Movement of Nutrients in Soil Moisture and Measuring Soil Water

September 7, 2023 Agric4Profits

Movement of Nutrients in Soil Moisture and Measuring Soil Water: Nutrients dissolved in the soil solution move with it, and so when moisture moves by capillarity to replace that which has been taken up by plants, a supply of nutrients may be moved near the roots.

Although this action takes place through short distances only, the net result in the course of a growing season may add materially to the food supply of the crop.

Vertical movement of soluble salts in capillary water has been observed more extensively than horizontal movement. During periods of drought, a considerable accumulation of salts may develop at the soil surface.

In some soils, this may be sufficient to be visible, especially in regions of neutral or alkaline soils. The appearance of small quantities of salt on the surface of clods is a familiar sight even in humid regions.

The movement of salts to the surface is much more pronounced in soils not occupied by growing plants because the roots absorb the moisture and so reduce the quantity evaporated from the soil surface.

Water from heavy rains has a tendency to displace the soil solution from the pore spaces and force it out of the soil into the drainage water. This process results in a considerable loss of nutrients.

There is also considerable mixing of the rainwater with the soil solution, and a portion of the mixture drains away while the remainder constitutes the new soil solution which dissolves fresh supplies of nutrients from the soil complexes.

Transpiration

There has been much discussion concerning the value of transpiration to plants. Some cooling of the plant leaves may result from the evaporation of moisture from them, but there is a difference of opinion as to whether the plant is benefited from this effect.

Some investigators think that transpiration is of assistance to plants in the movement of nutrients, particularly from the lower into the upper pars.

Nutrients must be in solution for translocation to different parts of the plant, and since water serves as the dissolving medium, it is of service in the growth processes aside from its role as food material.

Measuring Soil Water

People who design or use irrigation systems need to be able to measure the amount of water in the soil. They also need terms to name the amount of water present.

Four methods are common: gravimetric measurements, potentiometers, resistance blocks, and neutron probes. At the base of all these are gravimetric measurements.

1. Gravimetric Measurements

Gravimetric methods measure the percentage of soil weight that is water. The percent moisture by weight can then be converted to other useful quantities.

2. Weight Basis

To measure the percent moisture of soil, sample by weight, the sample is weighed and the weight recorded. The sample is then oven-dried, and the dry weight is noted. The difference between the two weights is the weight of water in the soil.

As an example, suppose one needs to measure the moisture percentage of soil at field capacity. A sample is taken two days after heavy rain.

3. Volume Basis

It is often more useful to calculate the percent moisture on a volume basis. However, it is impractical to measure the volume of water in the soil.

This problem can be solved by making a percent weight determination and converting it to percent volume using soil and water densities (density being weight per volume).

If we are using U.S Customary Units, then density can be expressed as pounds per cubic foot.

4. Soil Depth Basis

A meteorologist states how much rain falls in inches of water, and irrigation is measured in inches of water as well. In saying that one inch of water fell, the meteorologist means that if rainfall were caught in something like a cake pan, it would fill the pan to a one-inch depth.

Inches of water is a convenient, easily visualized unit that can also be used to measure the amount of water in the soil.

Let’s say one could take one cubic foot of soil and squeeze all the water out of it into a one-square-foot cake pan. How many inches of water would be in the pan? If there were two inches of water, the soil had two inches of water per foot of soil.

Inches per foot is a common measurement used in irrigation. Irrigation also uses the acre-inch, which is the volume of water that would cover one acre of soil one inch deep. In the metric system, the measurement equivalent to an inch per foot is a centimeter of water per centimeter of soil.

From these calculations, one can determine how much water a soil holds at each moisture constant or how much of each type of water a soil can hold.

For instance, if soil contained three inches of water per foot of soil at field capacity and one inch per foot at the permanent wilting point, then the soil holds two inches per foot of soil of available water.

5. Potentiometers

In practice, it would be a bother to make gravimetric measurements each time one wanted to decide whether or not the soil needed watering. Besides, from the plant’s point of view, the important thing is not how much water is in the soil, but the water potential it is being held at.

A device called a potentiometer, or tensiometer acts like an “artificial root.” It measures soil-moisture potential and so gives a “root’s eye view” of how much water is available.

If a grower wanted to use a potentiometer to measure the percentage of water, it would be necessary to calibrate the device. Calibration involves making gravimetric measurements at different gauge readings to prepare a calibration chart.

Calibration must be done for each soil because the same amount of water will be held at a different matric potential in different soils.

6. Resistance Blocks

Another device for measuring soil moisture is the resistance block or Bouyoucos block, named after the person who introduced the device. Two electrodes are embedded in a block of gypsum, fiberglass, or other material.

When in the soil, the device measures resistance to electrical flow between the two electrodes. It is more difficult for electricity to flow in dry soil than in moist soil, so the reading indicates moisture level. Resistance blocks work well between the field capacity and wilting point.

Actually, pure water conducts electricity very poorly. It is ions in solution that carry the electrons of electrical flow. Therefore, most resistance blocks sense both water content and salt content of the soil. These blocks must be calibrated for each soil to obtain moisture readings, because of the different salt content and matric potential of different soils.

Neutron Probe: A neutron probe is a long tube containing a radioactive material that emits a stream of neutrons. The tube is inserted into a steel tube in the soil so the neutrons stream into the soil the behavior of the neutron stream depends on the amount of water in the soil and can be measured. Like the other devices, the neutron probe must be calibrated for different soil types.

In conclusion, the useful portion of soil water for crops is the capillary and gravitational water whose molecules are held together by forces of adhesion and cohesion. Water is absorbed into plants because transpiration causes low potential inside the plants. The water absorbed by plants is used in photosynthesis, to transport materials within the plant, and to keep plant tissues turgid.