Soil fertility

Deficiencies of any of the essential nutrients can decrease productivity by decreasing crop or pasture yield.

Manganese deficiency causes split seed in lupin. Deficiencies in calcium and boron commonly cause deformities in fruit that downgrade their marketability. Low concentrations of a range of nutrients can decrease seed vigour and hamper plant establishment.

Deficiencies of a range of nutrients increase the prevalence of leaf lesions due to foliar fungal disease. Root fungal infections can also increase with deficiencies of silicon, boron or manganese.

Nutrient deficiencies decrease nitrogen fixation by affecting nodule formation or nodule function when the requirements exceed those for growth of the host plant.

Whilst there are 17 essential elements for the growth of plants, for sustaining human life, in addition, there are another 33 essential nutrients comprising amino acids, lipids, vitamins, and a range of additional elements.

Coarse textured sandy soils rely heavily on their organic matter content for their ability to hold nutrients. As the capacity of these soils to protect organic matter is constrained, sands are commonly associated with leaching loss of mobile nutrients. However, where soil organic matter can be built up, micronutrients such as iron, copper, zinc, and manganese are tightly bound by the organic matter and prevented from leaching.

The capacity of a soil to hold nutrients influences yield response. Soils that adsorb nutrients strongly can render them unavailable to plants – low sorption capacity soils will reach a high concentration of nutrient in solution and are generally more fertiliser efficient, but are also more susceptible to leaching.

Soils high in iron and aluminium oxides (particularly those high in clay) adsorb phosphorus, molybdenum and sulphate. Similarly copper, zinc, and cobalt are adsorbed and may precipitate (solid separates from the solution rendering the nutrient immobile) under neutral to high pH conditions.

Soil pH is dependent on the type and concentration of cations in the soil solution (and is estimated by measuring hydrogen ions in solution). The pH buffering capacity is determined by the amount of adsorbed hydrogen ions the soil can adsorb because this determines the ability of a soil to resist changes in pH. Many light textured soils have low cation exchange capacity and low pH buffering capacity.

Soil pH alters the availability of nutrients. As pH increases iron, copper, zinc and manganese become less soluble and precipitate out of solution and become less available; whilst as pH decreases the availability of nitrogen, phosphorus, potassium, sulfur, calcium, molybdenum and magnesium is reduced.

Soil compaction constrains crop growth by limiting access to stored soil water and nutrients. In compacted soil, roots have to apply more force to displace soil particles when they are growing. As seminal roots reach the compacted layer, their rate of downward growth slows, because the roots encounter greater physical resistance in the compacted layer. As the seminal roots grow through the compacted layer, the rate of downward growth starts to increase again, but the impact on the root system is clear – it has less roots in the deeper layers and will have less access to subsoil water and nutrients later in the growing season.

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. This information will help you to decide what type and quantity of nutrients you need.

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 (phosphorus, copper, zinc, etc). This is not only a waste of fertiliser and money, but it may also pollute ground water and surface waters and may contribute to the development of algal blooms. 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.

Non-mobile nutrients such as phosphorus stay close to where they are applied, and are moved only by cultivation and seeding. Fertiliser placement should be used to ensure optimum plant uptake during growth and development, and phosphorus should be positioned where it is likely to be in moist soil for longer.

Yield responses also depend as much on the growing conditions and crop demand as they do on the level of extractable nutrient. The principles of soil fertility and plant nutrition may be equally applicable anywhere in the world, but in any given environment the response will vary. It is important to ensure that all nutrients required for plant growth are available in sufficient quantity as Liebig’s Law states that growth is controlled by the most limiting factor. Soils in Australia vary enormously in their ability to supply satisfactory nutrition to the crops and pastures growing on them. Many of our soils are acid (ammonium-nitrogen fertilisers make this problem worse) and low in calcium, magnesium, phosphorus and micronutrients.

High amounts of soil organic matter buffer the effect of fertilisers on pH, increase cation exchange capacity (CEC) and provide a source of nutrients. Different crop types have varying residue quality, with the largest variable being their nitrogen content. Legume crops fix nitrogen and generally result in increasing levels of soil nitrogen which benefit subsequent non-legume phases. These residues decompose more rapidly than cereals and release nitrogen throughout the subsequent growing season, but contribute less to building soil organic matter than more stable residues such as cereals and grasses. Plants such as capeweed and canola are good at scavenging for nitrogen and can increase the requirement for fertiliser nitrogen in subsequent crops.

The role of arbuscular mychorrizal (AM) fungi is considered beneficial to agricultural production in some instances. These organisms (and others) form a mutually beneficial (symbiotic) relationship with plant roots to actively help plants take up phosphorus through exploration of a greater volume of soil or mobilisation of bound nutrients, in exchange for plant sugars which provide energy for growth. The contribution of AM fungi to phosphorus uptake is comparatively greater in low phosphorus content soils than under high phosphorus fertiliser when AM colonisation is suppressed. Thus the contribution of AM fungi could be considered nominal under most practical agricultural conditions.

Soil temperature and moisture are rate limiting factors influencing biological activity, mineralisation of organic matter and the release of soil nutrients. Rainfall can also influence the location of nutrients in relation to rooting depth.