Guideline Index

Chapter 8: Assessing Soil Nutrients

8.6 Visual symptoms of nutrient deficiencies in pastures

To help determine your fertiliser needs, an important step to take in conjunction with soil testing, is to visually assess the pastures. The main features to look for are overall colour, the presence of weeds and poor pasture species, and if applicable, legume density and leaf size. If symptoms are apparent on individual plants, then pasture production will have been below its maximum potential well before this stage. In fact, visual symptoms will not become apparent until the reduced growth has exceeded 30%. This is referred to as ‘hidden hunger’.

8.6.1 Identifying plant disorders from visual symptoms

The visual symptoms plants exhibit in response to nutritional disorders can be a useful guide for identifying the cause of a disorder. Common plant responses include unusual colours or patterns in the leaves, burns, distortion of individual plant parts, stunting or abnormal growth.

Several non-nutritional disorders (Table 8.1) can also produce similar symptoms, so careful observation is needed to ensure the diagnosis is reliable.

Table 8.1   Major causes of visual symptoms in plants
Table 8.1 Major causes of visual symptoms in plants

 

In pasture plants, nutrients move from the roots to other parts of the plant through a network of cells called the vascular system (veins). These cells specialise in moving water, nutrients and metabolic products throughout the plant. The arrangement of veins and the ease with which individual elements move within the plant (in other words, their mobility) have a strong influence on the way symptoms develop.

Symptoms that show patterns that align with the plant’s veins usually indicate a nutritional disorder. Non-nutritional disorders usually show no relationship to vein pattern.

8.6.2 Characteristics of nutritional disorder symptoms on leaves

Nutritional disorders produce characteristic symptoms in leaves. These include:

  • Symptoms are restricted initially to a single leaf-age class, that is, young, old or intermediate-aged leaves.
  • Patterns are symmetrical and closely related to leaf venation.
  • Changes in leaf colour and tissue death develop gradually (rarely overnight).
  • The boundaries between green and chlorotic (yellow) or necrotic (dead) areas on a symptom leaf tend to be fuzzy or blurred. Strong, definite boundaries are often produced by herbicides or viruses.
  • Leaf symptom patterns due to a nutrient deficiency are rarely blocky or angular. Such patterns can be caused by a pathogen or occasionally by nematodes.
  • Nutritional problems impair cell function and rarely cause mechanical disruption of the cuticle (outer layer) of the leaf. Thus, damage to the surface of a symptom leaf is not likely to be caused by a nutritional disorder.
  • Symptoms develop first in tissues most distant from the major veins of the leaf, such as the interveinal regions and the tips and edges of the leaf blade.

Visible changes in a crop, such as yellowing, small leaves and poor seedset, all begin as a breakdown in cell functioning and tell us that a nutritional disorder exists. For example, the distortion of new tissues or flowers or the death of growing points is typical of boron deficiency. These visual symptoms occur because boron is necessary for the proper regulation of cell division. Similarly, the leaves of nitrogen or magnesium deficient plants are pale because nitrogen and magnesium are components of the green plant pigment, chlorophyll.

Such links between an element’s function and a specific abnormality that results when it is deficient are common in plants. For this reason, the nature of the symptom can provide a useful guide to the identity of a nutritional disorder even in unfamiliar crops.

The two most important diagnostic features of a nutritional disorder symptom are:

  • Where the symptom is found on the plant (location).
  • Its appearance (colour and pattern).
8.6.2.1 Location

Nutritional symptoms generally develop irregularly over a plant but show first in specific organs, such as the leaves, roots, shoots or growing points. Depending on the mobility of the element, leaf symptoms can occur in the upper, middle or lower sections of a plant.

Mobile elements like nitrogen, magnesium or potassium (see Table 8.2) are moved about the plant relatively easily to satisfy local shortages, particularly in new shoots or developing seeds. When one of these mobile elements is deficient, the older leaves are the first to be depleted and the first to show symptoms.

