The most sensitive ‘scientific instrument’ is the plant itself. Cell expansion rate is highly sensitive to the prevailing environmental conditions. Soil water is the major environmental variable which affects cell expansion, the cumulative result of which is observed as plant height, leaf area and fruit size. Figure 1 illustrates how soil water affects cell expansion and photosynthesis. At the full point both cell expansion and photosynthesis proceed at their maximum rate. As the soil dries cell expansion is quickly affected but photosynthesis keeps going at close to its maximum rate. The result of this is that leaf size and node length are reduced by cell reproduction and dry matter accumulation continues unabated.
Figure 1. Soil Water Content as it Affects Plant Growth and Yield.
The refill point is defined as the water content below which the plant can no longer meet the evaporative demand. At this point stomatal closure occurs and wilting will become evident. This causes a reduction in photosynthesis. This is the reason for the rapid decline in photosynthesis when soil water content falls below the refill point. When the cotton crop is below the refill point cell reproduction and development is affected and development of squares and bolls will be reduced.
If the soil water content is maintained near the full point and/or soil physical conditions are excellent then excessive vegetative growth can occur. Maintaining soil water content near the refill point and/or poor soil physical conditions are present it may be difficult to produce row closure essential for maximising yield within the crop. Therefore it is very important to get the correct balance of vegetative and reproductive growth to try and maximise yield.
The plant height of cotton in late November directly reflects the previous cropping history of a particular field and degree of soil compaction. Plant height, depth of root extraction and daily water use are used in conjunction to determine the timing of the first irrigation.
A compacted field will generally have a lower daily water use and less readily available water for the crop to use. The plant in effect has to work harder to extract soil water from the profile. It is a under a mild degree of stress most of the time. This causes an overall reduction in the length of internodes and can cause abrupt reductions in internode length if any water stress occurs (see Figure 2). Fields in good condition generally show a gradual reduction in internode length as an irrigation approaches. This means it is much easier to time irrigations on good fields as the onset of stress is gradual rather than sudden. Compacted fields go into stress quickly which can cause loss of fruit and there may be problems with reduced growth preventing row closure.
Figure 2. The Effect of Soil Water Content on Internode Length.
Figure 3. Cotton Plant Description and Water Stress Analysis; Moree 1987/8.
The pattern of long and short internodes on a cotton plant can be used to determine degree of soil compaction, timing of and response to past irrigations and timing of wheel track passes during cultivation on wet soil. Figure 3 is an analysis carried out on a cotton farm in Moree showing how various factors affected growth of the crop. This information is very valuable for determining refill points for scheduling subsequent irrigations.