Soils and plant nutrition
What a soil is made of, why its acidity matters, and how to read a nutrient shortage in the leaves before it costs a harvest.
Home: Crops and soil, Level 0 · Triage, level 1 of 7 in its track. Serves 5 steps across 2 subjects.
Crops take nutrients out of the ground with every harvest, and the soil only gives back what it holds. This guide is the basic set: texture, acidity, the nutrients crops run short of, and what the shortage looks like, with where to put it right. It is the ground under opening ground and holding fertility.
Texture: sand, silt and clay
Shake a handful of soil in a jar of water and let it settle for a day. Sand drops in a minute, silt in hours, clay over a day, and the layers show the mix. The building guide uses the same jar test for earth walls.
- Sandy soil drains fast, warms early and loses nutrients to rain.
- Clay soil holds water and nutrients but is heavy, slow to warm and easily packed hard.
- Loam, a mix of the three with plenty of organic matter, is what most crops want.
Acidity
- Most crops grow best between pH 6 and 7.5. Below about 5.5, phosphorus is locked up and aluminium becomes toxic to roots.
- Test it with red cabbage: boil chopped leaves, and the purple water turns red in acid, stays purple near neutral, and turns blue-green in alkali. Stir in a spoonful of soil and let it settle.
- Raise the pH with lime, crushed limestone or wood ash: see lime.
Reading a shortage in the leaves
Where on the plant the yellowing starts tells you which nutrient is short. Nitrogen, phosphorus, potassium and magnesium move within the plant, so it robs its old leaves first. Sulfur, iron and calcium do not move, so shortages show in the young leaves and fruit.
| Nutrient | What it does | Shortage looks like | Put it back with |
|---|---|---|---|
| Nitrogen | Leaf and stem growth | Whole plant pale and stunted, oldest leaves yellow first | Manure, urine, compost, a legume crop before it |
| Phosphorus | Roots, flowers, seed | Stunted, dark green with a purple tint | Bone meal, manure; lime acid soil |
| Potassium | Water control, disease resistance | Older leaves scorch brown from the edges inwards | Wood ash, manure |
| Magnesium | The green of the leaf | Yellow between the veins of older leaves | Dolomitic lime |
| Sulfur | Protein | Young leaves pale yellow | Manure, gypsum |
| Iron | Making the green | Yellow between the veins of young leaves, often in chalky soil | Lower the pH; compost |
| Calcium | Cell walls | Blossom-end rot: a black patch at the base of tomatoes, usually from uneven watering | Steady watering; lime or gypsum |
Organic matter
- Organic matter holds water and nutrients and feeds the soil life that makes them available.
- Compost works fastest with roughly 25–30 parts carbon to 1 of nitrogen: about two to three parts brown, dry material (straw, leaves) to one part green, wet material (fresh plants, manure).
- Legumes add nitrogen through the bacteria in their roots: see the micro-organisms guide.
Where to go next
This page covers what you are most likely to meet. It is not a field guide, and on identification that difference can kill you. These are the works that go further — chosen because they can be held offline.
Still to come
Illustrations carry a technique further than prose, but some things only move well. Recorded here rather than left implicit.
- Photographs of phosphorus and magnesium shortage in a staple cereal