Isn’t Soil Moisture Good Enough to Estimate Crop Stress?

For decades growers and researchers have used soil moisture to guide irrigation of crops. It is logical that since plants get their water from the soil. But does the soil moisture really give a good estimate of the water status of the crop? Sometime yes, and sometimes no. There are several reasons for that especially with perennial trees and vines. So, when is soil moisture data useful and when is it not?


The key questions are:

• is the soil probe located by the roots and representative of the root system?

• does the measured soil moisture correlate well with the plant stress?


Locating soil probes - Soil moisture measurements depend on locating the moisture probes in representative areas of the root system of the crop. This is not difficult in many annual crops as the root systems are in a limited depth o soil and often very dense with few areas of the soil not explored as in the photo below. So pretty much anywhere in that zone will be a good location near roots.


[Root system of a planting of grass. Photo from earth.com]


However, with deep-rooted perennial trees and vines this is not true. Roots may extend one to several meters deep across several horizons of soil. Since soil hardness, water and nutrient availability can change markedly with depth and can affect root growth patterns, it is very difficult to know how far the roots extend (see apple root map below) or where to place soil moisture probes.


[Root map of an apple tree. Individual tree root maps are often quite variable. Typical location of soil moisture probes are shown by diagram of probes. From Thomaj and Kullaj, 2019]


Also, the root density in the soil volume (in cm of root length per cubic centimeter of soil) of many fruit crops, especially for apple and grape, is very low compared to annuals. Sometimes the root density of the tree or vine may be only 1/100th of that of competing grass. Additionally, with depth, there tend to be strong variations in soil horizons in texture, hardness, nutrition, water-holding capacity, etc. These changes in structure and chemistry can strongly affect the distribution of deep-rooting trees and vines. This makes locating soil moisture probes even more difficult.


[Distribution of grapevine roots in a uniform sandy loam soil (left) and with a compacted plow pan at about 30 cm (right - Figs from Van Huyssteen, 1988b). Root growth and useful soil volume was greatly limited.]


Does soil moisture correlate highly with plant stress?

Another complication with soil moisture measurements related to crop water status is that crops differ in their ability to transport water from soil to the leaves. Analogous to Ohm’s law, a high resistance (to water flow) will lead to a high potential gradient (from soil to leaf) when there is a high current or water loss rate. Annuals generally have low hydraulic resistance and transport water very easily, so the plant water status is primarily determined by the soil moisture. However, many woody perennials are larger and have more resistance to water transport through the wood. The result of this is that their water status depends on both the soil moisture AND the weather, that is the evaporative demand of the atmosphere (sunlight, temperature, humidity) (see below, left).


For example, the dashed line on the graph shows that the plant may have a water potential of -10 bars under conditions of wet soil but high transpiration (sunny, warm); or with soil water stress but lower transpiration due to cooler, cloudier conditions. So, with both soil and weather having effects on plant stress, measuring soil alone (or weather alone) cannot predict the plant stress very well.


Below is a plot of grapevine water potential versus soil water content at 15 cm, the depth with the best relationship. Clearly, a soil water content of 0.4 could correspond to -3 bars or -10 bars.


[Relationship of plant water potential to the transpirative water loss rates for a plant with fairly high hydraulic resistance. Note both soil and weather affect the potential.]

[Relationship of grapevine mid-day stem water potential to soil water content at 15 cm depth (data courtesy of A. Coniberti, INIA, Uruguay]


Soil moisture is a very useful tool to help determine the amount of water to apply and to monitor the decline in water availability. Together with weather data and direct monitoring of the tree or vine water potential with the FloraPulse microtensiometer, growers will have powerful tools to optimize water management.


This is summarized in a quotes from recent papers:


“I am also a firm believer that soil moisture monitoring is a valuable tool here also. We monitor this; and when the threshold is reached, we can note that on our moisture chart—we do it on our profile average moisture chart. This soil moisture level is usually a consistent indicator of the irrigation starting point each year, once it has been defined, reducing our need for the other measurements. However, monitoring of both plant and soil helps us to avoid mistakes “ (M. Greenspan, 2018)


“A plant-based measurement, such as the water potential, can be considered as the most straightforward indicator of plant water status as it integrates the effects of soil, plant, and atmospheric conditions. More precise interpretation of SWP values provides winegrowers with a tool to more adequately implement short- and long-term management strategies to adapt to drought in order to ensure yield and grape quality.” (Suter et al., 2019).


References

Greenspan, M. (2018) Water Stress Management: Do We Have it Figured Out? Wine Business Management May 2018, pp 68-72.


Suter B, Triolo R, Pernet D, Dai Z and Van Leeuwen C (2019) Modeling Stem Water Potential by Separating the Effects of Soil Water Availability and Climatic Conditions on Water Status in Grapevine (Vitis vinifera L.). Front. Plant Sci. 10:1485. doi: 10.3389/fpls.2019.01485


F.Thomaj and E.Kullaj. 2019. Scion genotype controls biomass allocation and root development in grafted apple. Acta Hort. 1261:93-98.


Van Leeuwen, C., et al. (2009) Vine water status is a key factor in grape ripening and vintage quality for red Bordeaux wine. How can it be assessed for vineyard management purposes? J. Int. Sci. Vigne Vin 43(3): 121-134.