#41 - Jun 2020 - Garden Stuff - © - Sandy Lang - slang@xtra.co.nz
NITROGEN June/July: Early/mid-winter. COVID’s gone. Or has it? (this was in the early days when we managed to contain the odd outbreak from managed isolation). Time to prune trees and shrubs. But do this in the dry (diseases spread in the wet). Time to lay tree chip mulch around trees to control weeds and improve the soil. Winter crops: No seeds in lockdown, so I’m late with my winter crops. But it’s still OK to sow peas and broad beans for a spring harvest. But start leaf vegetables from garden-centre seedlings to get a 4-week head start, to partly offset your 8-week (COVID) hind start. Legumes: Beans and peas are legumes. There are many others - clover, soybeans, chickpeas, lentils, peanuts. We’ve cultivated legumes for 10,000+ years. Today, they remain a vital source of food protein across the world. This makes me think of nitrogen… Nitrogen gas: The air you breath is 78% nitrogen (N2) and 21% oxygen (O2), the other 1% is a mix of other gases including 0.4% H2O and 0.04% CO2. But N2 gas is unreactive. On the other hand, you hear that plants need N fertiliser to grow. They need this N to make amino acids (NH2–), and amino acids to make proteins, and proteins to live. Mineral nitrogen: When N appears as a mineral usable by a plant, it’s not as N2 gas but usually as a nitrate NO3–. So, a plant can sit there surrounded by N2 gas but craving for some NO3– from the soil. Isn’t it a pity plants can’t turn N2 into NO3–. Nitrogen fixation: But there are some soil microorganisms that can turn N2 into NO3– these are called ‘N fixers’. They include bacteria of the genus Rhizobium. These soil bacteria invade and colonise the roots of legumes – forming visible nodules. There, they convert N2 gas into N compounds. They then trade these N compounds with the plant, receiving back energy compounds (sugars) from the plant. A great partnership between a bacterium and a plant. Pastures: So, legumes can thrive in a low NO3– soil. When they die, they release their NO3– to the soil where it’s available to other plants. Hence, there’s another great partnership, this one between clover and ryegrass on which our pastoral industry is based. NZ’s GDP from the ryegrass:clover partnership is worth $17b a year (cf. pine trees $4b, kiwifruit $0.8b and grapes $0.8b). And most of us get our dietary protein N fifthhand - (1) Rhizobium, (2) clover, (3) ryegrass, (4) sheep/cattle, (5) us… But thirdhand if you eat beans...! ________________________________________
NITROGEN June/July: Early/mid-winter. COVID’s gone. Or has it? (this was in the early days when we managed to contain the odd outbreak from managed isolation). Time to prune trees and shrubs. But do this in the dry (diseases spread in the wet). Time to lay tree chip mulch around trees to control weeds and improve the soil. Winter crops: No seeds in lockdown, so I’m late with my winter crops. But it’s still OK to sow peas and broad beans for a spring harvest. But start leaf vegetables from garden-centre seedlings to get a 4-week head start, to partly offset your 8-week (COVID) hind start. Legumes: Beans and peas are legumes. There are many others - clover, soybeans, chickpeas, lentils, peanuts. We’ve cultivated legumes for 10,000+ years. Today, they remain a vital source of food protein across the world. This makes me think of nitrogen… Nitrogen gas: The air you breath is 78% nitrogen (N2) and 21% oxygen (O2), the other 1% is a mix of other gases including 0.4% H2O and 0.04% CO2. But N2 gas is unreactive. On the other hand, you hear that plants need N fertiliser to grow. They need this N to make amino acids (NH2–), and amino acids to make proteins, and proteins to live. Mineral nitrogen: When N appears as a mineral usable by a plant, it’s not as N2 gas but usually as a nitrate NO3–. So, a plant can sit there surrounded by N2 gas but craving for some NO3– from the soil. Isn’t it a pity plants can’t turn N2 into NO3–. Nitrogen fixation: But there are some soil microorganisms that can turn N2 into NO3– these are called ‘N fixers’. They include bacteria of the genus Rhizobium. These soil bacteria invade and colonise the roots of legumes – forming visible nodules. There, they convert N2 gas into N compounds. They then trade these N compounds with the plant, receiving back energy compounds (sugars) from the plant. A great partnership between a bacterium and a plant. Pastures: So, legumes can thrive in a low NO3– soil. When they die, they release their NO3– to the soil where it’s available to other plants. Hence, there’s another great partnership, this one between clover and ryegrass on which our pastoral industry is based. NZ’s GDP from the ryegrass:clover partnership is worth $17b a year (cf. pine trees $4b, kiwifruit $0.8b and grapes $0.8b). And most of us get our dietary protein N fifthhand - (1) Rhizobium, (2) clover, (3) ryegrass, (4) sheep/cattle, (5) us… But thirdhand if you eat beans...! ________________________________________