2. Maintain soil fertility
Nutrient availability (influenced by soil pH), adsorption (nutrient complex with soil - insoluble), transport and function of nutrients are all critical factors in relation to plant growth. The interception of nutrients by the plant is also crucial; as is the relative uptake and translocation of nutrients from root to shoot, leaves, flower and grain. The amount of nutrients removed in produce will determine how fast the soil resource is depleted. Nutrient budgets indicate crop demand and removal of soil nutrients, and may be used to develop appropriate management strategies and balance the application of inputs to maintain soil fertility.
Inorganic fertilisers have been historically cheap and easy to use, and overcome the difficulties of matching biological nutrient supply with plant demand. Developing management strategies to improve the asynchrony between differences in the timing of supply and demand, and the synocation (i.e. relative positioning of plant roots in relation to nutrients) can be difficult but developing a better understanding of the biological functioning of soil can maximise benefits. Climatic conditions can alter the uptake of nutrients, with availability declining in dry soils and rapid mineralisation of organic matter under warm, moist conditions.
Apply nutrient budgeting and nutrient replacement with targeted inputs of macro and micro nutrients
Before starting a fertiliser program you need to consider your soil type, the existing pH and nutrient status, and the form and amount of fertiliser required to suit your farming situation. Organic fertilisers can be applied in larger amounts less often, as they release nutrients more slowly, but you still need to be careful how you use them. Fertiliser placement should be used to ensure optimum plant uptake during growth and development (interception), and P should be positioned where it is likely to be moister for longer. Soil related factors including temperature, pH, biological activity, nutrient concentration and soil water content influence nutrient supply. The ability of a plant to capture these nutrients is influenced by factors such as subsoil constraints, root length, growth rate and physiological status, and the rooting pattern and distribution, In managed crop systems, nutrients can also be applied directly to leaves.
Use appropriate ameliorants addressing the soil condition (e.g. gypsum, compost)
Soil organic matter is a major storehouse of nutrients. Where soil organic matter has declined, so too will the ability of the soil to provide adequate nutrients for crops over the long-term. Efficient recycling of nutrients within the soil can offset the cost of inorganic fertiliser, reduce energy costs, cut waste, reduce potential environmental impacts and improve soil health. Currently, conventional farming systems rely predominantly on energy intensive inorganic fertilisers to supply nutrients due to ease of application and ability to time applications in response to plant demand. However, increasingly organic wastes and manures as well as manure crops and natural forms of N fixation are being considered as a lower cost source of nutrients, with additional benefits in maintaining SOM. Even where nutrition is adequate, structural constraints such as compaction or hardpans that may limit root growth and nutrient uptake requiring soil to be managed appropriately.
Time application of inputs to reduce risk of losses and off-site impacts
Soil can hold only a certain quantity of nutrients. This amount varies with the soil type: sandy soils hold less than clay soils. Negatively charged colloids of clay and organic matter hold positively charged nutrients (cations such as Na, K, Mg, Ca, and ammonium). If there are too many cations for the colloids to hold, the rest stay in solution and drain away with the water (that is, leach). The rule to remember with soluble fertilisers is to apply small amounts regularly, rather than a large amount occasionally. Plants can use only a fraction of any large amount applied, so most will be leached away or react with the soil (P, Cu, Zn, etc). This is not only a waste of fertiliser and money, but it also possibly pollutes ground water and surface waters and may contribute to the development of algal blooms.
Manage rotations to assist in maintaining soil fertility
Rhizobia are soil bacteria that, after becoming established inside root nodules of legumes, fix atmospheric N in forms that the plant can use. Failure to nodulate results in N deficiency within the crop and a failure to produce N for use in subsequent crops. Free-living soil bacteria can also fix significant quantities of atmospheric N into forms that become available to plants. Arbuscular mychorrizal fungi are considered beneficial to agricultural production in many instances and form a mutually beneficial (symbiotic) relationship with plant roots to actively help plants take up P through extensive exploration of a greater volume of soil or mobilisation of bound nutrient, in exchange for plant sugars which provide energy for growth. Different crop types will use nutrients in different amounts and from different parts of the soil profile, so consideration must be made for changes in demand. Soil organic nitrogen or legume N can be more effective in raising grain protein through sustained release of N later in the growing season when inorganic sources may have been depleted.
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