Less mobile elements, such as iron, copper, boron or calcium, do not move readily from older to younger tissues; so when these elements are deficient, the symptoms appear in the newer or upper leaves or in the flowers or the seed.

Symptoms of nutrient toxicity generally show first in the oldest leaves. These leaves have the highest transpiration rates and receive most of the nutrients absorbed by roots as the nutrients move in the transpiration stream.

Table 8.2   The difference in mobility of nutrients within the plant
Table 8.2 The difference in mobility of nutrients within the plant

 

8.6.2.2 Pattern

Observe the size and shape of the plant, the overall foliage colour, the colour of symptom leaves and the pattern of chlorotic (pale or yellow) or necrotic (burnt, appears dead) areas in relation to vein pattern. Also note any irregular shape, splitting, cracking or corkiness of affected organs. All of these may help to establish the identity of the disorder.

8.6.3 Deficiency and toxicity

Are the symptoms indicative of a deficiency or a toxicity?

Deficiency symptoms typically occur on a single leaf-age class unless more than one problem exists.

Toxicity symptoms often develop rapidly. When this happens, the affected leaf tissue may change from healthy green to grey-green or dark brown without a transitional yellow phase.

Symptoms that appear on old and new leaves at the same time may indicate a toxicity. For example, when an excess of one element causes a nutrient imbalance, deficiency symptoms may be seen in the young leaves while older leaves may show burn or other symptoms of toxicity. Excess phosphorus, manganese or zinc can cause iron deficiency chlorosis in young leaves as well as symptoms of nutrient excess (toxicity) in the old leaves.

Diagnostic keys (Table 8.3) provide a framework for a visual diagnosis of deficiencies, but there are two major weaknesses:

  • A disorder is usually quite advanced before clear visual symptoms appear, and some loss of yield or quality will have occurred. Also, the absence of symptoms in a crop or pasture does not mean that nutrition is adequate. ‘Hidden hunger’ is the condition in which performance is limited, but no symptoms have been expressed.
  • Visual symptoms can be unreliable when more than one element is limiting or when some environmental stress has modified the normal pattern.

 

Table 8.3   Quick guide to nutrient deficiencies: What to look for.  Source: Adapted from Weir and Cresswell (1994).
Table 8.3 Quick guide to nutrient deficiencies: What to look for. Source: Adapted from Weir and Cresswell (1994).

Chapter 3 covers all the nutrient disorder symptoms of individual nutrients in more detail.

8.6.4 General paddock symptoms

As soil fertility declines, the grasses and legumes (clovers and Lucerne) become patchy and stunted. Gradually, weeds start to fill the gaps.

Dandelion, rib weed, white daisy, etc. are collectively called ‘flat’ weeds. They are regularly associated with a reduction in soil fertility, usually potassium deficiency but also phosphorus and molybdenum. This is especially evident in regular silage and hay paddocks or where insufficient fertiliser has been applied in the past.

Onion weed is an indicator of soils that are deficient in phosphorus.

Sorrel and moss are usually associated with low-pH (strongly acidic) soils but can also be acting as a filler species like those mentioned above. That is, coming into a pasture to fill in the areas vacated by the more productive grasses and clovers.

A good indicator of whether a pasture may respond to extra fertiliser is to examine the areas around the dung and urine patches – see Figure 8.3. If there are healthy pasture plants within these areas, but such plants are sparse or less healthy in the areas between the patches, then this pasture is indicating that ‘If you feed me (N, P, or K), then I’ll grow.’

Soils becoming saline undergo a change of species as the level of salinity increases over time – See Chapter 7.5.5 . In temperate regions, the initial changes are a decline in white clover and an increase in strawberry clover. Then buck’s horn plantain, toad rush, and windmill grass begin to invade. Yellow buttons, sea barley grass and annual beard grass indicate advanced stages of salinity.

Damage may also be caused by insects, such as lucerne flea and red-legged earth mite, and by various viruses.