In some countries crops are irrigated using water containing salt (sodium chloride). This makes the soil saline (salty) which means it contains sufficient neutral soluble salts to reduce the growth and yield of most crop plants. The responses of crop plants to salinity vary between two extremes: • sensitive - plants show the greatest reduction in growth and yield • tolerant - plants show the least reduction in growth and yield. Agricultural research organisations have investigated the effect of irrigation with salt solutions on the yields of a variety of crop plants. The data collected allowed researchers to determine, for each crop, the concentration of salt solution at which yield is reduced by 50%. From the data recommendations could be made about which crops were most suitable to grow in saline soils. Table 2.1 shows the crops tested. [Table 2.1: grasses (monocotyledons) and broad leaf crops (dicotyledons) for human consumption (Triticum aestivum, Hordeum vulgare, Solanum tuberosum, Brassica oleracea, Spinacia oleracea) and animal grazing (Festuca arundinacea, Agropyron cristatum, Medicago sativa)] • Seeds were germinated in salt-free conditions. • Young plants were planted into fields that were divided into standard test plots of the same area for each crop. • Each field was irrigated regularly with a different concentration of salt solution. • Control plots were irrigated with salt-free water. • Each crop was grown to maturity and harvested. • The dry mass of the edible parts of the plants was measured to find the yield for each concentration of salt solution.

1. (a)(i)Identify one variable that could be standardised once the young plants are in the test plots in the fields.[1]
2. (a)(ii)Suggest one other variable that cannot be standardised once the young plants are in the test plots in the fields.[1]
3. (b)Suggest why the researchers used dry mass to estimate the yield of the different crops.[2]
4. (c)The data collected by the researchers allowed them to determine, for each crop, the concentration of salt solution at which yield is reduced by 50%. Explain how the researchers determined the concentration of salt at which yield is reduced by 50%.[2]
5. (d)Fig. 2.1 shows the results of this investigation. [Figure 2.1] State three conclusions that can be made about the effect on the yield of these crops when irrigated with salt solutions.[3]
6. (e)Based on these results the agricultural organisations published recommendations to help farmers choose which crops to grow in saline soils. However, farmers in different parts of the world who followed these recommendations did not obtain the yields expected. Suggest two reasons for this. 1 2[2]