The Relationship Between Soil & Water in the High Desert

by MOLLY McDOWELL DUNSTON
of North of South Landscapes

A successful garden is not based on any singular component or attribute of the garden, it is a combination of the many factors that came together to create the garden. It could be the plant stock, seed variety, specialty fertilizer, the perfect exposure of morning sun and afternoon shade, or the aged manure compost amended soil, or just luck. There are so many factors that can influence the success of a garden or landscape that it can be overwhelming to take all of them into consideration so we will focus on the two most important factors: soil and water.

Soil can be thought of as the fuel tank for the landscape; it holds the water and nutrients that your garden and landscape need to keep grow a healthy and beautiful landscape. Just like with a vehicle, the fuel tank needs to be replenished before it is completely depleted. In order to refill your fuel tank, it is important to know what kind of fuel is needed and how big the tank is. Gasoline can be expensive and users avoid spilling or overfilling their tank. Irrigation water should be thought of in the same way, a valuable resource, which needs to be measured and monitored in order to know that what should be applied.

Soil is a mixture of clay, rock particles, and organic matter. The typical soil of the high desert has a very low organic material content, a low percentage of clay and silt with a higher percentage of sand. Each of the components in soil interacts with water differently, clay hangs on to water tightly while sand retains very little water. Also, the soil profile of the high desert is very shallow as the bedrock is often close to the surface. The soil in the high desert is very different compared to a lower lying floodplain soil. So what happens to high desert soil when they are over-irrigated?

With the shallow soil profile, the depth of the soil often does not have the capacity to hold the water applied to it. The soil can quickly become saturated and reach field-capacity, water will move past the plants’ root layer and not be available for uptake. At this point, the irrigation water will percolate through the bedrock and does not serve the landscape. Depending on the cost of the irrigation water, the cost of the power to move the water, and the cost of the nutrients applied to the soil that are washed through, the over-application of irrigation is expensive on multiple levels. Over irrigation can also cause runoff and other environmental impacts that can create hazards to neighboring streets and properties.

Also consider the over-irrigation of desert soils is the introduction of excess water to an environment. It can create ideal conditions for pest and disease that are not often found in a desert environment. When pest and disease are found in the garden and landscapes they are often treated with pesticides, fungicides, and or herbicides to control the issue. Altering the irrigation application and schedule to avoid overwatering can often alleviate a fungus, disease, or insect problem in the landscape and it is the more cost effective and environmentally responsible way to address the issue. Application of chemicals can often affect the native species of plants and insects that are beneficial and necessary for a balanced, healthy environment.

An understanding of the porous soil of the High Desert is critical to garden and landscape success in Central Oregon. Irrigation practices utilized in other regions with deeper soil profiles, and higher organic matter and clay content that can handle yearly rainfalls of 30 inches per year are ideal for deep and infrequent watering to develop strong roots. The rocky soils and shallow bedrock of the Bend area prevent that practice from being successful here. When referring to lawn care, many folks lean towards the deep and infrequent watering cycles that work in most other parts of the country. However, here in Central Oregon less frequent, longer irrigation run times do not provide the desired results that most homeowners are seeking. Lawns will often die out when overwatered and moss and other weeds will invade the area. It is important to maintain the sprinklers that irrigate lawns and landscape. Aging systems, winter snow removal activities, and our frequent freeze thaw cycles cause sprinklers to sink, raise, or tilt and can become blocked by foliage. When sprinklers are out of adjustment, the distribution uniformity of the application becomes skewed. Distribution uniformity can also be affected by a clogged or blocked nozzle. A tilted sprinkler will apply more water to one end of its performance arc than the other end. In this scenario, irrigation run times are turned up to apply more water to the area that is drying out and the other area is being over irrigated. After time an altered environment can develop that may be suitable to pests or diseases to move in. By adjusting a tilted sprinkler the water is applied as designed and will address the overwatering issue as well as the dry area. This simple fix will save water and prevent unnecessary applications of fungicides, herbicides or pesticides to the landscape.

One way to monitor the amount of water that is in your soil is with a soil moisture gauge. Soil moisture gauges can communicate with most irrigation controllers and will send a signal to tell the system when irrigation is needed. Agricultural irrigation systems use deficit irrigation which is similar to when the fuel light turns on indicating the tank is getting close to empty. When the fuel light turns on, there are 2 choices: either fuel up now or keep driving and run out of fuel. A deficit irrigation system allows the water to be depleted to the minimum amount of water needed by the plants before they reach permanent wilting point. By allowing the soil to almost completely dry between irrigation cycles the plants are forced to endure some stress that results in a stronger, more tolerant plant that can resist pest and disease. When plants are overwatered they can often become stunted in growth and susceptible to pest and disease easier.

Many irrigation systems are programmed to apply water based on evapotranspiration (ET). ET is the process by which water is transferred from the soil to the atmosphere from the soil surface and by transpiration from plants. Irrigation applications based on ET rates are very effective and is common practice in the irrigation industry. Monitoring the amount of water held in the soil will really take your irrigation to the next level. If the soil has been amended you’ll have improved water holding capacity in the soil and the ET rate might not be accurate for the irrigation system to irrigate for a given cycle. There are often rain events in Central Oregon that appear to have rained enough to suffice for the landscape but actually did not apply enough water and the soil is dry enough to warrant an irrigation cycle.

Over-irrigation can cause a multitude of issues in landscapes and gardens that can often appear as other issues. Reducing water applied through irrigation will reduce maintenance costs and avoid other costly issues that can affect plants and crops. Simple irrigation maintenance practices of adjusting sprinklers for proper alignment and allowing the sprinkler to apply water as designed will save water and improve the look and the health of the landscape. Fertilizer and nutrients will not be pushed down through the soil by the water and will be available for plant uptake. Success in the garden and landscape is achieved by finding the happy balance between all of the factors that influence plant health. Water and soil are the most important factors and a balanced relationship between the two is pivotal to the success of your garden and landscape.

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photos courtesy of North of South Landscapes

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