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9. Forage Production and Conservation
FORAGES ARE PLANTS AND PARTS OF PLANTS, other than harvested grain, that can provide feed for grazing
animals, or that can be sown and harvested for feeding. Sown forages are often the prime source of feed in
ruminant production systems, and the success of the system is dependent on how efectively the forages are
grown and managed.
9.1. The Soil
Soil is the supporting medium for all plants and forages, and as such soil texture and fertility are overriding
inluences on the productivity and persistence of forages. Several factors contribute to soil fertility including
nutrient concentration and nutrient availability, and are understood by examining the chemistry of the soil.
9.1.1. Soil Texture
Soil texture is the relative proportion of the various soil particle size fractions in soil. These fractions include
clay, silt and sand, and the proportion of each of these fractions in each soil determines the soil texture class
Fig. 9.1. In an agricultural perspective, soil texture classes are able to be used to describe soil with similar
behaviour and management needs. Soil texture is commonly determined in the ield by
assessing the behaviour of a small handful of soil when moistened and kneaded into a ball. Based on feel of the
soil and the assessment of the bolus, the soil sample can then be classiied into a texture class. For example,
a soil sample which can be formed into a long ribbon
Figure 9.1. The soil texture classiication triangle. Soils can be classiied based on the occurrence and proportion of particle sizes. Source: University of New South Wales.
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of approximately 50 to 75mm, is coherent, has a plastic feel and ine to medium sand can be felt and heard,
would be considered to belong to the Sandy Clay soil texture class.
Soil texture has a substantial bearing on the agricultural capacity of the soil, in that it determines the ability of
the soil to store both moisture and nutrients. Soils dominated by clay particles are largely impermeable
to water and restrict the movement of plant roots, whilst sandy soils have relatively poor soil moisture
holding characteristics, but allow free and unrestricted movement of plant roots through the soil.
9.1.2. Soil Chemistry
Two components of soil chemistry help to understand the agricultural potential of the soil. These include plant-
available nutrients and soil pH.
Plant Nutrients There are 15 essential nutrients that plants require in order
to meet their needs. Three of these nutrients may be obtained from the atmosphere through photosynthesis;
hydrogen, carbon and oxygen. The remaining 12 are obtained from the soil. Of these, the primary agricultural
macro-nutrients are nitrogen, phosphorus, potassium, sulphur, magnesium and calcium. These elements
have the most substantial efect on plant growth. The remaining essential nutrients, or micronutrients, are
required in much smaller quantities.
Whilst macronutrient availability drives plant growth, micronutrient deiciencies will limit the plants ability to
obtain its maximum potential yield.
Plant nutrients are closely associated with bound to soil particles and organic material. The mechanism of
adsorption and desorption of these nutrients with soil particles is important since it is only when they are
detached and in solution with water that they become available to plants. Conversely, when they are adsorbed
to soil particles, the leaching of these nutrients down through the soil proile is reduced.
The texture of the soil and also the presence of organic matter is therefore important to soil fertility, as diferent
soil particles have diferent abilities to absorb nutrients. The smallest particles, clay, have the greatest ability to
adsorb and ’store’ nutrients, whilst sandy soils have a comparatively smaller ability to adsorb nutrients, and
are more prone to nutrient loss as water passes easily through the loose soil structure, taking with it any
nutrients that have been released from the soil particles. Consequently, our most arable and agriculturally
productive soils tend to be composed of a balance of soil particle sizes that are able to store both high levels
of nutrients and soil moisture.
Soil pH Soil pH, or the measure of the level of acidity or alkalinity
in the soil strongly inluences the availability of essential nutrients in the soil, including elements that might be
toxic to plants.
Plant growth is usually best in neutral soil conditions pH 6 to 7, when most nutrients are readily available
Fig. 9.2, although species vary signiicantly in their adaptation to pH extremes.
Soil pH is easy to measure either in a laboratory using a pH meter, or in the ield using chemical indicators that
change colour as pH changes. Laboratory tests may be carried out using water or a bufer solution of calcium
chloride CaCl2. The CaCl2 determination is normally 0.5 to 0.8 lower than the water determination.
Soil pH can afect the availability of major elements to plants, particularly phosphorus. However, some
of the most important efects of pH lie with the trace elements and phytotoxic elements, such as aluminium
Al and manganese Mn. For example, Fig. 9.2 shows that molybdenum Mo becomes less available and
aluminium becomes more available as soil becomes more acid i.e. with decreasing pH. The former is an
essential nutrient, required in minute amounts for plant growth, and in somewhat larger amounts for efective
legume nitrogen ixation. The latter is toxic to plant growth.
Manganese, which also becomes more available with decreasing pH, is an essential nutrient at low
concentrations in the soil, but becomes toxic at higher
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levels. Hence, knowing the pH can give a guide as to what deiciencies or toxicities may exist at a particular
site Table 9.1. This is not to say that all acid soils will give problems with Mo deiciency or Al toxicity. It simply
points out that these might be considered if problems occur on an acid soil.
Figure 9.2. Change in nutrient availability with pH. Source: University of New South Wales.
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9.1.3. Testing Soil Fertility
