Part 1 of this series began a discussion of the significant importance of organic matter in soils and the role played in forage quality and quantity. The topic is becoming increasingly important as we continue to experience high fuel, fertilizer and feed costs. The need for cattle producers to harvest as much nutrient content as possible from the land has never been more important. Nor has the importance of building and maintaining the soil to enhance long-term productivity.
As discussed, the quality of soils and their productivity are closely correlated with their organic matter content. Soil organic matter (SOM) content has a direct effect on nutrient and water transference from the soil to the plant. The higher the SOM content the better these characteristics will be. Taking steps to improve SOM content helps improve the productivity of this “matrix” in which plants grow.
Let’s take a look at some characteristics of productive, healthy soils that can maximize forage production and increase nutrient availability to the grazing animal. These need to be particular areas of focus for the producer.
Maintenance of SOM Equilibrium
Soil organic matter consists of all materials found in, or on, soil that originate from organic material (plant and animal origin). It comprises both living and dead organisms and materials in various stages of decomposition and ranges in age from recent inputs to inputs thousands of years old. Of the SOM, approximately 15% of this is ‘living’ (made up of roots, fauna – insects and other animals – and micro organisms such as bacteria, fungi, molds, etc.). The microbial component of this ‘living’ pool changes rapidly and is considered essential for organic matter decomposition (especially microbial populations), nutrient cycling, degradation of chemicals and soil stabilization.
Organic matter decomposition regulates the flow of energy and nutrients in soil. It plays a key role in Carbon (C), Nitrogen (N), Sulfur (S) and Phosphorus (P) cycling and also acts to improve soil structure. Management and plant inputs influence both the quantity and quality of SOM, which in turn directly impacts soil productivity and the ability of soil to recover from stress. The amount of organic matter in a soil is often used as an indicator of the potential sustainability of the growing system.
The optimal level of soil organic matter for any given soil is one which supports the functional capacity of the soil to hold and supply plant available water, store plant nutrients, provide energy for soil organisms and improve crop/biomass yields.
Effects on Water Introduction, Storage and Supply
Generally, forage production is limited by the capacity of soil to store and supply water to the plant. Soil texture and structure influence the amount of water that can be extracted from soil. This is influenced by the volume of water held in the soil and the depth to which root growth can access this water. Enhancing plant nutrition results in vigorous growth and increases the uptake of available water, thus increasing the efficiency of water use between the soil matrix and the plant.
An important effect of improved SOM is modulation of water infiltration, permeability and storage. Poor soil structure (low SOM) can lead to excessive soil and water loss from the soil surface, reducing water entry into the profile. Poor soil condition and loss of structure also results in surface sealing or compaction, reducing water entry and storage. Protection of the soil surface and retention of organic matter will help maintain soil structure.
Enhanced Soil Biological Function
A huge number and variety of organisms live in soil; some of which perform beneficial functions such as organic matter decomposition and nutrient cycling, while others are often associated with plant diseases. They all contribute to SOM. Some of these organisms are visible to the naked eye (earthworms, insects, etc.), but most are microscopic (bacteria, fungi, molds, etc.). Although not easily identified, the “biomass” of the soil micro flora is a useful measure of soil health and directly affects biological activity of the soil. Other factors affecting biological activity in soil include soil moisture, carbon availability, aeration, temperature, pH and plant type.
This biological activity is one of the most important components of soil productivity and functions primarily within the SOM portion of soils.
To optimise biological function several objectives are sought:
- Development and maintenance of a diverse and resilient biological community. The total number of organisms, species diversity and activity varies with changes in the soil. Organisms carry out a wide range of processes that are important for soil health and fertility. They decompose and recycle organic matter, improve nutrient availability and soil structure, prevent disease, and degrade pollutants. A larger, more diverse microbial community decreases plant disease and ensures the recovery of essential soil functions after stress or disturbance.
- Support and enhance key functions of soil micro biota. Soil organisms recycle organic matter by feeding on dead plants and animals, manure, and other soil organisms. They break the SOM into smaller particles for further decomposition by smaller organisms and play a key role in the transformation of plant available nutrients such as N, P and S. Nutrients in excess of microbial requirements are released in forms that are readily plant available and include nitrates, phosphates and sulfates. Stored nutrients may also be released when organisms die.
- Optimal biological functioning. Soil biology is vitally important in forage production due to the impact on plant health, as well as soil properties and processes. An active soil biota is essential to improve and sustain agricultural production. A healthy soil is a complex and dynamic system that is teeming with life, and includes all the organisms to ensure optimal soil function.
It is obvious that the soil system and its many components and the plant life it supports are extremely complex. Attention to the soil and SOM is critical to plant growth, nutrient composition and overall productivity. To build or improve soil productivity requires a focus on the organic matter component and taking steps to build this soil fraction. Management steps such as rotational grazing, resting pastures, soil testing, and varying plant species including legumes, are all important to enhancing productivity and efficiency.