so here I hope to provide some background for those of you who are not yet familiar with them.

Soils are made up of mineral matter, organic matter, water and air. The space between the soil particles are referred to as pores, air and water occupy these pores. Macro pores allow water to filter through the soil and then drain out the bottom. Micro pores store water that is available for plants to grow. 

Soil texture is an important characteristic that influences water holding capacity, drainage characteristics and water infiltration rate. The finer the texture of the soil the greater volume of micro pores and therefore greater water holding capacity compared to coarser textured soils.

The total amount of water that a particular soil can store is the water holding capacity (WHC) of the soil. Coarse textured soils such as sandy and gravelly soils have a low WHC while silts and clays retain more water therefore have a higher WHC. WHC is usually expressed in miilimetres (similarly to rainfall) held per depth of soil e.g. Xmm/100mm.

The following terms define certain soil moisture characteristics:
Saturation – When all the macro and micro pores are full of water. If more water is added to a saturated soil it will either drain out the bottom, pond or run-off.
Field Capacity – Macro pores are full of air, micro pores are full of water. Silt and clay soils generally reach field capacity after 2-3 days of drainage from saturation, sandy and gravelly soils much faster. Field capacity may also be referred to as full point.
Stress Point – At this point the plant has to work to harvest the water from the soil, therefore plant growth is slowed and yield potential is reduced. The plant will survive beyond this point but will become increasingly stressed. Stress point is related to crop type, rooting depth and soil type. Stress point may also be referred to as trigger point or refill point.
Wilting Point – At this point although there is still water held in the soil the plant is not able to access it as it is held to tightly (hydroscopic water). The plant will therefore permanently wilt and die. Wilting point may also be referred to as permanent wilting point.
Water Holding Capacity (WHC) – Is a measure of the water that is extractable by plants. This can be calculated by taking the difference between the soil water at field capacity and at permanent wilting point. Water holding capacity may also be referred to as total available water or available water.
Readily Available Water (RAW) – Is a measure of the amount of water in the soil that supports optimum plant growth. This can be calculated by taking the difference between field capacity and stress point. As a general rule of thumb half of the WHC is readily available to the plant, therefore RAW = 0.5 x WHC.
Soil Infiltration Rate – Is the speed at which applied water can enter the soil. It is described as the millimetres depth of water infiltrated per hour (mm/hr).

Figure 1 below may help to illustrate the difference between saturation, field capacity and wilting point.

Figure 1
It is worth mentioning that while this theory is all very useful, nothing beats seeing like in the real world. So I’d encourage you the next time you’re doing a paddock walk to take a spade with you and locate what appears to be the driest and the wettest spots in a paddock. Dig a hole in these two spots and compare the soil type/texture, the depth of topsoil, depth of the roots and other obvious visual differences. Later posts will discuss how to use this information to help determine the soil water characteristics of your soils.

Posted by Sarah Elliot from Lindsay NZ