How is Soil Water Important to Developing Plants and Fertility?

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Published January 5, 2021 | By Mike Petersen


Couple three weeks ago I laid out some items that may provide clarification to a developing root system, first I shared about the fertility placement and then wrote to you how soil porosity (abundance, size and continuity) is so important. Soils that are massive or compacted will inhibit root development and depth of penetration for so may crops, perennials can adapt and push through over the years. For an annual plant such as corn, soybeans, dry edible beans, canola – oh we have problems.
So let us consider the water content of soils, where the water is in a soil profile, if there may be a dry sandwich layer (more moisture above a dry zone and wetter below), how much suction power (or not) does a plant exert to draw moisture into the roots, that ‘zone’ around the physical root diameter for water uptake, how far can a root dehydrate or drink water from the root itself, how much that is and briefly what mycorrhizae hyphae does in aiding water uptake. All of the above are factors along with climatic conditions; humidity of the atmosphere, heat of the day, wind and then crop stage.


Fig. 1: On the left is very young plant at V4, the right is a plant at VT


As a plant ages the amount of biomass that is alive has a certain demand for water; for instance, a V4 corn plant is only 13 grams but a fully mature plant with a developing ear may be 2450 grams – 190X more grams and surely a bigger demand to keep hydrated and functioning.


Images to your left represent what we might be looking at….
Facts of water holding capacity:


When the soil matrix is near field capacity for a medium textured soil (silt loam or loam) it can hold nearly 0.34-0.40 inch/cubic inch – what is available tho is 0.21 to 0.22 inch per cubic inch. So in a root zone of the V4 plant the root zone is fairly small, potentially in 144 cubic inches of root zone it could have 16 oz of water available to the root system. When a plant has reached maximum height, maximum root volume in the soil (VT-R1) we are seeing 5000 to 6500 cu. inches of soil volume that can hold water and make it available to the roots. If soil profile is at field capacity when plant is mature we are looking at 21.6 to 28.2 gallons of water available. Compare that to 1 cubic yard then we would have a field capacity 202 gallons available in a silt loam soil profile. Great potential, however those numbers at that stage of growth in a corn field are extremely rare.


Facts of Dryness:

Consider how a soil will be refilled by rain events or irrigation in a medium textured soil (bear with me), the profile has to over fill the upper horizons first to help push water downward through cracks, crevices and pores as well as gravity pulling it. A soil that may be very dry, down to near the hygroscopic range, the soil fills somewhat slow then seems to catch up and water moves into the soil profile. There are times when the soil is so dry a condition called “hydrophobicity” sets up and water will be shed and runoff (not infiltrate). This happens during long drought spells and when a fire has swept across the surface at temperatures up above 500 degrees F. Waxes, lipids, fats and resins in the soil organic matter are the usual culprits that cause such an effect. A 25mm or 1 inch rain when the soil surface is real dry will rarely fill the upper portion of the soil profile evenly. In a strip till environment we find that a 25mm or 1 inch rain may penetrate 30-35cm (12-14 inches) in the strip tilled area but only 10 to 15cm (4-6 inches) between the strips when it is very dry. Part of the reasoning, we have observed over the last 20 years in strip tilled fields the residue between the strips may shed water towards the strip zone, and the porosity and shallow roots under the residue absorb water very quickly. So I am telling you this to say the soil profile will have dry spots, shallow depth in parts and in strips deeper penetration – this gives the plants an advantage in strip till. The existing pores under the residue where the soil surface has not been disturbed in most cases will have contiguous pores under where last years crop stood and aid in water penetration unless as I said it is super dry like the 2020 late summer into fall and winter.


Fig. 2: Courtesy Gary Naylor – Intense rain on soils that puddled bad then soils flow and erosion is severe.


Compacted Soils – And Potential Soil Erosion

I have observed and also measured water movement when soils have a compacted layer within the upper 6 inches (15cm). Ugly effects. What happens? Because of what we call an “abrupt discontinuity” at the top of the compacted layer there is a smear zone. Pores, cracks, crevices, even some of the vertical ped faces can be cut off or truncated, as with a rapid rain events or large doses of irrigation (flood) the soil above the compacted layer has to fill to 130% of field capacity before gravitational water starts to pull water in and down. What does all that have to do with soil water? A bunch, when water is not able to penetrate the soil profile and refill pores so a growing plant can survive – we have problems; the results may well be runoff carrying away soil for a period of time unless the rain slows to a infiltration rate below the normal soil textural rate (ie: loam soils have a standard infiltration rate of 0.6 to 1.0 inch/hour, silty clay loam soils standard rate is 0.2 to 0.6inch/hour). When soils become that saturated and the rain continues to pelt down at a high rate (>1.5-2 inches per hour), the soil above the compaction usually turns into a gel like substance and then flow – water erosion becomes horrendous. Image to your left is a perfect example – losses may exceed 50 to 160 tons/acre.


