Where do nutrients go when you irrigate?

Irrigation is essential for container production and is typically applied at least once daily during the peak growing season. Scheduling irrigation to avoid both over- and under-irrigation will improve productivity and keep nutrients in place. Under-irrigating plants can result in reduced growth, a longer production period, increased pest pressure on weakened plants, and plant death from desiccation. Since the visible symptoms of under-irrigating are quickly apparent (wilting, desiccation, death), irrigators tend to err on the side of over-irrigating. However, the consequences of over-irrigating are just as detrimental. Over-irrigation can cause reduced growth, a longer production period, increased pest pressure, poor plant quality and even death. Over-irrigation in combination with heavy fertilization can cause overly vigorous plants, also reducing plant quality and often resulting in higher pest pressure on the lush growth.

While under-irrigating cause inadequate water uptake due to a lack of availability, over-irrigation can cause inadequate water uptake due to anaerobic conditions resulting in loss of proper root function, although this is rare.

More commonly over-irrigation leaches nutrients from containers thus affecting plant nutrition, delaying flowering and reducing plant growth and quality. If irrigation water has high alkalinity, as many groundwater sources do, over-irrigating can further exacerbate nutrition problems by increasing substrate pH above the proper range for nutrient availability.

Over-irrigation combined with heavy fertilization to counteract high leaching can lead to even greater problems. Leached nutrients are not only a waste of money but can result in significant environmental problems that increase the probability of regulatory action. Eutrophication is the proliferation of biological organisms in aquatic systems due to excess nutrients, particularly phosphorus and nitrogen, which can cause serious economic and environmental damage. For example, periodically over the past decade toxic algal blooms have negatively affected the drinking water of nearly half a million people who rely on Lake Erie for their water source.

Water is highly undervalued in most areas of the U.S. but that is quickly changing. Over-irrigation wastes water, often relatively high quality water. Although some areas of the U.S. pay a substantial price for irrigation water, the cost of water for most irrigators is the cost to pump it from its source. Low cost is another factor that makes it easy to over-irrigate, however, there are hidden costs to water. Over-irrigation can increase fertilizer costs, although this is often minimal. Most importantly over-irrigation can result in a longer production cycle and all of the costs associated with growing the same crop over a longer period such as more labor, more pesticides, more fertilizer, land costs (fewer crop turns per year), interest, and longer time for return on investment.

Drilling down

Some important considerations to keep in mind when implementing leaner irrigation practices are the source water quality (especially soluble salts and alkalinity), substrate properties, and local rainfall patterns. Routine monitoring of substrate electrical conductivity (EC) for soluble salts and pH (as an indicator of the effect of water alkalinity) using methods such as the Pour-Thru method (Link 1) is essential when using lean irrigation practices. Water with high soluble salts may require periodic leaching if EC exceeds recommended values of 0.5 to 1.5 dSiemens/m (mmhos/cm). However, there is no need to continuously leach salts (fertilizers) from nursery crops, at least in the eastern U.S., unless there is a problem with high salts in the irrigation water. Relatively frequent rainfall during the production season will usually cause sufficient leaching of outdoor-grown container plants. If ECs are consistently high and water does not have high soluble salts it is possible that the fertilizer rate is too high, consider saving some money by backing down on fertilizer rates. Water with high alkalinity will slowly increase the pH of container substrates, possibly above recommended ranges of pH 4.7 to 6.5, depending on the crop and substrate, resulting in the need to apply sulfur compounds or acid-forming fertilizers to reduce substrate pH. In this case irrigating less, if possible, reduces the problem.

It is important to know how water is held in container substrates when deciding how much irrigation water to apply. Some important terms are:

• Substrate Volumetric Moisture Content (SVMC): the amount of water in a container based on volume of water divided by the volume of substrate.
• Container Capacity (CC): the maximum amount of water a container substrate will hold after gravitational drainage.

1. Typically 45 – 60% SVMC

• Unavailable Water (UAW): water that is tightly bound to the substrate and cannot be taken up by a plant.

1. Typically 25 – 35%

• Available Water (AW): the amount of water that can be taken up by a plant.

1. = Container Capacity – Unavailable Water

• Readily Available Water (RAW): the amount of water that can be easily taken up by a plant.

