Most soil test reports arrive as a table of numbers with colour-coded bands and a few recommendations. If you've never seen one before, it's easy to either ignore everything or panic about every number that isn't in the green zone. Neither response is helpful. The skill is knowing what each number is actually telling you about your soil - and then deciding what, if anything, to do about it.
This article assumes you already have a lab test. If you're still at the "should I test at all" stage, the guide on how to get a useful soil test covers which tests are worth getting and how to collect a proper sample.
pH: The First Number to Check
pH measures how acidic or alkaline your soil is on a scale of 1 to 14, with 7 being neutral. For vegetable gardens, the target range is 6.0 to 7.0. This isn't arbitrary - it's the range in which nutrients already in your soil are most available to plant roots. Outside this window, nutrients become chemically locked up, and plants can't access them even if they're physically present.
Below 6.0 (acidic): aluminium and manganese become more soluble and can reach toxic levels; phosphorus becomes less available; calcium and magnesium are depleted. Common symptoms are stunted growth and yellowing that doesn't respond to nutrient applications.
Above 7.5 (alkaline): iron, manganese, and zinc become unavailable. Iron deficiency - yellowing between the veins of new leaves while old leaves stay green - is the classic sign of overly alkaline soil.
The fix for low pH is lime; the fix for high pH is sulphur or acidifying organic material like pine needle mulch. Neither works overnight. Budget 3 to 6 months for meaningful change, and re-test before applying again.
Organic Matter Percentage
This measures the proportion of your soil that consists of decomposed or decomposing organic material. It's one of the best overall indicators of soil health.
- Below 2% - genuinely poor. Soil will be hard to work, drain poorly or too fast, and won't hold nutrients well. A consistent programme of compost and organic mulching will improve this over 2 to 3 years.
- 2 to 4% - adequate but improvable. Most garden soils sit here. Keep adding compost annually.
- 4 to 6% - good. Soil biology is active and nutrient cycling is happening. Continue what you're doing.
- Above 6% - excellent. Rare in most gardens; common in long-established beds with years of consistent composting.
Organic matter improves almost everything: water retention, drainage, aeration, nutrient-holding capacity, and biological activity. It is the one number worth building toward over the long run.
Nitrogen
Nitrogen is what plants use to build leaves and stems. It's the nutrient most responsible for lush, green, vigorous growth - and the most mobile nutrient in soil. It moves with water, leaches readily, and fluctuates with temperature and microbial activity. A soil test gives you a snapshot, not a reliable fixed reading, because nitrogen levels can shift within days.
- Low nitrogen - plants grow slowly, older leaves turn pale yellow starting from the bottom of the plant upward, stems are thin. The whole plant looks underfed and washed out.
- Adequate nitrogen - steady, healthy green growth appropriate to the plant and its stage of development.
- High nitrogen - excessive soft leaf growth, often at the expense of flowers and fruit. Tomatoes go all-leaf and don't set fruit. Carrots produce spectacular tops and tiny roots.
Because nitrogen is so dynamic, soil-building approaches (compost, cover crops, organic mulches) are generally more reliable than single large applications. The article on fertility strategy by plant type covers how to manage nitrogen for different crops throughout the season.
Phosphorus
Phosphorus drives root development, flower initiation, and fruit and seed production. In early life, seedlings with good phosphorus establish faster; in maturity, flowering plants with adequate phosphorus set fruit more reliably.
- Low phosphorus - slow root establishment, delayed flowering, purple or reddish coloration on leaf undersides or stems (especially in cool weather).
- Adequate phosphorus - normal root and flowering development, reliable fruiting.
- High phosphorus - this is the most common finding in long-cultivated garden soils. Compost and organic matter naturally contain phosphorus, and it accumulates because it doesn't leach the way nitrogen does. High phosphorus doesn't directly harm plants, but it blocks the uptake of zinc and iron, and it suppresses mycorrhizal fungi - the fungal networks that extend plant root systems. If your phosphorus is high: stop adding phosphorus-rich amendments, focus on compost and organic matter instead, and give it time to be used down.
More phosphorus is not better. Over-applying it is one of the most common mistakes in home gardens, and it usually goes undetected without a test.
Potassium
Potassium doesn't build specific structures the way nitrogen builds leaves or phosphorus drives roots. Instead it regulates internal plant functions: water movement, enzyme activation, sugar transport, and stress responses. It's the nutrient most associated with resilience.
- Low potassium - leaf edges turn brown or scorched, plants look stressed without a clear reason, fruit quality is poor, plants are more susceptible to disease and frost damage.
- Adequate potassium - robust growth, good disease resistance, fruit with developed flavour and structure.
- High potassium - uncommon in most soils, but possible in heavily amended clay soils. Excess potassium can block magnesium and calcium uptake.
Using the Numbers Together
Soil tests are most useful when read as a system rather than reacted to one number at a time. A garden with low pH and adequate phosphorus needs lime - not phosphorus. A garden with high organic matter and high nitrogen needs a nitrogen-hungry crop, not more compost. Getting pH right first often resolves apparent nutrient deficiencies without adding anything else, because nutrients already in the soil become accessible once conditions are correct.
Once you know what your soil needs, the guide to fixing soil naturally explains how to address each issue in a way that builds the soil system long-term, rather than just supplying a short-term patch.