The amount of water that we hoped to penetrate and go into the soil and replenish what the plants have absorbed or evaporated will be way short of what could and should have refilled the soil. Now please do not think that moldboard plowing is the answer. Strip tillage shanks or coulter units follow the normal vertical structural units of the soil and disturb the soil without inverting, smearing, rolling and smashing soil structural units. All of those disturbances damage soil structure which can and will alter soils to become massive, reducing porosity and that results in less water content for root access.


Water is the essence of plant life. Without water and at the right times plant desiccate, wither and die. Water obtained and drawn into the plants by the root system move sugars, proteins, carbohydrates and basic nutrients into the xylem and phloem tubes to continue the cycle of photosynthesis. It is said and studied by plant scientists that 98%+ of the water in and used by plants come from the soil profile. I can hear several of you say – That is an understatement or in one word, DUH! My writing such is to note we are farmers of water and the soil profile is the bank account we manage with certain tillage practices. This discussion is to emphasize with what we promote with Strip Tillage we are helping you with water infiltration, where it gets banked, making a root system as big as necessary to grow a crop whether it is forage, grains, fruits, tubers, bulbs or nuts.


Effective zone around the root and root hairs can obtain water:


Fig. 3: Diagrams of zone where root of 1mm size pulls water. The red line indicates the distance water can be sucked into the plant (1cm to 2.5cm) on the left.


Roots have to compete against the tension water is held by organic matter and clay particles in the soil to pull water into the epidermis and then Xylem transports it up to the above ground portions of the plant for cooling, hydration and photosynthesis processes. The suction can be measured in atmospheres, pounds of force, or dynes.


In the diagram to the left there is a cross section of a root and then a red line around it giving you an idea of how far the root can pull or suck water when the soil pores are full of water, the tension soils hold water at that phase is 1/3rd of an atmosphere. That is easily obtainable water and the plant exerts little effort. For ease of reference to remember, that is 1 to 2.3 cm or approximately 1/4 to near 1 inch around the root. Finer root size can squeeze into smaller pores, the larger roots when growing fast and down deep do not send as many roots radially outward and may grow past drier zones for cooler and more moist environments.


FIG. 4: Depiction of an Orthman root dig with very fast vertical root development (red circled area) shows where very, very few lateral roots developed


With this drawing [Figure 4] of a root dig we at Orthman completed in 2012 with fast growing conditions of the hot summer, roots show little to no lateral root expression within the 18-32 inch depth. At the time of this root dig it was late August, the plants were mature and had finished pollinating. We returned near to the same site after harvest, dug new holes and discovered the zone 18-32 inches still had moisture and the rest of the soil profile was bone dry.


My intention is to illustrate that both lateral roots and root hairs can make a significant difference in water uptake. Then with the later soil-root dig we observed the remaining mosit soils.


Lastly, Mycorrhizae hyphae contribute to water uptake:
The ever emerging and broader knowledge of mycorrhiza is fascinating and a study in itself with how and what they contribute to crop production. I have written about this fungi before and will just briefly identify in this article what they contribute to plants in water uptake.


The hyphae are small filaments microns in size that grow out from the hosts root into the soils in the upper 4 to 12 inch depth of the soil profile. Aerobic in nature they do not live too much deeper into the soil. These hyphae can extend outward from the host plant up to 10cm (4in). As they do they can absorb water and nutrients to feed its host to continue growing more hyphae. Brassicas – plants of the mustard family do not have a symbiotic relationship with mycorrhiza. Nearly all other terrestrial plants do.


A very interesting feature of these symbionts that they aid the plant handle droughty conditions. Even cactii have a solid relationship with mycorrhizal fungi and thrive due to their interaction. Offering another quick fact about mycorrhizal hyphae – Glomalin is a glycoprotein produced abundantly on hyphae and spores of arbuscular mycorrhizal fungi in soil and in roots. Glomalin was discovered in 1996 by Sara F. Wright, a scientist at the USDA Agricultural Research Service. I had the honor of meeting Dr. Wright when she came to the Great Plains USDA-ARS Experiment Station near Akron, Colorado and she taught a field course about her discovery, identifying the microscopic filaments, the importance of glomalin as a “soil glue” and soil aggregate stability. Since that date the science of soil fungi has exploded and brought so much more to the science of soils.


FIG. 5: Computer image of a soybean root (in yellow) and the hyphae near the surface are very fine in size.


As I said water is absolutely essential and provides life or without soil waster most plant life does not exists. Offering you a small window into the realm of soil science that incorporates physics, biology, chemistry, and specific root relationships that are of recent findings.


Stay tuned as we explore more.


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