This document provides an overview of key concepts related to soil water, pH, nutrients, and relative humidity. It defines terms like field capacity, gravitational water, and permanent wilting point. It explains the pH scale and how pH affects nutrient availability and plant growth. The major plant macronutrients nitrogen, phosphorus, and potassium are described as well as calcium and iron deficiencies. Relative humidity is defined in relation to air temperature and water vapor holding capacity. Learning objectives cover soil water, pH, macronutrients, calcium and iron, and how relative humidity is impacted by temperature.

1. RHS Level 2 Certificate Week 16 – Review Week: Soil Water, pH, soil nutrients and relative humidity.

2. Learning objectives Explain soil water and the terms used to describe this including Field Capacity, Gravitational Water and Permanent Wilting Point. Describe the pH scale and the calculation of increases or decreases in pH Explain the effects on plant growth of the principle macro nutrients, Nitrogen, Phosphorous and Potassium, in normal quantities in the soil and in deficiency. Explain the effects on plant growth of calcium and iron in normal quantities in the soil and in deficiency. Explain the effect of air temperature on relative humidity and how RH can be increased or decreased.

3. What water is available to plants?

4. Soil water questions When irrigating the soil what state is ideal for plant growth? Field Capacity Why is adding water to a soil at Field Capacity wasteful? Additional water will drain away What harm might this cause? Leaching of nitrates into water table Is a soil at PWP completely dry? No, it contains hygroscopic water but the plants cannot access this. Why do sandy soils have a lower AWC than improved clay soils? Because the soil pores are larger in sandy soils, so they hold air and more water drains away.

5. pH Scale A measure of how acid or alkaline a soil is. Technically a measure of concentration of hydrogen ions. Scale runs 1 – 14 where 1 is very acid, 14 very alkaline pH scale is logarithmic – a way of showing large numbers in a small scale. Each point is ten times greater than the point before (1 x 10 = 10 for the first point, 10 x 10 = 100 for the second, 100 x 10 = 1000 for the third and so on).

6. pH Questions How many times more acidic is pH 4 than pH 7? 1000 How many times more alkaline is pH 10 than pH 7? 1000 What texture of soil would need most lime if it was acidic? Clay – high CEC = high reserve acidity What texture of soil would need least? Sand Apart from knowing the soil texture what test would you use to help you work out how much lime to apply to an acidic soil? pH test

7. Main Plant macro-nutrients Nitrogen – essential for production of chlorophyll and all metabolic processes. Deficiency causes chlorosis, older leaves first. Phosphorous – root development, germination, photosynthesis. Deficiency – small, purple or blue tinged leaves. Potassium – fruiting and flowering, cold and drought hardiness. Deficiency – brown, curled leaf edges, poor fruiting.

8. Nutrient questions Causes of nitrogen deficiency – removing crops/plant matter; applying high carbon organic matter e.g. sawdust to the soil (nitrogen locked up); decline in nitrogen fixing and nitrifying bacteria in acid soils; leaching. Remedies – application of nitrogen rich organic matter (composted green waste); chemical fertilizers (ammonium nitrate); liming soil to give neutral or near neutral pH; avoid over irrigation or applying nitrate fertilizers outside the growing season; use of cover crops in winter; green manures.

9. Other nutrients Calcium – essential for cell wall formation; usually plant deficiency is pH or drought related not an absence in the soil. Low pH (acidity) makes Ca unavailable. Deficiency – bitter pit in Apples, blossom end rot in Tomatoes. Iron – essential for chlorophyll; immobile in the plant; usually deficiency is pH related not due to lack in the soil. Alkaline soils may show iron deficiency. Deficiency – interveinal chlorosis, young leaves first.

10. Nutrient questions Why is testing for the presence of nutrients in the soil not adequate to find out if there is a deficiency of that nutrient in the plant? May be in the soil but not available to the plant What other tests might you do to find out? pH test and nutrient test on plant tissue e.g. leaf. Which nutrient deficiency is shown by a plant with chlorosis of the older leaves first? Nitrogen deficiency- unlike iron, nitrogen is mobile within the plant Do the terms ‘macro’ and ‘micro’ when used in relation to plant nutrients refer to the amounts needed or to the nutrients importance? The amounts needed not the importance.

11. Relative Humidity The warmer air is the more water vapour it can hold before it becomes saturated and the water begins to precipitate out. Cooler air can hold less water vapour and becomes saturated with a lower percentage of water vapour. Relative humidity measures the temperature of the air and what proportion of the total amount of water vapour that the air at that temperature can hold is actually present.

12. Learning outcomes Explain soil water and the terms used to describe this including Field Capacity, Gravitational Water and Permanent Wilting Point. Describe the pH scale and the calculation of increases or decreases in pH Explain the effects on plant growth of the principle macro nutrients, Nitrogen, Phosphorous and Potassium, in normal quantities in the soil and in deficiency. Explain the effects on plant growth of calcium and iron in normal quantities in the soil and in deficiency. Explain the effect of air temperature on relative humidity and how RH can be increased or decreased