1. Typically the first 10-15% of water below container capacity

• Permanent Wilting Point: the point where a plant has extracted all of the available water and is not able to regain turgor.

These terms are usually expressed as a percent and are calculated as the volume of water in the substrate divided by the volume of the substrate times 100, more on how to determine that later.

It is good to know how much AW is in a container so that you don’t over-irrigate. Irrigating more than the AW will cause leaching because the container cannot hold more water than this under normal circumstances. To calculate how much AW a container can hold is pretty straightforward if you know the actual volume (not trade size) of the empty container (usually provided by the manufacturer), the percent moisture at container capacity and the percent unavailable water. The latter two values can be provided by a good substrate supplier or from a substrate analysis by a substrate/soil testing lab. The available water is the difference between these two percentages. It is easy to convert this to irrigation rate drip or spray stake emitters: multiply the AW times the container volume and divide that by the emitter application rate. For example, with a AW = 25% for a 3 gallon container and emitter application rate of 2 gallons per hour:

• (0.25 x 3 gal)/(2 gal per hour / 60 min per hour) = 22.5 minute run time

If you’re using overhead:

• acre-inch to apply = gallons AW x 231 / pr2
• acre-inch to apply = (0.25 x 3) x 231 / (3.14159 x 5.52) = 1.82
• 231 is the constant to convert gallons to cubic inches, 5.5 is the radius in inches of the 3-gallon pots we use.
• Multiply acre-inch by 27,154 to determine the gallons per acre.

It should be obvious that these values are too high for irrigating 3-gallon pots.

We want to avoid not just the permanent wilting point but wilting as well, so we need to irrigate somewhere in the RAW range. RAW differs by plant species and substrate properties. Fortunately we don’t want to come close to using all of the RAW either. All water in a substrate is not equally available to plants. As water is removed from a substrate the remaining water becomes more and more difficult for the plant to extract. This is a function of the type of substrate and the plant. A good target is to irrigate somewhere between 5 and 10 percent below CC. Calculating how much water to add to replace 5 or 10 percent water loss below CC uses the same equations as above. For example, a 3-gallon container allowed to dry to 7 percent below CC needs 0.21 gallons (0.07 x 3). For a drip or spray stake emitter with a 2 gallon per hour application rate you need to run for 7 minutes to replace 0.21 gallons: 0.21 / (2 / 60). If you’re using overhead:

• acre-inch to apply = gallons to replace x 231 / pr2
• acre-inch = 0.21 x 231 / (3.14159 x 5.52)
• acre-inch = 0.51

The amount of irrigation needed to replenish AW and 10 and 5 percent water loss for various container sizes with a typical nursery substrate is shown in Table 1. In this example, substrate CC is 43 percent SVMC, UW of 18 percent, making AW 25 percent. Obviously irrigating to replace all of the AW is excessive, even the most extravagant irrigator will question irrigating #1 containers with 1.8 acre-inch of water. Irrigating daily at 5 to 10 percent below CC results in rates similar to those we have found over many years of research (links 2,3,4) for plant daily water use for a range of species with different watering needs from very high to low (Figure 1). Remember that these calculations do not take into account irrigation system distribution uniformity (DU). See links 5 and 6 for methods to determine DU. Divide the calculated irrigation rate by the DU for your irrigation system to determine the actual rate to apply. When using these rates you might be at a zero leaching fraction or possibly under-irrigating depending on the species. We have shown that plants can tolerate limited regular deficit irrigations with no detriment in growth as long as they are brought back to container capacity every second or third day. However, this should be done by skillful irrigation managers who are willing to monitor production systems closely.

Improving water management

Closer attention to irrigation practices will become more important as competition for water continues to increase. Additionally, the consequences of over-irrigation and runoff affecting surrounding water resources will be of greater importance. Link 7 has many additional resources to improve water management. So far we’ve only discussed how much water a substrate can hold and how much to replenish at various depletion levels but not how quickly plants use water. A better understanding of how to determine plant water use will allow irrigation scheduling to be based on the plants rather than a set volume of water. There are several methods to determine how much water a plant has used in a day or a certain time period. It can be done by weight, by determining leaching fraction or using substrate moisture sensors. These will be discussed in an article next month.

R. Thomas Fernandez is a professor in the Michigan State University Department of Horticulture, fernan15@msu.edu