RHS Level 2 Certificate

Year 2 Week 9 – Hydroponics.Review of soil water and of nutrients and pH.

Learning Outcomes – Hydroponics

State what is meant by the term hydroponics. Explain the role of aeration, nutrient supply, nutrient levels

and pH control, water supply and quality, pest and disease control and automation in hydroponic culture.

Describe TWO methods of growing plants in water culture, to include: NFT (nutrient film technique), substrate culture (rockwool).

State the situations in which water culture can be used, to include: green walls, vegetable production (tomatoes – detail week 11), interior landscaping (house plants - in week 14) .

Identify the environmental implications of growing plants in water culture.

Learning objectives – Review

To identify the three main macro nutrients and their deficiency symptoms

To identify two micro nutrients and their deficiency symptoms.

To identify the importance of soil and leaf testing for nutrient deficiency.

To state the mechanism of CEC To define pH correctly To identify the effect of pH on nutrient availability for

specific nutrients To state that excess of certain nutrients may make others

unavailable To apply this knowledge to the diagnosis of plant

problems

Hydroponics

Growing plants in a pH adjusted solution of water and nutrients.

Any rooting media – and this is not always present – provides only anchorage/ oxygen.

Usually used in protected environments. Provides precise control of nutrient availability

and pH for the selected plant species. Water availability is never a problem.

Provision of oxygen for roots must be addressed.

Nutrient Film Technique (NFT)

Constant flow of nutrient solution past roots.

Nutrient solution aerated to provide oxygen.

pH must be carefully controlled or nutrients will precipitate out of solution.

Solution is recycled – closed loop. Means nutrient mix must be monitored and adjusted as nutrients are used and evaporation occurs.

Substrate technique (rockwool)

Plants propagated in rockwool blocks

Blocks placed on rockwool slabs. Larger pores in rockwool hold air, smaller hold solution.

Nutrient solution drippers in each block provide constant flow.

Excess leaves via drainage holes into gullies. May be recycled. pH and nutrient solution adjustment as for NFT.

Green walls

Substrate of expanded clay aggregate or other low weight material, held in bags in a frame attached to the wall. Plants planted into this through the bag.

Nutrient and water solution dripped through the structure and recycled by a pump.

Environmental issues Water use – if not recycled hydroponics is

wasteful of water. Pollution – if not recycled nutrient solution is

potentially harmful for soils and ground water/watercourses (if drained to waste).

Pollution – used substrate materials may not readily biodegrade and contain high levels of nutrient salts. Risk of soil contamination if dumped.

Benefits – increases production for a given area of land (potentially freeing land for natural ecosystems), green walls useful for cooling and cleaning air in cities.

Plant nutrient review - macronutrients

Nitrogen – used by plants in two forms. Ammonium (cation) and nitrogen (anion). Vital for green, leafy growth – deficiency symptom yellowing of older leaves first.

Phosphorous – vital for root development and germination. Deficiency symptoms – stunted growth and bluish/purple tinged leaves.

Potassium – vital for fruiting and flowering and hardiness. Deficiency symptom – brown, often curling edges to leaves, poor flowering and fruiting.

Plant nutrient review – key micronutrients

Iron – vital component of chlorophyll. Deficiency symptoms – interveinal chlorosis, younger leaves.

Calcium – vital component of cell walls. Deficiency symptom – ‘bitter pit’ in apples, ‘blossom end rot’ in tomatoes, black spots on leaves.

Review Question answers

1. Removal by humans (harvest, cleaning up) – composting, feeding with nitrogen fertilizer. pH – too acidic or too alkaline. Adjust with lime or sulphur. Used by plant growth – feed the soil, adjust planting density.

2. Because nutrient presence in the soil may not mean it is available to the plant or that the plant can take it up. A leaf test may also be required.

3. Nitrogen – older leaves first because the plant can move nitrogen from plant tissues in the event of deficiency.

4. To the amounts needed by the plant not to the importance of the nutrients.

Cation Exchange Capacity

Clay and humus particles have a negative surface charge.

Certain nutrient (and other) ions found in soil water have a positive charge (cations).

Negative charges attract positive charges, so holding these ions in the soil.

Which cations are held depends on their relative concentrations in the soil water. The higher the concentration of an ion the more likely it is to be held by a given soil particle.

What is cation exchange?

The ability of one cation to displace another from the negative charge site on the clay or humus particle.

This means that the nutrient cations can become available to the plant as they are displaced.

Cation exchange

Soil pH

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

Plants grow best (with some exceptions) in the range 6.5 -7.5

Soil pH scale

Logarithmic scale – each point is 10 times greater than the one before. So pH 7 is 0 (balanced), pH 6 is ten times more acidic, pH 5 is 100 times more acidic (10 x 10), pH 4 is 1000 times more acidic (10 x 100) etc. Alkaline soils are less acidic so pH 8 is 10 times less acidic than pH7 etc.

Soil pH – link to cation exchange mechanism

Cations fall into two types – acidic and basic (or alkaline). H+1 (hydrogen) and Al+3 (aluminium) are acidic; Ca+2 (Calcium), Mg+2 (Magnesium) and K+1 (Potassium) are alkaline or ‘basic’ (meaning that they have acid neutralising capacity).

So the more acidic the soil the more H+1 ions will be either free in the soil water or attached to the negative charge points on clay or humus soil particles. This displaces the nutrient cations that can then leach from the soil.

Plant nutrients and pH

So high soil acidity can lead to soils being unable to hold alkaline (basic) nutrient cations. This means that these are subject to leaching and may become deficient.

Soil pH can also affect the way that plant nutrients form chemical compounds with other elements. If these compounds are less soluble in soil water then the nutrients may become unavailable to the plants.

This is called Immobilisation

Plant nutrients and pH

pH Worksheet answers

1. The concentration of hydrogen ions in soil solution

2. More alkaline (the higher the number above 7 the more alkaline it is)

3. Positively charged (cations)4. 1000 (7=0, 6=10 times, 5= 100 times, 4 = 1000

times)5. Hydrogen ions will be exchanged for calcium

ions on the surface of clay or humus particles. The calcium is displaced into the soil water.

pH worksheet answers6. Iron7. If the level of iron in the soil is within normal

limits and the soil is acidic then the likely cause is an over application of phosphates at some point which have formed an insoluble compound.

8. Nitrogen deficiency. Either the soil lacks nitrogen because no N fertilizer or organic matter has been applied for some time or the soil pH is below about 5 or above 8.5. To tell which it is carry out a soil nutrient test for N and a pH test.

Learning Outcomes – Hydroponics

State what is meant by the term hydroponics. Explain the role of aeration, nutrient supply, nutrient levels

and pH control, water supply and quality, pest and disease control and automation in hydroponic culture.

Describe TWO methods of growing plants in water culture, to include: NFT (nutrient film technique), substrate culture (rockwool).

State the situations in which water culture can be used, to include: green walls, vegetable production (tomatoes – detail week 11), interior landscaping (house plants - in week 14) .

Identify the environmental implications of growing plants in water culture.

Learning Outcomes

To identify the three main macro nutrients and their deficiency symptoms

To identify two micro nutrients and their deficiency symptoms.

To identify the importance of soil and leaf testing for nutrient deficiency.

To state the mechanism of CEC To define pH correctly To identify the effect of pH on nutrient availability for

specific nutrients To state that excess of certain nutrients may make others

unavailable To apply this knowledge to the diagnosis of plant problems