Mustard Cake Fertilizer: Boost Cucumbers And Tomatoes

Mustard cake (also called mustard oil cake, or khal) is the solid residue left after oil is pressed from mustard seeds. It’s long been used as an organic fertilizer in South Asian gardening in particular, valued for two things most fertilizers only do one of: it feeds the soil, and it helps deter certain soil pests at the same time.

Nutrient Profile

Mustard cake isn’t a precisely standardized product — nutrient content varies somewhat by seed source and how much oil was extracted — but it typically falls in this range:

That nitrogen level makes it a genuinely strong organic feed for leafy, vigorous growth — comparable to or stronger than many composts — which is exactly why it suits heavy feeders like tomatoes and cucumbers.

Why It Works Well for Tomatoes and Cucumbers Specifically

• Both are heavy nitrogen feeders during their vegetative growth phase, and mustard cake’s nitrogen content supports that stage well
• The natural compounds released as it breaks down (glucosinolates) have a mild biofumigant effect, which can help suppress some soil-borne pests and nematodes — a secondary benefit most synthetic fertilizers don’t offer
• As an organic, slow-release source, it feeds soil biology over time rather than delivering one fast spike, which suits a full growing season rather than a single feeding

Two Ways to Apply It

1. The Soaking Method (Recommended for Containers)

Soak mustard cake in water for 7–15 days before applying, stirring occasionally. This ferments it, which does two things: reduces the strong smell during decomposition, and prevents the ammonia-like burn that can happen if fresh cake contacts roots directly. Use roughly 1 part mustard cake to 10 parts water, then dilute the resulting liquid further (about 1:3 with fresh water) before applying to soil around the base of plants.

2. Dry Soil Mix Method (For Garden Beds)

Work dry mustard cake into the top few inches of soil 2–3 weeks before planting, giving it time to decompose before roots are established there. Avoid applying it directly into a seed-starting mix or right against transplant roots — fresh, undecomposed cake generates heat and releases compounds as it breaks down that can stress young roots.

For a full feeding schedule that pairs with this — including when tomatoes need more phosphorus and potassium instead of nitrogen — see our guide to raised bed and container fertilizer.

Important Safety Cautions

• Toxic to pets if ingested: mustard cake (and similar oil seed cakes) have caused poisoning in dogs that dig it up and eat it directly — keep it worked into soil or in a sealed container, not left exposed, if pets have access to the area
• Strong odor during decomposition: expect a noticeable smell for the first several days after applying, especially with the dry method — this is normal, not a sign of a problem
• Can attract rodents and insects if left on the soil surface: work it in rather than top-dressing loosely, or use the soaked liquid method instead
• Not for seed-starting mixes: too concentrated for germinating seeds — reserve it for established plants or beds prepared well ahead of planting

Application Frequency

If you’re growing in containers rather than a bed, this pairs directly with our guide to the best tomatoes for container gardening, since container soil depletes faster and benefits from the liquid method’s more frequent, lighter feeding.

Looking to correct soil pH rather than boost nitrogen? That’s a different amendment entirely — see our guide to using chalk and garden lime.

FAQ