Soil health is the foundation, but soil health alone doesn't tell you what each plant needs from that foundation. Different crops draw on soil nutrients in completely different proportions, at different stages of development, and in response to different growing conditions. Applying the same fertility programme to tomatoes, carrots, and fruit trees is the equivalent of feeding a toddler, a marathon runner, and an elderly person the same meal.
This article connects soil test results and amendment choices to the actual plants you're growing. If you haven't read the guides on reading your soil test and fixing soil naturally, start there - this article builds on both.
Leafy Crops: Nitrogen-Forward
Lettuce, spinach, kale, chard, arugula, Asian greens, and leafy herbs are all growing for one thing: leaves. Leaves are nitrogen-built. These crops need a consistent, steady supply of available nitrogen throughout the growing season, with less concern for phosphorus and potassium than for fruiting crops.
The practical approach: plant leafy greens in beds where compost has been added generously, or where a previous nitrogen-fixing cover crop grew. A high-organic-matter soil with good biological activity will produce most of what they need naturally. Where nitrogen is genuinely low, a liquid organic nitrogen source - compost tea, fish emulsion - applied every 2 to 3 weeks gives consistent support without the surge-and-crash of heavy synthetic applications.
What to avoid: excess nitrogen in late season or warm weather accelerates bolting. In hot weather, ease off nitrogen feeding for lettuce and spinach, and focus on moisture and shade instead.
Fruiting Crops: Balanced, with a Potassium Emphasis
Tomatoes, peppers, cucumbers, squash, and beans all need nitrogen to establish and grow foliage in early life - but as they transition to flowering and fruiting, the balance shifts. Too much nitrogen late in the season produces lush, vegetative plants that flower poorly and set fruit reluctantly. Phosphorus and potassium become more important.
Tomatoes are the most instructive example. In the first 4 to 6 weeks after transplanting, they need moderate nitrogen to build a healthy root system and canopy. Once the first flowers appear, taper nitrogen and ensure potassium is adequate - this supports fruit development, flavour, and disease resistance. A common mistake is continuing heavy composting or nitrogen feeding into mid-summer, producing enormous plants with few tomatoes. If your tomato plants are lush and dark green but not fruiting, nitrogen is almost certainly the issue.
Squash and courgettes are similarly heavy feeders early but need balance later. They respond well to potassium - squash with inadequate potassium often produces fruit that starts well and then collapses or rots from the blossom end.
Beans and peas don't need external nitrogen at all if your soil has active Rhizobium bacteria. They fix their own. Adding nitrogen fertiliser to a bean bed actually suppresses the nodule formation that makes this happen. Focus instead on phosphorus for root and pod development, and let the plants do their nitrogen work.
Root Crops: Balanced and Restrained
Carrots, beets, parsnips, and turnips are growing for the root, not the leaf. Excess nitrogen is directly counterproductive: it drives leafy top growth at the expense of root development, and often produces forked or misshapen roots as the plant responds to uneven nutrient availability. Fresh manure or heavy compost applications immediately before root crop planting are a classic cause of forked carrots.
What root crops need is balanced, moderate fertility - enough to support steady growth, with good phosphorus for initial root establishment and adequate potassium for root development and sweetness. They do best in beds that were well-composted the previous season rather than freshly amended.
Carrots prefer a fine, moderately fertile soil. Stone-free growing conditions and consistent moisture matter more than nutrient levels for good shape and size. If your soil test shows adequate NPK, don't add anything extra before a carrot bed.
Beets are more tolerant of nitrogen than carrots and handle moderate compost additions well. They do respond to boron, a micronutrient that affects root cell formation - if you're in a high-rainfall area where boron is commonly low, a very small application of borax can make a visible difference in root quality.
Perennials and Trees: Long-Term, Slow Feeding
Fruit trees, soft fruit bushes, asparagus, rhubarb, and perennial herbs have root systems that develop over years, not weeks. They don't benefit from the kind of in-season fertility management that annual crops need. Pushing perennials with heavy nitrogen feeding causes problems: excessive vegetative growth at the expense of fruit, soft tissue that is vulnerable to disease and frost damage, and disruption of the natural growth cycles the plant needs to harden off properly before winter.
The right approach is surface mulching and long-term organic matter building, with a single annual application of compost in late winter or early spring as the plants come out of dormancy. For established fruit trees in balanced soil, that's genuinely all that's needed in most seasons.
Where specific deficiencies appear - yellowing foliage suggesting nitrogen or iron deficiency, for example - diagnose from a soil or leaf test before applying anything. Iron deficiency in fruit trees is almost always a pH problem, not an iron shortage. Fixing the pH resolves the deficiency without any iron amendment.
Young trees in their first two to three years benefit from a light balanced feed to support establishment without pushing excessive soft growth. After that, let the soil system take over. Trees that have been over-fertilised for years often perform worse than comparable trees grown in balanced soil with minimal inputs.
The Summary
- Leafy crops - nitrogen-forward, steady supply throughout the season, ease off in hot weather to prevent bolting
- Fruiting crops - moderate nitrogen early, balanced then potassium-forward once flowering begins
- Root crops - restrained fertility, avoid nitrogen excess, plant into soil that was amended the previous season rather than freshly
- Perennials and trees - surface mulch, single annual compost application, no heavy fertilisation, always diagnose before amending
The soil testing guide tells you what is in your soil. The soil improvement guide tells you how to fix the system. This article tells you how to deploy that system for the specific plants you're growing. Together, they replace guesswork with a strategy that improves every season.