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The Ultimate Guide to Soil Moisture

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23 Jun 2018

Introduction

You need to get the right amount of water to crops at the right time. Water and nutrients are used most efficiently when your crops have access to the amount of water the crop needs and the soil can hold.

This article describes how to use the data provided by soil moisture probes to more effectively manage your farm. Observing and understanding the consequences of soil moisture fluctuations in your crops’ root zone will enhance your efforts to maximize yield and reduce wasted resources.This article will cover:

ConnectedCrops provides the latest advances in sensor technology, cellular connectivity, and mobile interfaces. You will have a whole new level of real-time visibility to what’s happening in your fields.

Why monitor soil moisture?

Knowing how much soil moisture (see definition) is available in real-time can lead to:
  • higher yields
  • better product quality
  • improved plant vigour
  • reduction in disease
  • more effective use of water (water efficiency)
  • reduced irrigation costs
Considering the time it takes to irrigate, it makes sense to invest in knowing soil moisture levels and improve irrigation decisions.
Growers report that using soil moisture measurements has led to a 10% increase in yield. University case studies have demonstrated yield increased by more than 20% for some crops, as well as faster growing cycles by using soil moisture measurements effectively.

What is a normal soil moisture reading?

Volumetric Water Content (“VWC”) is a numerical measure of soil moisture. . It is expressed as a percentage or ratio of the amount of water held by the soil. For example, one cubic meter of soil with 30% VWC contains 0.3 cubic meter, or 300 liters, of water. You should expect VWC readings between 5% and 35% depending on your soil type.

ConnectedCrops soil sensors provide research-grade accuracy in mineral soil – however they aren’t designed to work in pure air, pure water or custom soils.  If you are monitoring VWC in potting soil, or custom soil mixes with different levels of organic content, then contact support have your sensors recalibrated.

What is my ideal soil moisture level?

Your ideal soil moisture level is between field capacity and wilting point.  Once you get more experience tracking soil moisture readings you will be able to narrow this down based on your soil type, crop type, growth stage etc.  To start with, here are some key definitions you should be familiar with:
Field Capacity: As much water as the soil can hold, 2 or 3 days after a heavy rainfall, when the soil is fully saturated. At this point, there is very little downward movement of soil water due to gravity and very little suction due to capillary action.
Permanent Wilting Point: The amount of water remaining in the soil when the plant wilts in a humid atmosphere. The water remaining in the soil is held tightly by soil particles, and plant roots cannot easily extract water.
Saturation: refers to a soil’s water content when practically all pore spaces are filled with water. When the soil is fully saturated, the water will run off quickly via available channels until levels reach field capacity.
Available Soil Water: The amount of water in the soil between field capacity and the permanent wilting point. Generally, overhead irrigation should start before soil reaches 50% of available soil water. Drip irrigation should start before soil reaches 80% of available soil water

How soil type affects soil moisture

The amount of water available to the plants depends on the texture of the soil, for example sandy soils retain much less water than loam soils. Loam soils are usually highly valued for their consistent crop production. Note that the greatest amount of crop available water is in the loam-to-silt loam texture.

The field capacity and permanent wilting points are shown in the table below for various soil textures. You don’t need to remember these levels, the ConnectedCrops mobile app automatically configures Field Capacity and Permanent Wilting Point based on the soil type that you specified when you configured your sensor.
Soil Type Permanent Wilting Point Field Capacity
Sand 5% VWC 10% VWC
Loamy Sand 5% VWC 12% VWC
Sandy Loam 8% VWC 18% VWC
Sandy Clay Loam 17% VWC 27% VWC
Loam 14% VWC 28% VWC
Sandy Clay 25% VWC 36% VWC
Silt Loam 11% VWC 31% VWC
Silt 6% VWC 30% VWC
Clay Loam 22% VWC 36% VWC
Silty Clay Loam 22% VWC 38% VWC
Silty Clay 27% VWC 41% VWC
Silt 30% VWC 42% VWC

Viewing soil moisture data on your phone

When you launch the ConnectedCrops mobile app, you will see a dashboard view where you can see all of your stations and the current reading for each sensor. The mobile app will look like this:
Connected Crops Dashboard Screenshot Explained
The soil moisture reading will be color coded based on your soil type and the thresholds you have set. Colors:
Green – VWC reading is between field capacity and wilting point
Yellow – VWC reading is approaching either field capacity or wilting point
Red – VWC reading has exceeded either field capacity or wilting point.
For advanced features, the Connected Crops mobile app allows you to export the data to excel and do further analysis.