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Nutrient uptake from solution is driven by the use of nutrients within the plant, by the fixation of nutrients in new growth of leaves, shoots, stems, fruits, seeds and roots. Growth is thus the primary driving force for nutrient uptake, but growth itself is driven by environmental conditions and in particular photosynthesis and solar radiation, temperature and humidity. Most crops have phase of exponential vegetative growth, while they grow leaves, shoots and roots, and then move into a phase of reproductive growth when development occurs mainly and flowering and fruit or seed development.

Nutrient demand for vegetative growth is often quite different from the nutrient demands for reproductive growth. Seasonal changes in solar radiation and temperature impose seasonal variations in growth rates and hence variations in uptake rates. Crop nutrient demand is thus very dynamic, changing with environmental factors and crop development so that each situation of crop, season and environment, and each stage of growth has a unique nutrient uptake demand. Considerable research effort is being made in developing on-line nutrient concentration sensors which will enable continuous measurement of every nutrient in the solution with input to a computer and control of nutrient additions to perfectly match uptake demand and nutrient supply all the time. This is still a dream and for now nutrient additions can only be made to match an average uptake rate over some period of time. Use of a single solution for a particular crop throughout its growth cycle and through all seasons of the year  is obviously only a poor compromise, and growers who make and adjust their nutrient solutions according to the stage of crop and time of year and environment will obtain crops with higher yield and better quality.

 

This NFT lettuce growing system, and many other like it, are kept full with plants ranging from small seedlings to mature heads ready for cutting. The mix of plant age is approximately the same all the year round and the nutrient solution is formulated to provide fast vegetative growth. However seasonal variations in solar radiation and greenhouse temperature result in variations in nutrient uptake and in the average nutrient uptake concentration( mmol.equivalent per litre of water transpired).

   

Some greenhouse crops grow through a vegetative phase and then cease new vegetative growth while fruit or other reproductive or storage organs are developed. Melons are a good example, cauliflower would be another.

Tomato, cucumber and pepper crops grow over a long and extended season and thus start with rapid and often exponential vegetative growth, followed by a build in fruit load with a slower continuing rate of new vegetative growth.
 
 This crop is producing 1.5 kg of ripe fruit per m2 per week and growing 0.65 kg of leaf and stem per m2 per week.

Crops like these often need relatively high nitrogen, low potassium feeds during their phase of vegetative growth, followed by greatly increased potassium supply as they build to maximum fruit load. A steady phosphorus supply is needed throughout their growth, while calcium and magnesium may need adjustment in relation to competition with potassium uptake, and the proportion of nitrogen may need reducing as vegetative growth slows. As with all crops, seasonal effects on nutrition overlay the effects of growth phases. A tomato crop planted in early autumn and reaching maximum fruit load under low light in mid winter might use the nutrient solution adjustments in this table:

Nutrient concentrations in mmol/litre

Growth
stage
              CF
Planting to truss 1 13.1 1.4 3 0 0.5 6.8 4.5 2.1 20.5
Trusses  1 to 3 14.9 1.5 3.5 0 0.7 9.3 4.5 2.2 23.4
Trusses
 3 to 5
16.7 1.7 5.6 0.9 0.6 13.5 5.9 2.3 30.5
Trusses
 5 to 7
20 1.7 6.15 2.1 0.5 17.5 6.7 2.6 36.1
Above
Truss 7
24 1.7 6.6 2.1 0.5 21.5 7.0 2.6 41.0

This table is not intended to be used as guide to nutrient solution changes but rather as an example of changes which might be made as result of solution analyses and feed beck from the crop on uptake rates.

The extent and frequency of changes required for optimum crop nutrition and production of top quality fruit illustrate why growers should be making their own nutrient solutions.

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