Lesson 4 - Water
Soil properties, particularly texture and structure, strongly influence how water behaves in the soil – how it infiltrates, how it is retained in and how it drains through soil. Air and water in soil is held in the pore space between soil particles and aggregates, with the larger pores being mostly air and the smaller pores retaining more water. This means that soils that have a high proportion of large particles, such as sands, also have lots of larger, air-filled pores, meaning water drains freely through the soil and little water is retained after drainage. While in clays, where there are mainly smaller particles and mainly small pores, the opposite is true, the percentage of air-filled pores is much less than sands, so water drains more slowly and more water is retained. The total soil pore space, called porosity, is also much higher in clays than in sandy soils. The smaller clay particles fit more tightly together, creating lots of smaller spaces, which overall have a greater volume than in sands, meaning clay soil stores more water than sands. This characteristic of soil water is called water holding capacity. When rainfall saturates a soil and water drains freely there is a point when drainage stops and there is no further downward water movement, the soil now has maximum water available for plant use, we call this field capacity.
Knowing the differences between soils in terms of air and water also helps us understand aspects of drainage and when watering or irrigation might be needed. It also helps explain our aim to achieve a loamy soil, somewhere between the extremes of sand and clay soil. Loams have a balance of sand, clay and silt particles, meaning they have a good balance of aeration and water retention, making them ideal soils for growing.
For plants to access it, water first has to penetrate the soil through the soil surface. Water penetration into the soil can be problematic if the soil is compacted, if the soil surface has a crust causing water to pool on the surface or if the soil becomes resistant to wetting, effectively repelling water from soil entry. These problems of surface crusting, compaction and wettability can be found in quite different soil types, but the effect is the same – poor water infiltration and excessive drying of the soil. We discussed methods to alleviate these symptoms in Lesson 3 - Improve your soil
Then there is the issue of water movement through the soil profile after entry. The easier the soil is to dig, cultivate and till, the easier it is for water to penetrate. We call this permeability or infiltration rate. Sandy, open-textured soils have greater permeability than clay, finer-textured soils. This can be measured by the movement of a column of water (mm) per hour.
Plants also need air in the soil to function and grow properly, so we need water to drain through the soil and for sufficient pores to hold air as well. If soil retains too much water, then it becomes saturated or waterlogged. These low-oxygen conditions (anaerobic) result from having little air in the soil for plant roots to access and can give rise to other problems as well, particularly root-borne diseases.
We have already discussed methods of assisting drainage by improving soil properties, particularly clays, including additions of organic matter and sand, possibly gypsum, and by using raised beds. However, in some situations you may also need further methods to improve drainage. Installing physical barriers, surface drains or swales to intercept any stormwater before it reaches your garden is helpful, particularly in low-lying sites. Additional methods to improve drainage include trenching with mineral aggregate or using sub-surface drainage pipe. Please keep in mind that these require an end point where excess water can drain freely from the site.
Diggers tip
Test the drainage in your soil by digging a hole 20-30cm x 20-30cm deep with vertical sides, wet thoroughly and allow to drain, ideally overnight, then refill the hole with water and then record the time it takes for the hole to drain of water. Fast-draining soils drain within 10 mins, well-drained soils up to 30 mins, moderately drained soils up to 60 mins and poorly drained soils are those that take more than 60 mins to drain.
By contrast to poor soil drainage and waterlogging in the soil, we can also have conditions of low rainfall where water availability in the soil has dropped to a point where there is insufficient water for plant growth. We call this drought. Some soils, such as sands, are more susceptible to drought than soils that are able to retain a greater water reservoir in their profile. Similarly, all plants vary in their response to drought. Some large and thin-leaved species from cool, wet climates may be very sensitive and struggle after just a week or so of no rainfall. While succulent or small-leaved species from warm, dry climates may be able to tolerate many months of low water availability.
When water moves though a plant, it is transported from the roots to stems and leaves. But not all water taken up is used by the plant, some of it is lost to the atmosphere as water vapour, mostly through tiny pores on the leaf called stomata. This process of transpiration as it is called helps to cool the plant, in turn increasing humidity. Higher temperatures and lower humidity both raise transpiration, increasing water loss from the plant.
Water is lost from the soil in three ways – it evaporates from the soil, drains through the soil or is used by the plant. Plant water use includes transpiration, where water is drawn from the roots into the leaves and then moves out through the tiny pores of the stomata in the plant leaves to the surrounding atmosphere. The rate of transpiration increases with air temperature and wind speed and as air humidity drops. It’s on dry, hot, windy days that plants will use the most water, and supplementing that water through watering and irrigation is a task many gardeners are familiar with, particularly over summer!
Soil type and properties strongly influence water loss. Shallow, coarse, sandy soils that are highly water repellent and low in organic matter are particularly subject to ‘drought’ conditions. Of course, climate also plays a role in water loss, hot and dry conditions being the most dominant.
Plants can somewhat tell us they are suffering from water stress when they show varying degrees of wilting. Wilting is where plant stems sag and leaves droop or roll and is caused when the rate of transpiration through the leaves exceeds the rate at which water is taken up by the roots. It can be temporary, where plants recover their turgidity (water in the stems and leaves) when cooler conditions return, typically in the evening of a hot, dry day. But wilting can also be permanent, a situation where plants don’t recover after the heat subsides at the end of the day. To avoid plant damage or even death in some species supplementary water to the root zone of the plant is required, even then, some symptoms such as dead leaf edges or leaf defoliation may occur.
Knowing how and when to water is a challenge even for the most experienced gardeners. Watering effectively is all about knowing your soil, the plants you are growing and using practices that target the root zone, minimise water loss and are applied to maximise water uptake. As noted, ensuring that water can penetrate and infiltrate your soil is important to watering successfully. Once water is in the soil, methods that reduce evaporation and any runoff will be most effective. This includes slow, steady and infrequent hand watering, ensuring that water goes deep into the soil profile. Or highly efficient irrigation like drips or sprays that target water in the root zone over a long period.
Any watering is best done in the cooler part of the day, particularly early morning and evening. This assists water uptake by plants as transpiration will be less than during the sunnier and hotter periods of the day. Small things like making small wells or berms around each plant at planting will reduce surface runoff of water after application. Mulching is critical to reducing evaporation and improving soil water availability, we will discuss this more in the Module 3.
Diggers tip
Use a hose nozzle that emulates a soft shower. This enables you to water deeply and is gentle on your plants.