Soils, Soil Testing and Plant Nutrition

Soil is four things

It looks like one substance. It is really a mixture of **four** components, and roughly half of a healthy soil is not solid at all. **Rock particles** make up about 45 per cent. **Water** and **air** share the pore spaces between them, at roughly 25 per cent each. The remaining 5 per cent or so is **organic matter**: dead and decaying plant and animal material. Nearly everything in this topic follows from that. Change the size of the particles and you change how much air and water the soil can hold, which changes what will grow in it.

Clay against sand

Clay has the smallest particles and sand the largest. Every difference below comes from that one fact.

Match each soil to what defines it

Why does sand drain faster?

A sandy soil drains much faster than a clay soil. What is the actual reason?

• Its larger particles leave larger pore spaces, so water moves through under gravity more easily
• Sand is lighter, so water passes through it
• Sand contains more organic matter, which absorbs the water
• Sand is warmer, so the water evaporates

Words a soil report uses

Dig a pit in a field and you see the soil in layers. These are the terms for what you find.

Uplands and lowlands

Select the TWO statements that describe Northern Ireland's soils accurately.

• Upland areas tend to have wetter, peaty and more acidic soils, used mainly for rough grazing
• Lowland areas tend to have deeper mineral soils that support grassland and arable crops
• Upland soils are typically deep, sandy and free draining
• Soil type is much the same across Northern Ireland, so location makes little difference to farming

Why a farmer tests pH

Most crops grow best in a soil that is only slightly acid to neutral, around **pH 6 to 7**. Get it wrong and nutrients that ARE in the soil stop being available to the plant. **Potatoes** tolerate more acid ground than most crops, and are often grown at the lower end of that range. **Brassicas** such as cabbage and kale do better on limed ground nearer neutral, which also discourages clubroot. So the pH test is not a formality. It decides whether the field needs lime before anything is sown, and which crop is worth sowing in it.

State what liming does

Build a sentence that describes liming accurately.

Testing a soil, and one worked sum

Four tests you should know: **sieves** separate the particle sizes; a **pH meter or pH paper** gives the acidity; pouring a measured volume of water through equal samples compares **drainage capacity**; and **oven-drying** gives the moisture content. Burning off the dried sample then gives the percentage of organic matter. For moisture content, use: **moisture content (%) = (wet mass - dry mass) / wet mass x 100** A sample weighs 200 g fresh from the field. After oven-drying it weighs 160 g. The water lost is 200 - 160 = 40 g, so the moisture content is 40 / 200 x 100 = **20%**.

Work out the moisture content

A soil sample weighs 500 g when it comes in from the field. After oven-drying it weighs 380 g. Using the formula above, what is its moisture content? Give the percentage as a number only.

What plants take out of the soil

Three elements matter most, and fertiliser bags are labelled with all three as **NPK**. **Nitrogen (N)** drives leafy green growth. **Phosphorus (P)** builds roots and helps a young crop establish. **Potassium (K)** supports flowering and fruiting, and hardens a crop against disease and drought. Nitrogen also moves in a **cycle**. Bacteria in the root nodules of clover and other legumes fix nitrogen from the air into the soil. Other bacteria then convert it through **nitrification** into the nitrates plants can take up. In waterlogged soil, **denitrification** sends nitrogen back to the air as a gas, which is one more reason drainage matters.

Read the bag

A 25 kg bag of fertiliser is labelled 20:10:10, which means it is 20 per cent nitrogen, 10 per cent phosphorus and 10 per cent potassium. How many kilograms of nitrogen does the bag contain?

Follow the nitrogen

Put the stages in the order nitrogen actually moves through them, starting in the air.

• Nitrogen gas sits in the air, where plants cannot use it
• Bacteria in legume root nodules fix it into the soil
• Nitrification converts it into nitrates the plant can take up
• The crop takes up the nitrates and uses them for leafy growth
• In waterlogged soil, denitrification returns nitrogen to the air as gas

Say the topic back

About 45 per cent of a healthy soil is rock particles and only about 5 per cent is _____. Because its particles are the largest, a _____ soil drains fastest, while a _____ soil holds water and nutrients but warms up slowly. A farmer whose field tests at pH 5 would add _____ before sowing most crops. On a fertiliser bag, the letter N stands for _____.

Growing without a field

A grower is considering hydroponics, where crops are grown in a nutrient solution rather than in soil. Which pairing of an advantage and a disadvantage is accurate?

• Nutrients can be controlled exactly and soil-borne disease is avoided, but setup costs are high and the system depends on power and technical skill
• It is cheaper to set up than growing in soil, but the crop quality is always lower
• It needs no water at all, but the crops grow more slowly
• No nutrients need to be added, but it can only be used outdoors

Your turn: advise the farmer

A farmer has taken over a wet, heavy field that tests at pH 5. Advise them on what the soil is like, what they should do before sowing, and why.

• Say what a heavy, wet soil tells you about its particle size and drainage
• Explain what the pH 5 reading means and what should be added
• Name one soil test they should carry out and say what it would tell them
• Explain why poor drainage also costs them nitrogen