Dairy farm fertiliser and lime prioritisation after winter grazing
A 280-cow spring-calving dairy herd runs on 95 ha effective (3.1 cows/ha) near Matamata. The farm has just finished a fodder-beet winter crop grazed in situ on 12 ha of free-draining volcanic soil. A late-winter soil test shows Olsen P 22 µg/mL (target 30–35), sulphate-S 7 mg/kg (low), pH 5.6, and K within range. The effluent pond is applied to the same 12 ha. The manager has $18,000 for fertiliser and lime before the spring flush and proposes 150 kg N/ha as urea in three equal splits, 300 kg/ha of superphosphate, and 2.5 t/ha of lime. Pasture clover content is 8% and declining. Rainfall is 1,100 mm/yr and drainage is free. Cows calve in three weeks.
1. Which soil limitation should be corrected first, and how should that change the fertiliser order?
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pH is the first limit to address. At pH 5.6, aluminium activity increases and root growth of both ryegrass and clover is restricted. Phosphate is also less available because it is fixed by aluminium and iron, so 300 kg/ha of superphosphate applied now would be partly locked up and the money poorly used.
Lime at 2.5 t/ha is a sensible starting rate to move pH toward 5.9–6.2, but the actual rate depends on soil buffer capacity, so the laboratory recommendation should be followed. Order matters: lime can go on now; P should follow rather than be mixed with lime in the same application, because lime can reduce P availability if applied together. Sulphate-S is low at 7 mg/kg, so S should be included, for example through superphosphate or ammonium sulphate. Clover at 8% is low; correcting pH and S will help clover more than buying N. Nitrogen decisions come after these because an N response depends on a healthy, actively growing sward with adequate moisture.
2. Evaluate the proposed 150 kg N/ha in three equal splits, given the effluent block and the clover level.
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A blanket three-split programme is too crude for this farm. Effluent applied to 12 ha adds available N, often in the range of 20–40 kg N/ha per application depending on dilution, and the recently grazed fodder-beet block will also have elevated mineral N and a high leaching risk. Applying the same N rate across all paddocks ignores those differences.
Split N by paddock and by soil temperature and cover, not by calendar. The first N is best applied after calving when pasture cover is above about 1,400 kg DM/ha and soils are draining but not saturated. Avoid application if more than about 10 mm of rain is forecast within a day, and avoid autumn N on free-draining soil because nitrate can leach below the root zone. Reduce or skip N on effluent paddocks and on the beet block unless a measured deficit exists.
High N also suppresses clover. With clover at only 8% and pH being corrected, strategic N of 30–40 kg N/ha in early spring and after grazing is more appropriate than building the system's N dependence. The $18,000 should cover lime and S first, then targeted N.
3. Explain the nutrient-loss risk of applying P and effluent to the winter-grazed block, and what you would do instead.
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The 12 ha block is the highest-risk area on the farm. Fodder beet grazed in situ returns urine and dung in concentrated patches, and the soil is free-draining. Phosphorus moves mainly attached to sediment, so bare, winter-grazed ground is vulnerable to runoff and to preferential flow. Effluent adds both P and N on top of that, so Olsen P on this block could climb above target while the rest of the farm sits at 22.
A better judgement is to apply the P to paddocks with lower Olsen P and good pasture cover, where uptake is likely, and to defer effluent until the block has a cover crop or is not freshly grazed. Test the effluent for nutrient content rather than guessing, and prepare a simple nutrient budget to show whether P is in surplus on the block. The aim is to avoid over-application on one area rather than to hit a target with a blanket rate. If the block must receive effluent, apply at a low rate when the soil is not saturated and when a growing crop can take up the N.