Root and bulb vegetables — what the family shares
Quick answer. The harvested organ grows underground, which changes two things. First, the physical condition of the soil — depth, looseness, freedom from stones and from fresh organic matter — governs quality more than any fertiliser decision does. Second, excess nitrogen is actively harmful here rather than merely wasteful: it produces leaves at the expense of the root or bulb you are growing. FAO's removal figures for this family show potassium consistently exceeding nitrogen, which is the clearest signature of a storage organ.
What unites this family
Every member fills a storage organ late in its cycle, after a leaf phase that builds the capacity to fill it. The management shape is therefore consistent: get the leaf phase right and finish it, then let potassium-supported storage proceed with nitrogen restrained or stopped.
FAO's Table 29 makes the pattern visible in numbers. Carrot root removal is given as 3.9 kg N, 1.7 kg P₂O₅ and 6.6 kg K₂O per tonne, and potato tuber removal as 3.3, 0.9 and 6.2 — in both cases potassium runs well above nitrogen, and phosphorus well below both. These are Indian data (Tandon 2004) describing what a harvested crop removes; they are not application rates and must never be used as such.
Where its members differ — and why this page is not enough
This is the most internally varied family here, and the differences are consequential. Radish finishes so fast that no in-season fertiliser programme can act within its cycle. Potato's critical stage is underground and invisible — FAO ranks stolonization and tuber initiation ahead of both yield formation and vegetative growth. Onion and garlic are shallow-rooted alliums where placement matters more than quantity and where sulphur is a flavour nutrient, not just a growth one. Beetroot tolerates sodic soil far better than any of the others.
Read the individual crop guide before acting. A programme correct for potato would be pointless on radish and misplaced on onion.
The mistake this family invites
Two, and they compound. The first is nitrogen: a grower sees small roots, applies nitrogen, gets larger tops and smaller roots still. The second is fresh manure or lumpy compost before sowing, which causes forking and distortion in carrot and radish — a physical defect no nutrient corrects, and one that appears weeks after the decision that caused it.
Crops in this family
- Carrot
- Radish
- Beetroot
- Potato
- Onion
- Garlic
Each has its own guide in the crop library, filtered by family.
Salinity and sodicity are not the same measurement. Several members of this family appear in FAO's Table 26 of tolerance to exchangeable sodium percentage — beets at ESP 50–60, garlic 25–30, onion 20–25. ESP describes sodicity. It is not ECe, which describes salinity. A crop tolerant of one is not necessarily tolerant of the other, and reading the ESP figure as permission to irrigate with saline water is a serious and expensive error. Measure both. A visual symptom alone is not a confirmed diagnosis.
Sources
FAO — Fertilizer and Plant Nutrition Bulletin 16, Table 29 (removal, Tandon 2004), Table 26 (ESP tolerance; Tyagi 2000, Gupta and Abrol 1990), Chapter 6 (critical stage order for potato), Chapter 3 (sulphur and pungency in onion; manganese toxicity in potato), Chapters 4 and 7. Accessed 27 August 2026. No rate, planting date or irrigation quantity is stated.
Last reviewed 27 August 2026