Over time, high tunnels tend to accumulate salts, which can drive up the alkalinity and pH of the soil. Acidifying irrigation water is one way to address this problem, and it can be done with varying degrees of investment and technology.
Understanding the problem
When we apply fertilizer, compost, and manure to our soil, we are often adding salts. Calcium, magnesium, potassium, sodium, and ammonium are all examples of salts that we commonly add to agricultural soils. Typically, acidic rain water washes these salts out of the soil to keep things balanced. However, in tunnels where we lack access to rain, this washing out process does not happen. Further, sometimes the groundwater we use to irrigate in tunnels is also rich in salts.
When salts build up in the soil, the alkalinity of our soil also increases. Alkalinity refers to the ability of water to neutralize acids and bases, and thus maintain a stable pH. Over time, this can lead to an increase in the pH of soil in high tunnels. This is problematic, because essential plant nutrients become less available when the soil pH moves above 7. Even if the pH of the soil is within the optimal range, irrigation water with a high pH can lead to decreased nutrient uptake, especially in tunnels.
We often see this process start to happen in year 3 or 4 after installing a tunnel, especially in tunnels where heavy loads of compost have been applied. Your plants might look amazing immediately after a heavy compost application, but over time as the pH creeps up, the higher pH can inhibit nutrient uptake. Sometimes this means that while the soil has very concentrated levels of nutrients, plants may still exhibit deficiency symptoms. Adding more nutrients will not fix the problem; rather, growers in this situation need to focus on bringing the pH back down and reducing the salt concentrations in the soil. Some plants are more tolerant to high salt levels than others; review the table below for a comparison of vegetables and their tolerance to salts.
Low-tech solutions to manage salts and pH
There are some simple ways to reduce salt build-up in your high tunnel soil:
Be careful about over-applying compost. While tunnels are high value environments and it’s tempting to add tons of compost, this can have negative effects on salinity in the long-term. Test your compost to make sure the salt levels (measured in electrical conductivity, but also check calcium, magnesium, potassium) are not excessive.
In years when you plan to replace the plastic, remove it in late summer and add the new plastic the following spring. The rain and snow should help to leach out some of the salts in the soil.
Some growers with access to bobcat-type machinery will shovel large quantities of snow into their tunnels in the winter
You can also flood your tunnel with irrigation water periodically if your water source is not very hard. For growers with very alkaline water, or high pH water, this process should either be done with water from elsewhere, or water that has been acidified.
Source: umn.edu
Photo Credit: GettyImages-ImagineGolf
Categories: Minnesota, Livestock